Structures Congress 2009: Don't Mess with Structural Engineers: Expanding Our Role

This Paper presents the planning, design and construction experience of P.E. Structural Consultants, Inc. and URS Corporation structural design team on the 183A Turnpike Project in Austin, TX. An overview of the toll road project is provided and unique structural and aesthetic design solutions for the bridges and related structures are highlighted. Lessons learned through the Design-Build Delivery Method are offered.

Cross-frames are essential to the stability of steel girder bridge systems. They help resist lateral torsional buckling during construction as well as prevent failure under horizontal loading conditions. However, due to fabrication complexities, construction fit-up issues, and service fatigue problems, these braces often contribute a large percentage to the overall bridge cost. Furthermore, in bridge systems with skewed supports, cross-frames can transmit large forces between members due to differential deflection of adjacent girders.

This paper discusses performance based design issues that were considered during a Phase I study of structural safety associated with the proposed implementation of a high speed MAGLEV guideway. In the current paper, the concept of PBD implies explicit (as compared with implicit) imposition of probabilistic design limits, but does not preclude the possibility of partial safety factor design for certain limit states, provided the relevant factors are explicitly assigned based upon target reliability levels for those limit states. Neither is it assumed that only a partial safety factor format is applicable for all relevant limit states. In some instances, simulation based design assessment such as widely discussed in the literature may be the only reasonable approaches.

Trapezoidal steel box girders, commonly known as tub girders, have become more and more popular on the application of urban highway bridges due to their high torsional stiffness and the aesthetic appeal of their clean lines. Although much research has been done recently for better understanding the behavior of trapezoidal steel box girders, the findings are yet to be incorporated into design criterion. In addition, few publications were found in explaining the overall design process. Hence, with limited resources, designing a trapezoidal steel box girder for the first time can be challenging for a bridge engineer. The objective of this paper is to share with other engineers the authors' experience in the design of a simple span trapezoidal steel box girder bridge for the Spur 366 Extension Project located in Dallas, Texas. A project description and a design approach will be given first. A discussion of the framing considerations will follow and then the framing plan and typical section of the project will be introduced. Finally, the MDX software model and the designs of top lateral bracing, end support diaphragms and the elastomeric bearing will be briefly discussed.

Analyses of existing bridges are conducted for different reasons such as load rating and retrofit simulations. It is desirable to have full 3D finite element models for analysis purposes; however, development of such models may not be feasible since these 3D models require considerable time, effort and expertise. As a result, AASHTO provides practical methods for the analysis of the bridges. The basic approach which is commonly used by design offices, DOT engineers and other practitioners is the beam line analysis using distribution factors. The recent AASHTO LRFD Bridge Design Specifications provides formulations where span length, beam spacing, beam stiffness and slab thickness are used to obtain girder moment and shear distribution factors.

Rivers freeze in most North America areas in winter season. The thick ice sheets formed around bridge piers will not only change the constraint or boundary conditions of bridge structures, but also generate great forces on bridge piers under earthquake loading. The ice-pier interaction was deemed as the main reason that caused failure of bridges in South-central Alaska during the 1964 Great Alaska Earthquake. Figure 1 shows two examples of the damages occurred to the highway bridge piers. River ice increases the lateral constraint stiffness to bridge piers, hence reduces its free length and causes the formation of short piers. Short piers are notoriously known for their much weaker resistance to shear force than long piers. The interaction mechanism between ice sheet and bridge pier has to be understood in order to investigate its impact on the seismic performance of highway bridges in cold regions. This paper presents the interim results from the analysis of river ice-bridge pier interaction process.

The correlation properties of the wind (expressed as wind coherence in frequency domain representation) are important for precise prediction of response to wind for flexible, line-like structures such as long-span bridges, telecommunication towers, wind turbine towers and transmission lines. Although extensive research including field measurements has been carried out in the past, there is still a lack of data that defines complete correlation between the components of wind turbulence. Whereas correlations in the along-wind mean direction are more or less clearly understood and well defined, there is a certain shortage of full scale data in the cross-properties among along-, vertical- and across-wind directions. It is therefore desirable to obtain more information related to these important wind parameters based on field measurements. The objective of this paper is to provide more information on this subject based on two in-situ wind measurements. One set of data was obtained during the measurements undertaken for the New Cooper River Bridge, Charleston, South Carolina. The second set is the wind measured on the experimental line of Hydro-Québec Research Institute (IREQ). The first site is characterized by open water for wind directions normal to the bridge crossing, i.e., close to south and north winds, whereas the second site was an open terrain.

The design of curved I-girders is complicated due to the many performance stages that must be evaluated, including erection, construction, and in-service. Evaluation of girder stability during early stages of construction is particularly complicated due to the limited presence of bracing. In addition to stability issues during girder lifting, guidelines for the necessity of shore towers during erection are also not available. The Texas Department of Transportation has funded a research project to investigate girder stability during early stages of construction. Results from this research project will be used to develop guidelines for the evaluation of girder stability during early construction stages as well as proportion the girder cross sections for safe and economical construction. The research includes: (1) field monitoring, (2) a parametric study using finite element analysis, and (3) the development of a user-friendly finite element analytical software that will serve as a tool for designers. Field studies have been successfully conducted on a bridge that is a part of the SH 130 Turnpike Project near Austin, TX. Data was collected for the validation of a 3-D finite element model. Recommendations were made to provide guidance for safely lifting horizontally curved steel I-girders. This paper will provide a summary of results from the field studies that have been used to validate finite element models of the curved girders during girder lifting. A method for calculating the rotational deformation of curved I-girders during lifting and results from the parametric investigations that are currently underway will also be discussed.

The Tiaoyue Bridge in Dalian, China is a well-designed structure in which the cable system is designed as a space reticulate structure. For each cable plane, four main cables are combined by four huge cable clamps. Since this type of self-anchored suspension bridge was adopted for the first time, some innovative and unique problems were presented in the design and construction control. A new method named analytical element method (AEM) was introduced to obtain the optimum solution of the cable configuration, which is difficult to determine by conventional finite element method. It was also confirmed that the control method of stress-free status is appropriate in nonlinear elastic systems. As a result, the simulation of the construction control of this bridge was remarkably simplified in this way.

In November of 2007, Pennoni Associates was awarded a historic wooden covered bridge rehabilitation project by a State Department of Transportation. Originally constructed in 1866, the historic covered bridge is listed on the National Register of Historic Places and spans 104'–6" over a state classified scenic river. The original scope of the project included removing and replacing deteriorated wood members as well as repairing the limited capacity steel superstructure that had been installed in 1977 as part of a bridge rehabilitation project. During preliminary design, the project scope was expanded to include replacing the existing superstructure with one that is capable of carrying current LRFD AASHTO live loads and providing a geotechnical analysis to identify abutment and subsurface design parameters. All the proposed project work required review by the State Department of Transportation as well as the State Historic and Museum Commission for authenticity compliance. The proposed work was not permitted to visually alter the appearance or historic character of the bridge. Currently the project is in the final design phase with construction anticipated for the spring of 2010.

One of the most promising fields of finite element model updating involves using data obtained from the modal response of a structure to update a finite element model. The University of Central Florida (UCF) has constructed a scale model of a two span bridge. Using an impact hammer and accelerometers mounted on the structure, the first 17 natural frequencies and mode shapes were identified. Using two different residual error functions, a subset of the measured modal data was used to verify and update a finite element model of the structure so that it better reflects the behavior of the system. The results of the updating process are presented.

The Florida Department of Transportation owns and operates the largest number of movable bridges in the U.S. The 98 movable bridges in Florida are complex structures utilizing machinery to open a portion of the bridge allowing for the passage of waterborne traffic. The majority of the movable bridges in Florida are of the bascule type, having interior spans called "leaves" that rotate upward and away from the centerline of the waterway thus providing clear passage. Movable bridges are commonly used over the waterway especially in flat terrain. These bridges also present significant drawbacks and problems associated with the operation and performance. Movable bridge rehabilitation and maintenance costs are considerably higher than that of a fixed bridge. Deterioration is a concern since they are located over waterways, and often close to the coast, which constitute conditions suitable for corrosion, causing section losses. Deterioration and damage is also observed due to moving parts, friction and wear and tear of the structural and mechanical components. Fatigue can be a problem due to the reversal or the fluctuation of stresses as the spans open and close. If there are breakdowns, these cause problems for both, land and maritime traffic. Maintenance costs associated with the operation system and mechanical parts require special expertise, and may cause extensive repair work. Finally, difficulty in repair works is an issue for movable bridges. A minor or major malfunction of any component can cause an unexpected failure of bridge operation. Electrical and mechanical problems may require experts and may be difficult and time consuming to fix.

Structural identification (St-Id) offers wide ranging benefits to infrastructure owners by providing insight into the safety and performance of their bridges. St-Id generally refers to the construction of field calibrated models of a structural system by correlating simulated and measured responses. The Drexel Intelligent Infrastructure Institute (DI3) has applied St-Id to numerous bridges over the past decade. These applications have ranged from long span (arch, suspension, and truss) landmark bridges to undocumented short span reinforced concrete slab, t-beam, and arch bridges. Through these experiences, DI3 researchers have gained an appreciation of the benefits of St-Id related to decision-making, and the distinctions between applications to short and long span bridges. The application of any St-Id begins with the formulation of a specific objective that will drive the study along with the structure type, which may limit the experimental and analytical tools to be employed. St-Id, founded in the model of the scientific method, is applicable to a wide range of structures and can inform a range of objectives. Every bridge/owner combination presents a unique situation which will have its own characteristics, requirements and deliverables. There are several other factors that can affect the feasibility and reliability of any St-Id, and these include: (a) relevant responses used to assess safety and performance, (b) owner motivation and resources, (c) impact of structural and population scales, and (d) reliability of results and owner expectations. The objective of this paper is to highlight how a St-Id is defined by the constraints set forth through the owner, structural type (long or short span), and expected life of the structure, and how these constraints delineate what type of modeling, experimental methods and potential outcomes that are applicable. This comparison will be presented an illustrated through a discussion of two St-Id case studies: a concrete arch bridge and a long span truss bridge.

There are a number of performance stages that must be considered in bridge design. The critical stage for lateral torsional buckling of steel girder bridges often occurs during deck placement. The buckling capacity is improved by installing cross-frames at intemediate points along the bridge length. Many times, standard cross-frames are used, which can result in braces that are larger than required. These stiffer braces tend to attract larger live load forces and can lead to the development of fatigue cracks around the brace locations, particularly in bridges with skewed supports. Bridges with skewed supports are often necessary because of geometric constraints of underpasses or geological issues with the terrain. Though, a major concern with skewed bridges is differential deflection, which can intensify the propensity for fatigue cracks. Using lean-on bracing concepts can alleviate these concerns by allowing designers to reduce the number of cross-frames that are required as well as minimize live load induced forces in bridges for both skewed and non-skewed supports. The Texas Department of Transportation (TxDOT) recently implemented the recommendations of a research study on lean-on bracing [Wang 2002, Helwig and Wang. 2003] for three steel bridges with support skew angles of approximately 60 degrees. One of the bridges was instrumented with strain gages on the girders and braces prior to erection. The bridge performance was monitored during placement of the concrete bridge deck as well as subsequent live loading with two trucks loaded with sand. In addition to strains in the girders and braces, girder deflections and rotations were also monitored. This paper summarizes the results from the field monitoring and demonstrates the benefits of the lean-on bracing techniques.

Forty Foot Road Pedestrian Bridge is an 80-foot long by 40-foot wide, signature bridge completed in 2007 over a five lane, Pennsylvania state highway. The bridge is the centerpiece of a "context sensitive design" highway infrastructure project to create transportation improvements through a redeveloping town center. This project: 1) Incorporates aesthetics as a priority program element early in the planning phase. 2) Integrates aesthetics into the engineering design of the structure throughout the design process — as an alternative to "applying" façade treatments after engineering. 3) Exploits aesthetic capabilities of materials and individual features by specifically showcasing concrete as the preferred structural and artistic material to achieve economy, longevity, and a seamless aesthetic between the bridge design and site elements. 4) Offers innovative engineering to create a primary architectural "fascia" beam design. 5) Sculpts the approaches to create a seamless landscape-bridge aesthetic, featuring sloped unit "paver" retaining walls supported by geogrid reinforcement.

Over the past three decades, the Texas Department of Transportation (TxDOT) and other Texas entities have used segmental bridge technology. In the early 1970s, Texas embraced this form of bridge construction with the first precast segmental bridge in the United States in Corpus Christi. Since then, Texas usage has included large precast urban viaducts in San Antonio and Austin, a large precast interchange in Dallas, and cast-in-place bridges over navigable waterways along the coast. Segmental bridge construction involves multiple precast or cast-in-place concrete elements joint together by post-tensioning. In Texas, cast-in-place segmental construction has typically used the balanced cantilever method with form travelers. Most of these bridges are three-span continuous structures of variable depth whose primary purpose is to carry a relatively long main span over what is usually a navigable waterway. The navigable waterways usually have a relatively large vertical clearance that requires a substantial amount of bridge approach to reach the requisite elevation with appropriate roadway profile grade. The approach spans typically involve prestressed concrete I-beam spans typical of most Texas bridges. In recent years, prestressed I-beam bridge construction has represented 50 to 60% of the bridges constructed with costs around $50 per square foot.

The North Texas Tollway Authority (NTTA) saw the need for a roadway crossing over Lake Lewisville to tie together the fast-growing communities, Little Elm, Frisco, and Lake Dallas on the north and south sides of the lake. Lake Lewisville is a U.S. Army Corps of Engineers (COE) flood control lake with recreational parks at various points around the edges. The proposed 1.7-mile-long, four-lane bridge would be part of an electronically tolled roadway that will reduce existing travel times around the lake. The project was developed by NTTA under the direction of their managing consultant (GEC), HNTB. AECOM was selected to be the section designer of the portion of the proposed tollway that crossed the lake. This paper will provide information on the signature design feature, the Tied Arch Span.

The City of Dallas is embarking on one of the largest and most complex urban development projects in the country. The Trinity River Corridor Project covers 20 miles of the Trinity River through the heart of Dallas and was originally developed in 1965 to improve flood protection for the downtown vicinity, the adjacent south community of Oak Cliff and West Dallas. Beginning in the mid 90s, City planners realized that the Trinity River had been a neglected natural resource and began the development of a bold and vibrant urban revitalization project that includes flood protection, environmental restoration, an urban park with lakes, rivers, trails, and recreational facilities, major transportation improvements and economic development. In May 1998, the citizens of Dallas approved the development of the Trinity River Corridor Project. At the heart of the Trinity River Corridor are two (2) bridges designed by internationally acclaimed architect, engineer and artist, Santiago Calatrava. The design of the Margaret Hunt Hill (Woodall Rodgers) bridge is complete and under construction. The Margaret McDermott (IH 30) bridge design is nearly complete and will be let with the Texas Department of Transportation (TxDOT) Project Pegasus Phase I scheduled for completion in December 2010. These bridges will reconnect north and south Dallas by linking the Trinity's banks, promoting economic development and spurring neighborhood revitalization. The Calatrava bridges will serve as the Dallas landmarks for the 21st century.

The Wolf Creek Bridge is a three-span fully composite curved I-girder bridge, designed using MDX and completed in 2006. The superstructure consists of four continuous plate girders running the length of the bridge. The radius of curvature is 260 feet at the centerline, with an overall length of 188 feet along the centerline curve subtending a total angle of 41°24'. Interesting features include bolted channel diaphragms, whose degree of fixity to the girders was difficult to assess accurately without field study, and two intermediate hammerhead piers, which possess non-negligible rotational and translational flexibility. The superstructure is tree move tangentially at the abutments, but is restrained against motion in both radial and tangential directions at the piers.

Modeling a moving load, such as a tire rolling across a roadway, across discretized elements entails a correlation of nodal excitation shape and time between the various structural elements. Decoupling the roadway from the vehicle and geometry simplification leads to model loading complexities. For example, the context of this study is in the application of modular bridge expansion joints, where tires rolling across finite width beams are modeled using zero width FE beam elements. Similarly, though plate elements can be used to model a tire rolling over a plate (or roadway), the plate finite element itself is composed of zero width rows of nodes. If the nodes are closely spaced relative to the tire footprint, the problem becomes greatly simplified — a sine wave can be used as the excitation kernel. However, if the nodes are not compact the use of a simple sine wave excitation produces higher frequency overtones resulting in an unnatural model response. The tire-rolling problem, i.e. modeling a moving load over discrete nodes, is fundamental to transient finite elements. It is in essence the same problem whether modeling a rolling tire on an expansion joint, a cover plate, a roadway, or the sun as a moving heat source on the planet earth. The derivation of an excitation kernel that can be applied to rows of evenly spaced finite element nodes such that a rolling tire load is accurately simulated is addressed in this paper. Comparisons are made showing that the theory presented shows good correlation with field test results.

Mechanical behavior of cable-stayed bridges subjected to earthquakes is one of the key issues in bridge engineering. In this study, a three-dimensional Finite Element (FE) model of the Bill Emerson Memorial Cable-stayed Bridge is developed and validated with data recorded during the M4.1 earthquake event on May 1, 2005. The model takes into account the geometric nonlinear properties associated with cable sagging and the soil-foundation-structure interaction in the approach span of the bridge. Sensitivity analyses were conducted to understand the effects of various modeling parameters and conditions. The calculated natural frequencies and mode shapes are in good agreement with the measured data. The bridge structure was verified to behavior elastically as designed according to the original performance criteria except that the out-of-plane behavior of the two H-shaped towers is near yielding under a scaled-up design earthquake from the recorded ground motions.

This paper provides practical engineers with an equation to calculate the secondary moments of prestressed concrete continuous beams. Normally the prestressed secondary moments are calculated by deriving it from the net moment and the primary moments. This involves extensive calculations and sometimes complex integration techniques but the proposed equation described in this paper will give the secondary moment directly. The secondary moment calculated using the equation is compared with the secondary moment calculated using various existing methods such as support displacement method, moment distribution method and stiffness method, and also using the structural analysis software STAAD. For the purpose of analysis, two span, three span and four span beams are considered. This equation can be applied to multiple span beams/girders. Finally the equation is used to show the principle of concordant profile where the secondary moments are zero at the intermediate supports.

Integral abutment bridges (IAB) are common types of bridges that are designed to avoid maintenance problems occurring due to corrosion, creep and fatigue at connection elements such as expansion joints, bearings and roller supports. Deterioration of these connection elements may cause corrosion in structural elements and higher bridge maintenance costs. Performance of IABs is primarily governed by seasonal thermal changes, which subject abutments and piles to large deformations. These thermal movements are dependent on the superstructure material and geometry. Past research on this type of bridge has included real time field monitoring and finite element analyses on straight bridges with and without skew. Outcomes of these research studies have been used to form the basis for several guidelines on IAB design developed by state departments of transportation (DOT). Contrastingly, research on curved IABs are scarce and since, by virtue of their geometry response of the curved integral bridges to thermal loadings are complicated, most DOTs have restricted the design of curved girders for IABs. This paper presents results from finite element modeling of curved integral abutment bridges using the structural characteristics of a curved IAB in Vermont. The bridge models differed by varying the degree of curvature of the superstructure and backfill material properties. The influence of these two parameters on IAB thermal response were quantified through comparison of longitudinal and vertical deformations of the bridge deck, abutment displacements and rotations, weak and strong axis pile moments, and backfill pressures behind abutments. The models included five different degrees of curvature ranging from 0° (straight bridge) to 50°. For each model two different backfill properties (loose sand and dense sand) were used.

This paper presents a nonlinear finite element model applied to various types of structures using a new set of constitutive models that adopt the fixed angle soften truss model and the soften membrane model. These newly developed constitutive laws can predict the concrete contribution V_c which is produced by the shear resistance of concrete along the initial crack direction. A computer code was developed specifically for application to reinforced concrete structures by modifying the general purpose finite element program FEAP. The constitutive relationships of the RC element have been developed based on the smeared behavior of cracked continuous orthotropic material assumption of concrete with the inclusion of Poisson effect (mutual effect of the two normal strains). The concrete model accounts for the biaxial state of stress in the directions of orthotropy, in addition to degradation under reversed cyclic loading. The shear mechanism along the beam is modeled by using Timoshenko beam approach. Transverse strains are internal variables determined by imposing equilibrium between concrete and vertical steel stirrups. Element forces are obtained by performing equilibrium based numerical integration on section axial, flexural and shear behaviors along the length of the element. The predictions made by the new element are in good agreement with the experimental results.

This paper deals with the seismic design and ductility evaluation of partially concrete-filled steel box columns. The effects of residual stresses on seismic capacity prediction of partially concrete-filled steel box columns are investigated. Recently, Mamaghani and his co-workers have developed a seismic design method for ultimate strength and ductility evaluation of hollow and concrete-filled thin-walled steel tubular beam-columns [1]. The method involves an elastoplastic pushover analysis and definition of failure criterion taking into account local buckling ignoring residual stresses. In this study the method is modified for the effect of residual stresses. The application of the method is demonstrated by comparing the computed strength and ductility of some cantilever beam-columns with test results. The interaction between the effects of residual stresses and some important parameters, such as width-to-thickness ratio, column slenderness ratio and height of infill concrete, on the ultimate strength and ductility of thin-walled steel tubular beam-columns, are presented and discussed.

The hot-dip galvanizing process takes place in a molten bath of zinc metal at a temperature of 840 ± 20°F. The coating process takes only a short time at this temperature, usually less than 5 minutes, so this limits the potential changes to the mechanical properties of the steel that is being galvanized. This study examined steels that are commonly used for structural assemblies, A 36 and A 572 grades 50 and 65. Cold work processes and welding procedures were performed on these steels before the hot-dip galvanizing process to verify that the immersion in the liquid zinc bath would not affect the mechanical properties of the structural steels. This study follows the procedures and testing protocol established in a previous test of the hot-dip galvanizing process. The book was the culmination of four years of intensive research conducted at the BNF Metals Technology Centre in England under the sponsorship of the International Lead/Zinc Research Organization (ILZRO). The researchers examined 19 different steels from all over the world that were readily available in the 1970's. Many of the common fabrication practices were used on these steel samples and the mechanical properties of the steel were examined after hot-dip galvanizing. The general conclusion form this report is that the hot-dip galvanizing process had no major affect on the mechanical properties of the structural steel. The present work was to test the response of three grades of structural steel that is commonly used for structural assemblies in North America to cold work, bending and welding operations typically used in fabricating assemblies with these steels. Tensile properties were determined on the as-received steel, 10% cold rolled, 40% cold rolled, welded, hole punched and bent samples. All steel grades were tested in the uncoated and the fully galvanized states. Tensile test, Charpy impact tests and hardness testing were all performed on the steel samples.

A unique application of metal straps as soil reinforcement to support the complete lateral load on a thirty five (35) foot high Full Height Abutment was constructed in Mercer County, New Jersey on the Alexander Road Bridge, an 80 foot long single span steel bridge that spans over Amtrak rail lines. Using this bridge as an example, this paper demonstrates the advantages of using metal straps to support the lateral load over a more conventional system that uses piles for this purpose. By comparing the structural system of drilled shaft foundations originally bid by the Contractor to a Value Engineered solution of using metal straps, this case study will demonstrate clear advantages of using metal straps. These advantages include significant cost savings of up to 60 percent in the foundation elements, a more reliable structural system with 40 percent fewer piles, a greatly reduced time of construction, a more predictable construction schedule, reduced impact to the patrons of the bridge, fewer disruptions to rail operations, and minimal noise during construction. The design procedure used for these straps will be explained to show how the lateral load resulting from a full-height abutment can be fully supported by the straps, and this design philosophy will be compared to the similar methods used to design a standard Mechanically Stabilized Earth (MSE) Wall. Through this case study, readers will understand that metal straps can be used on full height abutments in a wide variety of applications beyond the case study presented in this paper.

This paper presents the results of an experimental and analytical study on the improvements gained in the axial compressive behavior of confined concrete. Among several parameters affecting compressive ductility of confined concrete, including confinement mechanical volumetric ratio, shape and size effect, number of confinement layers, strength of plain concrete and ductility of confining material, the latter was found to play the most important role in determining the ultimate strain and post peak behavior of concrete. Axial compressive tests were performed on small-scale circular or square section concrete columns, which were retrofitted either by CFRP jacket or by two types of high-strength steel strips with ultimate ductilities of 1% and 7%. The axial stress-strain of confined concrete specimens was obtained by measuring axial and lateral displacements and strains. Great values of concrete ultimate compressive strains, in excess of 6%, were recorded by using ductile high-strength strips, which is much greater than the values reported in the literature. A database of results of compressive tests on concrete confined with various materials was collected from the literature. An analytical model was proposed based on results of this study and the collected database to determine the ultimate strain of confined concrete. The proposed model takes the confinement ductility into account and shows good agreement with the experimental results.

In segmental box girder bridge construction, post-tensioning is used to connect individual bridge segments together to create bridge spans. Post-tensioning tendons transmit high compressive forces to concrete bridge segments. As a result, compressive and tensile stresses develop at and near the tendon anchorages.

The Central/Artery/Tunnel (CA/T) project cost roughly 15.6 billion dollars and is the largest public works project in the history of the United States. The project originated as a result of traffic forecasts for Boston, Massachusetts indicating that by 2010 there would be at least 16 hours per day of stop-and-go traffic along Interstate 93. In addition to traffic congestion, the exiting Interstate 93 elevated roadway effectively separated the North End and Waterfront areas from the remainder of the city. Planning for the CA/T project started in the 1980's with construction starting in 1991. Construction was substantially complete by 2007. The two primary objectives for the project were the relocation of the Interstate 93 underground and the extension of Interstate 90 east to Logan Airport. In order to meet these objectives, the CA/T project included a massive amount of infrastructure including on-grade roadways, bridges, tunnels, buildings, and ancillary structures. In all, the project included approximately 7.8 miles of highway. In order to support the physical structures, the project also included extensive electrical, mechanical, and communications systems. The scale of the CA/T project has been compared to construction of the Panama Canal, English Channel Tunnel, and Trans-Alaska Pipeline.

Scour in the vicinity of bridge piers may result in a reduced capacity of bridge foundations. A complete study of bridge stability under scoured conditions requires integration of water, soils, pile systems, and bridge superstructure analyses. This paper describes an analysis performed in which soil and pile group interaction was evaluated using the Group Equivalent-Pile (GEP) method. A pile foundation supporting Kansas Bridge 45 was studied to evaluate its lateral behavior under scoured conditions. Effects of scour depth and pile head boundary conditions on the total deflections of the pile head and pile deflection profiles were examined. Results showed that scour reduced the lateral capacity of the pile group, especially when the scour depth was below the depth of the pile head. In addition, scour changed the influence depths of the deflection, bending moment, and shear force of the pile group.

According to Better Roads Magazine's 2008 annual survey of state Departments of Transportation for bridge inventory data, 24.3% of the nation's bridges are either structurally deficient or functionally obsolete. This number increases from 24.1% in 2007. The statistics shows that the nation's infrastructure continues to crumble. The collapse of the I-35W Mississippi River Bridge on August 1, 2007 apparently made the headline news not only in the United States, but also the entire world. This bridge failure is known to the public because of the size of the structure and the magnitude of the casualty. However, this for sure has not been the only bridge failure case and it will not be the last one if the bridge engineering community does not do a better job. In addition to many long span bridges like the I-35W Bridge, We also have a larger number of smaller bridges. Many smaller bridges also contain deteriorations/defects that could lead to failure of the bridges if they are not fixed in time. Bridge engineers' ability to identify problems during bridge inspection is, no doubt, critical to the safeguard of our bridge structures. Reporting a critical defect in time to the authority is also very important to avoid any possible immediate failure of bridge structures. This paper uses two cases to illustrate the importance of visual bridge inspections. One case is the failure of a column in the I-95 Viaduct in the City of Philadelphia. The other case is the rapid deterioration of non-composite adjacent box beam bridges.

