Structures Congress 2005: Metropolis and Beyond

In this study, the steel plate prestressing method is applied using high strength steel plate to intermediate floor beams of a steel railway through girder bridge. The behavior of the reinforcing steel plates and reinforced steel floor beams is monitored during prestressing and live loading, in order to assess the effects of prestressing and reinforcement. The study confirmed that these effects are beneficial to the performance of steel railway floor beams.

Historically, bridge load ratings based solely on theoretical calculations tend to be overly conservative, due to the many assumptions made in modeling of the bridge. A controlled load test can be used to determine a more accurate load rating of the bridge. Presented in the paper is an overview of a typical controlled load test. Specific attention is given to determining the actual load distribution in the bridge, assessing support fixity, unintended composite action, and contributions from nonstructural components. The manner in which these factors are adjusted and applied to a simple beam model of the bridge for load rating is also discussed.

The subjectivity inherent in empirical based load rating methods for bridges can be significantly reduced by creating a calibrated finite element model (FEM). The calibration process uses measured structural response from controlled diagnostic load testing to quantify boundary conditions related to support, geometry and stiffness. The resulting calibrated FEM can then be used to predict behavior for other axle configurations as well as multiple lane loadings. This paper will describe the process of collecting strain data from diagnostic load testing and then show how this information is used for calibration of a FEM and calculation of load rating factors.

This paper presents four applications of load testing and monitoring for evaluation of bridges and bridge members. The first three applications are on evaluation of Fiber Reinforced Polymer (FRP) retrofit systems and the fourth is on monitoring of a bridge during superload permit trucks crossings. For each application, the paper briefly describes the objectives and the testing technique used for the evaluation.

A multi-response parameter estimation method, including an error function normalization procedure, is proposed to allow simultaneous estimation of stiffness and mass parameters for model updating. The initial finite element model (FEM) of the University of Cincinnati Infrastructure Institute (UCII) bridge deck was developed based on preliminary calculations using information gathered from available drawings. The unknown parameters were selected for the grid model based on the results of damage localization, and were successfully updated using structural parameter estimation methods. This paper presents both the updated structural stiffness and mass parameters of the grid model using a few subsets of static and dynamic non-destructive testing (NDT) measurements. The static and modal response from the updated model resulted in a closer match with the NDT data than the responses from the initial FEM of the bridge deck.

This paper discusses advances in evaluation and health monitoring of cable-stayed bridges and available methods. Bridge Engineers and Highway Administrators in the US are gradually becoming more comfortable with cable-stayed bridges, and the past few years has seen a significant increase in construction of these elegant bridges in many parts of this great nation. In the last decade, the author has contributed extensively to advances in cable-stayed bridge construction, design, and health monitoring. Information gathered from evaluation of the strength and reliability of stay cable systems for nearly twenty-five long-span bridges worldwide, and instrumentation, health monitoring, and inspection of more than eight cable-stayed bridges in the US and abroad has helped towards formation and evolvement of a unified approach for in-service evaluation and problem solving of these aesthetic structures. Additionally, discussed in this paper are a series of methods developed or tailored for evaluation of these unique structures. The paper also reviews the typical problems observed in the course of field evaluations for in-service cable-stayed bridges.

Virtual Reality (VR)/Three-dimensional computer graphics (3-D CG) is a technology that creates a virtual three-dimensional space in a computer to visually reproduce the texture and movement of objects. Due to the significant improvement of the performance of computers and price reduction of software and various peripheral equipment in recent years, VR/3-D CG is rapidly becoming widespread and used for various purposes including scientific simulation and entertainment. It is also widely used in civil engineering for such purposes as aesthetic evaluation during planning and design phases, analytical simulations, and damage detection with real-time health monitoring. In this study, using above-mentioned great advantage of VR, a damage process education system as an "on the job training system" is developed for education of unskilled bridge maintenance engineers related to deteriorating bridges. The proposed system will greatly help not only easy-to-understand but also enlighten them on the importance of safety evaluation and management of existing bridges. The deterioration sequence and damage progress in the future are reproduce for some actual bridges in the system, then it is found that the proposed VR system helps efficiency bridge engineers related to bridge management understand easily what are the major processes in the deteriorating bridges in the future, how to improve their performance, and what kind of repair/strengthening works is most convenient for the deteriorating bridges. Bridge monitoring system via information technology is capable of providing more accurate knowledge of bridge performance characteristics than traditional strategies. This paper also describes an integrated internet monitoring system that consists of a Stand-alone Monitoring System (SMS) and a Web-based Internet Monitoring System (IMS) for bridge maintenance. IMS, as a web-based system, is capable of addressing the remote monitoring by introducing measuring information derived from SMS into the system through Internet or Intranet connected by either PHS or LAN. Moreover, the key functions of IMS such as data management system, condition assessment and decision-making with the proposed system are also introduced in this paper.

An In-Service Bridge Monitoring System (ISBMS) has been developed to provide near real-time web-based monitoring of live load strains in a bridge. The monitoring system is small, battery-operated, can be rapidly deployed, and is programmed and interrogated via a user-friendly web interface. The ISBMS has been designed to be portable and used on an "as-needed" as a diagnostic tool or for health monitoring of ordinary bridges. The system is based on a small single-board-computer with analog inputs; it also includes a cellular digital packet data modem for communication via the existing cellular network. Strains are measured using either a full bridge strain transducer or a quarter bridge foil strain gage. The system has three modes of operation; the peaks program records peak live load strains that exceed a specified threshold, the time history program captures dynamic waveforms that exceed a specified threshold, and the rainflow program counts varying amplitude strain cycles. The selection and setup of the program, and retrieval of data is handled through a custom designed web interface. The system has been tested in the laboratory and in the field on a heavily traveled steel girder bridge. The data obtained from the ISBMS can be used for load rating, fatigue assessment, monitoring of stresses due to overload vehicles, and long-term health monitoring of bridges.

Wireless sensor networks offer several benefits over conventional sensors for civil engineering structures monitoring. Therefore, applying wireless sensor networks to health monitoring of large-scale civil infrastructures has received considerable interests over the past few years. This paper discusses practical issues emerged from implementing wireless sensor networks for large-scale civil infrastructure monitoring, such as sensor location identification, robustness of sensor network, and data compression techniques and their effect on vibration-based structural health monitoring. The proposed lossy data compression algorithm, termed BLS algorithm, can be used to reduce raw data size to facilitate data storage and save communication bandwidth for data transmission from wireless sensor to remote station. Numerical simulation results show that the BLS algorithm is capable of reducing the raw data size by almost 92% while having a minor effect on the modal parameters identified from sensor data.

In the field of non-destructive evaluation and damage detection, there is continued interest in the utilization of vibrational techniques. Structural damage will result in permanent changes in structural stiffness, distribution of stiffness, and relevant material properties. These changes may be detected by monitoring dynamic behavior of the structure. Because of the direct relationship of mass, damping, and stiffness of a multi-degree-of-freedom system to the natural frequencies, mode shapes, and modal damping values, many studies have been directed at using these properties for the purpose of structural health monitoring. The use of vibrational monitoring is very valuable in some cases, and is capable of assisting in both detecting and locating structural damage. Vibrational data is most useful when used in conjunction with other monitoring if a remote and robust damage detection scheme is desired. This paper includes a literature review and discussion of the basic approaches to vibrational monitoring, suggested guidelines for sensor selection and monitoring, and concludes with examples of two case studies.

The installing and debugging of a bridge monitoring system has been finished on the Wuhu Yangtze River Bridge in China in 2004. The system consists of strain sensors, acceleration instruments, displacement transducers, and other testing instruments which are all installed at selected locations across the bridge. The objective is to do safety evaluation for fatigue, vibration, and deformation of the bridge in operation. The Wuhu Yangtze River Bridge was constructed in 2000. It is a stayed-cable bridge with the use of both highway and railway. The fly-past configuration of the main crossing is 180m + 312m + 180m. The stiffening girder is a truss made of welded steel plates of H style and box style sections with thickness of up to 44mm. The top chord of the steel truss is made composite with the concrete deck. The monitoring system continuously records traffic-induced strains and transfers the data into stress spectrums. The structural details for fatigue evaluation include various welded details in truss members as well as in cross beams and longitudinal stringers in tension regions. Connections between the stayed cables and the floor system are also instrumented. Various features have been developed in the data processing software for satisfying specific needs of fatigue analysis. This is a key research project sponsored by the China Railway Ministry. The objective is to develop an effective and safe monitoring system and evaluating method to be applied to other extra-long bridges in China.

It is estimated that nearly 90% of all fatigue cracking is the result of out-of-plane distortion or other unanticipated secondary stresses at fatigue sensitive details. Interestingly, neither design nor evaluation specifications provide any guidance on how to evaluate the in-service potential for fatigue cracking at these details. Furthermore, the causes and driving forces producing these cracks are typically difficult to accurately characterize using simple design tools. Often, as a result, the effectiveness of various retrofit procedures is often questionable and ill fated. There are many examples where implemented retrofit procedures did not work and fatigue cracking continued. In such cases, details must be re-retrofitted and additional corrective actions taken to ensure a safe structure. Implementation of one or two prototype retrofits is attractive alternative approach to ensuring effective retrofits are developed. In situations where several dozen or even hundreds of details have to be retrofitted due to a common problem, this approach is most practical. The prototypes should be instrumented and the performance evaluated. Field instrumentation and testing has consistently been demonstrated as the most effective means to confidently determining the effectiveness of a given retrofit strategy. Unanticipated effects, due to modification of the detail, are also revealed thereby reducing the chance for future problems. Monitoring for a few weeks to months can also be conducted at little cost to help ensure the longevity of the retrofit and to identify how the retrofitted detail performs when subjected to the variable-amplitude load spectrum. This paper examines examples of where effective prototype retrofit strategies were shown to be effective as demonstrated through performance and field instrumentation.

Large numbers of conventionally reinforced concrete deck-girder (RCDG) bridges remain in-service in the national highway system. Diagonal cracks have been identified in many of these bridges, which are exposed to millions of load cycles during service life. The anticipated life of these bridges in the cracked condition under repeated service loads is uncertain. Stirrup strain data were collected from in-service RCDG bridges and experiments were performed on full-size girder specimens to evaluate possible deterioration in load-carrying capacity under repeated loading. Specimen variables included: T and inverted-T configurations, stirrup spacing, and flexural reinforcing details. Test results indicated bond deterioration increased diagonal crack displacements and analysis methods to predict the shear capacity of diagonally-cracked reinforced concrete girders subjected to high-cycle fatigue damage are provided.

With the aging of existing steel bridges and the accumulated stress cycles under traffic loads, assessment of remaining fatigue life for continuing services has become more important than ever, especially for decisions on structure replacement, deck replacement or other major retrofits. Experiences from engineering practices have indicated that fatigue analysis based on specification loads and distribution factors usually underestimates the remaining fatigue life of existing bridges by overestimating the live load stress ranges. Fatigue evaluation based on field measured stress range histograms under actual traffic load proves to be a more accurate and efficient method for existing bridges. This paper describes the application of such method in assessing the remaining fatigue life of bridge structures. Current AASHTO specifications for fatigue evaluation of existing bridges are reviewed and compared. Case studies of three major highway bridges are discussed. Finally a procedure for evaluating the fatigue life of existing bridges based on case-specific field strain measurement is proposed for practicing engineers and bridge owners.

There have been several new approaches and advances in sensing technologies for monitoring long-term durability of the infrastructure. With the use of new materials (such as fiber reinforced polymers, stainless steel and stainless clad rebar, etc.), identifying factors affecting and accelerating the growth of corrosion in highway bridges through monitoring has received significant attention. This paper presents a review of available systems for such purposes.

Current inspection procedures used to assess bridge conditions subjected to corrosion of reinforcing steel involve the use of sounding technique (e.g. chain drag for bridge decks), half-cell potential test, chloride analysis and depth of cover measurement. However, other non-destructive testing methods are available whose implementation in the evaluation process of bridge members subjected to corrosion provides a more comprehensive, scientific, and quantitative assessment of bridge conditions. It also provides the information needed to design and select the optimum rehabilitation system. Impulse-response technique and ground penetrating radar (GPR) are used in assessing the delamination of concrete at the concrete-steel interface. GPR is also used to provide depth of cover profile across the bridge member rather than measuring depth at selected points using Pachometer. Use of corrosion rate measurement using linear polarization technique gives more realistic indication of corrosion activity of reinforcement compared to half-cell potential measurement. In addition to chloride measurements at the level of steel which currently part of the conventional inspection, chloride profile at different depth levels is obtained and used for calculating the diffusion coefficient of concrete. These data are used in computer models to predict the service life of the bridge.

The impact on bridge reliability, of prestressing steel deterioration and tendon rupture created by corrosion mechanisms, is much more critical and rapid than that of any other component of a prestressed concrete bridge. Additionally, deterioration of embedded prestressing reinforcement in these structures may not necessarily be made visible through manifestation of external distress in the concrete. Initiating mechanisms and particular forms of corrosion affecting prestressed concrete structures are described based on the author's experience. These degradation mechanisms include: chlorides and corrosion; concrete carbonation effects; influence of concrete cracking; electrochemical (Macrocell) and pitting corrosion; stress corrosion cracking and steel embrittlement; fretting corrosion; corrosion and fatigue. Recently, prestressed, post-tensioned concrete structures have been thought to be threatened by certain premature performance impairments worldwide, predominately when defects were present. These problems may continue to manifest themselves. Therefore, corrosion preventative methods and need for scrutiny during construction and maintenance needs will be described. For existing structures experiencing evidence of premature distress, the author discusses case-study based investigative techniques and available rehabilitation strategies.

Fusion bonded epoxy coating technology has been the primary defense used by most state agencies, including the Pennsylvania Department of Transportation (PennDOT) and the New York State Department of Transportation (NYSDOT), to protect reinforcing steel in concrete from corrosion. However, results from recent research activities cast doubt on the ability of epoxy coatings to provide long-term corrosion protection to steel in concrete exposed to chlorides. The projected problem has been attributed to the number and size of breaks or defects in the coating and reduction in adhesion between the epoxy coating and steel substrate. Coating breaks or defects are represented by holidays, narrow cracks, mashed areas, and bare areas that occur during coating application, fabrication bending, transportation, handling, and concrete vibrating. Reduction of the adhesion bond between the epoxy coating and underlying steel has been widely observed. Occurrence of this phenomenon has been found to be independent of the level of chlorides at the reinforcing steel depth and may or may not be associated with corrosion underneath the coating. While there is a consensus on the occurrence of these problems, their significance with regard to the long-term performance of epoxy coated reinforcing steel in concrete bridge structures exposed to salt in-service is not completely understood, and thus, much controversy remains. PennDOT and NYSDOT decided to conduct a joint research effort to survey and evaluate in-service bridge decks constructed with epoxy coated reinforcing steel by way of a statistically based sampling plan. Project funding was provided through a regional pool fund and the Federal Highway Administration. This paper presents a summary of the project findings. A detailed report on the project is available from PennDOT.

From the very beginning of the design phase, construction engineering considerations were already being interjected into the project. The purchase order for the steel supply and fabrication was in place early, and fabricator input was sought and included throughout the design process, including fabricator presence at nearly all Task Force Meetings, with the intent of designing-in features that made fabrication and erection efficient. For shipping considerations, the segment stacking method, estimation of cumulative fatigue damage, and the use of passive damping and temporary connections between segments were considered important factors. The cumulative fatigue damage sustained during shipping was limited to 10% of the final Design fatigue threshold. Detailed studies were performed to evaluate the structural performance of major bridge components (main cables, suspenders, main load carrying superstructure elements, towers) under various erection stages, including dynamic and buffeting wind loadings for critical erection stage configurations. Detailed wind tunnel and mathematical models representing significant components of the bridge and temporary equipment, with their respective masses, were employed. This paper chronicles the construction support and engineering efforts performed for the Tacoma Narrows Constructors to streamline and integrate the design and construction of this unique structure.