Among the multiple highway structures such as box culverts, chambers, retaining walls and bridges, the latter are among the most critical components of modern transportation networks. Although these networks provide the foundation for vibrant economies, a lack of understanding persists about the role that individual bridges play in network performance when subjected to unforeseen natural hazards. This paper concentrates on bridge-soil system analyses and probabilistically investigates the complex behavior of multi-span continuous steel bridges (MSCS) typical of the central-eastern U.S. (CEUS) when exposed to earthquake-induced soil liquefaction. MSCS bridges are among the most vulnerable bridge classes owing to their bearing and abutment inability to accommodate excessive demands. Due to the large mass and relatively small bearing stiffnesses at the abutments, this bridge type experiences the largest deck displacements of all bridge types common to central-eastern U.S. Sophisticated bridge models developed in OpenSees -the computational platform of the Pacific Earthquake Engineering Research Center (PEER)- are coupled with liquefiable soil models. To account for the effects of soil-pile interaction on bridge response, nonlinear P-y springs sensitive to excess pore water pressure are embedded in the models. Uncertainties in bridge response propagating from soil liquefaction to structural performance of bridge systems are synthesized as parametric fragility functions. The developed fragility curves for complete damage show considerable amplifications on vulnerability of rocker bearings and piles. Moreover, depending on the soil profile, liquefaction decreases the fragility of bridge columns. These vulnerability assessments result in more accurate regional prioritization and maintenance programs that are useful to U.S. departments of transportation and related infrastructure agencies.

The Poughkeepsie-Highland Railroad Bridge is a 19th Century engineering marvel listed on the National Historic Register and currently under consideration by the American Society of Civil Engineers (ASCE) for National Landmark status. The first cornerstone was laid in 1873, and when it was completed in 1888, five years after the Brooklyn Bridge, it was touted as the longest bridge in the world (same time frame as Tay and Forth bridges in Scotland). As the first bridge spanning the Hudson River between Albany and New York City, it had an enormous impact on the transportation of freight in the Northeast, and played a key role in shaping today's transportation network. Railroad traffic over the bridge increased dramatically in the early 20th century, and by World War II train volumes exceeded 3500 cars/day. This rapid growth resulted in the need for immense improvements to the original bridge to accommodate longer and heavier trains. The original main span structure consisted of only the north and south trusses, but in 1906, a center truss was woven into the structure to increase capacity. A second strengthening project was undertaken in 1918 when the main span deck system was replaced.

Bridge engineers have long been accustomed to working within standards and guidelines, such as those promulgated by the Bureau of Public Roads (BPR), state highway commissions, and the American Association of State Highway and Transportation Officials (AASHTO). However, with the passage of the National Historic Preservation Act (NHPA) in 1966, engineers encountered a governmental element that was not necessarily in harmony with the existing process, nor was it managed by familiar agencies and professionals. The NHPA vastly widened the involvement of the federal government, and subsequently the state governments, in the protection of the nation's historic resources. Simultaneously, the Act inserted cultural resource management, in the form of the Section 106 process, into the established system of designing, building, and maintaining the nation's bridges. The NHPA proved to be a controversial addition to bridge management and, almost a half-century later, remains troublesome to some agencies, stakeholders, and projects. This paper presents an innovative approach to the management of historic bridges that was developed for the Minnesota Department of Transportation (Mn/DOT) by Mead & Hunt and HNTB. Mn/DOT found the task of working through the historic and engineering issues associated with managing historic bridges to be time-consuming, inefficient, and costly. Hoping to streamline this process, Mn/DOT worked with Mead & Hunt and HNTB to develop management plans for selected historic bridges that would incorporate a new collaborative approach by the historian and the engineer.

The National Historic Preservation Act of 1966, and Section 4(f) of the U. S. Department of Transportation Act of 1966 require federal agencies to have a long term preservation plan in place for historic bridges under their jurisdiction. As agents of federally funded bridge programs, state transportation agencies are tasked with implementation of a statewide preservation plan. The Texas Department of Transportation (TxDOT) is responsible for monitoring the condition of the 49,829 publicly owned bridges. Of these 32,647 are administered directly by TxDOT as part of the State Highway System (on system bridges), and 17,155 are under the jurisdiction of local entities (off system bridges). Approximately 127 on system bridges and 257 off system bridges, a total of 384 bridges, are designated historic. Because they must serve the ever increasing demands of the transportation infrastructure, most of these historic bridges are at risk of being abandoned or destroyed in the wake of federally funded efforts to eliminate structurally deficient and functionally obsolete bridges from the transportation system. In the mid 1990s TxDOT intensified the effort to implement a comprehensive and proactive program to preserve historic bridges endangered by TxDOT projects. In several initiatives, interdisciplinary teams of rank and file employees were utilized to define objectives, and develop procedures for meeting those objectives based on consensus. Different preservation strategies evolved for three basic outcomes of historic bridge rehabilitation: 1) full vehicular service; 2) reduced level of vehicular service; and, 3) adaptive (non vehicular) use. Because of these strategies, TxDOT has been able during the past decade to restore and extend the life of many of the best examples of historic bridges in the State. This paper will describe some of these successful projects to illustrate the implementation of preservation strategies.

The 1881/1910 Hays Street Bridge is a viaduct consisting of two wrought iron truss spans (one Phoenix Whipple 225-ft span, and one Pratt 130-ft span), and approximately 1000-LF of concrete approaches. This span is one of only six Whipple truss bridges remaining in the state, and one of the few remaining trusses with Phoenix columns in the country. In 1910, the City of San Antonio required the Galveston, Harrisburg & San Antonio Railway Co. (eventually part of Southern Pacific) to construct a viaduct over the railroad tracks at Hays Street. The railway company relocated the two older truss spans from elsewhere on its lines. The Whipple truss span is a Phoenix patent design using the now rare Phoenix segmental wrought iron columns with cast-iron joint blocks. It dates from 1881, when it was part of a bridge over the Nueces River west of San Antonio. The Pratt span also has Phoenix-branded components, including the floor beams. Both spans were widened in 1910 from about 16-feet to the current 25-feet. In the 1990s, Douglas Steadman, P.E., formerly president of W. E. Simpson Company in San Antonio, identified the bridge as historically significant and successfully obtained Texas Civil Engineering Landmark status for the two trusses. Mr. Steadman also led the effort to obtain grant funding and private contributions to save the bridge. The bridge is currently being rehabilitated as a bicycle and pedestrian facility by the City of San Antonio, using a Transportation Enhancement grant from the Texas Department of Transportation. Sparks Engineering, Inc. is the design consultant for the project. Plans and specifications were completed in September 2006, and the project was bid in early 2009.

In April 2002, the Commissioners of San Saba County, Texas closed the John H. Beveridge Bridge over the San Saba River on China Creek Road after 106 years of vehicular service. The Texas Department of Transportation (TxDOT) began construction of a new replacement bridge on a bypass alignment. The old bridge had been the last bridge remaining in service of dozens of wire cable bridges built in Texas by William Flinn between 1885 and 1904. Little was known about the builder before research conducted by the Historic American Engineering Record (HAER) during 1996 and 2000 uncovered documents and photos that illuminate the impressive scope and influence of his career. As mitigation for taking the bridge out of vehicular service, TxDOT agreed to restore the Beveridge Bridge on location as a heritage tourism site with pedestrian only access. Funding for the restoration was provided through a special nomination under the Statewide Transportation Enhancement Program. Under the program, construction was funded 80% federal funds and 20% state funds with the County of San Saba retaining ownership of the restored structure. TxDOT prepared rehabilitation plans, and managed construction. This paper explains the historic context of the bridge and describes the rehabilitation.

Ultrasonic testing can be useful in evaluating components of historic metal bridges, although obtaining meaningful data can be hampered by field conditions. Results from ultrasonic testing on the pins of an historic wrought iron bridge near Cimarron, Colorado are presented. This paper discusses the checkered history of the bridge, the current structural assessment, field problems of obtaining meaningful ultrasonic data, and recommendations for their solution.

The total rehabilitation of Kings Covered Bridge in Somerset County, Pennsylvania was completed in 2008, culminating an 11-year effort in which the non-profit Southern Allegheny Conservancy (SAC) established a private-public partnership to complete the project and secured 100% federal funding through two FHWA programs — the National Covered Bridge Program and the Transportation Enhancements Program. The timber bridge boasts a colorful history and provides examples of rare structural systems that are now conserved, including an unusual lattice floor joist system under the newly restored deck. Most the major historic members and much of the historic fabric remain in place. Arches that had been retrofitted earlier and tied to the lower chords of the original multiple kingpost truss system were rehabilitated using an innovative engineering strategy to extend the arches through the lower chords and create a hinged arch, that serves structurally as a Burr arch bearing on the abutment.

The Texas Department of Transportation (TxDOT) Environmental Affairs Division has developed a bridge project coordination process to ensure that bridge replacement projects comply with preservation laws and regulations and to facilitate project coordination with the State Historic Preservation Officer (SHPO). From this process, the TxDOT Historic Bridge Task Force was formed in 1996 for the purpose of developing a methodology to evaluate preservation options for on-system truss bridges that are listed or are eligible for listing in the National Register of Historic Places. Section 106 of the National Historic Preservation Act of 1966, as amended, requires TxDOT, acting as an agent for the Federal Highway Administration (FHWA), to coordinate all federally funded, licensed, or permitted bridge projects involving bridges 50 years of age or older with the staff of the SHPO. In Texas, the SHPO is the Executive Director of the Texas Historic Commission. In 2003, there were 38 metal truss bridges 50 years of age or older remaining on the State of Texas highway system. Of these 38 bridges, 33 are listed in the National Register of Historic Places. Many of these bridges do not meet current design criteria for rehabilitation because they have narrow deck widths, low vertical clearance, and substandard load capacity. In addition, the existing bridge railing systems on these bridges have not been shown to meet the current requirements for safety and strength.

Field testing of bridges is becoming more and more common these days, as the time, effort, and cost to conduct such tests continues to decreases and the benefit to be realized from the tests are demonstrated. On the other hand, numerical modeling and computation tools for the analysis of bridges continue to improve and to find their way into the mainstream of bridge engineering. The real advantage of testing and modeling comes when the two are used in combination — the test providing data for calibration of the numerical model, and the model providing insight into the behavior of the bridge that can be exploited in developing the test plan and in interpreting the test results. The paper discusses three actual projects in which testing along with modeling were or are being used together in the maintenance and rehabilitation of the structures. The examples include a steel tied-arch with fatigue cracks, an ambient vibration survey of a steel truss, and load tests and in-service monitoring of a viaduct.

Statistical analysis of the sensor network data is critical in processing the information and establishing frameworks for interpretation and comparison. Sensor networks in structural engineering applications produce small data sets in many cases, including during an event such as an earthquake when the duration of the event is limited to tens of seconds and hence the data sets are small. In the absence of abundant data, the bootstrap method is a statistical technique that can be used to generate the desired parameters. Additionally, compatibility between finite element models and the measured data is often used as a means to validate the model, the data, and the signal processing methods. In this paper the data from a scaled laboratory test of a reinforced concrete bridge specimen is used to demonstrate the effectiveness of both of these methods. A finite element model of the bridge, created in OpenSees, is calibrated such that the modal properties of the model match that of the data. Once the modal properties of the bridge are validated and a system identification model is selected that fits the data, the parametric bootstrap technique is used to generate statistical properties of the modes, including their confidence intervals.

Nondestructive testing on bridges is a large part of a comprehensive structural health monitoring program. Another key component of structural health monitoring is finite element models for evaluation of the measured responses through model updating and parameter estimation. One method of nondestructive testing is a controlled static load test, where a pre-weighed truck is placed at predetermined locations on the bridge and the response is measured via strain, rotation, and deflection measurements. The location, wheel weights, and time at each location are precisely recorded during the entire duration of the load test. However, there is another load that can have more of an impact on the bridge response than applied static load, and that is thermal loading due to temperature change. It has been observed in three different load tests at Rollins Road Bridge in Rollinsford, NH that temperature effects mask the load applied to the bridge over the duration of the load test. These temperature effects either need to be corrected for, using an empirical correction, or included into the structural predictive model to get accurate results for use in a parameter estimation algorithm and model updating protocol to determine the structural health of the bridge.

With the recent collapse of the I-35 Bridge in Minneapolis, MN, the societal challenges associated with our nation's aging infrastructures were thrust to the forefront. Throughout the recent election season, infrastructure issues, for the first time in recent memory, were mentioned alongside perennial political discussions related to health care and education. These discussions and the issues associated with infrastructure preservation and renewal have resonated at both the local/state and national levels. For example, Governors Edward Rendell and Arnold Schwarzenegger along with Mayor Michael Bloomberg formed a non-partisan coalition, backed by the Rockefeller Foundation, aimed at making infrastructure funding a national priority. In addition, in early December of 2008, President-elect Barack Obama indicated his administration would make the single largest investment into the U.S. infrastructure since the 1950s, when the interstate system was developed. While the call for increased infrastructure funding is welcomed by owners; the reality is that current civil engineering practices may not be capable of effectively leveraging such funds for preservation and renewal. The traditional practice of using conservatism in the face of uncertainty serves new design well, but provides owners with a poor picture of reality and has fueled record budget shortfalls. This challenge demands approaches capable of establishing accurate estimates of infrastructure vulnerability and risk to facilitate rational and informed decisions. Over the last decade the Drexel Intelligent Infrastructure Institute (DI3) has served numerous infrastructure owners through the use of structural identification (St-Id) to more reliably assess structural vulnerability and risk as they meet this challenge. The objective of this paper is to outline some of these experiences with a particular focus on the perspective of the decision-maker. The paper begins with an overview of St-Id, which is followed by a summary of the types of applications DI3 has been involved in. The relationship between St-Id applications and risk-based decision making is then discussed and the paper is concluded with a summary of the challenges related to using St-Id to support infrastructure decision-making.

The growth of the National Bridge Inventory database, the availability of new data from embedded sensors, in-situ tests and live load tests together with additional bridge related data, e.g. geospatial and weather data, represents an immense source of information helpful for a better understanding of bridge performance and deterioration. However, in order to efficiently exploit this overwhelming amount of information a new generation of data management and data analysis tools is needed. In this paper we describe an open, scalable, and extensible data management and data analysis infrastructure which will be established within the framework of the Long Term Bridge Performance Program (LTBP).

The Center for Advanced Infrastructure and Transportation at Rutgers University has been chosen by the Federal Highway Administration (FHWA) to execute the Long Term Bridge Performance Program (LTBPP). This program is intended to develop protocols and infrastructure for collecting and processing comprehensive bridge condition and performance data to support future bridge management programs. The resulting tools will assist managers of the nation's highway infrastructure in making better decisions in the stewardship of our highway assets. Phase I of the program involves the development of protocols and procedures for documenting long term bridge performance and demonstration of those protocols on a handful of selected structures. A significant task of this development phase is to identify the data elements that are most relevant to support the objectives of the LTBP program, examine the feasibility and economics of collecting reliable and quantitative data, and evaluate the potential for practical use of this data in evaluation of bridge performance. In this task, the team will determine what specific types of data are most relevant to the long-term objectives of the program from both the structural and the operational sense, identify relevant data that is already available and how it could be integrated, and then summarize types and selection criteria for additional data to be collected. The presentation will discuss the methodology and progress of the team in completing this program task.

The objective of the LTBP program is to collect, document, and make available high-quality quantitative performance data on a representative sample of bridges nationwide. As such, the LTBP Program will collect data, perform analyses, develop models and enhance education and practice aimed at extending bridge service life, optimizing structural systems, accelerating bridge construction, advancing the AASHTO specifications, monitoring bridge condition, contributing to national policy, and managing knowledge. Data will be collected through detailed inspections and evaluations, supplemented by a limited number of continuously monitored structures and forensic autopsies on decommissioned bridges. This paper focuses on the monitoring, testing and instrumentation portion of the program.

A comprehensive periodical bridge condition monitoring program by nondestructive evaluation (NDE) will be conducted as a part of the Federal Highway Administration's (FHWA's) Long Term Bridge Performance Program (LTBPP). The ultimate objective of the NDE monitoring will be to provide high quality quantitative data that will: 1) Enable objective condition monitoring that will be linked to bridge performance parameters, 2) Improve understanding of deterioration processes in bridges and bridge performance, and 3) Advance predictive methods and bridge management practices. The LTBPP is a far more complex program, and the stated objectives will be fully accomplished through integration of all the information received through continuous monitoring by instrumentation, periodical monitoring by NDE, full-scale load testing, bridge autopsies, data mining, etc.

Throughout the Midwest, there are a myriad of underground petroleum and gas pipelines connecting the source of the product (refineries) to distribution sites (terminals) via pump stations. Originally constructed in the 1920s to 1940s, there are isolated sections of 150 mm (6 in.), 200 mm (8 in.), 250 mm (10 in.) and 300 mm (12 in.) diameter pipelines carried overhead by cable-supported structures where the underground pipeline crosses a waterway. This work is based on the structural assessment of 52 pipeline crossings for a confidential client. The purpose of the project is to inspect the structural components comprising the support structures to determine the current condition and identify areas for future maintenance to ensure the safe operation of the overall pipeline distribution system. These steel structures consist of tower frames constructed of welded pipe tubes with vertical legs in the front and diagonal legs in the rear. Each pair of front and rear legs is connected with horizontal struts to each other and to the opposite leg on each side. The legs of the towers are supported on concrete foundations, connected through steel base plates on embedded anchor bolts. The two towers, located on each bank of the waterway, support two main catenary cables that are hung across the span with backstay cables anchored into the ground. Suspender cables hang vertically from the main cables, terminating at pipe hangers that directly support the pipeline. The structural components were inspected to determine the remaining useful life expectancy of each structure. With an accurate inspection and rating, the client can effectively manage their total inventory of 82 cable-supported structures located throughout a 14,500 km (9,000 mi) pipeline distribution system as part of an overall pipeline system-wide risk management plan.

The Texas Department of Transportation (TxDOT) has performed fracture critical inspections since 1990 using hands-on inspection methods. Up until 2005 TxDOT had used a risk based criteria that was approved by the Federal Highway Administration to set fracture critical inspection frequencies from 6 months to 60 months. It was felt that this methodology would allow the department to properly manage any risks associated with fracture critical structures and maximize the return on highway dollars. Since 2005 TxDOT has been inspecting fracture critical bridges at the frequency as mandated in the Code of Federal Regulations of 24 months. The state of the art for fabrication and design has come along way since the original concept of fracture critical inspections was proposed. Mandating rigid inspection schedules independent of magnitude of service stresses, age of the structure, material improvements, implementation of fatigue resistant details, better fabrication practices, and frequency of the loading cycles on a bridge is not a cost effective method for obtaining the desired safety. Other countries use risk based criteria to determine inspection frequencies for their bridge inventory. While TxDOT no longer uses a risk based approach to set fracture critical inspection frequencies work is still being done to show that risk based inspection frequencies are a valid and safe method for establishing evaluation frequencies.

The capacity of an existing highway bridge is traditionally quantified with a load rating factor. This factor, when multiplied by the design live load magnitude, describes the total live load a bridge can safely carry. Visual bridge inspections provide engineers with information to quantify the degradation in structural integrity of a bridge (i.e., the reduction in C). The trends in RJF over time can be employed by bridge owners to make decisions regarding bridge maintenance and replacement. For example, when a bridge is first constructed, RF=1.3 means that a bridge can safely carry 1.3 times the weight of its design live load (i.e., that C-D, the existing capacity after accounting for dead load, is 1.3 times the design live load L). If the RF decreases to 0.8 after 20 years of service, deterioration of the primary structural components has most likely occurred and rehabilitation or replacement should be considered.

In 2006 an unfortunate failure of a Lower Colorado River Authority (LCRA) bridge over an irrigation canal caused injury to an employee, a totaled vehicle, and a 100 gallon diesel spill into the waterway. An unstable foundation condition which could have been previously identified resulted in the bridge's collapse. As a part of the corrective action LCRA developed a comprehensive bridge inspection and management program. The goal of the program is to identify potential hazards and to extend the useful life of the structures through a documented inspection process. The results of inspections inform the area supervisor about the condition of the structures and identify preventive maintenance and repair requirements. This preventative program is a cornerstone for ensuring the safety of these structures which are used by LCRA personnel as well as the general public. While the program was loosely modeled on various federal and state inspection programs, this program is unique in that it considers all aspects of bridge ownership. Thus, the program incorporates legal, engineering, and maintenance aspects into one inspection and management policy.

In recent years concrete bridge structures in the USA have been experiencing an unusually high level of premature concrete deterioration. The most common characteristics of this deterioration are longitudinal cracking along the bottom flange at the prestressed beam ends, cracking in unreinforced areas of cast-in-place concrete, and general map cracking with discoloration on concrete surfaces. Further testing of these bridges has revealed the cause of the deterioration is due to Alkali-Silica Reaction (ASR). This paper addresses key aspects in the detection and management of premature deterioration due to ASR for concrete bridges. Detection methods discussed include crack and surface discoloration mapping and as well as procedures for excising samples for petrography and expansion testing. Detailed visual observations of the distressed conditions as well as laboratory test results are then used to develop a course of action to manage further premature concrete deterioration from ASR. Management methods discussed include long-term monitoring, repair strategies that reflect the severity of the deterioration, and measures to mitigate water infiltration into the concrete surfaces.

In 1968, following several highly publicized fatal collapses of bridges in various parts of the country, the Federal Highway Administration (FHWA) bridge inspection program was mandated as a part of the Federal-Aid Highway Act of that year. This mandate required that the National Bridge Inspection Standards (NBIS) be established to require states to identify bridge deficiencies in order to protect the traveling public. Over the next two decades, various modifications to the NBIS were made: the 1970 Federal-Aid Highway Act; the Surface Transportation Assistance Act of 1978; and the Surface Transportation and Uniform Relocation Assistance Act of 1987. This final federal act required states to have underwater inspections performed at a minimum of every five years if components of the bridge were in more than four feet of water. Just two years after the NBIS was established, the Occupational Safety and Health Act of 1970 brought about reform in protecting the safety of workers on the job. This act was in response to decades of poor on the job worker safety and several major fatal incidents that happened in the late 1960s. From this act, the Department of Labor established the Occupational Safety and Health Administration (OSHA) to develop worker safety standards to protect employees in the work place. Over the next several years, OSHA developed directives as part of the OSHA standard to address numerous areas of job safety. In 1978, OSHA published the directive 29 CFR 1910 Subpart T, which addressed commercial diving operations and specific safety requirements that these operations were to follow. Since 1987, divers have been following the requirements of the NBIS and adhering to the OSHA safety standards when conducting underwater bridge inspections. However, over the last two years (starting in 2005), both the NBIS and OSHA directive addressing commercial diving operations were changed. Divers must follow these new regulations in order to legally and safely perform underwater inspections of substructures of the nation's bridges. Of course, the changes make improvements on the requirements to help insure that the traveling public remains safe; that the deterioration of bridges is identified and mitigated; and those conducting the inspections perform them safely.

This paper summarizes an evaluation and cable replacement design program initiated by the Louisiana Department of Transportation and Development (LADOTD) to address deficiencies reported for the stay cables of the Hale Boggs Bridge, also known as the Luling Bridge, in Luling, Louisiana. Recent inspections of this bridge have found a variety of critical damages to the protective sheathing and exposure of the main tension elements of the stay cables. The rate of degradation is increasing and a timely action is necessary. A life-cycle-cost analysis was performed to assist the decision making for bridge maintenance and management. The results indicated that when all costs are added and both agency and users' costs are included, strategies involving repair or replacement competed closely while the cost for the Base Case was significantly higher. Based on the results of this investigation, LADOTD decided to replace all cables. The design of the complete 72-cable array replacement is believed to be the first occasion on which this process was attempted in North America. It is heavily influenced by the geometric restrictions of the existing anchorage locations, including erection restrictions and estimated costs. The replacement cables are designed for a 75-year design life incorporating the latest advancements in corrosion protection and vibration control. The new parallel-strand cable system allows individual strand installation, tensioning, inspection and replacement. Due to the suspected poor condition of the existing cables, the construction sequence incorporates temporary cables and detailed analysis of construction.

The Newport/Pell Bridge spans the east passage of Narragansett Bay and opened in 1969, connecting Jamestown and Newport, Rhode Island. The crossing, owned by the Rhode Island Turnpike and Bridge Authority (RITBA), was designed by Parsons Brinckerhoff to replace a former ferry line. The crossing consists of a suspension bridge with a 1,600-ft main span and 688-ft side spans, and approaches consisting of truss spans, dual steel girder spans, and AASHTO girder spans. Innovative methods used in the suspension bridge's design and construction included the first use of socketed shop-fabricated parallel-wire strands for the main cable, a patented anchorage block, and the use of a multilayer application of fiberglass and hardened methyl methacrylate syrup for the protective cable wrapping system. As part of RITBA's 10-Year Plan for Renewal and Replacement of the bridge, an in-depth investigation of the main cable wires and the addition of anchorage dehumidification was planned and carried out in 2007. The purpose of the investigation was to establish a baseline condition for the main cable wires by unwrapping, wedging, and examining the wires in nine main cable panels. Design and engineering services were provided by Parsons Brinckerhoff, and cable investigation peer review was performed by Ammann & Whitney. The general contractor for the anchorage dehumidification was Aetna Bridge Company, dehumidification shroud fabrication was performed by subcontractor North Sails, and main cable construction work was provided by Piasecki Steel Construction Company.

This paper reports on the replacement of nine stay cables of a cable-stayed bridge south of Dusseldorf, Germany, without traffic interruption. Due to the bridge's long service life, the stay cables showed some signs of damage and needed to be repaired. Nine of the cables were damaged beyond repair, with open fractures of single wires, so they had to be replaced. The replacement required enormous structural and technical skills on the part of all the engineers and craftsmen involved due to the necessity of keeping traffic flowing with no lane closures. This administrative requirement was successfully fulfilled. Other than slightly narrowed lanes, the bridge's traffic area remained unaffected by the replacement work, and traffic flowed smoothly throughout the project.

The Central/Artery/Tunnel (CA/T) project cost roughly 15.6 billion dollars and is the largest public works project in the history of the United States. The project originated as a result of traffic forecasts for Boston, Massachusetts indicating that by 2010 there would be at least 16 hours per day of stop-and-go traffic along Interstate 93. In addition to traffic congestion, the existing Interstate 93 elevated roadway effectively separated the North End and Waterfront areas from the remainder of the city. Planning for the CA/T project started in the 1980's with construction starting in 1991. Construction was substantially complete by 2007. The two primary objectives for the project were the relocation of the Interstate 93 underground and the extension of Interstate 90 east to Logan Airport. In order to meet these objectives, the CA/T project included a massive amount of infrastructure including on-grade roadways, bridges, tunnels, buildings, and ancillary structures. In all, the project included about 7.8 miles of highway. In order to support the physical structures, the project also included extensive electrical, mechanical, and communications systems. The scale of the CA/T project has been compared to construction of the Panama Canal, English Channel Tunnel, and Trans-Alaska Pipeline.

The collision of vehicles has been identified as one of the most probable causes of bridge failure [Wardhana and Hadipriono 2003]. Accidental collisions of vehicles with key components of bridges, such as piers, might lead to severe consequences. In particular, piers are most vulnerable to collisions from high speed heavy vehicles. Recent design guidelines have provisions to design for equivalent static loads (ESL), but the loads are sometimes underestimated [El-Tawil et al. 2005]. Furthermore, recent design guidelines and provisions for collision of vehicles with bridges do not take into account the uncertainty associated with the collision event [Staples 2007]. Key issues such as the failure modes, the actual dynamic demand on the bridge due to vehicle collision, the bridge capacity to sustain a vehicle collision, and the interaction between vehicle and bridge are not addressed in the current design provisions. This paper develops probabilistic dynamic shear capacity (DSC) models for the bridges subjected to vehicle collision. The current probabilistic models for the shear capacity of bridge piers do not take into account the effect of rate of loading [Choe et al. 2007]. The proposed model accurately predicts the shear capacity of the pier subjected to dynamic loading. The probabilistic DSC model accounts for the uncertainties in the material and geometric property of the pier. The model is assessed using data from detailed Finite Element (FE) analyses. The FE model takes into account the rate of loading, different modes of failure and realistic support conditions. The analyses also account for the variability in loading, the material properties, and stiffness properties of the bridge. The FE simulations also provide valuable information to assess the typical failure mode(s) of bridges due to collision. The developed probabilistic DSC model can be used for a reliability-based design of bridge. It can also be used to easily design or estimate the adequacy of a bridge to sustain a collision event.