The suspended superstructure for the new Tacoma Narrows Suspension Bridge consists of a continuous welded steel truss with an integral orthotropic steel deck. The truss elements are fabricated of both closed box and open I-sections. The bridge, including particulars of the stiffening truss, is designed to accommodate a future lower roadway or LRT without the need for component strengthening. This paper will discuss the design aspects of this 1,646m (5,400 ft) continuous steel truss superstructure with integral orthotropic deck, including the development of design details to satisfy serviceability, fatigue, wind and seismic performance. Global superstructure and local component computer modeling techniques are presented as they relate to force and displacement demands and overall structure performance for the required 150-year service life. Performance requirements for high seismic hazard and computer analysis techniques using ADINA computer modeling software is discussed. Wind performance and physical wind tunnel testing utilizing full-bridge aeroelastic models for the bridge in its completed configuration is also discussed.

The scope of seismic work included establishment of design criteria, extensive analysis and validation of the design. Analysis and design included both FEE and SEE criteria. The analysis was performed for the upper-level configuration and the future lower-level configuration, as well as half-scour and no-scour conditions, using detailed three-dimensional time history models that included geometric and material non-linearity. The desired post earthquake level of service was verified using stress, deformation and ductility performance criteria. This paper describes key issues related to the seismic design of the bridge.

Hydraulic and scour analyses were conducted to assure that the caissons for the existing bridge and new bridge foundations are adequate. The Tacoma Narrows Bridge is located in a unique hydraulic setting with daily tidal swings of 4–5m (12–17 ft) between high and low tides result in currents reaching 3.7 m/sec (7 knots). Large caissons in 38m (125 ft) deep water at these high velocities can produce significant scour. The caissons are skewed to the flow which also adds to the scour potential. However, the scour depths of observed scour holes that had occurred after 63-years of tidal flows for the existing structure are about 50% of the predicted scour depth using HEC-18 methods. To control the foundation costs, the hydraulic analysis included physical modeling, one and two dimensional hydraulic modeling and various site measurements. Site data was obtained to establish bed elevations, 3D velocity vector data to calibrate the hydraulic models and video observations to determine the channel bed material properties. Physical modeling used the large flumes at Colorado State University and new methods developed by D. Max Sheppard from Florida State University. This project was undertaken through a Design/Build contract and resulted in a need to determine the scour depth in a very short time frame and to reduce the depth of scour to the minimum reasonable amount. Additionally, approval of the methods and findings needed to be obtained quickly. Because of the size and complexity of the analysis, close coordination between the designers, construction engineers and project oversight reviewers was maintained throughout the design effort. The design effort between initiation of studies and determination of the scour depth took only 5 months and included physical and two dimensional hydraulic modeling. The analysis also showed a potential reduction of the scour potential from HEC-18 methods of about 10 meters (36 ft).

The suspension system of the New Tacoma Narrows Bridge consists primarily of two (2) main cables and 132 vertical suspenders. Each cable is constructed of 8,816 high-strength steel wires, resulting in an overall cable diameter of 20-1/2" (521 mm). These cables pass through splay saddles and divide into 19 strands each. The strands are held by strand shoes anchored with high strength anchor rods deep into the anchor block. The suspender groups consist of either two or four lengths of rope arranged with four rope terminations at deck level. The ropes are generally 1–5/8" (41.3 mm) diameter zinc coated steel structural wire rope. The suspenders are attached to the main cable by way of cast steel cable bands and the main cable is supported at the towers and anchorage splay points by cast steel saddles. The keyed-in gravity anchorages located on both ends of the bridge resist the pull of the main cable tension. The anchorages are constructed of cast-in-place reinforced concrete and each anchorage contains approximately 20,600 CY (15,750 m³) of concrete. The roof of the anchorage also serves as the roadway for vehicles traveling onto and off of the bridge. The roadway slab is supported by pre-cast I-girders with room for a lower level roadway that may be added in the future.

The process of designing a pedestrian bridge starts by identifying project objectives that have to be met within the project constraints. Project objectives are translated into functional, structural and aesthetic requirements. Functional requirements include bridge width, plan alignment, vertical profile, drainage system, railing dimensions and lighting. Structural requirements include providing sufficient strength and durability, complying with deflection criteria, and preventing excessive vibrations. The aesthetic goals are more difficult to quantify, however, as they are frequently stated in terms of satisfying the desires of the local community or creating a landmark structure of symbolic character. There are a wide variety of possible project specific constraints. General constraints include construction budget and schedule, environmental issues, geotechnical and hydraulic considerations, availability of materials, local construction practice and experience. Many of the objectives and constraints affecting pedestrian bridge projects are also common to roadway or railroad bridge projects. However, the fact that pedestrian bridges need to be built to fit the human scale and are often located in an urban environment makes aesthetic considerations much more relevant. Bridge designers, like design professionals in other fields, are confronted with the challenge of incorporating aesthetic goals into a complex set of functional requirements, structural considerations, and project constraints while keeping costs within budget.

Amgen Incorporated, one of the world's largest biotechnical firms, recently developed a new research center on Seattle's Elliot Bay waterfront. As part of the research center, a pedestrian bridge was built that connects the campus with a major transportation corridor. The owner's goal for the new pedestrian bridge was to create a signature gateway to Amgen's campus. The pedestrian bridge is a three-arch structure that depicts a three-dimensional helix form. The overall shape of the bridge reflects the helical DNA that connects all life and references the vital research underway at Amgen. The arches create the outline of the basic DNA helix shape, while the deck support and roof trusses complete the remaining components of the helix form. The abstract double-helix form provided many design and construction challenges. The slim profile, large enclosed areas and suspended deck meant wind and pedestrian vibration would be a critical design consideration. The unique shape made it difficult to apply the standard building or bridge design codes. The contractor was faced with erecting this twisting structure over active railroad tracks while meeting tight geometric specifications. The bridge stands today as a testimony to the owner's vision, teamwork, innovation in bridge design and creative construction techniques. The bridge combines both form and function to show how an ordinary pedestrian bridge can become an extraordinary statement while serving its intended function. The project has won broad community support in the few months that it has been open.

A new form of cable-stayed bridge has been developed to solve the problem of joining two parts of the campus of Rockefeller University in New York. The problem that had to be overcome was the inability of placing a support at one end of the proposed bridge. The bridge therefore had to cantilever across a street. Furthermore, the path of the bridge could not be in a straight line and the bridge would have to carry utility lines as well as pedestrians. The solution that was arrived at was a "Y" shaped plan with one branch of the Y for pedestrians and the other for utilities. The leg of the Y spans the gap in the campus joining a residence hall to the main campus. Since the suspension cables are not in a plane, the tower is shaped like a "V" with each leg of the V serving to hold the cables for each of the two legs of the Y bridge.

Laboratory measurement of local stresses was made on components of a full scale model of orthotropic deck panel of Bronx-Whitestone Bridge. Simulated wheel load of trucks was placed on the deck at various locations along longitudinal stiffening ribs of trapezoidal shape. The loads induced local stresses and local bending of diaphragm web plates and rib walls. The local stresses were moderately high in magnitude in all components of the model deck.

In an unprecedented undertaking to reopen the Waldo Hancock bridge to truck traffic. Parsons in collaboration with Maine DOT and the contractor designed and helped construct a new supplementary cable system to restore the bridge suspension system to its original load carrying capacity.

A number of construction problems have been observed in steel trapezoidal box-girder bridges in recent years. Failures have ranged from the buckling of bracing members to the complete collapse of a pedestrian bridge during construction in Marcy, New York, which killed one construction worker and injured nine others. These bridges met current design criteria, implying that constructibility may be inadequately addressed in the specifications. Software has been developed to investigate the overall behavior of steel trapezoidal box-girder bridges during the construction phase. Unlike currently available design software, the new software explicitly accounts for partially-composite behavior between the girder and the deck. In addition, the developed program utilizes a finite-element-based procedure in which the exact geometry of the cross-section is discretized for the purposes of analysis. Previous research has demonstrated that such an approach is required to accurately predict the response of trapezoidal box-girder bridges under construction loads. Results for the current study have shown that lateral torsional buckling of the entire girder, which is not currently a limit state for design, must be considered. In this paper, the developed software is used to investigate the failure of the bridge in Marcy. Computed results are shown to agree with observations from the forensic investigation. The potential of the software as a useful aid in the design of steel box-girder bridges is demonstrated.

Trapezoidal box girders are being used more frequently, particularly in urban interchanges, based on the aesthetics and substantial torsional stiffness of the completed bridge. The closed section formed by the combination of the steel box and the concrete deck is attractive in applications where the horizontally curved layout results in significant torsional demands on the completed bridge system. However, the open bare steel section requires a number of bracing systems to maintain stability during transport and erection, and casting of the concrete deck. Some of these bracing systems are permanent, while others may be removed before the bridge is put into service. This paper discusses results from a field instrumentation and FEA study of the bracing elements of a steel box girder bridge with a skewed support. The field investigation included monitoring of girder stresses, as well as the stresses in each type of bracing element typically used in box girder bridges, including external and internal cross-frames, the solid diaphragm at a skewed support, and the top lateral bracing system. The paper also discusses recommendations resulting from the field study and companion parametric studies using details found in the instrumented bridge.

This paper presents the results of a series of experimental field tests conducted on a continuous three-span trapezoidal steel box-girder highway bridge during construction. Concentrated loads were applied to the bridge using a construction crane. Permanent metal-deck forms were affixed directly to the top flanges using powder-actuated fasteners. Forces were measured in top-lateral truss bracing before and after application of the metal decking. Measured forces are compared with finite-element predictions using an equivalent flat-plate approximation to model the decking.

The failure of the Marcy Pedestrian Bridge during its construction raised concerns about design and construction practices. To address these issues, the current study examined the design of the Marcy Pedestrian Bridge, and the physical cause of the collapse. To reduce the potential for similar future catastrophes, specific recommendations are made.

Lateral torsional buckling of steel bridge girders is typically controlled by providing cross-frames or diaphragms spaced along the length of the bridge. The historic protocol employed by many states for the design of these bracing members has often included recommendations for the maximum spacing limits with typical sizes and details for the braces themselves. This procedure usually results in several cross-frames throughout the bridge. The State of Texas has sponsored a number of research studies focused on improving the understanding of the bracing behavior for steel bridge girders. One of the research studies resulted in recommended bracing details in which an individual cross-frame could be sized to brace several girders. The other study focused on improving the connection details between permanent metal deck forms (PMDF) so that the forms could be relied upon for bracing, thereby reducing the number of intermediate cross-frames required. Design methodologies have been developed for both of these bracing systems. The State of Texas is currently implementing results from both of these investigations in the designs of two bridge projects. This paper will provide an overview of the research investigations and highlight the recommendations. A discussion of the bridges on which the bracing recommendations are being implemented is also provided.

For horizontally curved steel I-girder bridges, not only must a suitable erection sequence be selected to minimize construction difficulties, but it is also imperative that the cross-frames and girders be detailed in a consistent manner that preserves the original design intent; especially in larger bridges. Nonetheless, inconsistent detailing of cross-frame members and girders is sometimes employed in order to force bridge components into some desirable geometric configuration: girder webs vertically plumb at the steel dead load condition, for instance. The bridge components are typically inconsistently detailed because the rotation of the girders and the subsequent lateral displacement of the top flange due to applied or dead loads are predicted to be "excessive." It is then hoped that by intentionally detailing the diagonal cross-frame members to be either too short or too long, the girders will be forced to be vertically plumb at the given loading condition. However, this is not the case, and will instead result in connection misalignments during the erection of the steel superstructure; or worse yet misfits of such a magnitude as to render the bridge un-constructible. The current paper is intended to quantity, qualify, and promote awareness of the issue of inconsistent detailing in horizontally curved steel I-girder bridges by studying two recently constructed horizontally curved steel I-girder bridges that were inconsistently detailed. The discussion highlights the fact that practice of inconsistent detailing can lead to very formidable and costly fit-up problems in the field; especially when girder sizes are large.

Brittle fracture results in unplanned loss of service, very costly repairs, concern regarding the future safety of the structure, and potentially loss of life. These types of failures are most critical when there is no evidence of fatigue cracking leading up to the fracture. Hence, the failure occurs without warning and the details are essentially non-inspectable. In these cases, it appears desirable to take a proactive approach and introduce preventative retrofits to reduce the potential for future crack development. These efforts will help ensure that the likelihood of unexpected fractures is minimized. This paper examines the behavior of two bridge structures in which brittle fractures have developed in recent times, discusses the causes of the failures, and offers suggestions preventative mitigation techniques. In situations where considerable uncertainty exists in the prediction of accumulated damage or in the ability to reliably inspect critical details, preemptive retrofit strategies appear to be highly desirable.

A significant crack was recently discovered on an I-95 bridge over the Brandywine River in Delaware. The steel girder bridge carries six lanes of traffic just north of downtown Wilmington. The crack was located on the fascia girder at midspan of the bridge's main span. The entire bottom flange was found to be fractured, with the crack extending upwards to within 0.3 meters of the upper flange. This paper will review the circumstances leading up to the crack, discuss the cause of the crack, review the repair strategy, and summarize the results of load tests performed prior to and during the repair.

The I-895 bridge over US 1 and Patapsco River is a three-span continuous bridge consisting of seven welded steel plate girders. The bridge has a skew angle of approximately 35 degrees and the girder webs are made of 5/16-inch thick A36 steel plates of 90-inch in depth. Two girders were found with web fractures in March 2003. One interior girder had a full-depth web fracture initiated from the top of a transverse stiffener which is also a connection plate for the cross frame. One exterior girder also suffered a web fracture initiated from the top of an intermediate transverse stiffener. Emergency repairs were made to the fractured girders and in-depth analyses were performed to investigate possible causes. The analysis included material property tests, fractographic examination of fracture surfaces, structural analysis for load distribution and cross frame member forces, and finite element analysis of local stresses in areas where the fractures initiated. This paper summarizes preliminary findings from this investigation.

Most timber bridges are currently designed in accordance with the American Association of State Highway and Transportation Officials (AASHTO) Standard Specifications for Highway Bridges, 17^th Edition (SSHB), which follows an allowable stress design (ASD) approach. However, the Federal Highway Administration (FHWA) will be requiring all highway bridges designed after October 1, 2007 to follow the AASHTO LRFD (Load & Resistance Factor Design) Bridge Design Specifications, 3^rd Edition. As the transition from ASD to LRFD nears, engineers need to be aware of the differences. Information about more recent changes to the design codes and differences between AASHTO's Manual for Condition Evaluation and Load Resistance Factor Rating (LRFR) of Highway Bridges and AASHTO's Manual for Condition Evaluation of Bridges, 2^nd Edition (MCEB) will be presented. This paper will provide an overview of the pertinent issues related to timber bridge design and load rating. An example is included that illustrates the dramatic reduction in rated load capacity when calculated by the LRFD method.

This paper describes an effort to develop a global dynamic testing technique for evaluating the overall stiffness of timber bridge superstructures. A forced vibration method was used to measure the natural frequency of single-span timber bridges in the laboratory and field. An analytical model based on simple beam theory was proposed to represent the relationship between the first bending mode frequency and bridge stiffness (characterized as EI product). The results indicated that the forced vibration method has potential for quickly assessing superstructure stiffness of timber bridges, but improvements must be made in measurement system to correctly identify the first bending mode frequency in bridges in the field. The beam theory model was found to fit the physics of the superstructure of single-span timber bridges and could be used to correlate first bending frequency to globalstiffness if appropriate system parameters are identified.