Structural deficiencies due to aging is a direct indication of the increase in susceptibility of the nations bridges to seismic damage and is a matter of prime concern today. Corrosion attacks on reinforcing steel is a major reasons behind the degradation mechanism that renders the bridge to be structurally deficient. The current study focuses on the development of corrosion models to evaluate the area loss of steel reinforcement over time in reinforced concrete columns. To represent the increase in the probability of damage of the components and the bridge system to seismic events, fragility curves are developed at various periods along the service life of the bridge.

A number of major bridges have been severely damaged or destroyed by hurricane generated storm surge and waves during the last few years. Examples include the I-10 Bridges over Escambia Bay near Pensacola, Florida during Hurricane Ivan in 2004, the US 90 Bridges over Biloxi and Saint Louis Bays in Mississippi and the I-10 Bridges over Lake Ponchartrain in Louisiana during Hurricane Katrina in 2005. With global warming and sea level rise some scientists are predicting that both the frequency and intensity of these storms will increase during the coming decades. It is important that these loads be considered in both the analysis of existing bridges and the design of new bridges located in coastal waters. Even though significant effort has been devoted to the development of predictive methods for wave loads on vertical structures such as piles, seawalls, etc. horizontal, bridge span like structures have received little attention. Existing work in this area has been mainly directed at offshore platforms or open coast piers and ocean wave conditions. Wave conditions in coastal waters are different from those in the ocean and on the open coast. The periods and, therefore, the lengths of waves in coastal waters are much shorter. This results in large variations in wave-induced water particle velocities and accelerations over the width of a bridge span with corresponding variations in wave forces. This paper reports the results of a study to develop a mathematical model for predicting storm surge and wave forces on bridge superstructures. Also included is a description of physical model tests conducted in a wave tank at the Coastal Engineering Laboratory at the University of Florida to determine: 1) inertia and drag coefficients in the model and 2) information needed to predict wave-induced slamming forces.

Achieving a target performance in the design of slender, wind-sensitive tall buildings is challenging, and often the process of optimizing a structural design scheme goes through several iterations. In this paper, the enhancement of the structural design of a very slender residential tower through the implementation of a supplemental damping system is presented, and the advantages of considering the integration of damping systems in the structural schemes of dynamically sensitive towers at early design stages are discussed.

The acceleration demands on Non-Structural Components located in ductile low-rise and high-rise buildings of reinforced-concrete moment-resisting frames and steel eccentrically-braced frames are analysed under earthquake records with two design levels representing life safety or ultimate limit state, and operational or serviceability limit state. The numerical results on the response of the components are derived using a floor response spectra approach. Finally, the adequacy of the current provisions of NZS 1170.5:2004 in terms of acceleration demands for NSC with short and long periods have been verified.

Achieving a target motion performance level in the design of supertall towers is challenging, and often the process of optimizing a structural design scheme goes through several iterations. In this paper, the enhancement of the structural design of a very tall, slender, and unique residential tower through the design of a supplemental damping system will be discussed.

Modern tall buildings are expected to meet certain wind-induced drift (serviceability) and acceleration (occupant comfort) criteria to preserve the integrity of their building cladding system, interior finishes, and operation of elevator system and to ensure occupant comfort due to building motions. In light of the significance of habitability requirements of tall buildings, this paper presents a probabilistic framework to evaluate a building's habitability performance at different recurrence interval winds under the influence of parametric uncertainties. An occupant comfort based checking procedure is introduced that may lead to a practical probabilistic design procedure for evaluating the habitability of tall buildings. The perception index or probability of failure for this assessment can be related to the occupants' reactions and consequences. The consequences of building failure to meet human comfort guidelines are addressed, leading to a risk-based decision analysis framework. This will aid in establishing a rational approach to ascertain the need of supplementary damping devices.

The 600 m (1968 ft) tall Russia Tower has an extremely efficient and intuitively clear structural concept. But its unique form raised a number of key structural challenges to be addressed — all related to stability. The following paper outlines Halvorson and Partners' approach to these: applying stability concepts from 2005 AISC Specification for Structural Steel Buildings (AISC, 2005), including the newly introduced Direct Analysis Method (DM), as well as material secondary affects for concrete from ACI to assess, analyze, and design for stability in this complex, composite structure — the Russia Tower.

The Direct Analysis Method (DM) is a new procedure to address stability requirements in steel framing systems. Because the DM has important benefits over conventional methods, it is expected that the DM will replace the conventional methods as the standard method for stability analysis and design. However, currently, the interface of the DM and seismic design requirements is not well established. Stability is an important design consideration both in the absence of earthquake loads when the system ductility demand at the ultimate strength level is minimal, and in the presence of earthquake loads when the design premise is to permit controlled inelastic deformation in the system and provide large ductility capacity. Nonetheless, previous studies of the DM have not examined in depth how the DM could address seismic effects. Therefore, a study was initiated to investigate the interface of the DM and seismic design requirements, as the first step to expand the application and benefits of the DM to seismic design. The objectives of the study are: (1) to clarify how the DM addresses seismic effects; (2) to evaluate how the DM including plastic analysis, termed "direct elastic-plastic hinge analysis," addresses seismic effects; and (3) to identify research needs related to the interface of the DM and seismic design requirements. This paper describes a pilot study that partially addresses objectives (1) and (2).

With the publication of the 2005 AISC Specification [AISC 2005a], US designers were provided with a methodology that eliminated the need to calculate effective length factors when assessing the stability of framing systems. The approach, labeled the Direct Analysis Method (DAM), was initially developed to answer a simple question: if we better model the attributes that can have an effect on the strength of lateral resisting systems, can the strength checks be simplified? The development of the method focused on including the effects of initial imperfections using a horizontal notional load and the effects of inelastic response, in the form of both a general stiffness reduction and additional localized stiffness reductions for columns subjected to large axial forces. In this development, it was clear that column behavior typically classified as three-dimensional, such as flexural torsional buckling or warping, were not intended to be included in the analysis portion of the design and would consequently need to be addressed in the member checks as appropriate [Surovek-Maleck and White 2003.]

Restrained steel beams, when exposed to fire, can develop significant fire-induced restraint force. In most previous studies, the location of the fire induced restraint force was always assumed to be situated at the center of gravity of the beam cross section. In practice, the location of axial restraint force can vary depending on many parameters, including the configuration of the connection at the supports of the beam. A set of numerical studies is presented to illustrate the response of unprotected steel beam-columns under realistic fire, load, and restraint scenarios. Results from the parametric studies indicate that fire scenario, beam span, and restraint conditions have significant influence on the behavior of restrained beams under fire. High intensity fires produce high axial forces at early stages of fire exposure, whereas in mild fires, significant axial force develops only at later stages of fire exposure. Presence of axial and rotational restraint enhances the fire resistance of beams due to the development of tensile catenary action. Overall, the fire response of restrained beams generally improves when the axial restraint is located in the bottom flange.

Single plate (shear tab) connections connect a beam to a girder or to a column, and they are designed to resist only shear loads. In a fire event, the axial restraint provided by adjacent structure creates unanticipated compressive and tensile forces in the beam and thus the connection. Using finite element (FE) models with contact elements, this study examines shear tab connections under fire. The model, validated by experimental data, represents the connection as well as a portion of the surrounding structure in a composite floor framing system. By extending the model beyond the connection region, the combination of shear, axial force and moment are properly represented in the connection during the fire. Previous studies have shown that large beam rotations and tensile forces result in bolt tear out (beam bearing) or bolt shear failure. This study shows how some simple modifications in the connection model affects the response (if at all).

This paper investigates the behavior of steel beam-columns loaded in axial compression and uniform bending at elevated temperatures. The finite element code SAFIR has been used to predict the failure temperature under a determined load (M+N) assuming in a first case that the temperature within the column is uniform and in a second case that a longitudinal thermal gradient exist. The failure loads have then been interpolated and extrapolated to obtain the failure loads at 400, 500, 600, 700, 800 and 900°C. The results obtained under uniform temperature have compared with two analytical formulations, i.e. the one available from the Eurocode 3 part 1.2 (EC3) and the one established after the Buckling Curves in Case of Fire (BCCF) research project. The results of this comparison show that the BCCF formulation provides a slightly more accurate ultimate load when compared to numerical results than the one obtained with the EC3. It has been found that the ultimate load decreases in a similar way as the yield strength coefficient (K_yθ) as the temperature increases. The calculations performed with a longitudinal thermal gradient show that the evolution of the ultimate load as function of temperature follows a similar pattern than the one observed under uniform temperature.

This paper is on the compressive stress-strain behavior of unreinforced concrete under elevated temperatures from fire. Relationships are developed for the temperature-dependent compressive strength, elastic modulus, strain at peak stress, and ultimate strain of concrete by conducting a comprehensive multiple least squares regression analysis on the existing experimental data, including the effects of aggregate type, test type, and compressive strength at room temperature. These relationships are then used to produce predictive compressive stress-strain models for concrete under fire. High-strength and normal-strength concrete with normal-weight as well as light-weight North American aggregates are considered in the study. Unlike previous models, creep deformations are not included in the proposed models, resulting in baseline relationships to which time-dependent creep strains can be explicitly added in the future. It is shown that the relationships developed provide a good statistical fit to the available experimental data.

Reinforced concrete structural members possess good fire resistance properties and their performance under fire condition is influenced by a number of parameters including concrete strength, fire scenario and load level. There is limited information on the behavior of high strength concrete (HSC) columns under realistic fire, loading and restraint conditions. A numerical model, in the form of a computer program, is presented for tracing the response of RC columns under fire exposure. The validity of the numerical model is established by comparing the predictions from the computer program with results from full-scale fire resistance tests. The validated model is applied in a parametric study to quantify the effect of fire scenario on the fire performance of normal strength concrete (NSC) and HSC columns. Results from the numerical studies indicate that the fire scenario have significant influence on the performance of RC columns and higher fire resistance can be obtained under most design firescenarios. Also, the results indicate that HSC columns can provide good fire resistance performance even under severe fire scenarios.

A simple approach is proposed for predicting inelastic deflections of simply supported steel beams subjected to fire conditions. In this method, the same equations used for calculating elastic deflections of beams at room temperature are used with an equivalent flexural rigidity. Simplified equations for the equivalent flexural rigidity are developed using moment-curvature relationships of steel beams. The effect of creep on deflection, which is significant under fire, is implicitly accounted for in the equivalent flexural rigidity equations. The simplified method is validated with deflections predicted using the ANSYS computer program for a range of load ratios and heating rates. The proposed approach is simple enough to use in any fire-resistant design specification.

This paper focuses on the stability behavior of steel building structures under fire loading. A 10-story office building is designed according to current U.S. building codes and design specifications. The structural layout consists of interior gravity frames and perimeter moment resisting frames (MRFs). A 3D finite element model of the building is developed and analyzed for two fire scenarios: (1) a corner compartment of fifth story is under fire, and (2) the whole 5^th story is under fire. Structural analysis is conducted using nonlinear implicit dynamic analysis in ABAQUS. The results indicate that thermal expansion of the composite floor system produces large out-of-plane displacements and inelastic stresses in the perimeter columns. These columns, however, do not undergo inelastic buckling failure or collapse. Failure occurs due to inelastic buckling failure of the interior gravity columns because they are much lighter sections than the perimeter columns. The normalized story capacity of the building structure can be predicted with reasonable accuracy using the normalized axial load capacity of a three-column subsystem.

Two types of coupling may emerge in wind-resistant designs of twin buildings connected by a skybridge. One is aerodynamic coupling due to a high level of cross-correlation between the wind loading components exerted on the buildings [Lim and Bienkiewicz, 2008a]. The other is structural coupling, introduced by the skybridge, synchronizing the motions of vibration of the buildings [Lim and Bienkiewicz, 2008b]. The coupling issues have stimulated wind tunnel studies utilizing an expanded high-frequency force balance (HFFB) technique, originally developed and utilized in laboratory investigations of single tall buildings. To date, only a limited number of efforts employing an enhanced HFFB approach have been published in open literature: [Xie and Irwin, 1998, 2001], [Boggs and Hosoya, 2000] and [Lim and Bienkiewicz, 2007]. Xie and Irwin [1998] developed a multi-HFFB system and proposed a generalized loading model formulated in the time domain. They combined the base wind loading, the nonlinear mode shapes, building eccentricity and an assumed wind pressure scheme. The proposed framework was applied in the estimation of the dynamic wind response of a generic twin-tower [Xie and Irwin, 2001]. Boggs and Hosoya [2000] studied a two-tower structure with coupled motions and they employed six (coupled) modes in the modal superposition analysis of the vibration of the buildings. Lim and Bienkiewicz [2007] developed a dual high-frequency force balance (DHFFB) system to measure the correlated wind loading which they subsequently used to predict the coupled building response. Of main interest in their study were the wind-induced rooftop accelerations of twin tall buildings with various aspect ratios. This paper is concerned with issues of the aerodynamic interference and structural coupling of two tall buildings connected by a skybridge. A refined HFFB approach to account for such couplings was employed. The base dynamic loading, measured using a DHFFB, was employed to calculate the rooftop accelerations. The aerodynamic coupling effects were evaluated by including and excluding the cross-correlations of the wind loading components. A parametric study of these effects was carried out for a range of the inter-building structural coupling and three representative mean return periods.

Common reactions when one first sees an image of the Museum Plaza project planned for Louisville, Kentucky are wonder, confusion, and excitement. For the structural engineer, the initial reactions tended more toward disbelief, anxiety, and even a little apprehension, and that's just considering how to make the building stand up under gravity loads. When the design team started considering lateral forces—primarily wind forces—the building's complexities grew tenfold. Complex building behaviors required extensive collaboration between the structural engineer and the wind tunnel laboratory to ensure that the many possible design wind load combinations were applied to the computer models such that maximum demands on the lateral system could be considered appropriately.

For wind-resistant design of super-tall buildings, the traditionally used wind engineering techniques need to be reviewed and improved in order to meet the challenge of the associated complications. The present paper highlights some wind issues that are especially important for super-tall buildings that include 1 upper level wind conditions and their effects on wind response; 2 higher-order modal responses; 3 aeroelastic effects; 4 Reynolds number effects; and 5 aerodynamic optimizations. The current techniques in dealing with these issues are discussed.

This paper will provide engineers with an overview of the technical aspects of standing seam roof design. Guidelines for the proper specification of standing seam roof systems will be presented. Applicable test procedures will be discussed, including those required by the building code to determine wind uplift capacity and to determine the lateral support provided to purlins.

Cold-Formed Steel Framing (CFSF), often referred to by engineers and contractors as "metal studs," has seen a tremendous growth in its use in the mid-rise loadbearing marketplace, and in some markets, renewed growth in residential construction. Engineers are being challenged with new configurations and high load conditions, as members and assemblies are stretched to the limits of their structural capacity. This paper discusses some of those challenges, and how engineers, architects, and specifiers can use recent research, industry standards and technical guidance to design and build safe, economical and sustainable structures.

Current model building codes, such as the International Building Code (ICC, 2006) and NFPA 5000: Building Construction and Safety Code (NFPA, 2006), require that the risks associated with seismic activity must be addressed in design and construction of buildings in all regions of the United States. The provisions in these model codes attempt to achieve a consistent minimum probability of life safety across a wide variety of applications and options in design and construction. This probability is influenced by many different factors, including the ground motion intensity, soil characteristics, occupancy characteristics, building type, structural system used, and the amount of seismic damage the system is capable of absorbing. To address these factors, both model building codes utilize the same set of reference standards to specify the requirements for design and construction of buildings in all regions of the United States. In all cases, multiple standards must be used together to determine the full set of requirements for design and construction. For structural steel systems, there are three primary standards that must be used: ASCE/SEI 7, AISC 360, and AISC 341.

Currently it is common for steel seismic force-resisting systems, particularly concentrically braced frames (CBFs), in moderate seismic regions to be designed using a response modification coefficient, R, of three, which allows for seismic detailing to be ignored. However, this procedure has no clear basis and there is no assurance that the amount of ductility inherent in these systems justifies the selection of R = 3. This issue was studied by Hines et al. [in press] using nonlinear time-history analysis and it was determined that there may be an unacceptable probability of structural collapse under maximum considered earthquake (MCE) level seismic hazard for buildings designed with R = 3. In order to mitigate the collapse potential of R = 3 structures, it may be more appropriate to develop an alternate design philosophy for moderate seismic regions where wind typically controls the design. In this approach, CBF braces and connections are designed for wind forces, but the brace-gusset connections are permitted to fracture under seismic demands. The resulting period elongation typically reduces seismic demands, but a semi-ductile reserve system is necessary to provide collapse prevention performance under these demands. One possibility for providing this reserve system is to employ the flexural strength and stiffness of the beam-column connections in the CBF after the braces are no longer active.

This paper will discuss the effects of large deformations and stiffeners on the behavior and design of gusset plates. It will also present a procedure to predict the inherent moment resistance of beam-to-column vertical bracing connections.

The authors' firm practices in the northeast, where steel frames are routinely more cost-effective than cast-in-place concrete frames, and ordinary concentric braced frames are the preferred lateral force resisting system where architectural constraints allow. This paper discusses how Building Code and Reference Standard revisions have affected the design, detailing, fabrication and erection of these preferred structural solutions, and how research into the influencing factors behind Code revisions enhances cost-effective solutions. The design case studies are all taken from the Commonwealth of Massachusetts which, for the time period of 1997 to 2008, permitted construction projects under the 6^th Edition of the Massachusetts State Building Code, which is based on BOCA 1997 (ASCE 7-95) and certain prescriptive exceptions which result in most buildings being designed for Seismic Design Category C, including special seismic detailing consistent with the 1992 AISC Seismic Provisions. Spectral design acceleration values for Massachusetts under the 6^th Edition are based on: Sa = .12g; T = H_n^.75/50; Site Factors = 1, 1.2, 1.5, and 2. Wind forces in the Mass Code 6^th Edition are based on fastest-mile wind speeds of 90 mph with standard exposure coefficients and no breakdown of leeward and windward forces.

Recent discussions related to the seismic performance of low-ductility steel systems designed for moderate seismic regions have generated new interest in the cost-effective design of ductile systems for such regions. Concentrically braced frames (CBFs) are prevalent in moderate seismic regions, both because of their high stiffness-to-weight ratio, and because of the ease with which they can be designed and evaluated by the equivalent lateral force method. Eccentrically braced frames (EBFs) can offer the same advantages as CBFs, while also providing significant ductility capacity, and greater flexibility with architectural openings. The office of LeMessurier Consultants designed several eccentrically braced frames for prominent buildings in the United States from the 1960's through the 1980's. For these structures, designed prior to modern seismic provisions, eccentric bracing was the most efficient means to developing a stiff lateral system that accommodated the architectural program. Since the branding of eccentric bracing as a high-seismic, high-ductility system in the 1990's however, use of these schemes has tapered off to almost non-existent—restricting the potential both for working creatively with architects and for achieving moderate levels of ductility in the lateral force resisting system (LFRS). This paper first discusses the recent transition from the 6^th Edition to the 7^th Edition of the Massachusetts State Building Code and its implications for the relationship between seismic loads and wind loads. Thereafter a theoretical 9-story EBF design is developed for Boston, Massachusetts according to the 2005 AISC Seismic Provisions for the purpose of comparing its tonnage to a lower ductility CBF. Special link details are introduced that allow reduced tonnage without compromising capacity design principles.

This paper presents an ongoing research project on the seismic performance of concentrically braced steel frames of the Conventional Construction category designed according to Canadian seismic provisions. A preliminary test program performed on typical brace connections indicated that a governing bolt bearing failure mode can lead to ductile connection response. Nonlinear time history analyses were carried out for 4- and 8-storey structures to assess the force and deformation demand from earthquakes on brace connections. The building height, the location in Canada, and the site class were varied. The median brace axial forces obtained from the analyses were found to exceed the code design force level specified for non ductile brace connections, except for the buildings located on site Class C in Montreal. These structures however possessed increased overstrength due to building code provisions. The excessive force demand observed in the other structures is mainly attributed to the difference in damping levels assumed in analysis and design. For the 4-storey structures, the computed median inelastic connection deformation values are small enough to be accommodated by properly detailed connections exhibiting a bolt bearing failure mode. The 8-storey structures experienced larger deformations, in excess of the tested connection capacity, as well as marked demand concentrations over the building height.

Incorporating a rocking mechanism into steel frames, concrete shear walls, or masonry piers has been shown to reduce the amount of seismic base shear that a lateral resisting system is required to resist. Rocking structural systems have therefore been developed primarily for high seismic regions. The benefits provided by allowing rocking are, however, also advantageous in moderate seismic zones. This paper summarizes previous research on the seismic behavior of steel-framed rocking systems and examines the application of the rocking mechanism to steel frames in moderate seismic zones by analyzing an example braced frame with and without allowing column uplift.

The research field of structural control has evolved from the development of passive devices since 1970s, through the intensive investigation on active systems in 1980s, to the recent studies of semi-active and hybrid control systems in 1990s. Currently semi-active control is considered most promising in civil engineering applications. However, actual implementation of semi-active devices is still limited due mainly to their system maintenance and associated long-term reliability as a result of power requirements. In this paper, the concept of functionally upgraded passive devices is introduced and clearly defined. They can mimic the function of their corresponding semi-active control devices for various applications. They have clearly-defined counterparts, conventional passive devices, but with superior performance in certain functions. They require virtually no maintenance in applications and expect to gain wide acceptance in engineering practices.

The concept and application of seismic isolation has been well accepted worldwide as a technique to improve the seismic performance of highway bridges by lowering seismic demands. Low horizontal stiffness isolation devices, such as rubber bearings or sliders, are inserted between the bridge superstructure (deck and girders) and substructure (columns and abutments) to decouple the massive superstructure from the horizontal ground motion. The isolation devices provide flexibility to lengthen the natural period of the bridge and additional energy dissipation which, together, reduce the seismic demands. As of 2006, more than 200 bridges in the U.S. have been designed or retrofitted using seismic isolation, ranging from short single or two span highway bridges (i.e. Carlson Boulevard Bridge in Richmond, California) to the long seven-span Benicia-Martinez Bridge in San Francisco Bay, [Buckle et al, 2006a]. In this study, partial isolation refers to the concept of installing isolators over part of the bridge. Specifically, isolation devices are installed at piers or columns to separate the bridge mass from the columns, but remain absent at the abutments. Partial isolation is similar to PRSI in Taiwan except that full separation is achieved in neither the longitudinal nor the transverse direction. In some retrofit applications, partial isolation has been achieved in essence by simply not providing the required clearance at the abutments [Buckle et al, 2006b].

In this study, the natural frequency of the water sloshing wave in a rectangular tank with various sloped bottom shapes is derived from its equivalent flat-bottom tank using the linear wave theory. In this equivalency, the bottom wet length of the tank was used to replace the flat length and the deepest water depth was taken as the water depth in the natural frequency formula of a flat-bottom tank. The calculated natural frequency was validated with experimental data. The rationale for the use of the bottom wet length in the frequency evaluation of a rectangular tank of various sloped bottom shapes is demonstrated based on the finite element coordinate transformation. Different bottom lengths change the travel distance and thus the damping ratio of the water sloshing wave in the tank.

In order to mitigate the excessive isolator displacement that is usually induced by a near-fault earthquake, two new types of seismic isolators with variable stiffness (SIVS) are introduced and investigated experimentally in this study. By properly choosing the geometric parameters of the two SIVS isolators, the isolation stiffness of the isolators becomes a function of the isolator displacement, rather than a fixed value. The geometries of both isolators are defined by using six-order polynomials, so that the stiffness of the isolators has a softening behavior followed by a hardening behavior when the isolator displacement increases. The softening and hardening behaviors aim to reduce the structural acceleration and isolator drift, respectively. In order to verify the feasibility and effectiveness of the proposed SIVS isolators, a shaking table test was conducted on the prototypes of both SIVS isolators. The test results are highlighted and also compared with the theoretical ones in this paper.

The design of a passive component to provide combined spring and damping output is strongly driven by scaling issues for the discrete mechanical elements used to provide the required output forces. Multiple design concepts and technologies are available to provide damping and/or spring forces. However, consideration of the relative force requirements, life cycle, operative environment, and space envelope available usually will result in a specific type of damper and a specific type of spring being optimum. Long-span bridges provide unique challenges in shock and vibration control for the design team and component manufacturers. Bridges of this type are normally intended for a long service life in the range of 30–100 years, with current trends favoring a 50–75 year life. The functionally upgraded spring-damper isolator described was designed and manufactured for use on the world's largest cable stayed bridge-the Sutong Bridge over China's Yangtze River, completed in 2008. Analysis indicated that added viscous damping would reduce seismic deck motions substantially. During the other events listed, analysis determined that the most cost-effective solution was to incorporate a snubbing type spring element in parallel with the damper that would only engage (become active) when the damper was approaching its end of travel in either extension or compression. The spring-damper isolators for Sutong Bridge were installed in late 2007.

A slope-bottom tuned liquid damper (TLD) was previously found to be an effective wave-energy dissipater in various tests of a 1/4-scale three-story model. In this study, to ensure that the nonlinear feature of a prototype TLD is properly represented, the 1/2-scale model of a three-story steel frame structure was tested on the shaking table to study the seismic performance of a large-scale TLD with three sloped bottoms: V-, W-, and Arc shapes. In comparison with small-scale model tests, the large-scale model tests more effectively demonstrated the seismic effectiveness of a slope-bottom TLD in the mitigation of structural responses under strong earthquake excitations. The V- and Arc shaped bottoms of a TLD are more effective than the W-shaped bottom due to more liquid engaged with sloshing motion.

In this paper a new adjustable passive fluid spring and damper (APSD) is studied experimentally. The APSD is capable of varying the stiffness and damping independently; thus, the device can be used as an adjustable damper or as an adjustable fluid spring or as a combined stiffness and damping device. This allows a range of possibilities to modify structural stiffness and damping properties by placing the APSD at appropriate locations in the bracing system of the structure. In this paper the experimental results of the APSD under cyclic and ramp loading are presented. The versatile properties of the APSD are shown by means of experimental for displacement loops.

Where architectural and engineering design approaches intersect and communicate with each other, usually a more innovative building structure can be created. We present three recent examples of tensioned fabric structure projects, where the two disciplines inspire each other. In these examples, structural engineering provides a basis for aesthetic design with form-finding and a clear definition of technical boundary conditions. Unlike conventional structures, which carry loads in bending and have a fixed given desired form, fabric structures take the shape according to the force flow and hence the architect or artist can not initially draw up the final shape of the project. Therefore, engineering support forms the beginning and various `design loops' need to be performed to give the design credibility and stay within given boundary conditions.

This paper is excerpted from the chapter on connection design in the draft report of the Tensioned Fabric Structures Task Committee. The full chapter contains additional commentary regarding the detailing of supporting masts and other forms of pretensioning mechanism.

Tensioned fabric structures are lightweight systems that generally have an uncommon ability to carry very high loads in relation to their self-weight. These fabric membranes are typically used as building enclosures and roofs and as such are subject to environmental rather than occupancy loads. Determination of design loads from ASCE 7 and building codes can be difficult due to unique and complex forms and the surface characteristics of these structures. Fabric membrane structures are typically non-linear in their load response, consequently superposition of load effects is not valid. The paper will review these and other challenges of applying ASCE 7 to these structures and review examples. The paper will provide insight into the topic in the second edition of the ASCE Special Publication, "Tensioned Fabric Structures".