Near infrared spectroscopy (NIRS) was used to study a set of Southern Yellow Pine boards exposed to natural weathering for different periods of exposure time. This non-destructive spectroscopic technique is a very powerful tool to predict the weathering of wood when used in combination with multivariate analysis (Principal Component Analysis, PCA, and Projection to Latent Structures, PLS). Absorbance spectra contained information about weathering behavior, and PLS models to predict exposure time were very strong, with the correlation coefficients above 0.9. These results, and the "non-destructive" nature of this technique, suggest the potential of NIR for monitoring the condition of in-service wood structures.

The Triborough Bridge and Tunnel Authority's (TBTA) Throgs Neck Bridge connects the boroughs of Queens and the Bronx, and opened to traffic in 1961. The structure consists of a suspension bridge with approach viaducts of simple spans using a girder/floorbeam/stringer system. The superstructure of the 37 approach spans of the Throgs Neck Bridge has experienced chronic problems and has been monitored closely by the TBTA. The viaduct consists of typical straight and curved spans of 58 m and 52 m, respectively, with a two-girder support system. Floorbeams are spaced approximately every 7.3 m. Soon after the bridge opened, fatigue cracks developed in the webs of the end floorbeams. After 20 years in service, the original concrete deck bad developed longitudinal cracking and break-up over the girders, where the deck surface experiences tension from negative bending induced by cantilever deflection of the floorbeam brackets. In the 1980s, the TBTA replaced the deteriorated concrete deck with an orthotropic steel deck, chosen for ease of installation in a staged construction program to allow full traffic flow during morning and evening rush hours. Orthotropic deck was chosen for its small decrease in dead load, and for its capacity to better resist the tensile stresses over the girders. After the orthotropic deck was installed, the TBTA found fatigue cracks developing in the webs of the new subfloorbeams installed to support the orthotropic deck. Also, some of the bolts connecting the deck ribs to the subfloorbeams began to break, and the cracking of the floorbeam webs continued. The TBTA enlisted the expertise of several consultants to provide retrofit solutions to solve the structural problems. Parsons ultimately provided retrofits that are currently being installed on the bridge for further testing.

The replacement of the 105-year old Third Avenue Swing Bridge is complete. The new 5-lane crossing will consist of 17 approach spans and a 350-ft long swing span for an overall bridge length of 1500 feet. Including on-grade approaches, the overall project length is 3500 feet. The scheme selected is an on-line replacement, requiring complex staged construction that includes a temporary bridge and float-in of the fully-assembled swing span, which has been barged roughly 1800 miles to New York City from Alabama.

Analytical evaluations of historic covered bridges routinely detect weaknesses of the timber structure. The trusses are found to be too weak to support desired vehicular loading — often too weak to even support just the weight of the bridge. The floor is too weak to safely support vehicles. The roof won't safely support snow loads. And the bracing is too weak to withstand wind loading. What to do? Maybe the structure really is in trouble. Maybe there is visible evidence of distress to support such analytical predictions. Engineers are expected to protect the public from unsafe structures. Hence, the call for closure to traffic or the recommendation for reinforcement or replacement components may be prudent. At least, more investigation is in order. But what to do if there is not visible evidence of distress? Should we recommend the same solutions, when the numbers say we should? Is that "crying wolf"? Another approach is to consider the nuances of extant covered bridges within the context of complex structures that have withstood the test of time. To consider the basis for the allowable timber stresses that we use in today's environment. There is need for more knowledge about these fascinating vestiges of our early bridge building history. Not enough is known about the statistical 95% exclusion rule for timber allowable stresses. Complex timber joinery is often not understood to the extent that it should be by those dealing with the project. While there are some interesting and hopeful developments in the use of modern materials to augment existing timber structures, there is arguably even more need for experience and sound engineering judgment by those challenged to care for these precious structures. This article delves into the complexities that confound engineers challenged to determine the safe load capacity of such structures and to determine just what ought to be addressed in rehabilitation projects. A number of popular (re)actions are considered and discussed.

Kings Covered Bridge over Laurel Creek in Somerset County, Pennsylvania is approximately 114-foot clear span multiple Kingpost Truss with nail-laminated arches. This timber bridge is historically significant because it retains its original features of the 1860's since the 1930s when it was spared from modernization by the construction of an adjacent steel highway bridge for vehicular traffic. Since that time Kings Bridge has a colorful history, including use as livestock barn and a proposed new use as a part of a "twin covered bridge" municipal park. Kings Bridge remains a museum-quality artifact. The bridge is currently supported by a temporary steel queenpost truss system pending full rehabilitation. The rehabilitation strategy for Kings Bridge is to minimize interventions, repair in-place, and to replace deteriorated members in-kind where possible. This paper documents the rehabilitation of Kings Bridge as of November 1, 2004. An updated presentation of the existing timber conditions; structural analysis; and final rehabilitation design will be presented at the ASCE Structures Congress in April 2005. Rehabilitation construction will begin fall 2005. The rehabilitation funding for Kings Bridge is 100% federal through the National Covered Bridge and the Transportation Enhancements programs.

Wooden covered bridges endure as symbolic reminders of America's past, of simpler times when crossing a body of water was not taken for granted and horses and carriages were urged to slow to a walk before entering. These structures also represent an important stage in our engineering history, as the development of wooden truss design transitioned from experimental handcrafted works to scientifically developed works of engineering. However, these bridges are a threatened resource. An estimated 10,000 covered bridges have been built in our country's history, but less than 800 remain. While many were removed due to improvements necessitated by the automobile and modern highway in the first half of the twentieth century, others continue to be lost every year due to floods, arson, and lack of proper maintenance.

Fire resistive materials (FRMs) are a critical component in the design of safe buildings. Current performance testing is strongly based on the ability of the FRM to adhere to and to control the temperature rise of its substrate. A fundamental understanding of the microstructure and performance properties of these materials is sorely needed to model their performance in real world systems and scenarios. While room temperature properties are more easily evaluated, it is the high temperature properties of the materials that are critical to performance during an actual fire. This paper will describe preliminary efforts in an experimental/computer modeling program being conducted at NIST to apply a materials science approach to characterizing the microstructure and properties of these materials. Three-dimensional x-ray microtomography is applied to obtain a representation of the microstructure of the materials. These microstructures can then be analyzed quantitatively to characterize critical parameters such as porosity and pore sizes, and the effects of these parameters on properties such as thermal conductivity. This analysis, along with characterization of the density and heat capacity of the FRM as a function of temperature, will provide the inputs needed for thermal performance models.

As part of the National Institute of Standards and Technology's building and fire safety investigation of the World Trade Center disaster, four standard fire resistance tests were conducted on the composite floor system used in the towers. The fire resistance tests were conducted to study three factors, the effect of (1) fireproofing thickness, (2) scale of the test, and (3) test restraint conditions. Two full-scale tests, 35 ft (10.7 m) span, and two reduced-scale tests, 17 ft (5.2 m) span, were conducted. Three tests were conducted under restrained conditions and one under unrestrained conditions. Additionally, two fireproofing thicknesses were applied, the specified thickness and the average applied thickness. Results showed that 1) a fire rating of ¾ h was determined from the reduced-scale restrained test with the specified fireproofing thickness which would not have satisfied 1968 New York City building code rating requirement of 2 h for the WTC towers, 2) the full-scale restrained floor system obtained a fire resistance rating of 1 ½ h, and 3 h) the full-scale unrestrained floor system achieved a 2 h rating. Past experience with the ASTM E 119 test method would lead one to expect that the unrestrained floor assembly would not perform as well as the restrained assembly, and therefore, would receive a lower fire rating.

Structural safety of high-rise steel buildings in fire circumstances depends highly on the behavior of their columns. Real behavior of large section columns which are commonly used in such buildings, however, is not known well because of a lack of appropriate fire test facilities to conduct full-scale experimental investigation. Behaviors of a total of seven large steel columns in fire are investigated experimentally using a high-performance column furnace in the Building Research Institute of Japan. Several findings to improve the structural redundancy of steel buildings in fire circumstances are discussed. They include that properly protected large steel columns show high fire resistance; however, if a portion of fire protection is damaged, even a large section column can surrender far earlier than expected; ceramic fiber blanket is one of the good options for robust protection; keeping axial load ratio of columns smaller is effective for fire resistance improvements; usage of high-performance steel grade, such as fire resistant steel, improves the fire resistance of columns greatly hence the fire redundancy of a building.

Fire and life safety in buildings is typically provided through several interrelated fire safety systems and features. These include egress, detection and alarm systems, suppression systems, smoke management systems, and compartmentation, as well as protection of structural elements. When designing buildings, these systems are intended to work in conjunction with each other to provide a certain level of safety, both to life, as well as to property. However, due to the interrelationships in fire safety design, as well as the role of the design fire(s), the failure of a fire protection system or feature, or the introduction of a design fire hazard that is different (i.e. larger, faster, etc.) than anticipated in the original design, may compromise the effectiveness of other systems. This includes the design of the structural system and its integration with the other fire protection systems. Failure to understand characteristics of the building occupants, as well as the characteristics of the credible design fire hazards and performance of the building, fire protection systems/features, and structural systems may lead to further difficulties in achieving stakeholder's goals and objectives in the event of a fire. Therefore, it is important to evaluate the overall performance of the structural and fire protection systems in concert, and to determine whether they will meet their intended objective(s). To accomplish the objectives and meet design performance criteria, it is also important to understand the interaction of various components such as the structural response, the response of occupants, and fire protection systems, as well as to what degree these systems may be damaged by an event. The "SFPE Engineering Guide to Performance Based Fire Protection Analysis and Design of Buildings" and "Introduction to Performance Based Fire Safety" are used as general guides for the overall approach discussed in this paper.

Standard methods have been developed and are used in current design practice to determine by calculation the fire resistance rating of structural steel elements protected with spray-applied fire resistive materials (SFRMs). These calculation methods are based on simplified analysis of heat transfer through the SFRM material to the steel substrate. This analysis assumes one-dimensional heat transfer in Cartesian coordinates. Based on this analysis, the ratio of the volume per unit length to the surface area per unit length, expressed in terms of the "W/D ratio," is the governing parameter for heating of the steel element. For cylindrical rods and other small structural shapes, this analysis is inappropriate because the surface area of the insulated element increases with increasing insulation thickness. Simplified and detailed numerical heat transfer analyses have been performed in both Cartesian and cylindrical coordinates that demonstrate the reduced level of fire resistance associated with a given thickness of insulation on a cylindrical rod relative to a wide-flange element with the same W/D ratio.

Recent events have shown that traditional design methods have not always provided sufficient protection against catastrophic failure when buildings are subjected to extreme events of an unpredictable nature. Hazards such as terrorist attack or accidental explosion followed by fire are difficult to deal with using the standard engineering methodology of designing structural capacity to match calculated loading. For one thing, catastrophic loading is virtually impossible to quantify due to its random and unpredictable nature. And even if such loading could be defined, the capacity vs. demand method would probably lead to structures too expensive to build. A design methodology is presented here that divides the structural design process into three phases based on three fundamental types of loading. The first level of loading consists of the "known" loads, such as those specified in building codes and those that can be accurately defined by engineering judgment. Structural design for these types of loads follows the usual demand vs. capacity methods. The second level of loading consists of the "knowable" loads, which can be visualized and quantified to some degree, such as earthquake loading or traditional fire exposures. Performance-based methods to design for these types of loading have been highly developed in recent years and have been very effective. The third level of loading consists of the "unknowable" loads, which are the catastrophic events that cannot be predicted or perhaps even envisioned before they actually occur. The most feasible way to protect a building from collapse due to a particular event of this type is to increase the overall robustness of the structure — its ability to survive all types of extreme loading. This paper offers a methodology to rationalize and quantify the third level of this approach — designing for unknowable loads. Characteristics of structures which contribute to robustness are identified and discussed. The concept of a robustness index to quantify structural robustness is introduced. Additionally, the application of such an index to evaluating robustness of new and existing buildings is discussed.

An analytical approach consisting of three sequential analytical steps, namely, fire analysis, heat transfer analysis, and stress analysis was developed for predicting the behavior of square CFT beam-columns under elevated temperatures from fire loading. Analytical models were developed for each of these analysis steps. These included 3D finite element models for nonlinear heat transfer and stress analysis. They were further used to analytically investigate the fundamental moment-curvature-temperature (M-Φ-T) behavior of CFT beam-columns subjected to combined axial, standard fire (thermal), and flexural loading. The effects of thermal insulation, axial load level, steel tube width (b) and width-to-thickness (b/t) ratio on the on the moment capacity and M-Φ behavior of CFT beam-columns were evaluated.

Hollow structural steel columns, which are widely used in the construction of framed structures in industrial buildings, are often filled with concrete in order to achieve an increased load-bearing capacity. In recent years, high strength concrete is becoming an attractive alternative, to traditional plain concrete, since it increases the load-carrying capacity of hollow structural sections (HSS).. In this paper the fire performance of high strength concrete-filled steel hollow structural section (HSS) columns is presented for various types of concrete filling. A comparison is made of the fire resistance performance of HSS columns filled with normal strength concrete (NSC) and high strength concrete (HSC). The factors that influence the behaviour of HSC-filled HSS columns, under fire conditions, are discussed.

The AISC Specification for Structural Steel Buildings 2005 (AISC 2005) will include an appendix that provides criteria for design and evaluation of structural steel components, systems and frames for fire conditions. The use of these provisions for the evaluation of performance of individual members at elevated temperatures during fire exposure is a new concept to most structural engineers. An approach to applying the AISC 2005 provisions related to structural capacity at elevated temperatures is addressed in this paper.

This paper presents a pilot study that was performed to develop design fires for an office compartment and to use these fires to analyze structural performance at elevated temperatures. Published information on office planning was used to define three model layouts and their furnishings. The CFAST computer program was used for parametric modeling of the fire conditions within each of the three layouts, subject to the following variables: ventilation, surface materials, heat release rates, and ceiling heights. The heat release rates were based on test data compiled and published by researchers at the National Institute of Standards and Technology (NIST). The results obtained from the fire models indicated that flashover occurred in a limited number of scenarios. The time temperature curves obtained for these flashover conditions were then used for combined thermal and structural analyses of both exposed and insulated steel beams. The ASTM E-119 standard time-temperature curve was also used for thermal and structural analyses as a benchmark. In addition to presenting the results of the study, the paper concludes with recommendations and proposals for future work.

With the advancement of metal production, a new type of steel plate. Fire Resistant Steel (FRS), has been developed to conquer inherent weakness of the conventional steel. FRS has been proven to have better mechanical properties at high temperature than those of the conventional steel. Also, the requirement of fire-protection in the fire-resistant steel can be released or relaxed in the fire condition. However, the knowledge related to the structural behavior of FR steel members under fire load is lack. In this study, a series of H-shaped steel columns, made of FR steel, were centrally loaded to their limit states to investigate their performance under fire load. The specimens examined in this study including stub columns, and columns within inelastic and elastic ranges, according current design specifications developed at ambient temperature. Stub columns with varying width-to-thickness ratios were designed to examine the local buckling behavior of FR steel columns under fire load; while, the columns with varying slenderness ratios were aiming at evaluating the global behavior. Based on this study, it was found that the current design criteria developed at ambient temperature is capable to prevent premature failure of local buckling of FR steel columns subjected to axial compression either at ambient temperature or at elevated temperature. Due to the limited effect of residual stress in fire, the governing failure mode could be changed from inelastic global buckling at ambient temperature to local buckling at elevated temperature. At 600°C, steel columns retain 60% of the ambient-temperature strength if the slenderness ratio less than 80. Design guidelines are proposed for FR steel columns under fire load.

This study investigates the thermal and structural effects of fire protection, the slab, and the connection fixity on the thermal and structural behavior of perimeter columns in a common steel-framed high-rise building exposed to fire. A model is developed based on a tall steel building (of common construction) that was subject to conflagration in the last decade. The progression of the fire from floor to floor as it moved up the building is considered in developing the time-temperature curves for the analyses. Structural analyses therefore consider that some floors are cooling down while others are heating up. Results show that the beams framing into perimeter columns expand significantly and induce large moments in the perimeter columns, which may lead to the development of plastic hinges. The development of many plastic hinges in a frame could lead to overall structural instability. This phenomenon is essentially independent of the beam-to-column connection fixity and the presence/absence of a slab; therefore fire protection designs for perimeter columns should consider the beams framing into them, which are integral to structural stability.

The Structural Engineering Institute of the American Society of Engineers has formed a Standards Committee to develop a new standard for blast resistant design of buildings. This paper discusses the proposed content of this new document and the current status of its development.

Making our daily engineering practice as sustainable as possible must become a matter of habit. How can we as practitioners take the lead and make sustainable engineering a reality in our lifetime? We must realize the tremendous power that we wield. We write the specifications for the materials and practices that govern every construction project—we can promote the cause of sustainability by putting those goals in the documents. The barriers practitioners encounter when they attempt to introduce sustainable practices, can be grouped within one of the three following categories—technical, policy and economic. Here we look at a specific example, a typical concrete specification, to illustrate how we can overcome these constraints.

Many in the building construction industry, including architects, building owners, and even structural engineers, fail to appreciate the significant contribution the structural engineer can make to reducing the environmental impact of a building construction project. This lack of understanding is one of the chief impediments to achieving the greatest possible reduction in the environmental impact of the structural system. Structural engineers interested in sustainability must combat this ignorance by educating themselves and others so that they will be brought into the circle of sustainability decision-makers and be able to make their contribution to reducing the project's environmental impact.

The moveable roof sections of Miller Park (the Milwaukee Brewers baseball stadium) exhibit several idiosyncrasies that make them unique in the United States. The triangular moveable panels each span approximately 183 meters (600 feet) over the field, and are approximately 43 meters (140 feet) wide at the outfield end. The panels are supported at three points: a "pivot" point and two "bogies". The pivot bearings are designed to resist forces along, yet allow rotation about, all three principal axes. Hydraulic jacking forces were applied to the different portions of the roof structure during construction and while performing roof repairs in the early spring of both 2003 and 2004. The response of the structure to the jacking is a function of the jacking process. This paper will serve to discuss the different movements anticipated and observed while jacking the roof, and how these movements were accommodated by creative engineering solutions.

An analytical study has been carried out to describe the effect of across-wind response on tall slender towers of circular cross-section. Since the across-wind response significantly influenced due to change in geometrical and dynamical properties of the structure, an attempt has been made by varying the diameter of the structure along its height to reduce maximum net response in terms of base bending moment in the specified range of wind speed.

While high-rise construction serves as one of the most challenging projects undertaken by society each year, tall buildings are one of the few constructed facilities whose design relies solely upon analytical and scaled models, which, though based upon fundamental mechanics and years of research and experience, have yet to be systematically validated in full-scale. In response to this deficiency, a full-scale monitoring project was initiated through the combined efforts of members of academe (University of Notre Dame), practicing design firms (Skidmore, Owings & Merrill LLP, Chicago) and commercial testing laboratories (Boundary Layer Wind Tunnel Laboratory, University of Western Ontario, Canada). The objective of this program is to monitor the full-scale response of some representative tall building structures and compare their actual performance to the predictions from wind tunnel testing and finite element computer models used in their design. As such, this program offers the opportunity to refine the design state-of-the-art based on any discrepancies revealed. As part of this full-scale evaluation, in-situ periods and damping ratios over a range of response amplitudes are being ascertained, which will prove vital for expanding the existing databases of full-scale dynamic properties. This paper presents a brief overview of the program.

Along with widely using high-rise building nowadays, much attention has been paid for the past several years to the conceptual design of high-rise building. From structural dynamics point of view, matters relating to wind loading are very important especially for high-rise building. Buffering forces on the structure are caused by fluctuating wind velocity. Due to the fact that structure is changing its boundary conditions of wind causing further turbulence and consequently additional dynamic forces called motion-dependent force will appear. For high-rise building, motion-dependent forces may become significant because building is very flexible and its natural frequency is very low therefore aeroelastic effects may have large contributions to the structural response. The wind induced response is usually estimated through analysis based on basic of random vibration theories. In such manner, response analysis is calculated by estimating modal spectral analysis in frequency domain and this approach was used in most of international codes and standards. International codes have simplified the random vibration theories by using algebraic expression and derived gust effect factor resulting in approach called "Gust loading factor". The "Gust loading factor" approach was first introduced by Davenport for computing the wind-induced responses in alongwind direction. This approach was later examined by Yin Zhou, et al. by correcting the unfavorable distribution of the static equivalent wind loads and the effects of the mode shapes and he also concluded that GLF approach will results in excessively conservative design when applying for high-rise building. Because GLF approach is based on frequency domain, therefore, the motion-dependent forces and non-linearity term of the fluctuating wind forces are not accounted. Experimental studies have been carried out by Katagiri, et al. and Yoshie, et al. using aeroelastic model of high-rise building with certain side and aspect ratios considering torsional and cross-wind vibrations. These studies have shown some characteristics of motion-induced forces for a certain types of model. However, the role of motion-dependent forces in along-wind direction and how it affects to response of building were not given out. Kareem predicted that the motion-dependent forces may become significant for flexible structure because of structural response velocity which is forecasted to become large and significantly affect the wind loading function. As some literatures mentioned above, the motion-dependent force in along-wind direction was either included qualitatively or not included and its role was still not clear clarified. In this paper, wind velocity filed has been simulated by using ARMA method, buffeting forces and motion-dependent forces were formed, analysis of high-rise building in along-wind direction was conducted in time domain considering the structural velocities and the non-linearity term of wind loading function. The results from this calculation will be compared with those estimated from buffeting force, in which the motion-dependent forces and non-linearity term of the wind loading function were not included, to evaluate quantitatively the role of motion-dependent forces and to clarify whether the structural velocity term could significantly affect loading function. The comparative study is also made for results from this study with that estimated from GLF method and with ones estimated from force formulation of along wind direction including three dimension responses, which is presented in work of Santos and Aas-Jakobsen, et al.

A new Manhattan skyscraper with lower-floor offices and upper-floor residences incorporates many special and challenging structural design features to accommodate very specific client needs. They include a mid-block atrium, restricted lower core and column locations to satisfy previously-established leasing documents, numerous column pick-ups and transfers, load transfers through outrigger systems, and a uniquely designed tuned mass damper to counteract wind motions.

Construction has commenced on One Bryant Park, a 55-story office building that will be the New York headquarters for the Bank of America. With 2.1 million square feet of office space and a height of 945 feet–1200 feet to its spire—it will be one of the largest buildings in Manhattan. Although simple in appearance, the structural design was anything but and presented challenges in all of its aspects, from the bottom of the foundation to the tip of its spire. The solutions to these challenges, however, ended up as elegant as its crystalline form would imply.

The New York Times Building, currently under construction in New York City, is a study of balance and compromise. The vision of The New York Times to create a building ahead of the times was balanced by the goals of real estate developer Forest City Ratner to develop an economically efficient building to maximize their leasable space on the upper floors. The innovative European-style architectural design of architect Renzo Piano Building Workshop was blended with the understanding of local building code and practice by New York architect Fox & Fowle Architects. One of the principal architectural features of the building, the expression of exterior steel, involved the compromise of aesthetic appearance, structural adequacy, and fabrication and erection practicality. The thermal movements of this exterior steel provided a great engineering challenge in attempting to balance the movements of interior and exterior steel to limit large differentials. The duality of each aspect of the building design from the ownership and design team partnerships to the overall balance between innovation and efficiency provided numerous engineering design challenges. This paper will outline some of these challenges, focusing on those driven by aesthetic, erection, and fabrication considerations of the exterior steel.

The award winning Rose Center for Earth and Space replaces New York's sixty-year-old Hayden Planetarium with its icon 80 mm thin concrete dome. The new building designed by Polshek & Partners with Weidlinger Associates as structural consultant, combines showmanship and science to provide a dramatic experience for the visitor. A 26m (87ft) diameter spotlighted sphere supported on three pairs of 18m (60ft) long tapered steel legs has the appearance of a space ship ready to rise toward a cosmic adventure. The sphere, that houses a sky theatre and a big bang exhibit, floats inside a six-story cube, transparent on two sides, that rests on a two-story concrete base. Completing the futuristic effect, a dramatic spiraling ramp, rises from the first level of the building, connecting it to the Big Bang exhibit space inside the globe.

The latest addition to Times Square, New York is the Times Square Tower office building. It stands on the one of the smallest sites in Times Square, a sliver of ground left between converging streets and subways. The Tower is surrounded by subways and sits atop a 19^th century structure. The final form and structure are a direct derivation of the site and its many constraints.

The history of the analysis of cable net structures is quite brief. The first prestressed cable net structures were built about fifty years ago. Cable nets are nonlinear structures although most of their nonlinearity is eliminated with prestressing. Several different methods of coping with the nonlinearity have been developed. As fabrics were improved, tensioned fabric structures (TFS) replaced cable nets. The methods of analysis developed for cable nets were modified and used for analysis of TFS. Most current available programs for cable nets and TFS are proprietary "black boxes" and hence do not give the user an understanding of their methods of analysis.

When we talk about cable nets, we normally think about large span roof structures. However, there is a class of cable net structures that have been largely ignored by the structural engineering community but have provided significant service in the area of research, forecasting, defense communication and public broadcasting. These cable nets provide the primary radiation source for the generation of radio waves over a wide range of broadcast bands. Very low frequency arrays may span over 300 m (980 ft) between towers and may be as tall as 350 m (1150 ft). Some horizontally polarized arrays span between mountain tops that are miles apart. These giant antennas are used to communicate with submarines at very low frequencies. Despite the use of satellite communications in military applications, the VLF antenna is still part of the Navy's communication system. Other cable net antenna arrays broadcast in the shortwave band and have served to provide uncensored news and information to people in countries who otherwise would only hear the "party line". Such broadcast arrays in Europe operated by Radio Liberty and Radio Free Europe played an important role in ending the Cold War. Similar antenna systems are in use today by the US International Broadcasting Bureau and the BBC, and provide worldwide coverage. Many other countries have similar system in their broadcasting system. Shortwave radio broadcasting is still the most efficient and least costly means of reaching a wide audience. Other uses of cable net antenna systems have more clandestine uses. These systems have been used for direction finding and over the horizon radar. Direction finding arrays are typically circular arrays for omni directional interception. Depending on the application, these arrays can have a diameter as large as 350 m (1150 ft) suspended from 40-m (130 ft) towers. Using the ionosphere and special computational techniques, shortwave antenna systems can pin point radio transmissions over 3000 km away. Over-the-horizon radar systems typically use vertically suspended cable net arrays. These systems can detect targets as far as 3500 km away in one "hop". OTH is also being used to monitor ocean-surface winds and current over data-sparse areas that would otherwise require thousands of widely dispersed instruments.

This paper examines the use of cable net structures in architecture. The time period of most of these buildings were in the 1950's through the 1970's The paper explains how this structure found its way into architectural design, the major achievement accomplished using this structure, and finally discusses why cable nets are rarely used today.

Vladimir Shukhov was a brilliant structural engineer who lived and worked in Russia in the late 19^th and early 20^th centuries. Besides the innovations he brought to the oil industry and the construction of numerous bridges and buildings, he was the inventor of a new family of doubly-curved structural forms. These forms, based on non-Euclidean hyperbolic geometry, are known today as hyperboloids of revolution. Shukhov developed not only many varieties of light-weight hyperboloid towers and roof systems, but also the mathematics for their analysis. Unfortunately Shukhov's work is poorly known today, particularly in the West; however, his direct influence may be seen in the work of two of the most important artist-architects of his time, as well as, more recently, two Western engineers.

Spatial cable-stayed structures are characterized by high efficiency, construction advantages and the architectural attractiveness they bring to the contemporary metropolitan environment. At the same time, such structures are characterized also by high deformability, which, under various conditions, particularly non-uniform loading, causes them to behave as complex mechanisms. Cable pretensioning increases the structural stiffness, but does not eliminate the possibility of large deformations under arbitrary loads. Furthermore, analysis of cable-stayed structures is immensely complicated by significant inherent geometrical nonlinearity. This paper describes a method for constructing geometrically stable spatial cable-stayed structures by introducing supplemental structural connections to cable meshes stretched on rigid contours.

Experience with large deflection non-linear structures has led the authors to a novel approach to structural design of more conventional steel truss systems using post -tensioning. A number of built examples of post-tensioned steel spatial truss networks which have been employed to cover sports arenas are presented. Four long-span structures are used as examples. The principles of designing post-tensioned steel structures is discussed, including prestress as a property of a structural system and the means of achieving reliable prestress in construction.

After steel wires and steel cables have been discovered for the construction of long span bridges in the middle of the 19^th century, it took another century to establish the use of cables for other buildings. In Europe the Olympic Game roof in Munich for the 1972 Olympics was not the first building with cables, but inspired many clients, architects and engineers to cover large spaces with cable structures. In the 1990s many stadium roofs have been designed using cable structures with only 20 – 30 % of the steel weight compared to conventional steel girders. Also for office buildings and skyscrapers stainless steel cables are now used as tension framework in transparent, almost invisible glass curtain walls. Exhibitions halls, airport terminals, railway stations, museums and concert halls are further applications for the extreme lightness and strength of structural cables. Still bridges challenge engineers and downtown locations ask for landmark designs with architecturally pleasing details, without visible threaded rods or other bulky anchorages. Advanced cable structure specialists can engineer and manufacture cable structures without length adjustment within the cable system. Due to the fabrication to very small tolerances in the factory, ready to install components can be supplied to the construction site.

ASCE Standard 19 "Structural Applications of Steel Cables for Buildings" was published in 1996. An update will soon be issued to keep this standard current with state-of-the-art cable design, materials, and fabrication. Several changes have been made, not the least of which is the standard's scope. The scope is now more specific in stating that the standard applies to statically loaded structures, and does not apply to guyed communication towers and vehicular bridges. So that the standard addresses only those products specifically manufactured as cables for use as permanent structural elements, some materials referenced in the earlier edition have been deleted. Required information to be shown on the contract documents has been clarified, and load combinations have been revised to maintain consistency with the latest edition of ASCE 7.

Light weight cable nets have been successfully used for spanning over very long spans, by using thin layers of steel cables. Under snow and wind loads, the nets are subjected to large deflections and both the static and dynamic behaviors are governed by the unusually high span to thickness ratios. It is therefore important that the deflections and forces in the net are controlled, by a careful selection of the design parameters, such as direction and magnitude of prestressing forces. Most nets utilize unidirectional post-tensioning of cables, which results in heavy anchorages and uneconomical supporting frames. A biaxial post-tensioning system will minimize the magnitudes of prestressing, the cable sizes, the reactions at anchorages and the use of lighter supporting frames and foundations would be possible. A rectangular net pattern will lead to a more efficient system than a skew pattern. Other important parameters are the spacing of cables, varying prestressing forces in the two directions, providing a composite cable and deck sheeting model (increasing planar stiffness of cladding), considering the effects of yielding supports under prestress due to anchor movement and the increase in axial stiffness due to closely spaced clamps. Also, results from an experimental study are discussed. Applications of current design code provisions (ASCE Cable Structural Standards-19, 1996) will lead to cost savings and improved structural performance.