Designers attracted to fabric structures are often intrigued with the wide range of forms which can be built. Although the range of possible forms is extensive, these are not "free-form" structures. They must rigorously conform to the physical principals which govern their behavior as limited by the characteristics of the materials from which they are built. Exploration and exploitation of the nature of tensile behavior is the basis for the design of fabric structures. The use of structural systems and materials which resist loads in tension govern their design. Ropes and cables are the simplest tensile elements. Structural forms may be generated by arranging cables between fixed boundaries. Compression struts or bending components may be incorporated into the web of tension elements. Membranes are generally made by combining or weaving numerous linear tension components. Membranes structurally act as tension surfaces. Loads applied to membranes must be resolved in their surface, as they typically have negligible compressive, flexural or shear strength. Cables may be used to establish membrane boundaries or "reinforce" a membrane by dividing the surface into manageable portions.

Fabric structures in the form of tents have been around for thousand of years, but it is only within the last fifty years that the design and construction of tensile membrane structures have begun to surface as a viable, permanent building method. In this paper I will discuss the three predominant architectural membranes used for tensile membrane structures. These being: (1) LDPE coated, woven HDPE scrim using tapes (2) PVC coated, woven or knitted polyester fabric (3) PTFE coated, woven fiberglass fabric.

This paper reviews and compares the results of ASTM Standard fabric tests and my Bi-Axial Rip Test (BART) to introduce a formula that can be further developed to provide for factors of safety (FOS) in designing tensile fabric structures that take into acount fabric tears and tear propagation. Tensile ruptures of fabrics are rare. Actual allures occur at stress concentrations near fixed locations, along seams or are the result of tear propagation subsequent to a puncture or snag. In practice tensile fabric structures are typically designed using a single set of FOS against fabric tensile failure, regardless of fabric type, of four (4) for snow load, five (5) for wind load and eight (8) for prestress. Using ASTM Standard test results, the calculated ratios of tensile strength per inch compared to the trapazoidal tear resistance (Sst/Rult) indicates that fiberglass fabrics are high (Sst/Rult varies from 6.5 to 12.5), low in polyethylene and ePTFE fabrics (2.0 to 2.8) and for polyester somewhere in between (2.8 to 6.5). The Trapezoid Tear Method ASTM D4857 (Old designation ASTM 5106) is typically conducted where a load is applied to a cut within the fabric plane at a test specified angle. A better test of the actual behavior of a fabric in a tensile fabric structure may be the mono-slit test in a uni-axially loaded cut fabric or even better a bi-axially loaded cut fabric. Comparing the mono-slit and bi-axial test results to the trap-tear tests may enable one to develop a formula for comparing fabrics when only given the strip tensile and trap-tear strengths of a fabric. This formula can then be used to chart the factor of safety required so that a tear of a certain length will not tend to propagate. Developing and using this formula should make for designing safer structures.

Buckling Restrained Braced Frames (BRBFs) have shown their value in building structures for many years. The benefits include reducing seismic demand for all structural components, providing a predictable seismic demand and simplifying repair in the event of a large earthquake. The non-building structures community and the power plant design community, in particular, have generally not utilized the BRBF because of concerns including lateral drift and excessive non-linear displacement. This paper will explore the application of this system for non-building structures and will discuss a specific application for a coal fired power plant boiler building in an area of high seismic demand.

Modular composite (MC) walls consist of steel plates on the exterior to serve as stay-in-place formwork and equivalent rebar. This type of construction is attractive for reducing the schedule and field labor associated with massive concrete construction projects (e.g., nuclear facilities). This paper presents analysis and design approach for MC walls subjected to simultaneous thermal loading and transverse applied loads. The applied loads can be due to accident pressure or a seismic event. Thermal loading can be due to severe temperature gradient across the wall thickness. Such gradient can be determined by performing heat transfer analysis. ACI 349 provides a simple treatment for determining the effects of temperature gradient on a conventionally reinforced concrete wall. For MC walls, similar techniques are not available that account for actual behavior under thermal and mechanical loads. Based on experimental and analytical research, the authors have developed an approach for determining the thermally induced moment on MC walls. Unlike the ACI method, the proposed method does not require determination of an equivalent (approximate) linear thermal gradient; rather, the actual thermal gradient, which has a sharp nonlinear profile within the first few to several inches of the wall thickness, can be directly used for determination of the thermally induced moment. For structural analysis of walls/compartments subjected to combined thermal and mechanical loading, the authors have developed an approach that takes into account the self-limiting nature of the thermally induced moment. The subsequent design of the walls is thus not unduly conservative.

This report looks at the benefits of using CFT braces and the relevant issues associated with their design, analysis, fabrication, and constructability. It is concluded that the use of CFT braces is an attractive option in terms of steel material cost and reduction of erection labor/time.

Natural draft cooling towers and chimneys of solar updraft power plants have many structural properties in common: They are dominated by thin ring-stiffened shell structures made of reinforced concrete, they transport by their internal updraft warm air into the atmosphere, and because of their height, gale actions play the most important role in the design.

Presently in the West of Germany, several thermal power plants are renewed to improve their efficiency. These new power stations are fuelled either by hard coal or by lignite; the maximum net electric capacity per power block is up to 1.100 MW. Also since the re-unification 1990, Germany had to renew a series of thermal power stations in its East, mainly fuelled by lignite. All plants are equipped with high-rise (wet) natural draft cooling towers (NDCT) up to heights of 200 m, mainly for environmental and for economical reasons. With these NDCTs, the net electric degree of efficiency of an entire power block lies closely below 45%. Forerunners of such high-efficient NDCTs serve at French, German and Swiss nuclear power plants. Consequently, Germany presently is the only nation worldwide with continuing design and construction experiences in NDCT-technology in the last 20 years. Many of these NDCTs are structurally designed by Kraetzig & Partners Consultant Engineers, Bochum, others verified by Kraetzig & Partners' State Authorized Checking Engineers (Prüfingenieure). All authors of the present contribution are cooling tower experts associated with Kraetzig & Partners, two of them CE professors at German Universities. Figure 1 introduces the reader to our world-largest lignite power block at the RWE Power Station Niederaussem, equipped with the world-highest NDCT of 200 m of total height.

There are a very limited number of case studies investigating the response of reinforced concrete chimneys subjected to strong ground motions. On August 17, 1999 Izmit earthquake, collapse of a 115 m (377 ft) reinforced concrete chimney in Turkey was a very rare experience. This study investigates the cause of failure. The seismic demands imposed on the collapsed chimney and a similar survived chimney at the same site was calculated using the ground motions recorded at a nearby station. Structural capacities of the inspected chimneys were compared with the seismic demand. Flexural capacity of the collapsed chimney was exceeded at the duct region. This prediction matches with the inspections after the earthquake. In addition to low ductility and inadequate capacity due to a large duct; splice failure, insufficient concrete cover, lack of confinement were the other significant factors potentially involved in the failure.

This paper evaluates chimneys designed for earthquake and wind forces per ACI 307-98 and the revised earthquake loading provisions of the ACI 307-08 standard. ACI 307 standard presents a comprehensive set of guidelines for analysis, design and detailing of reinforced concrete chimneys in the United States. Three chimneys of various heights and diameters with multiple openings of various sizes designed per ACI 307-98 are analyzed using 3D linear finite element analysis program GTSTRUDL/ANSYS to check adequacy of the standard. It is shown that chimneys designed per ACI 307-98 standard would invariably result in either insufficient strength and/or detailing around openings which can only be captured by a 3D finite element analysis approach [not by beam element analysis combined with sectional design approach proposed in the ACI 307 standard.] This overstress and lack of confinement detailing around openings will likely result in progressive degradation of structural performance of the chimneys as it responds to the multiple and reverse cycles of wind or earthquake forces. It is recommended that a 3D FEM analysis should be performed, in the most critical direction of the chimneys with openings, in order to estimate the possible stress overload and need for confinement of critical regions of RC chimneys.

In November 2008, ACI published a new document, "Code Requirements for Reinforced Concrete Chimneys (ACI 307-08)". (Ref. 1) This new document replaces the existing chimney design standard, "Design and Construction of Reinforced Concrete Chimneys (ACI 307-98)". (Ref. 2) The new chimney code incorporates a number of major technical revisions. One of the more significant changes is pertinent to seismic design provisions. The new seismic design provisions include several key changes from the previous edition: namely, selecting the design base earthquake as 2/3 of MCE (Maximum Considered Earthquake), including effects of soil amplification in design response spectra, use of a response modification factor (R) of 1.50, considering the P-delta effects for chimneys of seismic design category D, E and F, and adding a seismic clearance criterion between the concrete chimney shell and chimney liner(s).

During the Ismit (Kocaeli) Earthquake of August 17, 1999, a 115 m. High reinforced concrete chimney or heater stack, located at the Tüpras Refinery, collapsed. The falling debris cut 63 pipes, which contributed to interrupted production for more than 14 months. This stack was designed and constructed according to international standards and is representative of similar structures at refineries throughout the world, including those in earthquake-prone regions. It was distinguished from similar stacks at the site by a much larger rectangular opening for a flue duct, circumscribing a horizontal arc of about 50°. The opening was located about 1/3 of the height above the base and appeared to be the region of initiation of the collapse. The investigation is focused on the dynamic response of the stack due to anearthquake motion recorded at a nearby site. In this study, the results of a response spectrum analysis of the Tüpras stack and a generic U.S. stack are summarized. Then, a two dimensional nonlinear static pushover analysis of the collapsed Tüpras stack is presented using a demand-collapse comparison. Different pushover methods for the consideration of the higher mode effects, including traditional pushover procedures as well as the newly developed Modal Pushover Analysis (MPA) procedure, are evaluated. In order to consider three dimensional interaction effects, a new 3-D pushover analysis procedure is proposed and applied to the Tüpras stack. Finally, a full nonlinear dynamic analysis of the Tüpras stack is introduced to verify the pushover analysis and show more clearly the failure mechanism of the stack during the earthquake. Results are presented that show the effects of the opening and the orientation of the motion with respect to the opening. Higher mode contributions and three dimensional interaction effects are considered. The results confirm that the stack could readily fail under the considered earthquake and are also consistent with the debris pattern.

This study aims to qualitatively describe the seismic behavior of container cranes. Due to the nature of container crane construction, the portal frame in a crane is highlighted as the controlling aspect for seismic performance. Notably, the effects of the portal frame structural response and the uplift/rocking response are considered in depth. A detailed finite element model is used to consider a case-study container crane to verify and expand our understanding of the seismic behavior. It is found that the coupling between portal frame response and rocking is critical, especially when evaluating performance nearing ultimate. There is a narrow margin between first yielding and potential structural collapse. Several general strategies are proposed to design safer, more resilient cranes in the future.

Currently in California, typical stacking methods of 55 gallon Bordeaux wine storage barrels employ the use of portable steel cradle type racks to achieve stack heights upwards of six levels. Held in place by gravity alone, these stacks have no vertical connection between either the concrete warehouse supporting floor or any of the stacked barrels and racks. This current practice has been shown both in recent published research and recent earthquakes to not only violate the newly adopted California Building Code, 2007 [California Building Standards Commission, 2007], but also to pose a serious safety risk to the personnel working in wine storage facilities [Marrow, 2002; Chadwell et al., 2006; Brown, 2007; Chadwell et. al., 2008]. In an effort to mitigate hazard due to seismic excitation, research presented by Chadwell et. al, 2008 suggests the use of seismic isolation devices to change the effective response of wine barrel stacks during ground motion. These devices are in the form of steel ball bearings housed in a Teflon coated polynomial surface calibrated to maximize the protection of the stack against collapse during a seismic event. The research presented herein describes the state of an ongoing research program to provide wine barrel stack collapse hazard mitigation by use of seismic isolation bearings. What is unique to this particular application, unlike typical isolation where a balance between maximum displacements and force transmission is sought, is that seismic isolation of wine barrel stacks focuses only on collapse prevention. Excessive lateral displacements are not a main concern. The system is considered a success if after an earthquake, the stack remains standing without loss of property while maintaining life safety. This paper describes the development and details of the bearings used with emphasis on the general theoretical framework for housing of the ball bearings, the housing membrane to contain the ball bearings, and a proposed component model of the wine barrel stack Seismic Support System. With the component model parameterized from physical test data, an evaluation of the likelihood of success of an isolated wine barrel stack can be found for various seismic demands.

Earthquake-resistant design of essential infrastructure is paramount in areas with high seismic hazard and risk. Such critical infrastructure includes tall telecommunication masts which are highly nonlinear structures owing to the behaviour of their multi-level guy clusters. During earthquakes, their multi-support foundation is subjected to seismic wave delays that vary with the shear wave velocity of underlying soil. Such effects can be properly described in detailed nonlinear seismic analysis models, which are mainly used to explain exceptional situations (for example, mast failures) and for research purposes. However, the degree of complexity and sophistication these models require is too high for routine engineering design, and simplified procedures are necessary to perform design checks on the seismic vulnerability of tall masts. As a first step towards a simplified dynamic analysis procedure, this paper presents a method to obtain the equivalent dynamic properties of guy clusters. The method is based on rational cable mechanics and is verified with results from detailed finite element analysis of selected detailed models of real guy cables. However, the number of towers and the frequency range studied are limited, as this paper presents an ongoing research. More case studies are necessary to validate the method before it can be used in engineering practice.

Performance of a new full-depth precast overhang panel system for concrete bridge decks is investigated experimentally. In contrast to conventional cast-in-place deck overhangs, the proposed full-depth precast overhang system has the potential to speed up construction, reduce costs, improve construction safety and improve durability of the bridge system. To validate the capacity of the proposed overhang system, quasi-static monotonic tests are conducted on two precast overhang double-panel specimens and a conventional overhang. Factored design load limits are applied to the overhang to investigate the load-deformation capacity. Each overhang is then loaded near its edge to examine the collapse capacity and the associated failure modes—particularly the influence of panel-to-panel connections for the full-depth precast panels. When compared to the conventional cast-in-place overhang behavior, the experimental results show that the precast full-depth overhang introduces different behavior modes, largely due to the influence of the partial depth panel-to-panel connection. This reduces the ultimate carrying capacity by some 13 percent, but in spite of this reduction, behavior is satisfactory with a high margin of safety above the AASHTO LRFD design loads.

Fiber reinforcement is increasingly used to improve the strength and ductility of concrete, which is very brittle in nature. In this research, two types of hooked fibers and one type of twisted steel fibers were used as reinforcement in beam specimens. A series of experiments was carried out to investigate the effect of type and content of steel fibers on the mechanical properties of concrete, such as the compressive strength, first-crack flexural strength, and ultimate flexural strength, modulus of elasticity, flexural toughness, and ductility. Guidelines of ASTM C-1609 were used to carry out the bending tests. The compressive strengths of the concrete mixes were obtained from cylinder tests as per the guidelines of ASTM C-39. The experimental findings indicate that addition of steel fibers in concrete can slightly enhance the compressive strength and modulus of elasticity but remarkably improve flexural strength, flexural toughness and ductility.

The rapid pace of infrastructure growth in the State of Texas is responsible for the recent increase in the number of concrete structures prematurely damaged by alkali-silica reaction (ASR) and/or delayed ettringite formation (DEF). Common use of `high-sack' early strength concrete accelerated construction schedules, but also resulted in elevated alkali loadings, high curing temperatures, and substantially increased chances for ASR and DEF deterioration. The consequences of accelerated construction practices are apparent in the Houston District of the Texas Department of Transportation (TxDOT). A number of reinforced concrete bent caps, constructed within the last ten to fifteen years, have developed map cracking and spalls characteristic of ASR and DEF (Figure 1). Outdated material specifications were only recently updated to address the deficiencies and more cases are likely to be discovered in the future. Concerned that future damage may result in a loss of strength and require substantial resources for remediation, the Houston District commissioned the University of Texas at Austin to investigate the structural safety of bridge bents subjected to ASR and DEF.

Due to the increasing evidence from previous research results, the 2008 ACI Building Code allows engineers to use steel fiber reinforced concrete (SFRC) to replace the conventional shear reinforcement (i.e. steel stirrups) even if the design shear force is greater than half of the concrete shear strength. Though the new ACI provisions, marked a significant transfer from research to practice, beams constructed of steel fiber reinforced concrete are required to have a minimum amount of steel fibers of 0.75% in volume (100 pounds per cubic yards) and compressive strength not greater than 6 ksi. The ACI provisions are primarily formulated on experimental studies on non-prestressed concrete beams and majority of them had a cylinder compressive strength less than 6 ksi. However, in a prestressed concrete beam, the beneficial effect from prestressing forces could further relax the minimum required fiber volume fraction thus make the use of SFRC more economical. Further, concrete compressive strengths much higher than 6 ksi are commonly used in prestressed concrete beams to reduce the creep related issues as well as to provide larger loading capacity. Therefore, the current ACI requirements will hinder the use of SFRC in structures with prestressed concrete members made of high strength concrete. This paper presents preliminary shear test results of large scale prestressed concrete beams constructed of steel fiber reinforced concrete.

There is an increasing interest in the US in developing engineered approaches to structural fire safety as an alternative to conventional code-based prescriptive approaches. With an engineered approach, the response of a structure to fire is computed and appropriate design measures are taken to assure acceptable response. In the case of steel buildings, one of the key elements of an engineered approach to structural fire safety is the ability to predict the elevated temperature properties of structural steel. The properties of steel at high temperatures can be drastically different from those at room temperature, and accurate knowledge of these properties is a critical factor in predicting response of steel structures to fire. Although several past studies have examined elevated temperature properties of structural steel, there are some important gaps in the experimental database. Specifically, little data is available on ASTM A992 steel, the most common grade of structural steel currently used for wide flange shapes in the US. Furthermore, past studies have not adequately examined the important role of strain rate in tension testing at elevated temperatures. This paper will present preliminary results of a series of tension tests on A992 steel conducted at temperatures up to 900°C.

Loss ratio which is the ratio of the repair cost to the total replacement cost is an effective parameter to represent structural and non-structural damage caused by earthquakes. A probabilistic loss estimation framework is first presented that directly relates hazard to response and hence to losses. A key feature of the loss estimation approach is the determination of losses without need for customary demand-side fragility curves. Relationships between intensity measures and engineering demand parameters are used to define the demand model. An empirically calibrated loss model in the form of a power curve with upper and lower cut-offs is used in conjunction with the demand model in order to estimate loss ratios. Loss ratios for each of the damage states take into account epistemic uncertainty as well as an effect for price surge following a major hazardous event. The loss model is calibrated and validated for different types of welded steel moment frame buildings. The loss model is then transformed to provide a composite seismic hazard-loss relationship which is used to estimate expected annual loss for the different structures.

From October 2006 through September 2009, the US National Science Foundation is sponsoring a project on performance-based design of masonry and masonry veneer. This project deals with new masonry construction rather than existing masonry construction. In this paper, the project participants are introduced; our experimental and analytical work plans are reviewed; and the principal results to date are presented.

Radial Basis Function networks are extremely fast and require relatively small training data sets compared to other neural network methods such as back propagation. They are also less susceptible to problems with non-stationary inputs because of the behavior of the radial basis function hidden units. This makes RBF methods very attractive for realtime structural health monitoring (SHM) and damage detection. In this study, the aforementioned merits of the the RBF network will be exploited to perform realtime damage detection in buildings which are subjected to both sinusoidal and earthquake gound motion.

Development of a rational, standardized, and efficacious design methodology to produce blast-resistant designs for structural systems, such as those desired for buildings, sports arenas, bridges, air terminals, and other types of commonly built structural systems does not seem likely to happen anytime soon. The need for protective designs has engendered a host of issues needing resolution, starting with the difficulty of obtaining such a methodology. The difficultly stems from a variety of issues that need resolution, and the unsettled and embryonic state-of-the-art vis-à-vis protective design. Some of the key difficulties needing address are: the uncertainties related to defining suitable design basis threats and transforming them to design basis loads, the complexities of the structural behaviors likely to be involved for most realistic threats, and the difficulty of creating transparent and workable design methods for addressing them. The problem is further exacerbated and its resolution hampered by the absence of a governing agency or group with the charter, skills, funds, and organization to promulgate such a methodology. Efforts at developing a design methodology to date seem to be too biased toward simplicity and have too little of the rational, efficacious, and transparency aspects, and lack an over arching means to balance risk, likelihood, consequence, and cost with the development of new design technologies, methods, and engineering skills/tools that can work for a broad spectrum of risks and protection measures. This paper presents data to highlight some of the issues involved with developing such a design methodology, describing some of the elements and capabilities needed for both assessing the behaviors of an existing building's structural system that are engendered by blast, and generating protective designs for structural systems. This includes describing a range of failure modes and response behaviors not commonly considered in protective design and the design/analysis approaches needed to address them. The goal here is to foment discussion toward establishing a more rational, robust, effective, and transparent design methodology.

Building robustness can be adversely affected by local damage conditions that may initiate a failure sequence leading to disproportionate damage and/or collapse. Local damage can be defined as a loss of the load-carrying capacity of structural components that are part of the whole structure. Several steel moment frames with different connection types were studied, and important results were obtained.

Existing building codes contain some guidance on progressive collapse analysis and design. Unfortunately, in most of the U.S. building codes and standards that contain progressive collapse provisions, the available guidance is either vague, or does not define, the key issues that must be addressed. This lack of guidance has resulted in conflicting interpretations as to how one should approach progressive collapse analysis and design. The General Services Administration (GSA) and Department of Defense Unified Facilities Criteria (UFC) Progressive Collapse Guidelines [1–3] are currently the most complete sets of criteria in terms of providing usable guidance to the designer. However, only through experience can one identify the most appropriate modeling technique for a particular application.

Blast resistant design and retrofit of wall systems is of current interest. After standoff, mass and ductility are the most important parameters for blast protection. Concrete walls has the mass necessary, but could lack in ductility if not properly designed. PVC-formed concrete walls are suitable for rapid construction and expeditionary structures. The ductility provided by the PVC layers is expected to enhance the energy absorption capability, and thus improve the blast mitigation capability of the wall system. The paper discusses the mechanics of materials approach for developing the static resistance function for PVC-filled concrete walls. Static tests using a full-scale loading tree to simulate uniform pressure were used to verify the mechanics of materials models. Compared to concrete walls, PVC-concrete wall systems exhibited better energy-absorption capabilities. PI diagrams are developed in this paper to enable blast response prediction of the PVC-concrete wall system.

In response to recent terrorist activities, an increasing number of building projects specify design criteria for blast resistance, residual structural integrity, and survivability. Increases in structural system size, weight, and costs often are necessary to meet these criteria with conventional materials. However, using higher strength materials offers a way to mitigate these increases while achieving desired structural performance. The U.S. Army Engineer Research and Development Center (ERDC) investigated the effects of using vanadium micro-alloyed high-strength steel reinforcing bar coupled with high-strength concrete as a structural system. The combination of high-strength portland cement concrete and vandadium micro-alloyed rebar (HSPCC-Vr) meets or exceeds blast resistance criteria while allowing a more efficient structural design than can be achieved using conventional materials. Vanadium is widely used as an alloying element in steel production. Micro-alloying vanadium with steel reinforcing bar contributes to higher yield strengths than can be acheived with traditional Grade 60 rebar, without compromising ductility or formability. The ERDC team performed dynamic testing of one-third scale reinforced concrete panels using the Blast Load Simulator (BLS) at ERDC-Vicksburg. The panels consisted of double-mat conventional Grade-60 rebar or micro-alloyed vanadium rebar in combination with 4-ksi or 15-ksi concrete. Tests were performed using varying blast pressures and impulses to determine the performance of the different reinforced concrete panel combinations. During testing, the panels were constrained in the longitudinal (long) direction by simple supports, and were unsupported in the lateral (short) direction. Measured properties included center-span deflection, average blast pressure, and average impulse. The 4-ksi concrete in combination with micro-alloyed vanadium rebar provided the best alternative to the use of high-strength materials, but still had a lower level of blast protection. Permission was granted to publish this paper by the Director of the Geotechnical and Structures Laboratory.

Load-impulse (P-I) diagrams are graphic tools for quickly assessing the potential damage to building components caused by dynamic loading that were introduced in the early 1980s. The method presented in this paper is based on a fully nonlinear dynamic single-degree-of-freedom (SDOF) system approach and reliable dynamic behavioral and constitutive models. The damage levels in the approach are user-defined behavioral criteria within the program, such as, initial cracking, rebar yielding, strain hardening, or steel fracture. Reinforced concrete beams, reinforced concrete slabs, buried reinforced concrete boxes, and wood frame panels were evaluated under transient loadings to generate load-impulse diagrams for both flexural and various shear modes of response. This study showed that the method provided accurate load-impulse diagrams, as compared with test data.

Now more than ever, blast-resistant design is a major priority for structural engineers. Heightened terrorism has forced the need for advances in structural systems to resist the harmful effects of explosions. Much research has focused on the retrofit of existing structures located in threatening surroundings. However, for new structures, there is an opportunity to implement new technology from the start and more economically provide the required capacity. A structural system that has great benefits is reinforced masonry. One reason for this is that mass plays a large role in the resistance of a dynamic system, and masonry is very economical in terms of weight as compared to steel or aluminum. The other reason that reinforced masonry is a good choice, as opposed to unreinforced masonry, is that it provides more ductility. This allows the system to absorb a great deal of energy via plastic deformation.

This paper presents a new Blind Source Separation (BSS) method for the modal identification of structural systems. Independent Component Analysis (ICA), a popular tool to perform BSS, suffers performance issues at the levels of damping commonly encountered in structural applications. In this context, methods based on Second Order Blind Identification (SOBI) have shown significant improvement over ICA. However, traditional SOBI methods suffer due to the presence of non-stationary sources such as those that occur during earthquakes and other transient excitations. In this paper, a new technique, called the Modified Cross-Correlation (MCC) method, is developed to address the problem of output-only modal identification of civil structures. The conditions in which the problem of structural system identification can be posed as a BSS problem are discussed. The simulation and experimental results in terms of identified natural frequencies, mode shapes, and damping ratios are presented to show that the MCC method achieves better performance over traditional ICA and SOBI methods.

In this paper, a new health monitoring strategy has been suggested integrating NExT/ERA technique with a model based damage identification method. It consists of four major steps: 1) measurements of ambient vibration response; 2) modal identification using output-only techniques NExT/ERA in time domain; 3) damage localization using a new parameter subset selection method; 4) damage quantification formulated into optimization problems. Additionally, to distinguish physical true modes from non-physical comutational modes in the NExT/ERA technique, an improved consistency indicator that is better consistent in a wide range of damping ratios has been suggested through investigative parametric studies. A finite element model of a truss bridge structure has been built for numerical case studies. A series of numerical case studies with four different damage scenarios have been conducted under ambient vibration. Finally, the proposed methodology has shown potentials for either of damage diagnosis or finite model updating for model-based condition assessment of the remaining structural life.

The modal mass is defined by mass matrix and mode vector in the process of transferring a multi-degree-of-freedom (MDOF) system into a single-degree-of-freedom (SDOF) system in mode space. The modal mass has a relative value depending on the normalization of mode vector and mode vector is often normalized such that the value of modal mass is unit value for the convenience of analysis [Craig, 1981]. This modal mass is used as a representative value for the mass of structure corresponding to a specific mode in the design of mass type dampers [McNamara, 1977, Fujino and Abe, 1992]. Further, it is used to determine the magnitude of modal participation factor and to estimate how the ground acceleration is distributed to each mode in the seismic analysis [Paz and Leigh, 2004]. The modal mass of an idealized analytical model, however, is different to the actual one due to modeling and construction error. Therefore, it is required to estimate the accurate modal mass through experiments. Even though the modal mass is one of the most important coefficients that which constitute a governing equation of motion of a SDOF system like natural frequency and damping ratio, it has not gained many interests compared to natural frequency and damping ratio in dynamic property identification based on vibration test.