The limit states philosophy for structural design is being accepted by more engineers in the USA. The American Institute of Steel Construction [AISC] promulgated a specification for structural steel based on limit states concepts, denoted as Load and Resistance Factor Design [LRFD], with its first edition of the AISC LRFD Manual (AISC, 1986). Since then, LRFD-type specifications have been published for other structural building systems and materials. It has been suggested that an LRFD-type specification be developed for cable structures and components (steel wire strand and rope). Presently, the generally accepted basis for design of steel cables for buildings in the USA is ASCE Standard 19-96, Structural Applications of Steel Cables for Buildings. Provisions from that standard have been incorporated into USA building codes. Cable selection criteria in the Standard is based on a hybrid load factor/allowable stress concept. The object of this paper is to present some of the issues encountered in revising the basis of design for cables in ASCE Standard 19-96 to limit states design. Because a limit states specification for cable structures will likely be akin to the AISC version of LRFD, that specification will be briefly reviewed.

The 40,000-seat Jeju Worldcup Stadium has a lune-shaped fabric roof structure covered by Teflon-coated Fiberglass membrane for an area of 16,000 square meters. The structural system features a unique spatial steel trussed framing system suspended from six Masts by Stay, Side and Tieback Cables. It has twenty triangular Radial Trusses, diamond shaped Front and Rear Trusses, and planar Tie Trusses, all made of steel pipes. Between each Radial Truss, spans a Teflon-coated fiberglass membrane supported by steel cables. First, the unique geometry of the roof structure and its structural system are introduced. Comparison of design wind pressures based on the local building code and wind tunnel test is discussed. The design procedure is presented. And finally, the cables are discussed in detail.

Software systems available today give structural engineers the ability to come up with very detailed, efficient designs complete with CAD drawings and quantity take-offs in a matter of hours, not days. Moreover, these software systems provide the engineer with design options to help arrive at optimal structural designs quickly. This is important because during the early budget phase of a project, the structural engineer is often performing work he may never get paid for unless the team wins the project. In addition and in general, structural engineers are continually being asked to do more, often times with less fee, making it difficult to remain profitable.

The theoretical background, construction, and application of Economic System Maps (ESM) to several practical structural engineering problems are presented in this paper. ESM methods are applied to girder sizing in a uniformly-loaded steel floor system, three uniformly-loaded two-way reinforced concrete floor systems, and three steel shear connections. In general, ESM methods can be applied to a wide variety of top-level optimal system selection problems as well as very specific optimal parameter selection within a target system. Because optimal design data is pre-generated and presented in convenient 3-dimensional graphs, a designer can select optimal systems and parameters without needing to actually use optimization software. Sensitivity of an ESM to variable market and cost data is a significant challenge to its general usefulness. Work is currently underway to address this sensitivity and preliminary results are promising.

This presentation will introduce basic optimization tools available to structural engineers for solving a variety of everyday design tasks. Basic formulation of several common structural design problems will be discussed along with solution methods for those problems using the MS Excel Solver, Mathematica, and MathCAD.

In order to quickly run through often repeated calculations, engineers in design offices across the country routinely create electronic tools. Spreadsheets offer an efficient and extremely flexible environment for the creation of tools ranging from simple column load tables to design calculation sheets for composite steel beams. These tools supplement the range of commercially available design software for structural engineers. While the current programs are highly advanced, there are often gaps in there calculations for atypical conditions. The engineer-created tools can fill in these gaps and also help in the feasibility and estimating stages of a project. This paper discusses several design tools currently available to the engineer for economic steel design. The tools, which are Visual Basic-enhanced Microsoft Excel spreadsheets, are an alternative to back-of-envelope calculations and are designed to help engineers save time, especially in the early design stages of a project. Some sample design situations discussed include: minimizing the cost of the steel floor framing based on fabrication cost data; eliminating the need for column stiffeners by using a heavier column; comparing the total frame costs of various framing schemes to identify the most economical option; designing non-composite beams. It should be noted that each of the tools discussed in this paper can be obtained at no cost from the American Institute of Steel Construction (AISC), www.aisc.org/steeltools.

Neural networks are well-known universal approximators in function approximation but have been treated more or less as "black boxes" in engineering practice. In the application of system identification for structural health monitoring, neural networks have been adopted as powerful tools for problems that challenge conventional approaches as in many other situations. In particular, they have been treated as a superior fitting tool for any given input and output data set, however the critical steps of initial network architecture selection (network types, number of layers and hidden nodes) and interpretation of final trained parameters (weights and biases) are not thoroughly explored and understood especially by relating these issues to the underlying physics of the system to be modeled. This typifies a dilemma associated with black-box models that are highly adaptive but neither transparent nor parametric. Challenged by these controversial natures of neural networks and driven by the need of applying neural networks in damage detection, the authors have proposed a novel constructive approach in initial network architecture design. In this study, focus is given to modeling nonlinear hysteretic behavior typically observed in structural joints subject to extreme dynamic loads. Algebraic and geometric based analysis is carried out to reveal the "mechanisms" of how neural networks, which in turn serves as a practical guidance in designing neural networks. Numerical simulations are presented to demonstrate the efficiency and engineered feature of this approach. A training example is provided to show that this approach enables neural networks to carry some "meaning" (either physical or phenomenological) while remaining flexible and powerful in system identification.

In this paper, a novel approach to the characterization of structural damage in civil structures is presented. Structural damage often results in subtle changes to structural stiffness and damping properties that are manifested by changes in the location of transfer function characteristic equation roots (poles) upon the complex plane. Using structural response time-history data collected from an instrumented structure, transfer function poles can be estimated using traditional system identification methods. Comparing the location of poles corresponding to the structure in an unknown structural state to those of the undamaged structure, damage can be accurately identified. The IASC-ASCE structural health monitoring benchmark structure is used in this study to illustrate the merits of the transfer function pole migration approach to damage detection in civil structures.

Structural health monitoring can be defined as a statistical pattern recognition problem which necessitates establishing a decision boundary based on measured data. Choosing the decision boundary is often based on the assumption that the distribution of measured data is Gaussian in nature. This unwarranted assumption impairs the performance of structural health monitoring significantly by increasing false positive and negative indications of damage. This paper attempts to address the issue of the decision boundary establishment using extreme value statistics (EVS) so that the tails of a distribution associated with damage can be properly modeled. Encompassing three extreme value distributions (Gumbel, Weibull, and Frechet distributions), the generalized extreme value distribution (GEV) is adopted for establishing the decision boundary. A parameter estimation technique based on a nonlinear optimization scheme is developed to automatically estimate the model parameters of the GEV and choose the most appropriate domain of attraction. The validity of the proposed method is demonstrated in numerical studies using real sample data sets and in the context of delamination detection in a composite plate.

This paper presents recent developments and approaches (using GPS technology and real-time double-integration) to obtain displacements and, in turn, drift ratios, in real-time or near real-time to meet the needs of the engineering and user community in seismic monitoring and assessing the functionality and damage condition of structures. Drift ratios computed in near real-time allow technical assessment of the damage condition of a building. Relevant parameters, such as the type of connections and story structural characteristics (including geometry) are used in computing drifts corresponding to several pre-selected threshold stages of damage. Thus, drift ratios determined from real-time monitoring can be compared to pre-computed threshold drift ratios. The approaches described herein can be used for performance evaluation of structures and can be considered as building health-monitoring applications.

In recent years, a few structures have been instrumented to provide continuous vibration data in real time, recording not only large-amplitude motions generated by extreme loads, but also small-amplitude motions generated by ambient loads. The main objective in continuous recording is to track any changes in structural characteristics, and to detect damage after an extreme event, such as an earthquake or explosion. The Fourier-based spectral analysis methods have been the primary tool to analyze vibration data from structures. In general, such methods do not work well for real-time data, because real-time data are mainly composed of ambient vibrations with very low amplitudes and signal-to-noise ratios. The long duration, linearity, and the stationarity of ambient data, however, allow us to utilize statistical signal processing tools, which can compensate for the adverse effects of low amplitudes and high noise. The analysis of real-time data requires tools and techniques that can be applied in real-time; i.e., data are processed and analyzed while being acquired. This paper presents some of the basic tools and techniques for processing and analyzing real-time vibration data. The topics discussed include utilization of running time windows, tracking mean and mean-square values, filtering, system identification, and damage detection.

In 2001, the Southern California Seismic Network (a part of the California Integrated Seismic Network) installed a 3-component Episensor force-balance accelerometer on the top floor of Millikan Library (station name MIK). This accelerometer is recorded by a 24-bit Quanterra data logger, and the dynamic range of the system is about 10⁷. This means that ambient vibrations of the building can be recorded with a signal to noise ratio exceeding 100. Data from this system is recorded in a similar fashion to other more conventional seismographs that are installed in the ground throughout southern California. In particular, data is stored at 80 sps for any local earthquakes large enough to record on the seismic network, and data is also stored continuously after anti-alias filtering and resampling to 20 sps. We analyze a near-continuous three-year-long recording of the motion of the top floor of Millikan Library, which is a 9-story concrete building (a combination of shear walls and moment-resisting frames) built in 1966. The fundamental mode natural frequencies of the building (about 1.7 Hz N-S, 1.2 Hz E-W, and 2.5 Hz torsion) can be clearly observed from even short stretches of ambient vibration data. Using this data we have documented that changes in natural frequency are unambiguously correlated with the following factors. 1) There are diurnal variations in all natural frequencies on the order of 1–2%. 2) Heavy rain increases the E-W and torsional frequencies by up to 3%. The frequencies typically return to the pre-rain levels within about 5 days. 3) High temperatures (40° C) raise all natural frequencies by 1–2%. 4) Strong winds (50 kph) decrease natural frequencies. 5) Construction of non-structural office partitions on floors 3, 4, and 5 in 2003 increased natural frequencies, with the E-W frequency increasing by 5%. In addition to studying ambient vibrations, we also studied the apparent natural frequencies in a number of small to moderate earthquakes in southern California. We obtained the apparent natural frequencies by deconvolving the ground motions at a site 100 meters from the building (SCSN station GSA) from the Millikan roof records. We observed that the natural frequency dropped significantly for even small motions. For example, the E-W apparent frequency dropped from 1.18 Hz (ambient) to 1.06 Hz during a M 5.4 earthquake at a distance of 119 km (peak ground acceleration of 0.018 g). It returned to pre earthquake levels within several minutes.

This paper presents a unified approach for designing precast prestressed concrete piling in areas of high seismicity where soil liquefaction and/or loss of lateral support are design considerations. A general procedure for pile design is formulated based on governing codes, accepted practice, and relevant literature. The procedure is in compliance with the International Building Code 2003 (2002), "Recommended Practice for Design, Manufacture, and Installation of Prestressed Concrete Piling" (PCI, 1993), ACI 318-02 (2002), and the PCI Design Handbook (PCI, 2001). Using modified compression field theory, interaction diagrams (φP_n vs. φM_n) are developed for various solid square precast prestressed concrete piles that are fully braced by adjacent soil. By reducing fully braced nominal moment capacities by P-Δ effects, these curves are also modified to account for practical unbraced lengths resulting from nominal soil support afforded by marine clay and liquefied soil layers. This paper provides a simplified approach for designing precast prestressed concrete piling to include the effects of pile buckling, the development and sample results of interaction diagrams needed for the simplified procedure, an overview of other code requirements for precast prestressed piling in high seismic zones, and a practical application example.

Practical analysis and design of excavation support walls have been addressed in previous publications, which examined the use of computer-assisted analyses such as the continuous beam method and the beam-Winkler spring method in staged excavations. This paper examines the use of the finite element method for the analysis and design of excavation support walls. A multi-staged braced excavation was analyzed using the finite element method, and the computed wall bending moments and brace forces were compared with those obtained from the continuous beam method. A comparison of the computed earth pressure profiles obtained from finite element analyses with the theoretical active and passive pressures used in the continuous beam method provided valuable insight into the differences in predicted wall performance.

One of the most challenging problems in soil-structure interaction is the piled raft. Piled-raft foundations for important high-rise buildings have proved to be a viable alternative to conventional pile foundations or mat foundations. The concept of using piled raft foundation is that the combined foundation is able to support the applied axial loading with an appropriate factor of safety and that the settlement of the combined foundation at working load is tolerable. In some instances the piles are spaced uniformly and in other cases are spaced strategically to achieve a more uniform settlement of the mat. Two strong reasons dictate the use of the 3D Finite Element Method (FEM): (1) the problem is so complex that simplified methods cannot model the problem correctly; and (2) codes for the FEM are available, powerful, and capable of being run on the personal computer. Thus, the modeling of the raft, piles, and supporting soil as a soil-structure-interaction problem using FEM is a feasible method. Two piled-raft foundations were analyzed using the FEM program. Comparisons were made between experimental and analytical results and the FEM was shown to yield excellent results for the cases analyzed.

The effect of soil-structure interaction has often been neglected in seismic design and retrofit of structures in the past. For new construction, foundations are often not included as part of the primary lateral force resisting system. With pile supported structures, the lateral capacity is determined using an iterative procedure assuming a linear elastic response. Discounting effects of soil-structure interaction, such as increased energy dissipation and period shift, has lead to more massive foundations than may be necessary. For both new design and retrofit of existing structures, accounting for soil structure interaction can significantly reduce construction efforts and cost. With a greater knowledge of soil-structure interaction, the seismic behavior of structures can be more clearly understood. With a greater knowledge of the behavior of the combined structure and substructure systems, the design or retrofit may then be tailored to meet project specific seismic performance objectives. Accounting for soil structure interaction is important for design of new structures as well as providing seismic retrofit of existing structures. This paper describes a methodology used to analyze soil structure interaction for a seismic retrofit design. The retrofit uses calculated energy dissipation through soil structure interaction as an integral part of the seismic force resisting system. Seismicity, soil profile, and pile length vary along the entire length of the pipeline undergoing retrofit. To describe the methodology, pile capacities are established, soil springs developed and a combination of pushover, time history, and response spectrum analyses used to describe the expected behavior under anticipated seismic loading. The structure that serves as the basis of this paper is an existing large diameter above grade pipeline. Pile supported pipe bents are spaced at approximately 60 feet (18.3 meters) on center and massive pile supported temperature anchors are located at approximately 1000 foot (305 meter) intervals. An expansion joint is located midway between the temperature anchors. Design data was obtained from as-built drawings. Each pile group consists of at least four 16-inch square pre-cast concrete piles, with each pile driven on a 3 vertical to 1 horizontal batter, in directions 30 degrees from the perpendicular to the pipeline. In addition, there are 19 pile groups located at bends in the pipeline and road crossings and 44 pile groups located at temperature anchors. Each of these are supported on groups of 10 to 12 pre-stressed concrete piles. The pipeline is located in a moderate seismic zone. There are three elements to consider in the soil structure interaction study: the physical capacity of the pile as a structural element, the capacity limiting equilibrium of the surrounding soil, and the seismic input. All three components play an important part in establishing the overall structural behavior.

Analysis of raft foundation and superstructure is usually performed considering only soil-raft interaction. In this work, an extensive parametric study, using field tests and finite element analysis, is conducted to study and quantify the effect of raft thickness, column and beam rigidities and soil behavior, on the interaction process. Results of both experimental and numerical techniques are analyzed then input to an artificial neural network, ANN, to be capable of predicting redistributed column loads. A simplified analysis procedure is recommended and its accuracy is verified.