Development of performance-based earthquake engineering has made consideration of seismic performance of nonstructural building components necessary. Even if the structural components of a building achieve desirable performance level after an earthquake, failure of its nonstructural components can seriously endanger the performance of the entire building. For the purpose of seismic design and assessment of nonstructural components, determination of realistic seismic demand has a critical role, and should be studied carefully. Different parameters like characteristics of the nonstructural component, its location on the structural system, structural characteristics of the building that the nonstructural component has mounted on, and also site-specific seismicity are all important issues that affect seismic demand expected for a specific nonstructural component. In order to evaluate the seismic performance of nonstructural components, the seismic demand should be determined through a systematic procedure that encompasses all mentioned parameters. The seismic demand can be illustrated by loading protocols that are employed for seismic response evaluation of nonstructural components through laboratory experiments.

Structural behavior of beam-column joints in a RC frame under seismic loadings differs from RC beam-column joints as subassemblies because neighboring members affect each other. The effects of beam's eccentricities on the seismic performances of RC beam-column joints are investigated by 3-D nonlinear FEM Analysis. The verification of experimental results is conducted using FEM analysis. It is indicated that the torsion effects of beam-column joints caused by eccentricities is restrained more, if the contact area of slabs on the joint panel is larger.

Built-up box section members are becoming a popular alternative to steel open section members, particularly due to lack of production of large H-sections in steel profile factories. In the former braced frames with H-shaped columns, using a gusset plate was the most convenient way to connect the bracing to the column. This practice is now continued to be used in connection of brace to the box face. In this case, the gusset plate is welded at the center of the connecting flange, so the axial load is transferred directly to the web of the member. But the seismic configuration is completely different for built-up box columns because in this case the axial load must be carried into the two remote webs of the built-up box section member. A comprehensive investigation on seismic performance of columns with box sections is provided in this paper. The obtained results from finite element analysis of 9 different cases are compared with a similar column stiffened by through plate. The advantages and disadvantages of study connections are provided by a detailed parametric study.

Two main tasks of a structural engineer, as for many other branches of engineering, are analysis and design. Among these two, the latter needs more knowledge, skill and experience. It even comprises completely the first one, that is, a designer must already have the capacity of analysis. There are two technical tools to be used in these two tasks. One of them is finding solution(s) to equilibrium equations of the form Ax = b where A and b are known, x is unkown. For nonlinear problems this equation becomes nonlinear without changing the main character of the problem: find x satisfying the nonlinear equations put forth. The second tool is optimization, i.e. to find x making Φ(x) optimum, either a maximum or minimum. If one is asked to allocate tools to tasks, the answer generally will be such that two pairs are to be formed: (analysis, equilibrium conditions by root finding), (design, optimization). In fact, this allocation can best be shown by forming a two by two matrix. In this matrix, the two locations corresponding to the pairs mentioned above will be the ones that have the highest usage. The location corresponding to (design, equilibrium) will not be empty because most design problems are solved after some successive equilibrium applications. The least used location will be (analysis, optimization). In fact, there seems to be a mistake in this emptiness of the location corresponding the the analysis of structures by using a formulation which based on optimization, because we all know that a structure, loaded in whatever manner, will take the configuration having the least total potential energy. Thus, to analyze a structure under some loading, it suffices to find its shape with the least potential. Once this shape is determined, all the displacements, deformations, strains, stresses, reactions etc. can be determined to complete the analysis. This idea is now gaining importance and enabling engineers to solve problems which were very difficult to solve by the accustomed methods. This means that optimization is enlarging its range of applicability from design part to analysis part. If one considers in this context the advantage of new techniques of optimization, like metaheuristic methods, one will realize that optimization will increase its importance in structural engineer's life both in analysis and design.

Mitigation techniques are currently being sought to ensure public safety in the event of intentional or accidental explosions. Building material fragmentation is a major cause of human injury during such events. One of the most common methods of construction in buildings is the use of unreinforced, ungrouted, infill concrete masonry walls. Concrete masonry provides a fast inexpensive way to construct buildings of various heights; however, these walls are extremely vulnerable to blast pressure resulting in collapse, fragmentation, and severe injury to occupants. Much research has been conducted using full scale blast tests as well as high fidelity computational methods to study the behavior of membrane retrofit masonry walls, but the cost is prohibitive. Design tools developed by other investigators in the field have been available for the past few years; however, they lack a direct definition of the membrane retrofit and their accuracy remains questionable when compared to actual blast test data. The research presented in this paper developed resistance function definitions for three different scenarios of membrane retrofit unreinforced concrete masonry walls. These functions include the bonded and unbonded membrane retrofit scenarios as well as the arching behavior of the masonry wall. The resistance functions were further coupled with single degree of freedom systems to predict the wall response to blast loads. This research gives the structural engineer a practical tool for the design of membrane retrofit masonry walls to resist blast pressures.

This paper presents a novel type of heat transfer finite element that can be used to capture the three-dimensional thermal response of structural beams and columns subjected to realistic fire conditions. The element is a 3-node heat transfer element that uses a fiber discretization to account for both transverse and longitudinal temperature variations within a structural member. This fiber heat transfer element is explicitly formulated to be compatible with any fiber-based beam-column element in a sequentially coupled thermal-mechanical analysis of structural frames subjected to fire. While the element combines finite element and finite difference approximations of the thermal response, the element equations are in a form that can readily be implemented in an existing finite element analysis program. Because the element is discretized into longitudinal fibers, the fiber heat transfer element can account for temperature-dependent material properties and can also be used to model members comprising more than one material (e.g., steel-concrete composite members). To demonstrate the capabilities of the fiber heat transfer element, analyses are performed on a rectangular steel bar with various types of thermal boundary conditions. Results indicate that the fiber heat transfer element offers excellent accuracy and computational efficiency, making the fiber heat transfer element a valuable tool for modeling the thermal behavior of structural frames in fire.

Physics based collapse simulations of moment resisting steel frame buildings are presented with an emphasis on the development of energy flow relationships. It is proposed that energy flow during progressive collapse can be used in evaluation of moment resisting, steel frame building behavior and specifically, localized failure. If a collapsing structure is capable of attaining a stable energy state through absorption of gravitational energy, then collapse will be arrested. Otherwise, if a deficit in energy dissipation develops, the unabsorbed portion of released gravitational energy is converted into kinetic energy and collapse propagates from unstable state to unstable state until total failure occurs. The energy absorption of individual members provides very transparent information on structural behavior as opposed to oscillating internal dynamic forces in structural members. Therefore, critical energy absorption capacity is hereby proposed as a stable failure criterion in progressive collapse analysis. Energy flow quantification is shown to be readily available from the dynamic finite element simulations. The proposed dynamic, energy based approach to progressive collapse, provides insight and a simple yet robust analysis for producing structures capable of resisting abnormal loadings and/or unexpected hazards.

The USGS has reported that in terms of number of deaths and financial losses, floods were the number one hazard in the U.S during the 20th century. Of the 32 flood events characterized, storm surge events were responsible for three — the 1900 Galveston Hurricane, whose surge was responsible for 6,000+ fatalities; a 1938 unnamed hurricane that struck the northeast U.S., responsible for 494 fatalities; and 1969 Hurricane Camille, responsible for 259 fatalities. Katrina's surge claimed over 1,700 lives, devastated coastal Mississippi and led to the levee failures in New Orleans, Louisiana, with heights of 28 feet above sea level. Damage estimates make Hurricane Katrina the costliest hurricane in U.S. history with over $80 billion in total losses. Recent Hurricane Ike is poised to become the third costliest hurricane, with damage estimates between $13 and $21 billion. NOAA reports that much of the Atlantic and Gulf Coasts are less than 10 feet above sea level, making continued threats from storm surge a major issue for coastal communities.

Concrete masonry unit (CMU) walls are a common building component that is vulnerable to blast loads created by terrorist attacks or accidental explosions. Reinforced, grouted CMU walls in particular lack sufficient flexural strength to resist the violent out-of-plane loads and typically exhibit a failure mode highlighted by the formation of a plastic hinge, which could lead to loss of stability and result in catastrophic collapse. The work described in this paper investigated the efficacy of carbon reinforced fiber polymer (CFRP) composites for retrofits of CMU walls to resist blast loads. The retrofit adds strength and stiffness to the wall allowing the wall to protect occupants behind it from the blast loads. Furthermore, the retrofit can improve the performance of load bearing walls by limiting displacements during a blast which increases its ability to resist the design gravity loads. Studies such as those by Muszynski and Purcell [2003], Myers et al. [2003, 2004], Tan [2003], Baylot et al. [2005], and Stanley [2005a, b] have conducted explosive tests on CMU walls retrofitted with various types of reinforced polymers. These types of tests are the most accurate method for demonstrating a structural response to a blast load; however there are issues that arise from testing with actual explosives. First, the test programs typically define a level of threat and only conduct the test to show that the retrofit does or does not work at that threat level. Also, the data measured during the test is often incomplete due to loss of instrumentation and cables during the blast. Finally, the behavior and failure modes of the specimen can only be determined through post-test investigations because it is difficult to view the specimen during a test due to the fireball and debris field created by the explosion. The study described in this paper overcame these issues by using an innovative testing device called the "blast simulator" to investigate the behavior of the retrofit walls for a range of simulated blast loads. This paper will show the effectiveness of CFRP retrofitted walls to a range of impulses while demonstrating that the blast simulator is valuable for validation of constitutive models used by numerical tools, specifically finite element (FE) analysis.

Structures along the coast, especially in low lying areas, are susceptible to a variety of natural hazards such as wind, storm surges, and tsunamis. These hazards must be fully studied and well understood in order to effectively and safely design and construct hazard resilient structures. The load effects associated with each hazard on its own are unique and correlate with specific damage characteristics. For example, as wind encounters a structure, it pushes against the windward face and is subsequently deflected around and over the structure creating suction over the sides and roof. The suction forces result in the peeling off of the siding and roofing materials and, in extreme cases, the dislodging of the walls and roofing system. In contrast, surge and waves generally result in the pushing of ocean-side walls inwards, thus detaching the bottom wall connections from the foundation and leading to the formation of hinges at the top of the wall connections with the roof. Similar action results at the back walls as the surge and waves pass through the house. Therefore, damage generally proceeds from the bottom upward, destroying the structural integrity of the building. In addition to this lateral load, surge and waves induce a buoyant upward force that may lead to uplift and eventually the pulling of a structure from its foundations. In the case of tsunamis, loads are characterized by the surging action of the wave and the impact of waterborne debris. Like the storm surge and wave action, this is accompanied by buoyant forces that may uproot an already compromised structure from its foundation.

For accidental or terrorism-related blast loading, typical structural design practice allows substantial inelastic deflections and rotations to be considered for reinforced concrete and steel flexural members, provided that the members are well-detailed and the structure is not required to remain operational after a blast event. Such practices have been based on a wide body of experimental test data, largely compiled by military engineers. Flexural members made of prestressed concrete, conversely, are not permitted to undergo large inelastic deformations when exposed to blast loads. One reason for this disparity in code provisions is a lack of experimental test data for prestressed concrete elements under blast loading. Four prestressed concrete panels were tested under the simulated blast loading of a shock tube as part of an experimental program at The University of Texas at Austin. Post- and pre-tensioned panels both with and without polypropylene fiber reinforcement were tested; the focus of this paper is the response of the two normal-concrete panels.

Recent severe tropical storms such as Hurricanes Katrina (2005) and Ike (2008) have resulted in questions of whether wind or water (storm surge) is responsible for severely damaging or destroying structures along the coast. Because of a separation of insurance for wind and storm-surge (flood) damages in the U.S., the determination of whether hurricane damages are the result of wind or of storm surge becomes a crucial issue in the settlement of insurance claims. Disagreements as to the cause of such damages have led to an unprecedented series of legal challenges between homeowners and insurers. Engineering and meteorological reports directed at the performance of residential buildings have been prepared in support of both sides of this issue. Many such reports employ failure theories that are implausible and that can be potentially misleading. The failure theories in question appear to stem primarily from a misunderstanding of wind and storm-surge loading of structures, from a misunderstanding of the behavior of structures under extreme loads, from selective or incomplete use of physical principles, from the use of incorrect or non-standardized wind and surge data, and/or from incomplete analysis of clues provided by the damaged structure and surroundings. Physical principles that are mistakenly or inaccurately applied include fluid-structure interaction, buoyancy, and transport of structures and debris. Among the data improperly employed in recent analyses are unverified and non-standardized wind speeds and data from preliminary (non-calibrated) numerical models. Improper application of the EF-Scale relating tornado damage to wind speed ranges to hurricane damage also has been frequently observed. The supposition that tornado damage is responsible for the "selective" destruction of structures has also been mistakenly used in place of considering variations in structural resistance. Our experience has prompted the following recounting of physical principles, anecdotes, observations, and recommendations. This paper examines common misconceptions and provides guidance for application of the appropriate physical principles.

Economical design of curtain walls to resist extreme out-of-plane loads implies the curtain wall will suffer significant damage. Modeling curtain wall systems after the onset of glass cracking or mullion yielding is a significant challenge, requiring consideration of nonlinear material behavior and often nonlinear geometry. Further, the structural behavior of the curtain wall's individual components and their connections must be well understood. UNC Charlotte researchers [Weggel, Zapata, and Kiefer, 2007], [Weggel and Zapata, 2008] have previously developed a calibrated, elastic curtain wall model capable of simulating static and dynamic responses before cracking or yielding of system components when subjected to general loading; linear and nonlinear geometry was considered. The following analytical phase of that research program is to extend the model to the post-elastic regime as well as to develop a much simpler model that would be more appropriate for design purposes. The experimental test results of full-scale curtain walls subjected to extreme out-of-plane loading presented in this paper will be used to calibrate the nonlinear (post-elastic) model.

This paper presents an experimental study on the ultimate behavior of unbonded post-tensioning (PT) strand/anchorage systems subjected to a variety of loading parameters that can affect the performance of the strand inside the anchor. While unbonded post-tensioned gravity load systems (e.g., floor and roof slabs) are common throughout the United States, the use of this construction technique for seismic resistance can put the anchorages (where all of the PT forces are transferred from the strands to the structure) under extreme demands. Strand wire fractures can occur inside the anchorages, limiting the lateral strength, stiffness, ductility, and self-centering capability of the entire structure. The research described in this paper focuses on the strand stresses and strains at wire fracture considering the following parameters: (1) loading rate; (2) eccentricity between the strand ends; (3) post-yield cyclic loading; and (4) initial stress in the strands. Six representative cast and barrel anchor configurations with 0.5-in. and 0.6-in. nominal strand diameters are included in the investigation. While the effects of the loading parameters greatly depend on the strand/anchor configuration, the test results show that seismic loading conditions can cause a significant reduction in the strand strain at fracture as well as an increase in the standard deviation of the fracture strains. Current industry requirements for the testing and acceptance of PT strand/anchorage systems are assessed based on the investigation.

This paper presents the results from eight half-scale experiments of unbonded post-tensioned precast concrete coupling beams under reversed-cyclic lateral loading. Each test specimen includes a coupling beam and the adjacent concrete wall pier regions at a floor level. Under lateral loads, the nonlinear displacements of unbonded post-tensioned coupling beams are governed by the opening of gaps at the beam ends. Steel top and seat angles are used at the beam-to-wall joints to yield and provide energy dissipation. The test parameters include the beam post-tensioning tendon area and initial stress, initial beam concrete axial stress, angle strength, and beam depth. The results demonstrate the lateral stiffness, strength, and ductility of the test specimens under cyclic loading, with considerable energy dissipation concentrated in the angles. It is shown that the residual displacements upon unloading of the specimens are small due to the restoring effect of the post-tensioning force.

Earthquake damage is not only a disaster, but also an examination for the present structure engineering technique and a lesson for the future one. By collecting and investigating damage data, the correlation between ground motion and damage can be established directly and applied to earthquake loss estimation and hazard mitigation. ATC-13 report presented a methodology that has been widely used for estimating damage/loss caused by earthquake and collateral hazards. The major procedure includes facility classification, selection of ground motion characterization, and damage estimates for different types of structures under specific ground shaking intensity. Mostly the motion-damage relationship developed by using investigated damage data during a past earthquake is in the form of vulnerability function, also termed fragility curve. It predicts the probability of reaching or exceeding specific damage state for a category of buildings with given earthquake intensity. The vulnerability functions are feasible for damage loss estimate in a large region, such as a city or a state, yet scatter caused by insufficiency of detail in structure classification exists generally. The applicability of the functions to regions outside the area where it is originally developed is unclear as well. Seismic evaluation that concerns about localized structural behavior and ground motion characteristics is utilized to establish the motion-damage relationship for individual buildings. While detailed information and complicated calculation are required, higher accuracy and applicability may be expected. Vulnerability functions can also be developed by performing seismic evaluation procedure on representative type of model buildings.

Many large industrial projects are located in remote areas where the local labor pool does not include a significant number of skilled welders. As a result, field bolted connections are favored over field welded connections. Special Concentrically Braced Frames (SCFB) are a preferred structural system for many industrial structures. However to meet the detailing requirements for a SCBF structure in which member performance dictates the connection design loads, bolting can become complicated. The decision to use bolted versus welded field connections in industrial projects needs to be carefully evaluated before proceeding. A comparison of utilizing field bolted versus field welded connections for vertical braces is explored. A recent fossil power plant project employed a SCBF in a seismic design category D. It became evident in preliminary reviews of this project that the complexity of bolted bracing connections would create a challenge for the erector. The final conclusion for the project was that field bolted connections would be employed. However the actual design of the connections has cause the authors to revisit this decision and to closer evaluate alternatives for the next project that will require SCBF detailing.

A self-centering moment resisting frame (SC-MRF) is a viable alternative to a conventional MRF with welded beam-column connections for seismic resistant buildings. An SC-MRF is characterized by gap opening and closing at the beam-column interface under earthquake loading. The beams are post-tensioned to columns by high strength post-tensioning (PT) strands oriented horizontally to provide self-centering forces when gap opening occurs. For the SC-MRF investigated in this research, energy dissipation is provided by beam web friction devices (WFDs) attached to the columns at the beam-column interface. Both the PT strands and WFD contribute to the moment capacity of the SC-MRF connections. The SC-MRF in this study is designed to meet several seismic performance objectives. These include no damage under the Design Basis Earthquake (DBE), leading to immediate occupancy performance following the DBE. In addition, under the Maximum Considered Earthquake (MCE) the structure is designed to have minimal damage and achieve the collapse prevention performance level. A 7-bay, 4-story SC-MRF prototype building located on stiff soil in the Los Angeles area was designed with WFDs using a performance-based design procedure with the above performance objectives. A 0.6-scale model of two bays of the SC-MRF was developed and tested under simulated earthquake loading at the NEES equipment site located at Lehigh University using the hybrid simulation method to include the remaining parts of the building in the simulation. This paper presents an overview of the performance-based design procedure, the test results, and an assessment of the design procedure.

This paper focuses on the role that a selected yield mechanism plays on seismic design and response of structures. A parametric study was carried out by selecting different mechanism types for design of an example steel moment frame. The results of inelastic dynamic response analyses are presented and discussed. It is concluded that complete sway mechanism of the frame with strong, non-yielding columns appears to be most efficient in terms of required design base shear for a given target story drift.

Fragility analysis for highway bridges is essential for risk assessment of highway transportation networks exposed to seismic hazards. This study uses detailed three-dimensional nonlinear analytical models of a typical California multi-span concrete box girder bridge without and with protective systems to develop fragility curves which can be used in regional seismic risk assessment of transportation networks. The analytical as-built bridge model is calibrated using responses recorded during the 1979 Imperial Valley earthquake. Fragility analysis for the as-built bridge model and two retrofitted bridge models is carried out. The fragility curves for the reference bridge indicates that the bridge is robust compared to many other bridge types found in other parts of the United States. Also, compared to the as-built fragility, the steel jacketing retrofit reduces the fragility slightly, while the elastomeric isolation bearing retrofit also significantly reduces the fragility of the bridge.

Damage indices based on energy concept have been proposed in the past to measure structural damage while compensating for the inadequacy of ductility criteria. However, these indices are either based on implicitly defined energy as cumulative effect or unable to consider the energy in a quantitative manner. In this paper, damage is quantified based on the plastic energy ratio by analytically calculating the plastic energy. This calculation uses the force analogy method to evaluate the structural response and energy in the inelastic domain. Fragility analysis based on 25 adjusted earthquake records with similar 5% damped response spectra is employed to correlate the plastic energy ratio with the interstory drift ratio at the life safety limit state. Inelastic structural deformation is expected to occur during major earthquakes, and active control based on optimal linear control algorithm is used to improve the structural performance.

Seismic risk is a concern in the Mid-America region because of the series of severe earthquakes that occurred in the New Madrid Seismic Zone in 1811–1812. Many existing concrete structures in this area are considered to be vulnerable in a moderate to significant seismic event because they were not designed for the seismic provisions of the current International Building Code. Seismic performance assessment and fragility analysis are used to evaluate the seismic vulnerability of existing structures. Based on these analyses, possible retrofit techniques are considered to reduce structural damage and corresponding losses. Based on available inventory data for Memphis, Tennessee, tilt-up concrete buildings are the most common type of concrete structure in Memphis. Tilt-up construction is commonly used for low-rise industrial buildings and has several advantages, including a large open space and low construction costs. However, many tilt-up concrete buildings were damaged during previous earthquakes in the Western U.S. and other regions. Typical damage included collapse of wall panels and roof diaphragms due connection failures. To improve seismic performance of tilt-up concrete buildings, several seismic retrofit techniques are currently used including the addition of anchors to strengthen the roof-to-wall connections.

A self-centering steel moment resisting frame (SC-MRF) for earthquake resistant structures has the attributes of ductility, requires no field welding, and returns the structure to its pre-earthquake position. Post-tensioned high strength strands run parallel to the beam and compress the beam flanges against the column flange to develop the resisting moment to service loading. The behavior of a SC-MRF is characterized by a gap opening and closing at the beam-column interface under earthquake loading. Energy dissipation is provided by supplemental elements that deform under the gap opening behavior. This paper provides a literature review of steel self-centering systems. Then the steel SC-MRF behavior and existing design procedure is described.

Conventional concentrically-braced frame (CBF) systems have limited drift capacity before brace buckling and related damage leads to deterioration in strength and stiffness. Under the NSF funded NEESR-SG research program, a new type of CBF is being developed with increased drift capacity before damage and decreased permanent drift under seismic loading. These self-centering CBF (SC-CBF) systems are intended to provide significant non-linear drift capacity while limiting damage and residual drift, and are motivated by the goal of minimizing structural damage under seismic loading. At low levels of lateral load, the response of an SC-CBF is similar to that of a conventional CBF. At higher levels of lateral load, the fundamental lateral load behavior of the SC-CBF system is rocking about the base of the compression column, which occurs when the column under tension from overturning moment decompresses and uplifts at the foundation. To control the uplift, high strength post-tensioning (PT) bars, oriented vertically over the height of the SC-CBF, prestress the frame to the foundation. The SC-CBF is designed to decompress at the base at a selected level of lateral loading, initiating a rigid-body rotation (rocking) of the frame. The PT bars provide a restoring force to return the CBF to the foundation (to self-center the CBF). The rocking response substantially influences the member forces that develop in the frame members (beams, columns, and braces) and induces deformation and eventually yielding of the PT bars. To minimize structural damage before PT bar yield, the frame members are designed to resist the internal forces that develop at PT yield. The paper presents the results of an analytical study of SC-CBFs. Dynamic analysis results demonstrate the expected drift capacity and self-centering behavior of the system. Modal decomposition of these analysis results shows that the first mode response is effectively limited by the rocking behavior; however, the higher modes are also excited by the rocking response. Based on these analysis results an improved design method is presented that accounts for the higher mode contributions to the member forces of the SC-CBF.

Procedures for the structural engineer to properly analyze, design and specify steel joist and Joist Girder moment frames to resist seismic lateral loads will be illustrated in this paper. To accomplish this the reader is assumed to be familiar with the design and analysis procedures of ANSI/AISC 360-05 Specification for Structural Steel Buildings, ANSI/AISC 341-05 and ANSI/AISC 341s1-05 Seismic Provisions for Structural Steel Buildings Including Supplement No. 1 and also familiar with the requirements of ASCE/SEI 7-05 Minimum Design Loads for Buildings and Other Structures. The design methodology described in the paper will be limited to single story structures subjected to seismic loads; however, these procedures are also directly applicable to single and multistory moment frames subjected to wind loads.

The Steel Slit Panel Frame (SSPF) system is a new Lateral Force Resisting System (LFRS) intended for zones of high seismicity. Its main components are: steel beams and columns simply connected, and steel slit panels (SSPs) which provide the strength, lateral stiffness and energy dissipation. SSPs are made from a steel plate; rows of vertical slits at equidistant spaces are cut, forming a series of links in between the slits. When the panel is subjected to horizontal displacement, the links behave as beams in double curvature, dissipating energy in flexure. When properly designed, the SSPs do not require heavy stiffening to control out-of-plane deformations. This paper provides an overview of the design process required for using a SSPF.

The Lotte Super Tower is a unique form that transforms from a square, 70-meter square footprint to a 40-meter circle at 555 meters above ground. The shape of the building has been wind engineered to reduce wind forces and accelerations. A dual system comprised of an exterior steel diagrid and an interior reinforced concrete core is being employed for lateral resistance to wind and seismic effects. Each of the components of the structural system have been optimized to provide material efficiency and geometrically rationalized to increase constructability.

In the late 1990's, through a mutual acquaintance, the author met Sir Norman Foster, now Lord Foster, in connection with a project in Chicago, the author's home base. Since then, the two Principals and their firms have collaborated on a number of projects — each one unique in form and in structural detail, but each fitting into the fundamental pattern of efficient tall building structural design. The tallest example of this collaboration is the Russia Tower, a 118 story tall mixed-use building proposed for Moscow. Before addressing the Russia Tower in more detail, some background on the recent history of tall buildings will allow the building design to be placed in the context of the broader evolution of efficient tall building systems.

The design of supertall buildings is typically governed by their interaction with lateral winds and gravity loads. Selection of the building shaping and structural system can greatly affect the wind and gravity behavior of the structure. The following case study will demonstrate the design process and philosophy utilized in the design of supertall buildings, by highlighting specific challenges faced by the design team, and by explaining how the latest material, construction, and analysis technologies assisted in successfully overcoming these challenges.

Integrated Architectural Design relies upon a multi-disciplinary and collaborative team, making decisions based on a shared vision, a commitment to success and a holistic understanding of the mission — the project. Architectural Design Teams today embody expanded in-house expertise and services in order to engage the most effective methodologies in the integrated project process. This is achieved over some time and through focused application on large complex multi-use projects and can be especially effective in the design of tall buildings. Adequate resources to support an integrated design process can be developed through training and professional development, strategic hires and/or selection of recognized experts. Expertise required beyond traditional architectural and engineering design and documentation in tall buildings includes, for example, building siting, daylighting design, energy modeling, landscape design, water and energy system design, ecology, renewable energy systems, security and information technologies. Diversification of expertise fosters creativity, thinking and a cross-disciplinary approach to design. This is required to produce exceptional quality architectural design as well as truly sustainable tall buildings. The entire integrated Design Team must be viewed as Experts and Stakeholders throughout the process, rather than periodically engaging, then disengaging them. This ongoing integrated approach will generate a greater commitment to and an understanding of each project's systems, materials, complexities, sustainability goals and opportunities. This also will enable Clients and project teams to develop innovative ideas and opinions which may not have surfaced by following a traditionally-linear project approach. A truly integrated project process will also ensure the achievement of higher performance goals.

Architectural design and structural engineering have a symbiotic relationship: a close and essential union in which one heavily relies upon the other. In the ideal structure, the lines between science and aesthetic are blurred. By reviewing the development of Burj Dubai Tower and the cable wall of the New Beijing Poly Plaza, this paper will highlight examples of how the successful interplay between architects, structural engineers, MEP engineers and their various consultants influences the design process and ultimately, the structure as a whole. In particular, it will explore how a structural engineer approaches this essential collaboration which, in the best instances, results in situations where one cannot describe the structure without also describing the architecture, and the architecture without its structure.

Traditional as well as performance-based seismic design requires accurate simulation of building response to determine component deformation and load demands. This is particularly challenging for reinforced concrete structures in which concrete cracking prior to yield may change component stiffnesses substantially and for existing concrete structures in which inadequate design details may result in component strength loss prior to yield. In this study analytical models, which are practical for use in design and compatible with commercial software such as SAP2000, are developed for one type of concrete component: planar interior beam-column joints. A dataset comprising 45 joint sub-assemblage tests is used for model development. First, a simple, center-line model is proposed for elastic analysis in which the beam-column joint region is modeled as semi-rigid. Experimental data are used to determine the joint stiffness that minimizes the error in predicted inter-story drift and the new ASCE 41 recommendations are evaluated. A second model is developed for use in a nonlinear pushover-type analysis. For this model, joint flexibility and beam flexural response are represented by plastic hinges at beam ends. A simple empirically-based model is used to determine, on the basis of joint shear stress demand and bond stress demand within the joint, if system response will be ductile or brittle. A joint shear strain limit determines when strength loss will occur in brittle joints. For ductile joints, the behavior of the sub-assemblage is dominated by the nonlinear response of the beams.

Headed bars have become increasingly popular as longitudinal and transverse reinforcement of relatively large concrete structures (e.g., tall buildings, storage tanks), as use of headed bars provides an adequate solution to steel congestion particularly at beam-column joints. New code provisions for headed bars have been added to the 2008 edition of ACI 318, which detail the development of headed and mechanically anchored deformed bars in tension. These provisions include requirements for development length (l_dt), material properties (f'_c and f_y), reinforcing bar size, and net bearing area of the head (A_brg), as well as clear cover and bar spacing (c_cb and c_s). Here, f'_c is the specified concrete strength, f_y is the specified steel yield strength, and A_brg is defined as the gross head area (A_head) minus the larger of the obstruction area (A_obs) or the bar area (A_b) (see Fig. R3.5.9 of ACI 318-08).

Floor systems composed of two-way slabs without beams are frequently used because they offer several advantages, including relatively simple formwork and a shallow profile that can accommodate shorter story heights. Slab-column frame systems are common in regions of low to moderate seismic risk, where they are allowed as a lateral-force-resisting system (LFRS), as well as in regions of high seismic risk for gravity systems where moment frames or shear walls are provided as the main LFRS. However, past earthquake damage has shown that slab-column frames are not a suitable main LFRS in regions of high seismic risk because of their relative flexibility and potential for brittle punching shear failures in the slab-column connection region.

Reinforced concrete (RC) frame connections with the beam centerline offset in plan from the column centerline are commonly used in practice, especially in perimeter RC building frames. A few design provisions have been suggested for this type of eccentric RC beam-column connection in both the state-of-the-art report entitled "Recommendations for Design of Beam-Column Connections in Monolithic Reinforced Concrete Structures" and in the current "Building Code Requirements for Structural Concrete and Commentary". However, much recent laboratory testing and other analysis of this sort of connection have not necessarily been fully reflected in those design provisions. Therefore, this paper first summarizes all of the emerging research becoming available on the subject of behavior of eccentric (edge and corner) RC beam-column connections and then uses it to critically examine current design recommendations. A particular emphasis is placed on procedures for estimating the nominal joint shear strength of connections of varying eccentricity, but other aspects of their structural behavior (including joint shear deformation, energy dissipation, slab effects, and numerical modeling) are also examined and analyzed in this paper.

This research program was funded by the Texas Department of Transportation (TxDOT) to determine the cause of flexural cracking of prestressed beams at release. Girders that exhibited cracking were 54-inches deep, relatively short in length (60-feet and less), and contained highly eccentric strand configurations.

The maximum allowable compressive stress limit at prestress transfer was first introduced by the American Association of State Highway and Transportation Officials (AASHTO) in the 1961 Bridge Design Specifications. The limits prescribed by AASHTO in 1961 have not changed since their inception. Over the last decade, a substantial amount of research and discussion has been published related to increasing the allowable release stress in compression. In 2007, Dolan and Krohn conducted a survey of Prestressed Concrete Institute (PCI) professional and producer members [2007]. Thirty of forty-four respondents reported regularly using compressive stresses at release in excess of 0.60f'_ci. The majority of respondents reported no difficulties with higher release stresses. However, some respondents associated the following problems with higher release stresses: excessive camber, concrete splitting around the strand end, excessive strand slip, minor cracks at the ends, short horizontal cracks at the top of the web. Also, the results of the survey indicated that greater compression transfer stresses were typically used in building products than in bridge products. The authors recommended increasing the allowable stress in ACI-318 to 0.70f'_ci. Also, they stated that refined camber and prestress loss calculation methods may be needed as a result.

Freyssinet's original design concept for prestressed concrete maintained the concrete in compression under full service load [Billington, 2004; D'Arcy et al. 2003]. In fact, Billington [2004] notes that "for Freyssinet, `the fields of prestressed concrete and reinforced concrete have no common frontier.' Either a structure is fully prestressed or it is not to be called prestressed concrete." Prestressed concrete design has evolved over time to allow a nominal amount of tension in the concrete under full service load. Even with this nominal amount of tension, the design is still considered fully prestressed provided the tensile stresses do not exceed the modulus of rupture for the concrete. As part of the evolution of prestressed concrete, designers, code writers, and researchers have begun to examine the potential for utilizing even lower levels of prestressing that allow the concrete to crack under full service load. This type of prestressing is often referred to as partial prestressing. One of the primary reasons for the potential use of partial prestressing is that ultimate strength generally never governs the design of fully prestressed sections. Instead, the design is controlled by tension stress limits at full service load. However, that approach may be overly conservative, even with respect to serviceability concerns, if the full live load is infrequently reached during the life of the structure.

The ACI Building Code, first issued in 1910, is the primary knowledge asset of the American Concrete Institute and the concrete construction industry in the U.S. For nearly a century, the Code, by its adoption into virtually all US model building codes, has been the leading document in many areas related to concrete, including education, design, materials, construction, and repair. It is arguably one of the most influential concrete codes in the world. The Code is a primary reason for ACI's reputation as the leading source of technical information on the use of concrete. The framework of the current ACI Building Code has remained essentially unchanged since 1963. In 1963 ultimate strength design was introduced, prestressed concrete was in its infancy, and there were no provisions for seismic design or connections to concrete. The Code served to inform, educate and provide guidance for the introduction of strength design methods. Knowledge expands with time through practice and research. In the case of structural engineering, knowledge is also gained through lessons learned from behavior of structures in catastrophic events such as earthquakes. The Code captures change through this insight into behavior, addition of new materials and construction techniques, and advancement of technology. It is the addition of new knowledge that underlies the growth of ACI 318 Code and Commentary from 60 pages in 1956 to 471 pages in 2008. The Code has grown from 750 provisions and 250 primary cross-references in 1971 to over 2500 provisions and 1500 primary cross-references in 2008.

This paper reviews the steps taken to develop consensus for the structure of the revised ACI 318 Building Code and the progress to date developing the reorganized Code. The principals used to revise the Code are discussed. The block diagram of the revised code structure is introduced and a discussion of upcoming work is presented.

The current ACI 318 building code provides a mixture of behavioral based and member based design. While the current code format has served the design profession well over the years, it has grown more cumbersome as new advancements continue to be integrated. Furthermore, the current format often leads designers to question whether they have missed any of the code requirements. One of the primary goals in the next generation of the ACI 318 building code is to provide a more user friendly design document that can also provide a framework for future innovations. For this reason, the code is being reorganized for the 2014 edition and will be constructed using a member based framework. This paper provides an overview of the proposed organizational structure and discusses how structural design will be approached using this new framework.

The contribution of concrete to shear resistance is an important factor considering both the economic and safe design of structural concrete members. While this contribution is often taken for granted in design, there continues to be considerable debate on the internal mechanisms that resist shear and for that matter the shear strength provided by the concrete. For this reason, multiple design expressions can be found in various codes of practice and even within the same design code. Over the past several years, a reevaluation of the shear strength provided by the concrete and in particular the uncracked compression zone has been conducted. This has led to a simple methodology, now adopted in ACI 440 design guidelines, for calculating shear capacity that has proven remarkably accurate for a wide range of beams including those constructed with FRP reinforcement. This paper highlights the benefits provided by this approach and delves into several of the primary factors influencing shear strength including flexural reinforcement ratio, concrete strength, and size effect.

While the parallel chord truss model is a universally accepted model for evaluating the contributions of stirrups to the shear capacity of slender beams, codes-of-practice provide very different mean of evaluating the angle of diagonal compression of the struts in this truss model. This angle controls the contribution of shear reinforcement as the number of stirrups "n" to be counted as lifting the diagonal compressive force up a vertical tie in the truss is proportional to the cotangent of the angle of diagonal compression relative to the longitudinal axis of the beam. ACI318-08 assumes that this angle of diagonal compression is 45 degrees, while the AASHTO LRFD Bridge Design Specifications and EuroCode2 allow much flatter angles of diagonal compression to be used. The effect is that the contribution of in-place stirrups in a particular beam by one approach can be close to 3 times that calculated by another approach. The basis and appropriateness of different approaches are explored.

Understanding the effect of depth on the strength of deep beams can be essential to ensure their satisfactory performance in the field. In Houston, Texas, significant diagonal cracking (maximum crack width approx. 0.035-inches) was observed in several large, reinforced concrete bent caps in service. An expensive retrofit was implemented to strengthen all of the bent caps in the intersection. One reason for the poor performance of the bent caps was the incorrect association of deep beam strength to the depth of the member. In this paper, experimental results are presented that support that deep beam strength is more appropriately determined through a single-panel, strut-and-tie analysis.

Engineers designing concrete structures use many different types of elements including walls, beams, columns, and slabs. Slabs are often designed so that they do not need to contain transverse reinforcement. While such reinforcement can significantly increase member ductility and strength, it can add sufficient cost and time delays during construction that its removal can be an efficient design decision. To safely be able to avoid the use of such reinforcement, it is important that the engineer have a good understanding of how reinforced concrete members without transverse reinforcement behave so that problems can be avoided. This paper will briefly present the one-way shear design methodology used in the Canadian CSA building code as well as the most recent updates to the AASHTO LRFD bridge design specifications. The principles behind the method will be graphically demonstrated against individual data series as well as a larger database of about 1100 published sectional shear failures from the past 60 years.

Real-time hybrid simulation combines experimental testing and numerical simulation, and thus provides a viable alternative for the dynamic testing of civil engineering structures. Actuator delay compensation is vital to achieve a stable and reliable real-time hybrid simulation. An adaptive inverse compensation scheme for mitigating the effects of actuator delay is presented in this paper. A tracking-error based adaptive control law is developed to adapt a compensation parameter to minimize actuator control error due to an unknown time-varying actuator delay. Real-time hybrid simulations of a single-degree-of-freedom moment resisting frame with an elastomeric damper are conducted to experimentally demonstrate the effectiveness of the proposed adaptive compensation method. The adaptive compensation scheme is shown to improve actuator tracking capability by effectively negating a poorly estimated actuator delay.

Fiber-reinforced cementitious systems for concrete structures are increasing in popularity. However, fiber-reinforced mortar mixtures for masonry applications have not been extensively studied. In this study, various FRM mixtures using polyvinyl alcohol (PVA) fibers and FRM-clay unit prisms are tested and resulting mechanical properties are discussed. The FRMs are developed specifically for masonry applications such as the rehabilitation, reconstruction, and strengthening of existing masonry structures; therefore, mixtures with low compressive strength and high ductility are desired. Results show that increased toughness, ductility, and energy absorption can be achieved using FRMs in masonry joints without significantly altering the compressive capacity or aesthetics of the structure. The study provides valuable insight into effective FRM mixture design for masonry applications, suggested empirical formulas for modulus of rupture and modulus elasticity, and directions for future research.

Increased container shipping has lead to larger, more complex cranes to serve the ever-larger ships. The increased size and mass of modern container cranes makes them potentially more seismically vulnerable then their smaller predecessors. Container cranes are essential to the operation of a port, and the economic impact from their downtime can be substantial. In order to better understand the seismic vulnerability of modern container cranes, shake table experiments are conducted on a 1:20 scale model of a typical container crane found in the ports on the west coast of the United States of America. During previous seismic events, three main failures were observed: derailment, where the wheels lift up off the rails; local buckling of the legs; and collapse of the crane. During this study, the focus was on derailment and the uplift/pounding phenomenon associated with it. In order to better understand uplift and derailment, a 1:20 scale model is constructed. The model is designed such that the members would remain elastic during design earthquakes. The model crane is tested on a six-degree-of-freedom shake table at the University at Buffalo. The movements of the legs are tracked and strains in the sections of the crane that were determined to be vulnerable by the analyses were measured. The results of the study are presented in this paper.

The main purpose of this study is to investigate experimentally granite masonry specimens as well as its constituent components, granite stone units and cement mortar, separately under compression so that the results can be used for numerical modeling and analysis of granite masonry structures. The compressive behaviour of granite stone units and cement mortar are characterized and their effects on the strength and deformation characteristics of the masonry structure are examined. The experimental results indicate that granite stone specimens exhibit high peak strength values and very fragile behavior when subjected to compressive loading. The stress-strain relation of the mortar specimens show a strain hardening behavior with more ductility than the granite stone. The strength of granite masonry is much higher than the mortar but less than the average strength of the granite stone units. By using an in-situ optical technique called Digital Image Correlation, quantitative measurements of strain localization in granite masonry are achieved in both deformation and failure modes. It was found that cracks initiated at the vertical mortar joint interface and formed multiple shear bands. Further deformation occurs by extending the shear bands into the mortar joint layers, and finally the granite unit split vertically due to the laterally biaxial tensile force induced by the confinement effect of the mortar joints.

This paper focuses on the stability behavior and design of wide-flange steel gravity columns under fire loading. Their behavior was predicted using analytical models and approaches that were validated using existing experimental results. The validated models were used to conduct parametric studies to evaluate the effects of column slenderness, axial load level, and rotational restraints on column stability at elevated temperatures. These results were used to develop comprehensive design guidelines for wide-flange steel columns at elevated temperatures. This paper presents the results and design guidelines for steel columns considering the effects of various parameters. The proposed guidelines are simple modifications of the existing AISC column design equations, and are continuous with them at ambient temperatures.

Conventional reinforced concrete structures typically employ steel reinforcing bars that are embedded within the structure. In the event of a proximate explosive detonation, such structures are ineffective in providing adequate protection because they are prone to disintegration under blast pressures that may be in excess of thousands of pound per square inch. Protection of people, buildings, bridges, etc. from car or truck bombs, remote controlled explosives, etc. is of increasing importance and necessity. It has been previously suggested that wire mesh may be employed on or just beneath the front and rear surfaces or inside the structural elements to mitigate "scabbing" (i.e. creation of craters on the target face due to a blast load) and "spalling" (i.e. separation of particles of structural element from the rear face at appropriate particle velocities) for light to moderate blast loads. It has also been previously suggested that a wire-mesh reinforcement system not only prevents scabbing and spalling for light to moderate blats loads but can be designed to deflect elastically and plastically in response to large explosive loads to absorb the energy of the blast.

Shearwalls constructed with wood structural panels, such as plywood and oriented strand board (OSB), have been used to resist combined shear and wind uplift forces for many years in the U.S. For example, the Southern Building Code Congress International (SBCCI) published SSTD 10, Standard for Hurricane Resistant Residential Construction, in 1999, which provided the shear resistance table for wood structural panels. When wood structural panels are used in combined shear and wind uplift, SSTD 10-99 also tabulated the wind uplift resistance of wood structural panels with a minimum thickness of 12 mm (15/32 in.) when used in conjunction with the shear resistance table. Working with researchers at the National Home Builders Association Research Center (NAHB RC), Norbord Industries sponsored full-scale combined shear and uplift tests, showing that the cross-grain bending of the bottom plate, which is a brittle failure mode, could be avoided by using 5.8 x 76 x 76 mm (0.229 x 3 x 3 in.) plate washers with anchor bolts. The NAHB RC tests were conducted in lateral shear and tension (uplift) separately, and the effect of combined shear and uplift was evaluated based on an engineering analysis. After reviewing the NAHB RC study, APA and Norbord jointly conducted full-scale combined shear and wind uplift tests at Clemson University to gather more data on this subject. The test setup at Clemson University was capable of increasing the shear and wind uplift forces simultaneously until failure was reached by using a pulley system controlling the bi-axial forces in both lateral and vertical directions. Results of the Clemson study were used to support the development of engineering standards and changes to the national building code in the U.S., and are reported in this paper. In 2007, APA constructed new combined shear and wind uplift test equipment that is capable of bi-axial loading in both lateral and vertical directions with independent but synchronized loading mechanisms. The vertical load can be applied as either an uplift force or a downward gravity load. Research results using this new equipment are utilized to enhance the understanding of the design on the bi-axial combined shear and wind uplift. This paper describes the latest finding from this research.

This paper presents an ongoing research project on the behavior and design of "hybrid" precast concrete wall structures for use in seismic regions. Hybrid precast walls utilize a combination of mild (e.g., Grade 60) steel and high-strength unbonded post-tensioning (PT) steel for flexural resistance across horizontal joints. The mild steel reinforcement is designed to yield in tension and compression, providing energy dissipation. The unbonded PT steel provides self-centering capability, reducing the residual (i.e., permanent) lateral displacements of the wall from a large earthquake. Both the PT steel and the mild steel contribute to the wall lateral strength, resulting in an efficient structure. The paper introduces a seismic design approach that conforms to ACI 318 (2008) and ACI ITG-5.2 (2008) for the validation of hybrid walls as special reinforced concrete shear walls based on ACI ITG-5.1 (2008). This design approach is used to design a series of test specimens investigating the following parameters: (1) relative amounts of mild steel and PT steel; (2) wall height-to-length aspect ratio; (3) concrete confinement details; and (4) presence of openings within the wall panels. A pre-test study is conducted to evaluate the design of the test specimens based on nonlinear reversed-cyclic lateral load analyses. It is shown that the specimens satisfy all of the validation and design prerequisites set forth in ACI ITG-5.1 and ACI ITG-5.2. The subsequent testing of these specimens is expected to ultimately support the code approval of the hybrid wall system for moderate and high seismic regions.

A bracing system with tension-only elements such as cables is easy to install and has an advantage of requiring minimal on site construction without the need for heavy equipment, and results in minimal environmental impact and noise. The proposed system herein consists of eccentrically connected elastic cables and a rotating central energy dissipater (CED), where all cables are intended to be in tension under lateral loads so that the system dissipates energy through a bi-linear hysteresis curve. The CED dissipates energy through the cyclic bending of steel plates which are replaceable after a severe earthquake event. The full scale test of a prototype specimen successfully showed a stable behavior without pinching.

Several prototype steel moment-frame buildings have been designed for the purpose of assessing their vulnerability to progressive collapse. The buildings were designed for moderate and high seismic regions. This paper summarizes the development of three-dimensional finite element models of these prototype buildings, with a focus on the modeling approach used for the connections and the composite floor system. Initial simulation results under a column removal scenario are presented to illustrate the model capabilities. Ongoing assessments of reserve capacity and progressive collapse vulnerability using these models are part of a larger study aimed at quantifying and comparing the relative robustness of different structural systems.

Given the complexity and computational demands of conducting detailed finite element simulations of complete structural systems, a macromodel-based approach to simulate the large-deformation response of typical RC frame structures is developed. Accurate yet simplified analytical models of beam-column joint behavior were developed to represent essential and critical actions in the floor beams and the transfer of these forces through the joint region to the vertical elements. The validity of the developed macromodels was evaluated through comparison of simulated response with those obtained from detailed finite element analyses and with observed experimental data. Finally, the proposed macromodel approach is utilized in simulation of progressive collapse response of complete RC frames.

This paper presents an experimental and analytical assessment of the performance of steel beam-column assemblies with two types of moment-resisting connections under vertical displacement of a center column, simulating a column removal scenario. The connections considered include (1) a welded, unreinforced flange, bolted web connection and (2) a reduced beam section connection. The test configuration in both cases consisted of two beam spans and three columns. The two exterior column bases were anchored to the strong floor of the testing laboratory, and two diagonal braces were rigidly attached to the top of each exterior column. A downward vertical displacement of this column was imposed using a hydraulic ram, to simulate a column removal scenario. Load was applied under displacement control until connection failure occurred. The study provides insight into the behavior and failure modes of the connections, including their ability to carry tensile forces that develop in the beams. Both detailed and reduced finite element models of the connections are developed that capture the primary response characteristics and failure modes. The detailed models are capable of resolving the nonlinear behavior and failure in great detail, while analyses with the reduced models can be executed much more rapidly, facilitating implementation in models of entire structural systems. Good agreement is observed between the experimental and computational results, providing validation for the detailed and reduced models.

In the three years since Unified Facilities Criteria (UFC) 4-023-03 Design of Buildings to Resist Progressive Collapse was first published in January of 2005, various omissions, ambiguities, and opportunities for improvement were identified by civilian and government designers and engineers. A significant revision to the Progressive Collapse UFC was initiated in the Fall of 2006 and was recently completed, during which a number of significant improvements were made. Occupancy Categories (OCs) similar to those in ASCE 7 Minimum Design Loads for Structures are used to define a building's progressive collapse design requirements; previously, military definitions of levels of protection were used. Indirect design methods for enhancing load redistribution capacity with tie forces and direct design methods using alternate path and specific local resistance continue to be employed but with revisions. The alternate path method now includes dynamic and load increase factors that are based on careful analysis of the inertial and nonlinear aspects of load redistribution. Structural response criteria are specified in terms of force- and deformation-controlled actions, similar to ASCE 41-06 Seismic Rehabilitation of Existing Buildings. A non-threat specific, local hardening procedure was developed and implemented, to insure ductile behavior of critical elements without significant additional cost, for new construction. Finally, a brief overview of three example problems is provided.

The Department of Defense (DoD) Unified Facilities Criteria (UFC) 4-023-03 Design of Buildings to Resist Progressive Collapse was recently revised and a number of significant improvements were implemented, particularly in regards to the direct and indirect design approaches. Direct design explicitly considers progressive collapse during the design process and includes the Alternate Path method, in which the building bridges over a missing structural element, and, the Specific Local Resistance method, in which the building, or parts of the building, are designed for a specific load or threat. In indirect design, resistance to progressive collapse is incorporated implicitly through prescriptive requirements for strength and continuity, typically in the form of Tie Forces, which insure a minimum tensile strength in horizontal and vertical structural members. During the revision of UFC 4-023-03, the effectiveness of the indirect and direct design methods used in existing design requirements was evaluated and research was performed to improve these approaches. For indirect methods, tension membrane and catenary behaviors were used to develop improved Tie Force requirements. For the Alternate Path method, the linear and nonlinear analysis procedures were improved, through adaptation of the overall approach provided in ASCE 41-06 Seismic Rehabilitation of Existing Buildings. In addition, the load and dynamic increase factors were revised, to better account for inertial and nonlinear effects in linear static and nonlinear static models. The research, analyses, and improvements for the indirect and direct methods are reported in this paper.

Over the last 10 years, two United States government agencies have developed guidelines for the design of their structures to resist progressive collapse: 1. The General Services Administration, "Progressive Collapse Analysis and Design Guidelines," (GSA Guidelines) and 2. The Department of Defense Unified Facilities Criteria 4-023-03 "Design of Buildings to Resist Progressive Collapse" (UFC 4-023-03). Although both documents incorporate some of the same approaches, there are notable differences in the application of these procedures. Within both approaches, the main direct design procedure is the Alternate Path (AP) method, in which a structure is analyzed for collapse potential after the removal of a column or section of wall. Different analytical procedures may be used, including Linear Static (LS), Nonlinear Static (NLS), and Nonlinear Dynamic (NLD). Typically, NLD procedures give better and more accurate results, but are more complicated and expensive. As a result, designers often choose static procedures which tend to be simpler, requiring less labor. As progressive collapse is a dynamic and nonlinear event, the load cases for the static procedures require the use of factors to account for inertial and nonlinear effects, similar to the approach used in ASCE Standard 41 "Seismic Rehabilitation of Existing Buildings" (ASCE 41). It is important that the design requirements incorporate appropriate dynamic and nonlinear factors such that the linear static and nonlinear static designs are more representative of the actual nonlinear and dynamic response of the structure.

In the three years since Unified Facilities Criteria (UFC) 4-023-03 Design of Buildings to Resist Progressive Collapse was first published in January of 2005, various omissions, ambiguities, and opportunities for improvement were identified by civilian and government designers and engineers. A significant revision of the Progressive Collapse UFC was initiated in the fall of 2006 and was recently completed, during which a number of significant improvements were made. Readers are encouraged to refer to the newly completed document and its commentary for a discussion of revisions to the tie force procedure as well as alternate path dynamic and load increase factors, structural response criteria and other important topics. The revised Progressive Collapse UFC incorporates three example problems to demonstrate the application of the UFC requirements. This paper discusses example problems using steel, concrete, and wood structures, which illustrate the implementation of Occupancy Categories (OC) and both direct and indirect methods of providing resistance to progressive collapse.

The concepts of structural robustness, disproportioned failure and progressive collapse are usually associated to damage suddenly provoked by accidental actions and abnormal loads. However, damage could also arise gradually in time from aging of structures. Depending on the damage propagation mechanism, such kind of damage may also involve disproportionate effects. For this reason, it is of great interest to develop suitable measures of lifetime structural robustness. In this paper, the time-variant robustness of deteriorating concrete structures is investigated with respect to the ultimate limit state of collapse. The effects of the damage process are evaluated by using a proper methodology for lifetime assessment of concrete structures exposed to diffusive attacks from environmental aggressive agents. The evolution of the collapse load is evaluated by means of a time-variant limit analysis. The percentage residual carrying load capacity of the structure is related to the total amount of structural damage and, in this form, assumed as suitable lifetime measure of structural robustness. This measure is finally used to formulate a robustness criterion to verify if a structural system is robust or weak. The proposed approach is illustrated through the application to the lifetime robustness assessment of an existing reinforced concrete arch bridge.

The investigation of structures concerning their susceptibility to progressive collapse has attracted increased attention in the last decade. The topic is still largely a field of research at present. Neither is there a uniform theory of progressive collapse and collapse resistance, nor exists agreement about the used terms. Enhancing collapse resistance — the short form for resistance against progressive collapse — is the overall design goal in progressive collapse design. According to collapse resistance is defined as the insensitivity of a structure to accidental circumstances, that is, to unforeseeable and low-probability events. Accidental circumstances and their possible effects on the structure as well as insensitivity must be defined and quantified by design objectives. Collapse resistance is a property that depends on both local and global structural features as well as possible causes of initial failure (accidental circumstances). Collapse resistance can be addressed by event control, protection, local resistance or structural robustness.

Robustness of a structure, system, or component (SSC) is its ability to withstand types and magnitudes of demands that are significantly greater or different from the design basis demands without losing its ability to function or without suffering disproportionate damage. The need for robust SSCs has been recognized in view of a number of failures resulting from conditions that were not anticipated during the design developments of SSCs (e.g., human errors in design, fabrication and construction, deliberate attempts to destruct, and fire events). A robust SSC can also be defined as an SSC that is capable of coping well with changes in its operating environment, with minimal damage, alteration, or loss of functionality. In essence, the robustness indicates reliability of an SSC to perform its intended function. Therefore, it is an important characteristic in an SSC that is required for protection of public heath and safety and environment.