Conventional soil models with single spring and damping components can not properly simulate the frequency-dependent behavior of foundation soils. In this study, the foundation soil is modeled by a two-degrees-of-freedom mechanical model (2DOF) with eight integrated constant parameters. The 2DOF model provides a reasonable basis for the nonlinear dynamic analysis of soil-structure interaction (SSI) system in the time domain. To determine the values of the integrated parameters in the 2DOF model, a system identification technique using the extended Kalman filter (EKF) is developed in this study. To improve the computational accuracy and simplicity, the first order polynomial approximation is applied in the EKF procedure. A realistic identification example is given for two soil sites in Anchorage, Alaska. The recorded seismic data from two relatively long-period earthquakes, the Nanana earthquake (23^rd October 2002) and the Denali earthquake (3^rd November 2002), are used for the identification example. Results of the example demonstrate the feasibility and practicality of the proposed identification technique. Based on the identified integrated parameters, the site impedance functions and predominant frequencies can be estimated. The 2DOF soil model and the identification technique can be used in time-domain for nonlinear analysis of soil-structure interaction in various complex soil conditions. The identified parameters can be stored in a database for other similar soil conditions. Developing a broad database may provide an useful insight for investigating effects of SSI.

The objective of this paper is to investigate the seismic response of an actual bridge pier founded on a group of 64 steel piles in extremely soft clay (N_SPT = 0), which has been thoroughly instrumented by the Institute of Technology of Shimizu Corporation. Several earthquake motions have been recorded, with peak accelerations in the free field as high as 0.11g. An attempt is presented herein to reproduce the field measurements using analytical tools. The relatively low levels of peak shear strain in the soil (γ < 10^–4) and the high plasticity index of the clay justifies the use of linear elastodynamic methods. The effects of soil amplification, kinematic interaction, foundation compliance, and superstructure inertia on seismic response are considered. Group effects are treated using the superposition method in conjunction with wave-propagation-based interaction factors between piles. It is shown that records and predictions compare reasonably well for the response of the pile cap and bridge deck. Implications of valley effects, multiple-support excitation and structural connections on seismic response are discussed. The study complements and extends earlier studies by Tazoh et al and Gazetas et al.

Pile foundations are used widely as supporting elements for buildings and bridges. Their dynamic responses are affected significantly by pile-to-soil and pile-to-pile interactions. While most recent studies have studied the soil nonlinearity and its effect on pile response, the pile shaft is normally assumed to be linear elastic in the analysis. Such assumption is, in many cases, contradicting to the field observations after major earthquakes, where extensive pile damages have been observed in the form of plastic hinges and fractures etc. This paper attempts to study the pile yielding and its effect on kinematic response of pile-soil system under vertically propagated shear waves. Emphasis will be put on quantifying the demand placed on piles during strong shakings. The Beam-On-Nonlinear-Winkler-Foundation (BNWF) model is adopted here to study the response of pile embedded in a homogeneous soil layer. The pile is modeled as nonlinear beam element whose moment-curvature relationship is bilinear. The surrounding soil is modeled as distributive hysteretic springs and frequency-dependent dashpots, whose parameters can be obtained from standard geotechnical parameters. Such simplification enables the pile-soil system to be analyzed using one-dimensional finite element formulation. Both pile head response (displacement and rotation) and bending moment induced in the pile by strong shakings are investigated. The study shows that the pile nonlinearity does not have significant effect on pile head response under most frequencies. However, a number of analyses have ductility demand on the pile greater than the pile capacity, which implies the pile failure as observed in past earthquakes. Such failure will have significant effect on structural response since partial or full capacity of the pile can be lost due to the shaft failure induced by strong shakings.

Existing models that describe the behavior of reinforced concrete columns that are confined by transverse reinforcement have an empirical or semi-empirical basis. Many of these models refer to the uniaxial stress-strain relationship of the confined concrete, assuming that the transverse steel has yielded. This paper proposes a theoretical model of circular RC members confined by transverse reinforcement. The analysis is based on formulation of an equivalent, fully confined, concrete column. The equivalency is based on a preliminary elastic analysis of a cylinder confined by a sequence of equally spaced lateral elastic-perfectly plastic stiffening rings. Results of worked examples show that within a Reduced Cylinder Radius (RCR) there is a zone of uniformly distributed stresses in which the tangential stress is equal to the radial stress and the shear stresses are equal to zero. This finding leads to application of the concept of RCR that was observed in elastic solutions without leaving out the confinement influence near the cylinder surface. Thus, the discrete steel ties can be replaced by an equivalent tube and the equivalent confined cylinder can then be solved in the plastic range of the concrete material with the relatively simple boundary conditions at the concrete-tube interface. Using the Imran and Pantazopoulou (2001) concrete plasticity model, a computer program was written to solve this problem. Initial results of circular columns that were analyzed with this model show a very good agreement with published test results.

This paper studies the axial behavior of reinforced concrete columns using a new confinement technique that the first author has developed. Two opposing spirals (cross spirals) are used to confine reinforced concrete circular columns in order to enhance their strength and ductility or to increase the spiral spacing (pitch) to facilitate the flow of concrete during construction. The influence of the new confinement technique on the axial strength and ductility of reinforced concrete circular columns is experimentally investigated and compared to columns confined with conventional single spiral. Ten small-scale reinforced concrete circular columns confined with different spiral patterns were constructed and tested subjected to concentric axial compressive loading. The axial force, axial displacement and strains in the concrete and confining and longitudinal reinforcement were measured during the testing. This study showed that columns confined with two opposing spirals behaved almost similar to columns confined with single spiral when same amount of confining reinforcement is used. The study also showed that the new confinement technique could significantly improve the axial strength and ductility of circular columns without hindering the flow of concrete during construction.

Experiments for flexural strengthening of reinforced concrete (RC) beams with rectangular section using epoxy-bonded continuous horizontal steel plates are carried out. Three reference beams corresponding to flexure are loaded monotonically up to maximum load capacities in order to define the preloading conditions of damaged beams. These beams are then repaired/strengthened using different plate configurations as defined by variations in plate curtailment length (S1, S2 and S3). From the experiments, it is confirmed that continuous steel plates bonded externally to the beam webs are effective for the flexural strengthening of RC beams. It is observed that the flexural strength increases with increasing plate area. When considering the maximum load, the strengthening beams show 2.36, 1.39 and 1.27 times that of the corresponding reference beams for strengthened sample of S3, S1 and S2, respectively. Experimental results show that this strengthening method is effective and the specimens' flexural strength is improved.

The consideration and computation of secondary moments are important in the design analysis of continuous prestressed concrete structures. Such moments are secondary in nature but not in magnitude. Based on a comprehensive review of available methods of computing secondary moments such as equivalent load method, an alternative methodology of computing secondary moments is developed in this study. The proposed method discretizes an indeterminate structure into individual structural elements with end constrains. The concept of secondary moments under constraints is introduced in discrete structural elements. And they are computed as the function of primary moments in the structure. Secondary moments in the structure are then computed by applying basic principles in structural mechanics. The computing procedure is shown to be simple, prompt and accurate. The advantages of the proposed method over other existing methods are discussed and illustrated through numerical examples. In addition, the proposed computing method can be readily integrated into existing finite element codes of structural modeling and design. The application of the proposed method in practical design practice is demonstrated through a design case with complex tendon geometry.

As wet concrete cures, loss of moisture causes the concrete to contract. This contraction of the concrete, referred to as drying shrinkage, can create stresses than may cause the concrete to crack. Components can be added to concrete mixes to produce controlled, expansive chemical reactions that reduce or eliminate this drying shrinkage—in effect producing a shrinkage-compensating concrete. This paper provides a general overview of shrinkage-compensating concrete (SCC) including factors that influence the final concrete product and parameters for construction. A comparison of properties between SCC and regular portland cement concrete has been developed and is presented for SCC formed both with expansive cement and an expansive component. Early deterioration of concrete caused by delayed ettringite formation (DEF), which can occur with the use of expansive cement, is investigated, as well as alleviation of DEF by use of an expansive component.

This paper presents research documenting the local damage progression in rectangular concrete-filled steel tube beam-columns and connections in composite frames. Local damage is quantified through the development of damage indices based upon results of worldwide experiments. A fiber-based finite element formulation is also introduced for assessing global capacity and seismic demand.

Concrete filled steel columns have been used widely in structures throughout the world in recent years especially in Australia and the Far East. This increase in use is due to the significant advantages that concrete filled steel columns offer in comparison to more traditional construction methods. Composite columns consist of a combination of concrete and steel and make use of these constituent material's best properties. The use of composite columns can result in significant savings in column size, which ultimately can lead to significant economic savings. This reduction in column size can provide substantial benefits where floor space is at a premium such as in car parks and office blocks. The use of stainless steel column filled with concrete is new and innovative, not only provides the advantage mentioned above, but also durability associated with the stainless steel material. This paper concentrates on the axial capacity of the concrete filled stainless steel columns. A series of tests was performed to consider the behaviour of short composite stainless steel columns under axial compressive loading, covering austenitic stainless steels square hollow sections filled with normal and high strength concrete. Comparisons between Eurocode 4, ACI-318 and the Australian Standards with the findings of this research were made and comment.

This paper evaluates and compares between the seismic behavior of hybrid coupled wall systems that are 12- and 18-stories high. System response is studied using validated, transient finite element simulations that account for the effects of concrete cracking, crushing, and steel yielding. Suites of analyses are conducted to investigate pertinent parameters including interstory drift ratios, wall rotations, roof displacements, dynamic base shear magnification, strain demands, and coupling beam plastic rotations. The results for both systems confirm that hybrid coupled walls are particularly well suited for use in regions of high seismic risk. Although the responses of both structures were essentially dominated by the first vibration mode, higher mode effects manifested themselves by causing substantial base shear magnification. The effect was mildly higher in the taller structure.

This paper deals with the finite elements analysis of monotonic behavior in pull-out tests. The tests were developed using 10 mm reinforcing bars and concrete compressive strength of 30 MPa in RILEM pull-out test. In numerical study, the concrete and reinforcing bars are represented by non-linear materials behavior, combined with a model of the interaction between reinforcing bars and concrete (contact surface). A numerical approach with MEF (Finite Elements Method) was used by the softwares Abaqus® and Ansys® to represent bond-slip relationship. The simulations were carried out with several variations of the main parameters to get a better representation of the bond phenomena. The results show a satisfactory similarity between the numerical models and the experimental tests.

The theme of this conference is Metropolis and Beyond and this paper provides a state of the art review of practical applications, research and design into the use of high strength steel in buildings which may provide guidance for multistory and tall building designs of the future. Many current international standards and codes of practice do not cater for the use of high strength steel and thus much needed research is required to give structural engineers the confidence to use this new and innovative material in a safe and efficient manner in construction. Previous research on the use of high strength structural steel for composite columns is outlined herein and a summary of previous applications in multistory buildings is provided. This paper provides a comprehensive summary of experimental results on the uses of high strength structural steel in composite columns specifically for multistory building construction. The results and outcomes of short and slender columns tested concentrically, uniaxially and biaxially are summarized in this paper. Calibration of these experiments with theoretical models and the various international codes such as the American Institute of Steel Construction-LRFD, Australian Standards and Eurocodes have highlighted some of the differences that need to be implemented in order to design building structures safely with this material. Suggested design recommendations as well as further research are therefore summarized in this paper.

The interactive behavior of un-bonded post-tensioned structural components, when repaired and retrofitted with composite materials, is not yet properly understood (both short and long term). The current research is done to understand this subject. Eleven 12'-0" long beams B1 thru B11, were loaded to reach close to their ultimate values. As the beams got noticeably cracked, or the deflections reached twice the allowable deflection, or the post-tensioning forces reached critical values, the beams were unloaded. The cracked beams were repaired with varying amount of composite materials (carbon and e-glass) adhesives, and tested again. The tests were stopped based on the same three criteria stated above. Various repairing schemes were adopted. Attempt was made to find the minimum usages of the composite materials, prevent diagonal shear cracking (by providing shear patches), and sudden compression failures (by confinement). The changes in post-tensioning forces at ultimate stage, effects of having mild steel in the tension zone, and crack propagations were recorded and reported. The load carrying capacities and the deflections of the retrofitted and repaired beams were compared with that of the original un-cracked beams. The repair work was extended to continuous post-tensioned beams. Photographs of two-span post-tensioned beams (total span 30-0") are shown in the appendix.

The fiber dominated stiffness and strength properties of fiber reinforced polymer (FRP) composites make them most effective when used under global in-plane stress demands in shape-resistant structures such as membranes and shells. A two level uncoupled approach for the shape and laminate optimization of FRP shell structures was developed where shape and laminate-property optimization is followed by laminate design optimization of free-form shells for maximum structural stiffness. The approach was evaluated through the solution of optimized laminated FRP shell structures. Results show that the approach is dependable and provides a useful tool for designing optimized laminated free-form FRP structures.

Experience in the design of steel and concrete floor systems to control vibrations suggests that a floor system's vibration characteristics are often fundamental to its acceptable performance. In such cases, serviceability drives the design instead of strength. This is particularly true for laboratory floors that support sensitive equipment and floor girders that fall within certain spans. This paper discusses the initiation of a long-term collaborative research program between Tufts University and LeMessurier Consultants to investigate the vibration characteristics of a variety of floor systems. Examples include new concrete-steel composite floor systems, concrete floor systems, and irregular floor systems, and floor systems that have required remediation. The paper introduces well-known basic floor vibration equations and parameters, and discusses the possibility for developing further the concepts inherent in these equations for the purposes of improved conceptual design of floor systems to control vibrations.

In structural dynamics, the problem of finding oscillation natural frequencies of a structure can be solved using the mass and elastic stiffness matrices of the structure. On the other hand, the problem of structural instability of a structure and the critical buckling load is a problem, which can be solved using the elastic and geometric stiffness matrices of the structure. By including the geometric stiffness or in other words by taking into account stiffness conditions with respect to the deformed structure, a new more realistic representation of stiffness results. This influences the oscillation frequencies of the structure. This is why in general structural cases there will be a relationship between the vibration and buckling behavior. Both of these phenomena are sensitive to end boundary conditions and so if it is possible to make natural frequency measurements under different loads on a structure which take account of end conditions it may be possible to predict the critical buckling load. Stress analysis and structural analysis are the key requirements for the determination and improvement of the strength and stiffness of structures. The important problem of stability and, in particular, buckling behavior must also be considered and is necessary to complete the analysis. In stability, the importance of experimental studies in many categories of problems is evident. It has been found in many cases that the biggest problem is in defining the boundary conditions to which the buckling load is sensitive. This introduction aims to provide details of the buckling characteristics of various structural elements and to present the different analytical methods used in the solution of stability problems. To predict the critical load of a structure and then to find the relationship between the critical buckling load and the fundamental frequency of a structure, it is necessary to investigate the stability of the structure acting as a single unit. The problem of calculating the natural frequencies and corresponding mode shapes of all the kinds of variations of common structural components requires the development for each element in the model structure of a mass matrix which will represent the effect of the dynamic loading which is set up during vibration. Structural instability problems are geometrically non-linear, and a basic case of such problems is the problem of the lateral buckling under axial compression of a perfect pin-ended column or the classical Euler analysis of buckling load. In common with the vibration problem, the linear stability problem is all eigen value problem and the eigen values are the critical values of loading magnitude at which buckling occurs. Usually, only the lowest (fundamental) of these is of practical interest. A number of design examples show how the formulas can be used in practical applications. Also, these theoretical and practical examples help to understand the significance of the subject of this work and the checking of the safety in every kind of load that is applied. Also, the comments made are useful to the designers and engineers when trying to create safe structures. It also worth's noting that the engineer must design a structure, which will satisfy the demands for a functional performance, aesthetic appeal and the critical element of design.

The St. Luke's Medical Center Cardiac Center and Patient Tower Project in Milwaukee, Wisconsin consists of constructing a multi-story structure over an existing parking garage. Structural Affiliates International, Inc. was retained to analyze the structural system conceived by the structural engineer of record and evaluate the vibration potential. The study consisted of accurately modeling the structural system so that the dynamic behavior of the structure, such as the natural frequencies and natural modes of vibration, could be determined. This paper presents the results of the vibration study.