Following several recent failures and catastrophic collapses due to man-made and natural hazards, there has been increased interest in upgrading the bridge design philosophy from a member-oriented to a system-oriented approach in order to ensure that bridge structures possess adequate levels of structural redundancy and robustness. To account for uncertainties associated with the quantification of the levels of redundancy of complex structures and be consistent with the reliability basis of recent bridge design code developments, system reliability methods must be applied. In this paper, the authors review the applicability of Markov-based advanced simulation techniques to study the reliability of bridge structural systems. To evaluate the probability of structural collapse due to different damage scenarios, a simulation approach referred to as "DRSCAM-SS" is used. The method combines the benefits of the Subset Simulation (SS) and those of the Delayed Rejection (DR) and Single Component Adaptive Metropolis Samplers (SCAM). The proposed approach is efficient and robust and can be used to obtain approximate expressions for the limit state equations for the pertinent failure modes. Examples are provided to illustrate the applicability of the procedure for evaluating the reliability of bridge systems and quantifying their levels of redundancy and robustness.

Efficient management of transportation infrastructure requires the ability to accurately predict its lifetime performance. The uncertainties associated with deteriorating structures require the use of probabilistic methods to properly asses their lifetime performance. Existing and used lifetime performance indicators thus far are mostly safety based, such as the reliability index and the probability of failure or survival. The lifetime performance of existing structures may also be characterized by survivor functions. These functions allow the consideration of the effect of aging on the decrease of the probability of survival of a structure. Redundancy is an additional structural performance indicator that measures the adequacy of warning available prior to system collapse. Probabilistic measures of redundancy are usually formulated in terms of the reliability index or probabilities of failure or damage occurrence. Survivor functions provide an alternative basis on which lifetime redundancy can be evaluated and its measure can be formulated. The objective of this paper is to present a novel approach for the evaluation of the lifetime redundancy of structural systems. Measures of lifetime redundancy based on survivor functions will be investigated. The lifetime redundancy of various structural systems will be explored, including series, parallel, and series-parallel systems.

This paper provides a comparative evaluation of a recently proposed ductility-centred approach for progressive collapse assessment against more conventional design approaches based on nonlinear static analysis with adjusted load-factors. Several typical structural systems are considered for this purpose, incorporating characteristic elasto-plastic responses with/without tensile catenary and compressive arching actions. It is shown that the ductility-centred approach offers a more rational treatment of sudden column loss scenarios in progressive collapse assessment, particularly when reliance is placed on strength reserves such as due to catenary and arching actions. This approach requires additional knowledge of the ductility supply in connections, though it is contended that ductility parameters should indeed be part of the design specification for structural robustness, particularly given the significant variability in the ductility supply of components which might otherwise have similar strengths. While some recent load-factor approaches also attempt to incorporate ductility considerations, these approaches lack generality and can be unsafe depending on the nature of the underlying nonlinear static response. Importantly, it is shown that the explicit treatment of ductility issues can be undertaken within a practical design-oriented framework which is not overly complicated in comparison with load-factor approaches, thus offering a strong case for the adoption of the proposed ductility-centred approach in design practice.

Recent events such as natural catastrophes or terrorism attacks have highlighted the necessity to ensure the structural integrity of buildings under exceptional events. Design requirements are proposed in some codes but are generally not satisfactory. In particular, it is not demonstrated that, even if these requirements are respected, a structure subjected to an exceptional event will really behave properly. A European RFCS project called "Robust structures by joint ductility" has been set up in 2004, for three years, with the aim to provide requirements and practical guidelines so as to ensure the structural integrity of steel and composite structures under exceptional events through an appropriate robustness. The investigations performed at the University of Liege, as part of this European project, are mainly dedicated to the exceptional event "loss of a column in a steel or steel-concrete composite building frame"; the main objective is to develop a simplified analytical procedure to predict the frame response further to a column loss. The development of this simplified procedure is detailed in two complementary PhD theses: the thesis of Demonceau J.-F. and the thesis of Luu N.N.H. Present paper describes experimental and analytical studies carried out in [Demonceau, 2008]. In particular, a simplified analytical procedure for the prediction of the global frame response when significant membrane forces develop further to a column loss will be described; it allows: (i) to predict the development of the catenary action in a frame with joints subjected to combined bending moment and tension loads and (ii) to compute the requested rotation capacity at the joint level according to the loads applied on the frame.

Recent studies of progressive collapse have sought to move the design basis from one of the simple following of prescriptive requirements to approaches based on understanding, modelling and quantitative assessment. A key requirement for such approaches is the definition of a suitable failure criterion — expressed both in physical terms and in a way that accords with traditional views of structural analysis and design. The method developed at Imperial College London checks the ability of the damaged structure to attain a new equilibrium state expressed in terms of available connection rotation capacity. The effects of variations in connection type (and therefore properties) in improving resistance to progressive collapse may therefore be examined explicitly and quantitatively. Illustrative studies of this type — which employ new developments in connection modelling and frame analysis — are reported herein. The results show which strategies are most effective in providing enhanced resistance to progressive collapse.

Joints can play an important role in ensuring the robust performance of framed structures. Their adequacy is not only related to their resistance and deformation capacity but also to their capability of following without collapse the load-deformation path imposed to them by the accidental damage of the frame. This paper presents some of the results of a study of flush end-plate joints and T-stub components under combined forces.

European guidance in EN 1990 and EN 1991 provides preliminary guidance for determination of appropriate design approaches for mitigation of progressive collapse. This preliminary guidance is somewhat devoid of detail with respect to applicability and analysis. It also does not lend itself well to consensus based professional and industry approaches extant in the US. Recent updated guidelines for the US DoD incorporate prescriptive approaches as a function of Building Occupancy Category, related to similar categories in existing IBC[1] Section 1604 requirements. Additionally, recently adopted IBC changes now incorporate "tie provision" requirements for IBC Section 1604 Occupancy Category III and IV buildings that are greater than 75-ft in height. The SEI Progressive Collapse Standards and Guidance Committee is developing guidance more applicable to the US approaches, and has developed preliminary thoughts towards the eventual adoption of mitigation strategy applicability.

SEI's Progressive Collapse Standards and Guidance Committee is developing a guidance document to address collapse resistance in buildings. This comprehensive guidance document will support existing design standards and building codes and may be applicable to buildings and structures only as dictated by local statute. The guidance document will provide a variety of different approaches for design to resist disproportionate collapse. These approaches include the alternate path approach for bridging over a damaged zone, hardening of structural elements through specific local resistance and segmentation through compartmentalization. The guidelines will address the rigorous evaluation of damage resulting from a pre-determined threat or the potential for progression of collapse due to a threat independent prescription of initial zone of damage. Rigorous analytical methods will be presented along with the simplified approximate methods. These analytical approaches will help guide practicing engineers who must evaluate the behavior and stability of a wide range of structural configurations to varying extents of initial damage or specified threat. To help illustrate application of the guidelines, examples will demonstrate the implementation of the analytical procedures.

At lower levels of risk of disproportionate collapse, prescribed levels of robustness could be incorporated into the structural elements and structural system, even in the absence of specified threat or probability of occurrence. As a minimum, robustness can be added to an element or system through the prescription of integrity requirements such as element-to-element connectivity, balanced minimum requirements for multiple response mechanisms, and enhanced stability requirements (assumption of increased unsupported lengths, etc.) At higher risk levels, robustness requirements might include "tying" requirements, "bridging" techniques, prescriptive key element hardening, or, for certain form factor buildings, compartmentalization.

The ASCE SEI Committee on Progressive Collapse Standards and Guidance is developing a guidance document to address collapse resistance in buildings. The document will provide a variety of approaches for design to resist disproportionate collapse. In addition to providing design approaches, the document will also include guidance for owners, architects, and engineers on how to use risk assessment to determine the likely loads on the building and the recommended design approaches to reduce the potential for disproportionate collapse. Risk assessment provides a rational basis for decision-making related to design against disproportionate collapse, particularly when there is currently no consensus as to when such design is needed and to what level of resistance. The risk assessment informs the design process by focusing on: defining the desired performance level, identifying the hazards (including likelihood of occurrence and corresponding loads), and ultimately determining the specific design criteria that will — for the likely loads — achieve the desired performance. This process has been developed and vetted in recent years for specific hazards (e.g., seismic design), but has not been fully developed for collapse prevention for buildings subjected to multiple hazards, particularly intentional hazards. The focus of this paper is on the development of this process, i.e., use of risk assessment for identifying the hazards (as a function of building use, occupancy, location, criticality, size, structural vulnerabilities, architectural features, etc.), the likelihood of hazard occurrence (including the options for hazard avoidance), the anticipated loads, and the recommended design approaches to reduce the potential for disproportionate collapse.

A disproportionate (or progressive) collapse of a building or other structure is one that initiates from local damage and subsequently propagates to encompass a major portion of the facility. Current concerns with such collapses stem from changes in building practices and sociopolitical challenges that have highlighted the importance of hazards that historically either have not been viewed as significant (explosions or detonations) or have been dealt with through deemed-to-satisfy clauses rather than through formal structural calculations (severe fires). Practical strategies for mitigating the risk of disproportionate collapse of buildings can be evaluated using principles of structural reliability and probabilistic risk assessment, supported by advanced computational tools. This paper summarizes a number of these strategies, critically appraises major design issues — scenario-based vs non threat-specific analysis; direct vs indirect design; alternative path vs specific local resistance — and discusses prospects for implementation of criteria for collapse mitigation in ASCE Standard 7.

In response past terrorist events and to actions required by executive order, the U.S. General Services Administration (GSA), Office of the Chief Architect (OCA), designed and executed a steel frame bomb blast and progressive collapse (PC) test program (2004–2007) ["the GSA Test Program"]; enhanced by its integration into the `Discrete Leto' Blast Effects Test Series conducted by the U.S. Defense Threat Reduction Agency (DTRA), to experimentally evaluate the behavior of varied beam-to-column connection types subjected to such severe loading events. The broad objective of the GSA Test Program was to develop appropriate PC design criteria for steel frame construction, including identification of desirable connection characteristics for blast and PC mitigation that result in reliable high-capacity performance.

The internal forces increase in structural components neighbouring a member, usually a column, removed or destroyed as a result of manmade or natural hazards, such as an explosion. If the additional internal forces created by an initial small or local structural failure can not be efficiently redistributed within the structure, damage spreads and partial or total collapse (progressive collapse) of the building occurs. Progressive collapse has been of an increasing concern in the structural engineering community, especially since the collapse of the World Trade Center towers in 2001. The terrorist attacks showed that well-designed and robust modern buildings can be susceptible to progressive collapse. The Ronan Point collapse of 1968 in London, and the bombing of the Alfred P. Murrah building in Oklahoma City in 1996 are the most publicized examples of progressive collapse [1]. Therefore, it is necessary to design of buildings to resist progressive collapse. In this study, progressive collapse performance of an actual building was investigated. The Ohio Student Union building, located on the Ohio State University campus, was also tested by physically removing four first story columns from one of the long perimeter frames prior to building's scheduled demolition. Our field experiments and analytical studies provide both practical and fundamental information on the collapse response of an existing building with a regular structural configuration. A commercially available computer program, SAP2000 was used to model and analyze the building following the General Services Administration (GSA, 2003) guidelines. The elastic static and nonlinear dynamic analysis results are presented and their implications are discussed.

Structural robustness is recognised as a desirable property of structural systems which mitigates their susceptibility to progressive collapse. However, there is some confusion in the literature regarding the usage of the terms structural robustness and collapse resistance as well as competing expressions such as prescriptive vs. performance-based, threat-specific vs. threat-independent or indirect vs. direct design. This article tries to distinguish between the different meanings of these four pairs of terms, which are important concepts in the field of progressive collapse. In this context, design strategies and associated methods to prevent progressive collapse are briefly explained.

Mechanisms of progressive collapse resistance are described which help identify new design requirements that effectively improve collapse resistance of structures. Criteria must be established to identify structures for which progressive collapse resistance may need to be enhanced through the implementation of additional design requirements. Enforcing disproportionate collapse-resistant design criteria beyond current practice for all structures may not be justified because most buildings are not at significant risk. Simplified approaches may be sufficient for buildings that require explicit evaluation of their progressive collapse resistance following column removal. One such approach is based on a comparison between the strength of the structure under a downward force at the location of removed column and the axial compressive force in the column. If the structure is identified to be susceptible to collapse, more advanced analysis is justified. Such analyses may be required more often in evaluation of existing structures than in design of new structures. The results of such analyses involve significant uncertainty and their reliability heavily depends on the modeling and analysis assumptions. One of the more important assumptions that require further investigation is the deformation capacity of elements subjected to interacting actions (such as tension combined with flexure and/or shear). Features that are important in modeling structures for collapse analysis as well as parameters that are important in developing progressive collapse resistant mechanisms require further studies.

Collapse analysis can be used to assess the performance of a structure following the sudden destruction or removal of one or more columns or wall segments. The sudden removal usually takes place in a few milliseconds, with high local strain rates. This phase is not considered here. As forces are redistributed within the structure, it deflects dynamically, reaching a maximum deflection after roughly one second, with strain rates similar to those during an earthquake. This is the phase that is considered in this paper. The concern for design of the building is whether a substantial part of the structure is likely to collapse. This is commonly termed "progressive" or "disproportionate" collapse. The term "disproportionate" is used here. One way to assess the potential for disproportionate collapse is to use nonlinear structural analysis to simulate the dynamic behavior of the structure. This paper uses simple energy balance to explain the essential nonlinear behavior of a frame structure following sudden removal of a column. Based on this explanation, the paper argues that for most structures it is a futile exercise to use nonlinear analysis to assess the potential for disproportionate collapse. Instead, linear analysis can, and should, be used.

A time-domain parameter estimation algorithm for direct estimation of the stiffness properties of a torsional shear building has been developed. The algorithm decouples the banded global stiffness matrix into story stiffness matrices, and therefore is capable of identifying the lateral and torsional stiffness parameters of each story independent of others. Covariance matrices of the structural response to unknown sources of excitation are used in order to eliminate the external forces from the equations of motion. The method can be applied to ambient vibration data gathered before and after a severe event for damage identification and localization up to the story level. The proposed method is verified using analytical data from Phase 1 of the IASC-ASCE benchmark problem.

There is need to develop and evaluate new strategies for structural health monitoring, focusing on efficient problem decomposition to rapidly and accurately determine the occurrence, location and level of small changes in the underlying structural characteristics of a densely and continuously monitored structure based on its response to known and unknown forces. Under certain conditions found in many mid- and high-rise buildings, one can model a structure using a chain-like MDOF model with story masses connected above and below by single springs. If complete set of response sensors are installed at every floor, one can then apply a robust time-domain identification technique for chain-like MDOF systems in which the identification of each link of the chain is performed independently to estimate inter-story restoring functions. These are the mass-normalized local stiffness and damping values, and can be linear or nonlinear. This study presents the methodology applied to a full-scale 17-story building located on the UCLA campus. The Factor Building is permanently instrumented by the USGS Advanced National Seismic System (ANSS) with a state-of-the-art dense array of 72 accelerometers with four accelerometers on each floor. High-resolution, 24-bit data are being continuously recorded and archived through ANSS. In this study, we considered 50 days of continuously-recorded ambient vibration data and also segments of elevator-induced vibration data. Chain system identification was successfully implemented using these output-only and input-output data. Probability distributions of the estimated coefficients of displacement and velocity terms in the inter-story restoring functions are presented. Variability of the estimated parameters due to environmental conditions is investigated for the 50 days' data to understand the underlying real-world statistical variability. Results show promise for the continuous monitoring and detection of structural changes due to earthquakes or other extreme loads if dense, high-resolution data are available.

Finite element models are idealistic representations of actual structures and need to be updated in order to represent accurately these real structures. Multiple solutions may arise from the model updating process depending on the complexity of the structural model, the amount of variables involved in the updating process, and the uncertainties in the system identification part. This paper focuses on studying the effects of the uncertainties of modal parameters on the appearance of multiple solutions of posterior density functions used for model updating. In this study different sensor configurations and standard deviations for the natural frequencies and mode shapes are used, in order to show that the appearance of multiple solutions is largely influenced by the system identification uncertainties. The methodology used is based on Baye's probability theorem, which allows us to update a probability distribution based on data obtained and a prior knowledge of the system.

Evaluating up-to-date structural properties of an existing bridge is essential to improve the accuracy of the assessment for the structural performance. In this study, we propose a methodology to update the structural properties of a bridge using modal parameters considering measurement error. Modal parameters are usually obtained from a vibration testing conducted on a bridge. Time domain decomposition (TDD) is used in this paper to extract modal parameters from time history acceleration responses. The measurement error from the extraction process and from the noises in the acceleration sensors are estimated and considered in estimating the structural properties. There are five steps in the proposed framework: 1) extract modal parameters from a vibration test; 2) build a preliminary finite element model (FEM) based on the bridge design drawings; 3) estimate the measurement error by simulating vibration tests; 4) update the preliminary FEM to a baseline model using a Bayesian model updating approach with mode shapes; and 5) identify local structural stiffness damage using the damage index method with model shapes of the existing bridge and of the baseline structure found in the previous step. The proposed framework is constructed using the Bayesian model updating and the damage index method. The proposed framework is illustrated with a simulated finite element model of the Lavic Road Overcrossing bridge, a typical box-girder RC highway bridge in California.

The use of un-bonded post-tensioned (PT) reinforcement in concrete structures has been popular for many decades in parking ramp structures. The use of PT reinforcement in parking ramp structures greatly reduces the amount of cracking and possible leakage in the structure and also allows economical long-span construction. The boom of parking structures in the United States began in the 1960s which coincides with the start of heavy use of deicing products. Many of these early PT structures have been found to be in a deteriorated condition with varying degrees of damage to the PT systems. This paper presents a detailed explanation of the deterioration mechanisms that affect post-tensioned ramp structures addressing the most common types of PT systems: button head and sheathing strand systems including current testing techniques utilized in the in their evaluation. Detailed case studies are presented to illustrate the detrimental effects of deterioration mechanisms on post-tensioned structures providing specific repair recommendations in order to mitigate and prevent structural related problems for un-bonded post-tensioned parking ramp structures.

The Lakewood Condominium is a residential building consisting of three timber-framed stories supported on a structural concrete slab above the parking garage. The slab is supported on reinforced concrete columns and walls. The homeowners experienced multiple problems with their units, including water intrusion, deflection of the floors, jammed doors and windows, cracks in the drywall finishes, etc. The primary issue at Lakewood Condominiums was found to be the structural slab, which is deflecting due to deficient design. Water entry at the deck-to-wall transitions collects in the `bowls' of the non-planar slab, resulting in water migration through the damaged slab. This paper covers the forensic investigation, structural evaluation of the existing situation, several proposed repair alternatives, and the final structural repair recommendation.

When deficiencies in load carrying capacities are discovered in reinforced concrete elements, various options for stregthening the deficient members are evaluated by engineers in consultation with the owners and other stakeholders. These include removal and replacement of the deficient elements, strengthening by adding conventional reinforcement and encapsulating the new reinforcement by formed and placed concrete or by shotcreting or using externally bonded reinforcement. Externally bonded reinforcement can comprise of either epoxy bonded steel plates or epoxy bonded fiber reinforced polymer (FRP) composites. Some case histories of projects where externally bonded reinforcement was used to restore structural capacities of deficient structural members are discussed in this paper.

The Second Avenue Subway will be the first major expansion of New York's subway system in over 50 years. As shown in Figure 1 below, it will start from 125^th Street in Harlem and will connect directly to the Financial District near Wall Street. Phase 1 is already under construction and consists of twin bored tunnels, three new stations at 96^th Street, 86^th Street and 72^nd Street, and major modifications to an existing station at 63^rd Street. The new station structures will be located directly under the roadway of Second Avenue and are either cut and cover construction or mined rock blasting depending on ground conditions. The entrances will be formed within adjacent existing high rise buildings. This paper will focus on the issues associated with the design of three of these new entrances. For the purposes of this paper these are referred to as Entrance A, B and C. These entrances will incorporate new large open spaces at street level and below for lobbies and banks of escalators and elevators. This will require the removal of existing high load columns, shear walls and diaphragm slab areas. New load transfer structures, underpinning and strengthening will be required to ensure structural integrity is maintained.

The Antelope Valley Indian Museum is a historic one- and two-story wood-framed structure, built in the late 1920s through the 1930s near Lancaster, California. Given that the museum collection was inadequately protected from temperature fluctuations, the State of California embarked on an ambitious program to insulate the building, heat and cool the museum using geothermal heat exchangers, and structurally and seismically strengthen the building. The existing structure was not engineered and was built in several phases with a minimum of structural framing components and connections, even considering standard construction practices in the 1920s and 1930s. The addition of insulation and associated wood framing to the roof of the museum adds a significant amount of weight — nearly doubling the weight of the structure. As the interior finishes are actually the exposed structural elements, structural strengthening to protect the historic elements of the building can result in the diminishment or destruction of the very historic elements one is trying to protect — particularly if the strengthening is not performed judiciously; consequently any structural intervention must be carefully weighed against the potential impacts on the historic fabric and against the cost of the work. This paper addresses the stabilization measures that are currently in the process of being implemented, focusing primarily on the structural aspects of the project. In terms of structural strengthening, the authors selected an unusual arrangement of angled and inclined external stainless steel wire ropes (tension guy wires) to provide increased lateral stability, and horizontal internal wire ropes (tension collar ties) to increase the vertical load capacity of the gabled roof areas. The tension guy wire ropes lend themselves fairly well to bracing this light structure; since they connect directly to the roof joists, the wire ropes take lateral forces directly down to either stainless steel eyebolts epoxied into the surrounding rock or small concrete anchor blocks. The tension collar ties are used to create trusses out of the inclined gabled roof joists. Since the wire ropes are relatively unobtrusive, since they are clearly not part of the original construction, and since they are almost wholly reversible, the addition of wire ropes appeared to be a reasonable solution for the majority of the museum.

The retrofit and repair of pre-stressed elements has been a difficult issue to address both from a structural diagnosis standpoint and from a method of repair point of view. Difficulties such as identifying structural problems, quantifying the existing capacity of damaged pre-stressed elements, and finding the soundest yet most cost effective repair method are some of the difficult challenges encountered when analyzing the possibility of repairing damaged pre-stressed elements. To illustrate the aforementioned concerns, a case study recently completed will be discussed in depth. The observation of large diagonal cracking in the pre-cast pre-stressed double-tee beams led to the realization that a structural problem requiring attention existed. In order to mitigate the issue, a number of repair methods were explored. Due to the uncertainty of the structure's existing condition, in-situ testing was relied upon to verify the capacity of the cracked elements found by structural analysis and to validate the proposed advanced fiberwrap composite repair alternative. In the end, advanced fibrwrap composites were the strengthening systems chosen for the repair.

The objective of this paper is to provide analytical expressions for the elastic cross-section local buckling stress, including element interaction, of hot-rolled steel structural shapes. The cross-section local buckling stress is determined by finite strip analysis. Local stability of each cross-section is considered in axial compression, as well as positive and negative bending about the major and minor axis. All sections in the AISC shapes database are considered, with the exception of pipe sections. The local buckling stresses are converted into plate buckling coefficients (k's) and simplified expressions provided for all observed k's. The plate buckling coefficients (k's) from the finite strip analysis are also compared to the values inherently assumed in the AISC Specification. Significant differences between assumed (AISC) and calculated k values are observed.

The subject of this paper is the structural assessment and repair of multiple large pile cap foundations. A combination of high temperatures during early hydration and inadequate steel reinforcement resulted in large cracking that compromised the structural integrity of the caps to the extent that repair and modification were necessary. New pile caps, placed on top of the existing caps, were designed to support the existing loads and superimposed loads that would result from a future buildout. The new pile caps were designed in accordance with the strut-and-tie provisions of the 2002 ACI 318 Code and constructed with a high-strength, self-compacting concrete.

This paper focuses on the realistic fire behavior of floor systems. An analytical approach based on the finite element method was used to model and analyze the composite floor system of a 10 story office building. A set of nonlinear analyzes were performed to evaluate the behavior of unprotected floor systems subjected to realistic design fires at three different locations, i.e., corner, exterior, and interior compartments. The results show that primary and secondary beams undergo large structural deformations and develop large axial forces at the beam ends during the heating and cooling phases of the fire. These axial forces are greater than the tension capacities of the beam end shear connections at ambient and elevated temperatures. This leads to the need for structural fuse elements that can limit the axial force transfer to the beam shear connections.

Structural collapse may be initiated by numerous hazards, including earthquakes, hurricanes and blasts. Steel gravity framing systems, present in virtually every steel building constructed in the United States, may be particularly susceptible to collapse, especially when a column suffers damage that compromises its ability to carry gravity loads. In order to assess the ability of current gravity framing systems to prevent collapse when column damage occurs, deformation and strength demands on connections must be evaluated in the context of overall system behavior, including the development of large vertical deformations. Once known, means to enhance the threat-independent robustness of gravity framing systems can be developed, preventing future collapses. This paper proceeds a parametric study developed to assess the behavior of typical steel gravity framing systems bearing various layouts, member sizes, and connection configurations under loading consistent with column loss scenarios. Connection rotation and displacement demands are examined for systems utilizing various connection types including i) single plate shear connections ii) bolted web angle connections, and iii) bolted top and seat angle connections, under typical gravity loading to assess the development of large vertical deformations.

When designing for the limit state of collapse prevention, engineers must be able to compute the ways in which structural components fail and how the failure of one or several members can affect the response of an entire structural system. Past research on the response of structures under extreme loading conditions, including progressive collapse, has focused on structural frames with only limited attention given to other structural components. Though often not modeled explicitly, concrete floor slabs can contribute significantly to overall structural performance during extreme loading events, and the ability to efficiently include nonlinear slab behavior in structural models is important for assessing the collapse potential of a structure. In this paper, the authors address the importance of accounting for floor slab contributions in carrying out collapse simulations of structures. Modeling approaches include simplified "equivalent frame" models and high-fidelity finite element models. Preliminary findings suggest that floor slab contributions to collapse resistance are significant and should be accounted for when assessing the performance of structures.

Hydrocarbon vapor-phase explosions have long been known to create destructive overpressure and impulse loads when ignited both inside and outside buildings. This creates a hazard to both building structural elements and personnel. We have recently implemented the Baker-Strehlow and Baker-Strehlow-Tang curves into our Blast Analysis Module (BAM) code which computes the performance of structural members exposed to air-blast to determine component level of damage. A case study on the design of an internal laboratory cell to an accidental vapor-phase explosion of jet-fuel will be presented. The lab is 3150 sq. ft. with a 31 ft. high roof, and it is internal to a larger multi-story structure (approx. 35,000 sq. ft.). This cell must be designed to survive an explosion with a level of damage that prohibits collapse and personnel fatality, and it may be vented to the atmosphere to reduce the internal gas pressure as described by ASCE. The case-study will document the overpressure and impulse time history inside the lab cell, demonstrate the implications of structural hardening of the walls and roof with deflagration venting, and the final design with the response of the system.

ABS Consulting, Inc. has developed a new comprehensive building database to enhance the fidelity of the screening level facility siting study. The building database includes 113 potential industrial building configurations, which are classified according to their structural frame, wall and roof systems. Building damage levels are determined based on the predicted damage levels of the primary and secondary components. A single-degree-of-freedom structural dynamics program (SBEDS) was used to develop the component damage level curves. The result is a more comprehensive building type database.

This paper provides an overview of assessment of building hazard in petroleum and chemical facilities. A brief overview of explosion hazards is also provided. Then, the paper focuses on two specific aspects, that of Detailed Building Structural Analysis and Building Upgrade Conceptual Structural Design.

Structures that are typical of the petrochemical and process industries have structural forms that confound the application of standard wind load estimation techniques. The exposed framing, partial cladding, complex arrangements of ancillaries, and serpentine piping arrangements common in these facilities are beyond the scope of standards such as ASCE 7 (2006). Nevertheless, these structures must be designed to reliably resist wind loads. In 1997 an ASCE task committee released a guide publication entitled, Wind Loads and Anchor Bolt Design for Petrochemical Structures which reported the current state of practice for estimating wind loads for these structures. This document also recommended techniques for estimating wind loads that were based on the combined experience and research of the committee members. This task committee is underway again, and a new version of the publication is expected to be produced shortly. This paper summarizes the results of a series of wind tunnel experiments and analytical investigations performed at Louisiana State University over the past few years, which are relevant to estimation of wind loads for petrochemical structures (Amoroso, 2007). Results from these studies have led to some pending amendments and revisions to the recommendations contained in the original 1997 document.

Engineers working in the petrochemical industry are commonly required to deal with changes and modifications to an existing facility. These changes and modifications must be recognized and evaluated for their impact on the existing equipment support structures and foundations. The updated ASCE report "Wind Loads on Petrochemical and Other Industrial Structures" provides references and suggested guidelines that should help the practicing engineer who is involved in an engineering evaluation of an existing structure or equipment foundation subject to extreme winds. The evaluation of existing structures for extreme wind loading should be approached differently than when designing new facilities. Conservative design loads and a conservative application of the design loads are common practices in the design of new petrochemical facilities, and this conservatism usually has minimal impact on the installed cost of the facility. But, this same conservatism in the evaluation of an existing structure or foundation could result in unnecessary upgrades and increased costs for the owner. The following sections provide guidelines, references and special considerations that should be used when evaluating wind loads on existing petrochemical structures and foundations, and the focus is on eliminating the conservatism associated with the design of new structures. The topics to be covered include the wind exposure category, the wind load combinations, special considerations for hurricane prone regions, and other considerations and that are unique to an evaluation of an existing structure, such as the potential for past modifications or deterioration of the structure. Finally, some references to model building codes that address modifications or additions to existing buildings or structures are provided.

This paper discusses the dynamic modeling of a wind turbine and gravity base foundation system in order to understand the foundation motion during a turbine transient event. This information is used to evaluate the potential for turbine generated inertia loads, such as those caused by an emergency stop event, to cause liquefaction of the soils beneath the foundation. The transient time varying loads obtained from the turbine model are input into a finite element dynamic system model to accomplish this task. The first step is to calculate the Eigenvectors and Eigenvalues of the system; then apply the turbine loading to obtain the results for the motion at the bottom of the foundation footing. Classical methods are then used to evaluate the liquefaction potential.

This paper discusses the fatigue design of steel tubular towers serving as the support structures for large power-generating wind turbines. Unlike wind and earthquake analysis for most routine structures, fatigue analysis is not a mainstream concern for structural engineers. However, wind farm tower structures are subject to high-cycle fatigue on the order of several hundred million cycles over a typical 20-year design lifetime. In fact, fatigue loading often governs tower design. This paper focuses on the S-N curve approach using the Miner's Rule damage summation method as implemented in the Eurocode. European fatigue guidelines serve as the current standard of the wind industry worldwide and even in the U.S. market. This paper will discuss and illustrate fatigue concepts such as the following: 1) Fatigue loading basics including the popular "Rainflow" cycle counting method 2) The fatigue load spectrum extracted from the Markov matrix 3) The fatigue damage-equivalent load and its calculation from the fatigue load range spectrum 4) Construction details and fatigue Detail Categories 5) The "hot spot" stress approach in accordance with International Institute of Welding (IIW) guidelines that provide methods to calculate stress concentration factors from finite element analysis models. 6) Fatigue calculations using a load-to-stress transfer function that is nonlinear.

In this paper we mainly focus on using nanoplatelet reinforced polymers using exfoliated graphene nano platelets (XGnP) from XG-Sciences and Polyhedral Oligomeric Silsesquioxane (POSS) retrofitting masonry walls against blast loadings. The polymer used in polyurea Line-XS 350 from Protective Coatings. Nanoplatelet reinforced polymers are used because they have great potential to be low cost, high performance structural materials. It was observed that POSS enhanced polyurea significantly enhances the performance of masonry walls. On the other hand, minor improvement was observed for the case of polyurea enhanced with graphene platelets.

Data collected from intentional acts of terrorism, as well as tragic accidents leading to bridge collapse, have demonstrated the significant impact the loss of a key highway component can have on a community, region, or even nationally, both in terms of financial implications and societal concerns. Such events, while infrequent, need to be considered by structural engineers in order to provide safe and resilient bridges for the travelling public. This paper presents design and detailing guidelines for reinforced concrete bridge columns subjected to blast loads. The guidelines are based on an extensive research program involving half-scale bridge columns subjected to close-in blasts as well as detailed finite element modeling. To evaluate how design approaches for other loading cases respond when subjected to blast, column specimens included those based on typical seismic detailing in addition to "gravity-only" cases. Furthermore, columns designed specifically for blast were also tested. Based on the findings from this research, design provisions for columns subjected to extreme loads have been developed.

The present study presents a methodology for evaluating performance of steel moment resisting frames subject to extreme lateral loads such as earthquake, blast, tornado, and surge. Current analysis and design procedures specialized to individual hazards are viewed in a broader context in which multiple hazards are considered together. The premise is that the multi-hazard view will more effectively identify trade-offs in design choices that meet a specific hazard only and will lead to a better appreciation of both safety and economy especially in regions not dominated by a single hazard. This paper focuses on the design of a steel moment resisting building frame using AISC 360-05 composite beam flexural strength and beam-column interaction equations. As an example of the multihazard approach, beam and column sections are designed to satisfy a seismic load case and associated load combinations provided in the International Building Code 2006 and by reference ASCE 7-05. The frame is then analyzed under demands from an accidental blast pressure load case and load combination developed according to TM 5-1300 guidelines. Conclusions regarding potential instability are established using AISC beam-column interaction ratios applicable to the seismically resistant columns accounting for moments estimated using finite element models of gravity response of the frame system to a post-blast state of the damaged system. Observations are offered on how the blast analysis might affect the design process if brought in at a pre-construction rather than a retrofit stage.

Investigation led by the National Transportation Safety Board (NTSB) suggests that inadequate design of gusset plates played a key role in the collapse of the I-35W Bridge in Minneapolis, Minnesota. The particular gusset plate connection was at a panel point designated as U10. Further understanding of the behavior of the U10 gusset plates is essential. In the present study, the U10 gusset plates were analyzed using detailed nonlinear finite element models. The forces delivered to panel point U10 were reproduced using available information of the bridge. These forces were, in turn, used to calculate possible stress and strain states of the U10 gusset plates. The results indicate that substantial portions of the U10 gusset plates were yielded at the time of collapse. Weight increase due to deck reconstruction, staged construction materials, and thermal effects, along with insufficient strength of the gusset plate, seem to be the main contributing factors to the substantial yielding. The results also suggest that the interaction of large compression and large shear should be addressed. This interaction is not well understood.

This paper introduces a compliance ratio (CR) method for the estimation of energy release rates in structural steels. In the method, the elastic and plastic components of energy release are calculated from the area under the load versus load-line displacement assuming no crack extension that can be constructed directly from the standard unloading compliance curve. The method is used to investigate the energy release rates in the compact tension, C(T), fracture specimens fabricated with both conventional and high performance steels used in bridge structures. The conventional steels consist of the ASTM designations A572, A588, and A913. The high performance steels are the ASTM designation A709 grades HPS 345W, HPS 485W, and HPS 690W. The results indicate that (1) the total energy release rate is large at an early stage of crack growth and approaches a constant value in specimens with long crack extension; (2) the elastic energy release rate is significantly smaller than the plastic energy dissipation rate during crack extension for all steels; and (3) the elastic energy release rates in most HPS are higher than those in conventional steels. The paper also shows that the elastic energy release rates obtained from the method are in agreement with those calculated by the stress intensity factor equation in ASTM E399-90.

Post-weld treatment by various peening methods (i.e. needle or hammer peening, ultrasonic impact treatment) has received recent attention as a promising means for extending the fatigue lives of existing steel bridges or for enhancing the fatigue performance of new welded civil infrastructure. Peening treatments work primarily by introducing compressive residual stresses near the treated surface, which have the effect of reducing crack growth rates at the smaller crack depths. Recent research [1–4] has examined the effects of variable amplitude loading conditions on the fatigue performance of welds subjected to various peening treatments. This research has included fracture mechanics analysis using various models and test-based studies considering a variety of stress histories. In general, it has been observed that stress histories containing periodic compressive overload events can result in a reduced treatment benefit. The benefit of the treatment is predictable though and may still be substantial. Herein, a strain-based fracture mechanics model, validated elsewhere [2], is used to study the effects of needle peening to enhance the fatigue performance of steel bridge welds under realistic service loading conditions. To do this, a typical transverse fillet weld detail is analyzed under several load histories generated by simulating measured truck data passing across influence lines for the critical locations on bridge girders with two typical configurations (simply supported and two-span continuous). Following this, an additional series of analyses is performed to investigate the influence of stress histories for highway bridge welds subjected to periodic overload events that may occur, for example, due to the passage of trucks with axle loads that exceed the legal limit or convoys of heavy trucks. Based on these analytical studies, it is found that peening treatments can result in a significant fatigue life increase under realistic service loading conditions, including those containing periodic overloading events.

Composite concrete and steel slab-on-girder systems are used for the superstructures of highway structures, including underpasses and overpasses, throughout North America. In certain jurisdictions, it has been reported that large numbers of these structures show evidence of serious corrosion in the webs and the bottom flanges of the girders. The cause of the corrosion is believed to be a combination of moisture from melting snow, road salt, and sulphur dioxide. In a number of observed cases, the most heavily corroded regions of these bridges appear to coincide with the splash zones that are present due to the passage of large trucks. In order to facilitate the probabilistic structural analysis of these deteriorating structures, an analysis program has been developed, which enables the calculation of their diminishing structural reliability with the passage of time. Specifically, the effects of corrosion on the shear, moment, and bearing resistance of the girders are evaluated in accordance with the limit states outlined in the Canadian Highway Bridge Design Code [1]. Statistical distributions are then applied to the load- and resistance-related input parameters, including those associated with various corrosion models. A Monte Carlo simulation is then performed to generate curves of bridge reliability versus time for various assumed corrosion rates. The focus of this study is lifespan prediction of weathering steel highway structures. Weathering steel is a type of high-strength, low-alloy steel which has been found to behave favourably with respect to atmospheric corrosion resistance. The steel contains small amounts of nickel, chromium, and copper; it is available as Type A or Type AT, as designated in CAN/CSA G40.21-M92 [2]. Under repeated cycles of wetting and drying, weathering steel forms a thin, adherent oxide patina, which afterwards protects it from further penetration of oxygen and moisture that leads to corrosion. This patina is supposed to form in 18 to 36 months [2].

This paper describes an investigation of prefabricated modular floor and roof panels comprised of a thin-walled Engineered Cementitious Composite (ECC) slab and a steel truss substructure. The design, manufacturing and structural behavior of the composite panels are described and their structural behavior is compared to that of conventional hollow core decks. The features of the composite panels include light weight, high loading capacity and adequate stiffness, a modular manufacturing process with adaptability to various loading requirements, and the efficient utilization of material resources and industrial byproducts. The work described in this paper is a continuation of previous activities on composite floor and roof panels in which light gage steel joists were integrally cast with the ECC slab. The modular concept introduced in this paper aims at improvements in the manufacturing process of the panels by casting the ECC slab separately and subsequently joining it with the steel trusses. The focus of this paper is on design and manufacturing of a prototype modular panel and on its structural behavior under service conditions and at ultimate.

Engineered Cementitious Composite (ECC), a highly ductile concrete material with a tensile strain capacity several hundred times that of normal concrete, is establishing itself as a new construction material that can lead to substantial improvements in structural performance. A number of full-scale structural applications of ECC in the US and in Japan indicate that various unique characteristics of ECC, including its tensile ductility and self-controlled crack width, can be exploited to enhance structural safety and durability. This paper reviews the unique characteristics of ECC important to structural applications. Specifically, the tensile, shear and flexural ductility of ECC, and the distinctive nature of microcracking during inelastic deformation will be discussed in the context of structural member damage tolerance and energy absorption capacity under earthquake loading. The ductility of ECC will also be discussed in the context of resistance to penetration and fragmentation in protective structures subjected to impact loads. Finally, it is suggested that the ability to systematically tailor ECC for strength, ductility and stiffness, as well as other material characteristics provides expanded design space for structural member performance optimization much beyond that with normal reinforced concrete. This paper draws heavily from that contained in a book chapter on ECC by the author and from an article on impact resistance design of ECC.

This paper reviews recent research on the durability properties of Engineered Cementitious Composites (ECC), a special type of high-performance fiber reinforced cementitious composites designed with micromechanical principles, under various environmental and mechanical loads. The durability subjects include (a) ECC cracking and transport properties (permeability, absorption and diffusion), (b) corrosion resistance (c) freeze-thaw and salt scaling resistance, (d) performance under hot and humid environment, and (e) performance under high alkaline environment. The research results indicate that due to intrinsic self-control tight crack width and high tensile strain capacity, many durability challenges confronting concrete can be overcome by using ECC. The enhanced performances of ECC under mechanical and environmental loads are expected to contribute substantially to improving civil infrastructure sustainability by reducing the amount of repair and maintenance during the service life of the structure.

More than ever, engineers are realizing the importance of sustainability-oriented design, construction, operation, and demolition of the built environment. This is best demonstrated by the recent widespread adoption of LEED (Leadership in Environmental and Energy Design) criteria for building construction in the US and CEEQUAL standards for infrastructure in the UK. But exactly how to incorporate sustainability into design and evaluate the effectiveness of sustainability efforts through analytical, quantitative methods remains unclear to many engineers. This research details the steps a design engineer would take to engage in "sustainable" engineering of building systems and civil infrastructure that integrates green materials engineering with design of structures for prolonged service life and strategic management of infrastructure and building systems to minimize life cycle cost, social impacts, and environmental burdens.

For decades, homes and other structures in the southern Gulf States of the US suffered considerable damage and loss through the ravages of hurricanes and windstorms. The American Forest & Paper Association's (AF&PA's) Wood Frame Construction Manual (WFCM) and other building code provisions sought to remedy the situation through regulation to effectively change the way these structures located in high wind and hurricane prone regions were designed and constructed. A significant education program over a number of years launched by the insurance, wood, and homebuilding industry has resulted in far fewer homes with high wind damage, millions in dollars of savings, and significantly less disruption of people's lives as these storms become more commonplace today. Savvy home consumers and building owners in the south today know what wind resistant construction is, and increasingly demand it in a newly built product. This paper briefly describes what the WFCM is, educational tools developed, the training effort extended, and finally the beneficial results that realized.

The engineering profession is at a cross roads. Some professional associations are proposing that a Masters degree be required to enter practice while certain sectors of the structural engineering community feel a Bachelors degree should continue to be the requirement for entering the engineering profession. Regardless, both sides are trying to identify the minimum level of competency required to solve today's problems safely, economically, and efficiently (SEE). Engineering curriculum should be established that not only prepares graduates to SEE design projects, but also prepare students to face the problems of tomorrow and enable them to articulate those solutions to building owners, building designers and building contractors. The engineer of the future must possess the same skills as those of today, engineering dexterity, a bit of vision, and an ability to communicate with a variety of professionals. This requires a solid understanding of basic engineering principles, a solid understanding of how buildings behave, and a solid understanding of how a building is pieced together. Additionally, the engineer of tomorrow must look outward; beyond the confines or comfort zone of the traditional engineering world. The engineer of tomorrow will need to be prepared in the art of communicating: writing, verbal, and graphical and more importantly how the engineering profession is incorporated in the business world. Engineering education will continue to emphasize the basics of structural fundamentals, but there is also a need to expose students to those facets of the profession that enable one to bridge the gap between other designer professionals and between engineers and owners.

Currently, a significant number of civil engineering programs do not offer Design of Wood Structures course, offer it inconsistently, or offer it as a part of another course. The report [Barnes 2007], presented at the 2007 NCSEA Annual Conference on US higher education institutions that offer degrees in Civil Engineering, demonstrated this deficiency in education as related to the use of wood as structural material. This was further supported by the informal survey by the Wood Product Council in 2007 as well as numerous comments by the participants of the 2008 Structures Congress Wood Research Needs Pre-Congress Workshop. These reports, surveys, and workshops simply re-affirmed what industry insiders have voiced for the last two decades. In contrast, other structural materials such as, Design of Steel Structures and Design of Concrete Structures, are offered with regularity at almost every university and as part of a core curriculum. While there are a number of reasons that have led to this current trend of educational practice, it is conjectured that the main contributor is the lack of research funding. The research opportunities available for wood design related activities are lagging far behind the opportunists related to other materials. Consequently, university professors that study and teach wood engineering are becoming scarce. The educational materials related to design of wood structures are limited and scattered and consequently many questions related to wood design remain unanswered. The Wood Education Institute (WEI), described herein, intends to be a catalyst of change helping to stop and ultimately reverse this unfortunate trend by focusing on improving basic wood education at the undergraduate and graduate level and by providing resources and guidance to the universities interested in offering cost-effective educational options related to wood design.

The process to enact/implement a separate structural engineering practice act (not title act) reference [7] in a given State can be categorized into four separate steps/tasks. First, "know the Rules" prepare a proposed/draft State legislation/law/regulation (this maybe a new law or a revision to an existing law) that defines structural engineering, defines a structural engineer (i.e. what does a structural engineer do?) defines a structure and defines those structures that only a structural engineer is permitted to design or evaluate, see attached definitions. Educational, experience and examination requirements including provisions for grandfathering/transitioning shall also be included as part of this legislation.

Structural Engineering is a profession. A profession is an occupation that requires training and the study and mastery of specialized knowledge. Professions usually have: (1) a professional association, (2) an ethical code, and (3) a process of certification or licensure. Most structural engineers work for organizations that provide professional services to others. They may be consulting engineers who provide services to owners, developers or architects. Or, they may work for an agency that provides services to the other user agencies. In either case, the structural engineer is providing a professional service. The purpose of a code of ethics is to guide and support day-to-day decision making in the practice of the profession. A code of ethics has many uses, but I want to focus on the judgment provisions which encourage ethical decision-making.

The format of this paper will include first a section on professional rules of conduct. This will be followed by a discussion of bias and how certain aspects of both human behavior and the engineering enterprise impact decision making. The next section will focus on an ethics case study. Specifically, the case study involves technical assessment of the Hurricane Katrina disaster, the interactions of the assessment teams, and the local New Orleans grassroots organization Levees.org. Finally there will be discussion of the findings and recommendations of the American Society of Civil Engineers' (ASCE's) Task Force on Engineering Review.

Evolution of technology has continuously engaged the engineering profession over the past several centuries. From the wood covered lead pencil, which was first mass-produced in 1662 to the recently surfaced Building Information Technology (BIM) all note down the revolutionary impact of technology on structural engineering practice. Currently, increasing complexity of our codes and standards as we delve deeper in the information era makes conforming accurately to the specifications a cumbersome process. The concept of intelligent digital design system within the context of structural engineering in the 21^st. century will provide key solutions in simplifying engineering design practice. The information era shows that a proper information infrastructure offers many benefits that help produce and share high quality engineering data and decision management systems more cost-effectively and more efficiently. The intelligent structural design system described in this work focuses on the relationship among the various Engineering Information Management Systems and the Building Information Modeling (BIM). By providing more accurate information and interactive structural elements, intelligent design code offers engineers and consultants efficient and unambiguous methods to validate design assumptions against specifications and alternatives in a digital environment to achieve design objectives.

The fact that concrete cracks under the influence of everyday service loads is a truism that engineers, architects, contractors, and building owners often forget or fail to recognize. Three very common types of cracking — cracking in concrete fill on metal decks, cracking in flat plate concrete slabs, and cracking in concrete walls — are discussed in this paper. In the case of concrete fill on metal decks, cracking is often erroneously attributed to excessive dead and live loads, design or construction errors, or even earthquake loads. Actually, cracking in concrete fill is most commonly related to restraint against shrinkage and to the location of the supporting steel members. Due in part to curvature of the section induced by drying shrinkage of concrete, cracking typically occurs directly atop the supporting beams and girders, even when the decking is not shored during construction. Cracking at that location reduces the local moment of inertia at the crack, leading to incremental rotation of that cross-section and widening of the crack.

The current design method for stiffened seated beam connections is conservative for two reasons. First, the connection capacity is calculated based on an elastic analysis of the weld group which does not account for ductility and potential load redistribution in the weld group. Second, the additional strength of the portion of the weld group in tension is not accounted for. This paper will explain the assumptions underlying the current design method, illustrate why these assumptions are conservative, and provide alternative design procedures using safe, acceptable engineering methods to deliver a more economical connection.

The St. Regis Atlanta, one of the two five-star hotels located in Atlanta, Georgia, is a $150 million mixed-use building with 151 hotel rooms and 53 prestigious residences designed to meet the high expectations of the owner and architect. The distinctive architectural precast exterior combined with one-of-a-kind interior amenities creates a landmark building. With 26 stories including three underground levels, a two-story lobby with monumental staircase, and a tropical-paradise pool above an expansive hotel ballroom area, diverse structural analysis and design were required for the building. A 3D finite element model was developed to compute the dynamic characteristics of the building for both wind-tunnel tests and earthquake loads. This paper discusses the analysis and design of the post-tensioned concrete structure including the transfer beams, which accommodate transitions between the different occupancies of the building. It also presents the design challenges met for the design coordination of a mixed-used high-rise building and the challenges that occurred during construction.

The events of September 11, 2001 had a profound impact on the building design community. Although the weapons employed that day were fully fueled commercial aircraft, the industry turned considerable attention to the most common tactic historically employed in terrorist attacks around the world: the improvised explosive device. While information for addressing this threat existed, it was largely confined to military and other government publications that were neither readily available nor directly applicable to facilities constructed by the private sector. Recognizing this, in 2002 SEI began the process of establishing a new Codes & Standards Committee on Blast Protection of Buildings. Chaired by Donald O. Dusenberry, P.E., M.ASCE, of Simpson, Gumpertz & Heger in Waltham, MA, the committee set out to develop a new ANSI-accredited Standard for the planning, design, construction, and assessment of new and existing buildings subject to the effects of accidental or intentional explosions.

This presentation will discuss the work to date of the Task Committees on Structural Systems, Protected Spaces and Exterior Envelopes. This portion of the Standard will address the features of structural analysis and design that may be used to improve the performance of structures in response to blast loading. It will also include the design of the exterior envelope, which is typically the weakest part of a building as well as the costliest to construct because of its large area.

This paper presents the work of the Blast Protection of Buildings Task Committees on Materials Detailing and Performance Verification. The paper covers the last two chapters of the Standard that deal with design and detailing of structural components constructed of steel, concrete, masonry and fiber-reinforced plastics (Chapter 9), and performance qualification of site perimeter components, structural and nonstructural components and shielding structures (Chapter 10). The materials chapter references existing US standards where applicable and identifies more stringent design and detailing requirements for blast effects as needed. Prescriptive detailing requirements are scaled according to four Levels of Protection (LOP). Each LOP, from I to IV, provides increasing blast resistance beyond the nominal resistance provided by conventional design methods. The chapter on performance qualification address issues related to verification of the performance characteristics of manufactured products by analysis and design, and peer review of blast-resistant designs and performance qualification.

This paper highlights some of the issues that the author has found to be somewhat unclear and/or for which no specific guidance is provided in the ACI 318 Building Code. The issues that come up can be broadly divided into two categories: (1) issues arising as a result of applying ACI 318 design provisions, which have been developed for typical commercial/residential buildings, to nonbuilding concrete structures/components and (2) issues arising as a result of lack of guidance or clarity within the ACI 318 provisions themselves. The paper discusses some of the issues that come up under each of these categories. Based on this discussion, some thoughts are presented on how some of these issues may be addressed and/or need further clarification by the ACI 318 Committee.

In current seismic design practice, AASHTO, AREMA and IBC are the most popular specifications for highway bridges, railway bridges and building structures, respectively. Theoretical speaking, when earthquakes occurred, the structures would be damaged with no exception regardless of their functionalities. But the seismic design approaches and requirements in these three specifications demonstrate significant differences in many aspects. It is necessary to make a systematic comparison of these three specifications in terms of seismic design in order to carry out reasonable seismic analysis on some multi-function structures which are including building structure and bridges for vehicles or trains, or both. A good example of this type of structures is multi-model transportation center which is normally appearing in metropolitan area. In the seismic analysis of this type of structure, some additional coordination work among these specifications is critical in order to achieve correct results. Theoretical parametric investigations have been made to compare these three specifications in terms of seismic design. Some recommendations are provided for seismic analysis of this type of multi-function structures.

Within Europe, the Construction Products Directive (CPD) governs the sale of goods in the construction sector. Among other things, it requires that products approved for sale in the meet minimum standards of performance, safety and quality control, thereby promoting free trade of approved materials across the member states. The public procurement rules within Europe will mean that the use of these standards and approved "CE-marked" products will become mandatory for much of the construction sector. Concrete repair products and systems fall under the CPD, meaning that any repair product or system made available for sale must be pre-approved to meet minimum standards. This paper summarizes the latest progress in the development of these standards for concrete repair. The enclosed tables list progress with standards to April 2008. The paper describes the basic scope of the standards and the philosophy which has been adopted by the CEN Committee, and outlines the likely operation of the standards and their implications in practice. The paper also describes the interaction between the standards and the Construction Products Directive. A summary of the objectives of the ISO committee working on a global standard for repair will also be given.

The use of pre-stressed and post-tensioned wire strand for the strengthening of concrete structures has been used in many countries since the 1950s. Both construction methodologies have proven to provide an efficient and economical solution for a wide range of structures and conditions. The technique has grown in popularity because of the increased speed of construction and the efficient advantage of providing greater strength with less mass to the structure. The main structural element of any pre-stressed and post tensioned structure is the wire strand. In spite of its widespread use and technical and economic advantages, there appears to be an indifference to the need for quality oversight with reference to the manufacturing of wire strand and how the quality of the manufacturing process for wire strand is controlled. Strand that is not manufactured from a single controlled source of rod material can have varying physical properties of elongation, relaxation, yield strength as well as ultimate strength. Strand that is not correctly and consistently heat treated can have different properties of relaxation and undesirable consequences in a structure. Additionally, wire strand that is not correctly manufactured and tested to be resistant to stress corrosion cracking (SCC) may not be suitable for bridges or other structures exposed to an aggressive environment.

The International Building Code (IBC) recognizes the Steel Joist Institute (SJI) Standard Specifications for the design, manufacture and use of open web steel joists and Joist Girders. While the reference to the SJI Specifications limits the amount of wording in the IBC that is devoted to steel joists, there are a number of important passages in the IBC that relate to steel joists, both defining the responsibilities and process for utilizing steel joists and for loads and conditions that affect the selection, design and use of steel joists. This paper will examine the pertinent links and interaction between the IBC and the SJI Specifications, and how both documents work together to define the proper manner to specify steel joists and Joist Girders.

The new Liverpool Football Stadium will be an instantly recognizable sports complex that both represents the future, and honours the legacy, of one of the oldest football clubs in the United Kingdom. The new stadium will have a 72,000 seat capacity and will highlight the historic Kop which is the traditional stand for the passionate home fans. The structural concept of the roof has been developed as a response to the plan and elevation of the seating bowl and to create a visual dynamic for the stadium as a whole. To reflect the history of traditional British football stadia and deal with different stand uses, the stands are designed as four separate structures, rather than a continuous seating bowl which is the recent trend in modern stadia design. The roof structural system takes advantage of this architectural feature by introducing roof support in the four corners between the stands. These supports are inclined to provide thrust in three of the main span elements. These main span roof trusses in the east, west and south stands share a common form and feature steel, fabricated, stiffened arches. The east and west (sidestands) trusses also include catenary cables as an inverted mirror geometry to the arch spans which creates a unique hybrid system.