The identification of vibration properties for individual structural members of tabletop-type turbine-generator foundations is discussed. A procedure is outlined for the accurate determination of local natural frequencies using the harmonic analysis technique. Case studies are presented which involve applications of the proposed approach to the design of large steam-turbine-generator foundations.

Memorial Sloan-Kettering Cancer Center (MSKCC), the world leader in cancer research and treatment, has nearly completed the featured tower in a multi-million laboratory development project on its main campus in Manhattan. The tower is characterized by bold architectural design incorporating state-of-the-art exterior wall systems that express the functions of the internal spaces. The tower was designed to be energy efficient and will receive a silver rating from Leadership in Energy and Environmental Design (LEED). The tower, at 402-feet high, is the tallest research laboratory building in the world.

The AASHTO (2004) and AISC (2005) provisions for flexural stability design of steel I-section members have been revised in their entirety relative to previous Specifications to simplify their logic, organization and application, while also improving their accuracy and generality. This paper provides a brief overview of these provisions and summarizes the background to the corresponding flexural resistance equations. The base flexural resistance equations of these Specifications are, with minor exceptions, fundamentally the same. However, the organization and format in AASHTO (2004) emphasizes the streamlined design of various I-section member types common for typical bridge spans of about 30 m (100 ft) or larger, while AISC (2005) emphasizes the streamlined design of compact doubly-symmetric I-section members.

Developments in analytical software and computer hardware over the past few decades provide engineers with powerful tools for more realistically considering the behavior of steel structures. More sophisticated methods of analysis offer significant advantages in steel frame design by eliminating the need to calculate effective length factors and more directly including factors that affect system and member strength. One such method, the Direct Analysis approach, accounts for the effects of member inelasticity and frame imperfections in the assessment of both member and system strength. The latter is achieved by directly including these effects in calculating the distribution of forces in the structural system. This approach is applicable for use in the design office using commercially available software and it is applicable to a wide variety of structural problems including braced frames, moment frames and mixed systems. Just as importantly, the approach it allows for a natural transition between current elastic analysis procedures and the future availability of second-orderinelastic analysis programs suitable for use with an advanced analysis-design approach.

Civil structures are subjected to lateral forces during their life span. Earthquakes are an example of these lateral forces which excite the system and produce large inter-story drifts. Strong ground motions may crack or yield the resistant structural elements of a building. Cracking and yielding dissipate energy which in turn, could be considered as damage. Hysteresis cycles show the highly nonlinear relationship between displacements and restoring forces and give a measure of the energy dissipated during an earthquake. The present paper uses a class of proportional high gain observers to estimate the unknown nonlinear restoring forces of a single degree of freedom structure. Simulations are carried out using the El Centro (1940) time history acceleration record as ground motion input for a single degree of freedom system.

In this paper we describe how to detect the location of defects in the foundation of structures and buildings by the anti planar vibrations of the elastic layer with variable density along its depth. The problem has been investigated on the basis of the Lame equation for the elastic layer with the free upper and fixed lower edges. By the separation of the variables we obtain the inverse spectral problem for the Sturm-Liouville differential operator with the variable coefficient, and derive its eigenvalues for the given density distribution. The next step is the solution of an inverse spectral problem, i.e. the restoration of the heterogeneity parameters by the resonance frequencies. In the case of the heterogeneity symmetry relatively of the middle surface of the layer, the solution is obtained in the analytical form. The numerical solution of the inverse problem is carried out for the specific data. It is shown that the restoration of the density distribution for the elastic layer can be done with accuracy of less than 7% if the four lowest resonance frequencies are known.

An overview of the factors impacting the wood engineering/design education in the U.S. has been presented. The actions that need to be taken by the engineering departments and the wood products industry to ensure that wood engineering continues to be covered in the engineering programs have been discussed. The authors recommend a multi-faceted approach to promote wood engineering education in civil engineering programs in the U.S.

Acquiring knowledge of wood building design outside of formal post-secondary education training has been difficult for many prospective learners. The wood industry, through the American Forest & Paper Association (AF&PA), over the years has funded development of many design tools and education programs directed to design professionals to meet this need. Recently, the effort has been expanded to the digital realm to employ web technologies in combination with an effective direct contact education effort aimed at reaching more people with information they need to facilitate design of wood structures. All of these efforts to comprehensively deliver current wood building design educational programs are outlined in this paper.

While there would be little argument that wood plays a significant role in construction today and should be part of the standard offering within most civil engineering programs (as well as other programs), few engineers actually receive formal instruction in wood engineering and design during their undergraduate or graduate study. The reasons for this apparent disconnect between need and availability could certainly be argued. Is it that faculty are not appropriately trained to and therefore comfortable to teach wood design? Is a lack of research funding to blame, and without research funding is it that faculty trained inwood simply do not have the interest or time to teach wood? Is it, perhaps, that engineering students are not interested in taking a wood design course? Or, are employers not interested in hiring or giving any preference to new graduates with wood design on their transcripts? The speculation could go on. The issues facing wood education in engineering is presented from the academic administrative perspective.

This paper presents case studies to illustrate the importance of wood-engineering education in actual engineering projects and applications. The case studies are based on real projects from our engineering practice and include evaluation of wood biodeterioration (decay), in-situ stress grading, projects related to hygroscopic nature of wood, and analysis, design, repair, and rehabilitation of wood structures.

The paper describes a project which uses a novel non-linear computational method to teach structural topics ranging from elementary statics to plastic analysis of beams, and frame buckling. The project centers around a computer program, called Arcade, developed using a computational method, called a physics engine, which has been widely used in computer games. This method has two special characteristics with respect to structural behavior: it can model unstable structures, and it performs all calculations with respect to deformed geometry. In addition, the method can perform calculations in real time, so the on-screen structure responds instantly to loads and other actions induced by mouse clicks and key strokes. These characteristics allow the program to show the effect of forces on free-floating bodies, which is ideal for demonstrating free-body diagrams. Combined with elastic beam and truss elements, the method can model cable structures, elastic column buckling, and kinematic mechanisms that result from unstable configurations of supports and internal releases. Used with inelastic beam elements, the program can model plastic collapse mechanisms of beams and frames. The paper describes examples and exercises which have been used with the program in architecture and engineering courses at the University of Virginia.

This paper describes a unique project-based course used successful by the ABET-Accredited Civil Engineering Program at the United States Military Academy (USMA) to greatly enhance the academic program. The three general project classifications available within this senior course are service-based (i.e., USMA, the Army, local community, etc.), competition-based (i.e., steel bridge, concrete canoe, timber bridge, Big Beam), and research-based. The exceptional student work provided at little to no cost to the client has opened up a ground swell of service and research projects now constantly offered to the program coordinator by prospective clients. We believe these independent study projects are successful due to our students being prepared through open-ended design in nearly every course we offer and through some initial project-based learning in earlier courses. The mix of these open-ended projects usually ensures that each student can list a minimum of 3–5 project choices that meet their individual needs for a challenging, yet rewarding academic experience. Approximately 90–98 percent of the senior CE majors each year choose to work on one of these open-ended projects. With 45 to 60 CE majors graduating each year, it is easy to understand that one faculty member cannot advise the required 15–20 projects each spring. So many, if not all, of the CE program's 17 faculty must buy-in to the project-based senior program from the onset. It will be shown through student assessment that this form of experience not only challenges, but also motivates the students like no other aspect of their academic experience. The students are providing a solution to a real world problem for a real client. The assessment will show that the students find project-based learning demanding, but enjoyable and worthwhile because it forces them to push the boundaries of their knowledge through initiative, self-study, perseverance, and creativity.

This paper describes a new Columbia University Graduate Course on the modern challenges in the design of large scale building structures. Tall buildings, large convention centers, and major sports stadiums present major opportunities to teach creative solutions and leadership to engineers. This course is designed to expose the students to this environment by having them undertake the complete design of a large structure from initial design concepts on through all the major design decisions.

This paper presents a novel approach for teaching and learning structural analysis called the Continuous Segment (CS) Method. The nontraditional analysis procedure utilizes an organized bookkeeping methodology that results in a simplified means for solving structural analysis problems by direct integration. Irrespective of problem complexity, free body diagrams are not needed to develop shear and bending moment diagrams or to calculate deflections of beams and frames when using the Continuous Segment Method. Most importantly, the methodology is not only directly compatible with advanced topics in structural analysis such as deflections using virtual work and the analysis of indeterminate structures, but it also makes application of these traditional analysis methods much easier for students to understand and apply. As a result, the senior author has been able to teach the continuous segment method in addition to traditional methods covered in typical structural analysis textbooks in only one structural analysis course. This paper provides a derivation of the CS Method, a simple outline of rules and procedures, and two application examples for beams. A student survey has been performed, and students at The Citadel strongly prefer this nontraditional method of structural analysis to traditional methods used in the same and other courses.

The paper presents the outlines of a university course on the subject of forensic structural engineering developed and taught by the author. The objective of the course is to introduce students to the practice, technical and legal aspects of the investigation of structural/construction failures, as well as to expert witnessing.

Development of a new curriculum and supporting materials in the new field of Hurricane Engineering is well underway at Louisiana State University (LSU). A multi-disciplinary project team with expertise in many aspects of Hurricane Engineering is actively developing the new curriculum. The team consists of over 20 faculty from civil, environmental, chemical, and mechanical engineering, coastal science, landscape architecture, and environmental studies. The goals of the project are: to develop curricular materials in Hurricane Engineering; implement several new courses and create a hurricane engineering minor at Louisiana State University; and broadly disseminate this information to engineering faculty and the profession at large. Five new courses have already been created and offered through the Hurricane Engineering program at LSU (4 undergraduate and 1 graduate). Two of these courses focus primarily on wind engineering, and two more incorporate some aspects of wind engineering. Powerpoint presentations have been developed to accompany many of the lectures, which will be made available for download via a web site. A book is also being developed titled "Hurricane Engineering: Planning, Analysis, Design, Response, and Recovery of Civil Engineering Systems."

A unique multidisciplinary program in wind science and engineering for graduate studies leading to a doctoral degree has been developed. Students are required to take core courses in the disciplines of engineering, atmospheric sciences, and economics. In addition, they are required to fulfill a one-semester internship. Six courses in one field provide a degree of specialty. The four-year curriculum attracts students of diverse academic and social backgrounds. A specific plan of assessment of the program has been developed for improvement on a continuing basis. The program was implemented in the fall semester of 2003 and has 12 students pursuing doctoral degrees. With the goal of admitting 5 students each year after an on-campus interview, we hope to award 27 doctoral degrees by the eighth year of the program.

The ASCE/SEI Wind Effects Booklet Task Committee is developing a booklet, which will familiarize structural engineers, architects, builders and inspectors with the various ways in which the wind can influence the design and construction of their projects. The common queries that are often posed by people new to the various aspects of wind engineering will be readily addressed in this brief and simple booklet. The intent is that the text and figures will illustrate the complex phenomena of wind/structure interaction in a less technical fashion than ASCE members are typically used to seeing in their professional lives. In this manner, it is hoped that the booklet will be useful in explaining wind-related issues to clients, architects, builders and owners. This will allow the structural engineers' requirements to be more readily understood by other members of the project team. It is likely that the ASCE/SEI Wind Effects Booklet will be of value in the educational engineering and architecture arenas as well. The reader will see the nature of various extreme wind types (hurricanes, tornados, downbursts etc.) and how designers try to codify their impact on the anthropogenic environment. The following discussion will outline the topics to be presented in the ASCE/SEI Wind Effects Booklet and give some credit to the authors who have contributed to it. At the time of the 2005 Structures Congress in New York City the Wind Effects Booklet should be going through the peer-review process.

Steel structures in chemical plants often are adversely affected by harsh environmental conditions in the operating areas. Many times corrosive damage to an important main steel member may progress unnoticed. This can occur in areas of the plant where it may be difficult to repair or replace the damaged portion of a main structural beam or column. In these cases it is usually necessary to shore up the structure while cutting out and welding in a replacement for the damaged structural element. Shoring of the structure can be difficult due to congestion of piping and conduit. Additional difficulties may necessitate that the member be repaired as cost effectively as possible to prevent losses due to shutdowns caused by expensive dismantlement. The method to be demonstrated in this paper allows for the replacement of such corrosion damaged members by using the repaired member itself as shoring.

The unique design aspects of the 12-story, 430,000-s.f. (40,000m²) Cardiac Center and Patient Tower Addition in Milwaukee, Wisconsin led to innovative construction procedures. A major challenge of the project was erecting the superstructure over and through a seven-story, 1,500-car parking structure while maintaining the parking structure's functionality during the 3½-year construction process. To perform this work, the parking deck was extensively shored to support a 65-ton (60mton) drill rig. Drilled piers were then placed from the top of the parking structure and extend to end bearing in bedrock approximately 100 ft. (30m) below grade. Above grade, the majority of the new structure is supported by 85-ft. (26m) tall un-braced columns and 32-ft. (11m) deep steel trusses, which bridge the west portion of the parking structure. The design-build project delivery method was successfully used on this project and assured that the construction restrictions of the hospital's limited campus size were integrated into the design process from the beginning.

The recovery and cleanup operation following the September 11, 2001 terrorist attacks on the World Trade Center was a project unlike any other in history. Rising to the challenge, and working around the clock for nine months, engineers and construction managers directed the removal of 1.6 million tons of material. Remarkably, this was accomplished ahead of schedule, below budget, and without any serious injury. The success of this operation makes it an excellent candidate for further study. This study investigates decisions concerning the placement, use and management of debris removal equipment, particularly cranes, in the first one hundred days following the attacks. We explore the impact of risk on these decisions, and how decision making about this equipment influenced the project's effectiveness and efficiency. This analysis is supplemented with a case study that further illustrates how new procedures and management structures emerged during the operation. Data sources for the study include engineering field reports, maps of equipment locations, and periodic estimates of the quantity of debris removed from the site. The paper concludes with a discussion of "lessons learned" that can be applied to the management of any large scale construction or cleanup project. Decision-making strategies, encouragement of innovative problem-solving, and the role of information technology are all addressed.

The Greater Vancouver Water District (GVWD) owns and operates the water transmission system that serves approximately 2 million residents in the Greater Vancouver area, located in British Columbia, Canada with an average water consumption of 1.2 million cu. m per day, making the system one of the largest in Canada. The Little Mountain Reservoir located in Queen Elizabeth Park, in the City of Vancouver is the largest and most important of the 22 reservoirs in the GVWD system. The reservoir was originally constructed as an open basin in 1910 and a roof structure was added in the mid 1960s. Because of seismic and structural deficiencies of the structure, it was decided to demolish the reservoir and construct a new one on the same site with increased capacity and enhanced operational flexibility. Demolition began in September 2002 and construction of the new facility was completed by December 2003 on schedule, within budget. This paper summarizes the options evaluated before deciding to reconstruct the reservoir and details of GVWD's seismic performance criteria. Design features of this 175 million litres (38.5 million gallons) capacity reservoir will be presented with the challenges and constructability issues faced by the project team in building a major infrastructure in an urban park setting. The paper will show how the principles of sustainability were applied effectively by addressing social, environmental and economic issues in a balanced manner.

Two International Finance Centre (2IFC) is the latest and tallest addition to the Hong Kong skyline. At 420m, it is the fifth tallest building in the world providing 88 storeys of grade A office accommodation. The building is the centrepiece of the multi-phase US$ 3 billion IFC Development. The paper describes the design and construction of the tower focusing on some of the construction-led innovations that were incorporated into the design. At the time of the tragic events in New York, the 2IFC tower was part way through construction. Given that the tower was to be of similar proportions to one of the World Trade Center towers, there were obvious questions and concerns raised by the developer — how robust is the tower? Should the design change? Can people evacuate from the building safely? The paper describes the nature of studies that were undertaken to investigate the questions posed and validate the original design.

The three-lane Lions' Gate suspension bridge is a landmark structure at the entrance to Vancouver, British Columbia, Canada. The 472 meter (1550 feet) center span marks the entrance to one of the busiest harbors on the west coast of North America. Constructed in 1938, the bridge was privately financed, and as such, long term durability was often sacrificed in favor of initial capital cost savings. The bridge was designed with 2 traffic lanes, and the only concession the original designers made to the future was to design the bridge such that three narrow lanes could be squeezed between the curbs. In 1954, after the bridge was sold to the Government, the lane markings were changed to allow a middle passing lane. In 1963, as traffic volumes continued to increase, a reversing lane control system was added, which resulted in three full lanes of traffic. By the mid 1990's, the deck had deteriorated to the point where maintenance was costing in the order of US$2 million per year. In 1997, Buckland & Taylor Ltd. was retained to design the replacement of the entire suspended structure (deck, sidewalks, stiffening trusses, and suspension hangers). Because the new structure was significantly different from the old, and because there was lack of moment continuity at the working front, the design had to consider every stage of construction. Investigations were key to the viability of the project. These included derivation of long span traffic loads and load factors, and wind-tunnel testing in turbulent flow. Maintaining the 70,000 vehicles per day on the bridge during the replacement was a critical requirement that influenced every aspect of the design. After the design was completed in December 1998, three bids were received, and the contractor, American Bridge / Surespan A Joint Venture, was selected in April 1999. Between September 2000 and September 2001, the suspended structure of the 60-year old bridge was replaced during 10-hour night time closures plus a few longer closures on weekends. The bridge was open to full traffic at other times. This project is believed to be the first time an entire suspended structure has been replaced while traffic continues to use the bridge each day. The result is a wider, safer, more durable bridge that citizens consider an aesthetic improvement.

When the Millennium Bridge first opened it experience unexpected lateral vibrations. Arup investigated the cause and derived models for both lateral and vertical pedestrian excitation. This paper describes some of that work. These excitation models were used successfully in the design of a retrofit damping system.

The city of Portsmouth lies in the county of Hampshire on the south coast of England. It sits on a small island connected to the mainland by a causeway and a number of short span road and rail bridges. This is an area that boasts a long and impressive history with a Roman fort on the doorstep and the influences of English kings, notably Henry II and Henry VIII, still very much alive in the city's history. It was here that Henry V mustered his troops before the battle of Agincourt in 1415. The city was also the birthplace of Charles Dickens. However, it is as an historic naval port that Portsmouth is best known. HMS Victory, Nelson's flagship at the Battle of Trafalgar in 1805, and the Mary Rose, Henry VIII's Tudor warship, both lie in the Historic Dockyard and act as major tourist attractions, providing an historical context unsurpassed by almost any other naval port in the world.

If you pick up and read any corporate brochure or capability statement, from engineering firms through to management consultants, you are almost certain to find the word `innovation' written many times over. Innovation is clearly regarded by all businesses as the next big issue — their primary focus — the hot ticket. We are bombarded by offers of `an innovative approach', a confirmation that innovation will be at the core of what is offered and I have even read that one firm has been offering the wonderfully titled `innovation engine' as a driver for its work. This daily bombardment by so many firms across so many markets and disciplines has desensitised many to the real meaning and benefits of innovation. As a result of this, it is important to describe what innovation really is and why so many people have now become so desperate to demonstrate that they can do it.

By and large engineers are happiest dealing with deterministic events. They feel comfortable with mathematical structural analyses, prediction of member stress and definition of acceptance against codified criteria. In the routine of day-to-day design that makes for an efficient process giving us all an impression of certainty and predictability. But really, the structural world is not like that and we cannot insulate ourselves from the wider aspects of those projects on which we all depend for a living and to which we contribute for the greater human good. In reality things are much less certain. The forces we design structures to resist can defy prediction, the complexity of projects and the inter relationship between design and construction is such that it's hard to see at the outset what the totality of `design' means and failures still occur. At a more mundane level, project uncertainty and inappropriate risk management results in much cost and programme over run. We in the UK are aware that we share this problem with you in the US and the opinions and experiences expressed in Reference (1) have a strong resonance in the UK construction market. As I write this paper, the UK government auditors are reporting on why the new Scottish Parliament building ended up ten times over budget and, of course, late.

This paper outlines the principal issues of sustainable construction as it relates to conditions in the UK, reviews some of the progress which has been achieved, and suggests ways in which this can be further improved.

For more than a quarter of a century the European Union (EU) has been working to produce a set of common structural design codes, covering both the loading side and the response side for all the main structural materials. This has been an enormous task, involving hundreds of experts working in Committees and Groups; development has taken place by a partnership between those directly responsible and representatives from each of the EU member countries facilitated by the National Standards Organisations. The final documents (ENs) are now starting to be issued and each individual country is faced with the prospect of assisting its structural community to migrate from working on the basis of National Standards to working on the structural design basis of the Eurocodes. Given the associated infrastructure that supports the design process nowadays, plus a natural reluctance to change, plus the cost of such a change, considerable thought must be given to managing this process. Within the UK, the Institution of Structural Engineers was given responsibility by the Government for producing the National Strategy. The author, as President of the Institution and against a long history of personal involvement with Code producing activities, chaired the task. This paper discusses the nature of the challenges and indicates how it is proposed that they be addressed.

To facilitate modernization, China has been investing heavily in the construction of the country's infrastructure. Many bridges, large and small, have been completed in the last two decades. Many more are under construction and under planning. Several of them will have world record spans.

The paper reviews the experience from the bridging of the Danish straits, starting with the 1 km Lillebælt bridge, opened in 1935, and culminating with the 18 km Storebælt Link, completed in 1998, and the 16 km Øresund Link, operational since 2000, focusing upon the construction methods and the management approach.

Structural engineers face significant challenges in the 21^st century and among them, global environmental challenges must be a priority for our profession. On a planet with finite natural resources and an ever-growing built environment, engineers of the future must consider the environmental, economic, and social sustainability of structural design. To achieve a more sustainable built environment, engineers must be involved at every stage of the process. To address the broad issue of sustainability for structural engineers, this paper is divided into three sections: (1) Global environmental impact, (2) solutions for today, (3) challenges for the future. In summary, the paper identifies the global sustainability challenges facing our profession and suggests possible solutions. The conclusion is that structural engineering has an enormous global environmental impact and our profession should work harder to offer solutions to society. Working for a more sustainable built environment is in the best interest of our profession as well as the interest of future generations.

Our transportation infrastructure is aging. Bridges are an essential asset of our economy. Within the network of the Federal Highway Agency (FHWA) 590.000 bridges are serving, of which 160.000 are rated deficient. In order to avoid replacement costs of 7 billion US$ annually over 20 years innovative assessment procedures are required. This contribution highlights the state of the art of monitoring and assessment in combination with newly developed decision support methodologies. The benefits of ambient vibration monitoring in combination with traditional and high end applications, such as frequency decomposition, non linear acoustic emission or vibrational pattern recognition are demonstrated using real cases from practice. It is estimated that 30% of the deficiency ratings can be re-assessed to be within acceptable, no further action requiring, limits.

The paper presents the main objectives and results of a series of static and dynamic load tests performed on the bridges and viaducts of the Rosario-Victoria highway connection across the Parana River in Argentina that was completed and opened to traffic in May of 2003. The work reported in the paper was carried out by the authors as independent consultants to the link operator. A brief summary of the structural components of the project illustrates the type and scale of the project: a 350 m central span cable-stayed bridge, access viaducts on both sides of the main span, and a series of Gerber type bridges over secondary arms of the Parana river with a total length of approximately 12.5 km. The main bridge and access viaducts were designed by Leonhardt Andrä and Partners Gmb of Stuttgart, and the bridges over the secondary water courses by INCO Consultants of Milan. The contractor and operator of the link is Puentes del Litoral S.A., a consortium of construction companies working under a BOT contract agreement with the Federal Government of Argentina. The paper addresses the objectives and main features of the specifications for the tests within the terms of reference of the bid documents for construction and operation of the project. These documents required that load tests be performed before opening it to traffic, although they left the details and scope of the tests to be proposed by the contractor for approval by the supervisory board set forth by the Argentine Federal Government for control of public services provided by private operators. The test specifications are discussed, from definition of loading schemes and instrumentation, to acceptance criteria and test results. Beyond the obvious advantage of the purpose of the tests as a means to define a "zero or reference state" of the structures before opening them to the full traffic load, the paper focuses on the use of the test results to comply with a contractual requirement by the terms of reference that the bridge operator develop before commissioning, a "deterioration model" of all the major structures of the project describing the parameters and variables to assess their performance during the 25 year period of concession. This particular requirement was a direct consequence of recent experience with similar projects where such requirements had not been included in the tender documents.

Opened to traffic in August 2004, the Rion-Antirion Bridge crosses the Gulf of Corinth near Patras in western Greece. It consists of an impressive multi cable-stayed span bridge, 2,252 m long connected to the land by two approaches. An exceptional combination of physical conditions made this project quite unusual: high water depth, deep strata of weak soil, strong seismic activity and fault displacements. In addition a risk of heavy ship collision had to be taken into account. The structure has been designed in view of challenging the earthquakes and ensuring the every day serviceability of the link as well. To make the bridge feasible, innovative techniques had to be developed: The strength of the in-situ soil has been improved by means of inclusions; the bridge deck has been suspended on its full length, and therefore isolated as much as it can be. Due to high water depth, construction of the main bridge of the Rion-Antirion Crossing had to face major difficulties. In relation with this, foundation works, including dredging, steel pipe driving, but also precise laying of the required gravel bed under the pylon bases, were forming an impressive work package requiring unusual skills and equipment. To achieve this task, the conceptual design of the entire structure made it possible to simplify, in terms of implementation and reliability, the concept of huge shallow foundations and to prefabricate the major components of a bridge in the most favourable conditions, combining the latest technologies available in the construction of concrete off-shore oil drilling platforms and large cable stayed bridges.

With his knowledge of spatial geometry, shell theory and manufacturing process, the structural engineer is able to further develop light and transparent glass roofs. Double curved surface structures with a triangular mesh shape provide favourable prerequisites to single layer membrane shells. This paper describes personal experience with freely shaped glazed shells using triangular and quadrilateral glass panels. A geometric strategy for generating quadrilateral planar facets closely matching the architect's design is presented.

Egypt has been the cradle of western civilization and has enjoyed a strategic location, being in the center of the old world. It was Alexander the Great's dream of unifying the world that sparked the thought of creating a great library that gathered the cultures and civilizations known at that time. Bibliotheca Alexandrina appeared on a site near the famous lighthouse of Alexandria, one of the seven wonders of the ancient world. The library was founded in the year 295 BC. The objective of the library was to copy, translate, interpret and store books, in order to lead the progress of human society and culture. The ancient library was destroyed, some say, by Julius Caesar' fire ship in 48BC. With an aim to revive the world's most famous ancient library, the New Bibliotheca Alexandrina has been constructed in Alexandria, Egypt. The new complex lies almost at the same site as its predecessor. On the side ancient Royal quarter, the new Alexandria Library will give modern expression to an ancient endeavor. It will join the international information networks, with information stored in a database accessible to researchers and readers all over the world. In 1990, the Snohetta/Hamza Consortium was established between Snohetta of Norway and Hamza Associates of Egypt to plan, design, issue the tender documents, and supervise construction. The design concept, the critical ground conditions and the aggressive saline environment, due to the proximity to the sea, required remarkable foundation and structural works. This paper presents the foundation and structural design of the Bibliotheca Alexandrina. The various elements of the library building are described and the structural design challenges are summarized. The sophisticated geotechnical and structural analyses that were carried out during the design stage are also described.

The paper deals with the essential studies and works undertaken in the years 1990 to 2001 for a permanent stabilization of the Leaning Tower of Pisa and namely with their structural aspects. The present pathology of the Tower is described, as well as the non destructive testing and the structural analysis leading to the definition of the strengthening. The structural upgrading is briefly described and the results of the tilt reduction are reported.

The previous edition of the ACI Code, ACI 318-02, contained the largest number of technical changes in more that thirty years. For the 2005 Code, Committee 318 made a decision to delay most technical changes until 2008. The most significant change for 2005 was to improved the consistency of the notation used throughout the Code.

ACI 318-05 contains a new provision aimed at reducing the potential for punching shear failures at columns in two-way slabs located in regions of high seismic risk or in structures assigned to high seismic performance or design categories. The provision requires that slab-column connections in two-way slabs without beams, where the slabs are assumed not to contribute to the lateral resistance of the structure, shall be provided with slab shear reinforcement unless: (1) it is shown that punching will not occur under the design shear and the moment induced between slab and column at the design displacement; or (2) the design story drift does not does not exceed a limiting value that is a function of the ratio of the design shear to the nominal shear strength of the connection under gravity loading. The reason for this new provision is described along with recommendations as to how to calculate the lateral load stiffness of two-way slab structures in accordance with ACI Code 318-05 requirements. Also described is how to utilize the provisions of Sec. 11.12.6 (shear resulting from moment transfer to columns) and Sec. 13.5.3 (reinforcement requirements for moment transfer to columns and moment redistribution between columns) to determine if the first alternative can be satisfied. Finally, the relation between these provisions and those of Sec. 21.12.6 for intermediate moment frames consisting of two-way slabs without beams is discussed.

The need for a size effect factor on the shear strength of reinforced concrete beams and one-way slabs without stirrups where permitted by the ACI 318-05 Code is investigated through the examination of an extensive database compiled elsewhere and thoroughly evaluated by the ASCE-ACI Joint Committee 445 on Shear and Torsion. The database is examined in this paper under the light of applicable requirements in the ACI 318-05 code in an effort to represent current practice. The main findings in this paper indicate that the requirements in Sec. 11.5.6.1(c) to a great extent invalidate the specimens with ratios of measured to calculated shear strength less than 1.0 in the database. However, there is a paucity of data on members with depths greater than 36 inches without stirrups and without skin reinforcement. Also there is a lack of information on the shear strength of beams without stirrups having a ratio of overall depth to width less than 0.5.

Unlike previous editions, ACI 318-05 contains a unique definition for each term used in the code. Committee 318 believes that the building code will be easier to use, but current users of ACI 318-02 are warned that extensive editorial changes appear throughout ACI 318-05 as a result of the notation changes.

In the ACI 318-99 building code, new design provisions were introduced for the control of cracking through reinforcing bar spacing requirements. With the introduction of the ACI 318-05 code, the direction of controlling cracking through reinforcement spacing with be further extended. The new provisions provide for the control of flexural cracking along both the tension and side faces through a unified design approach. The presentation will provide an overview of the background regarding the control of cracking through reinforcing spacing as well as the development of the new specifications. In particular, effects of changes in the load and resistance factors on crack control and the resulting specifications will be discussed. Finally, design examples will be presented illustrating the use of the new design provisions.

This paper reports on the key features of the "Guide for the Design and Construction of Concrete Reinforced with FRP Bars" issued by the American Concrete Institute (ACI). For new construction, FRP bars have been used as the internal reinforcement in concrete members to replace conventional steel rebars for a host of reasons. The principles for design and construction were first established and proposed to industry in 2001. The third revision of the guide is now under preparation.

This paper reports on the key features of the "Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures" issued by the American Concrete Institute (ACI). Fiber reinforced polymers (FRP) systems have emerged as an alternative to traditional materials and techniques for the strengthening of existing concrete structures to resist higher design loads, correct deterioration-related damage, design or construction error, or increase ductility. Structural elements strengthened with externally bonded FRP systems include beams, slabs, columns, walls, joints/connections, chimneys and smokestacks, vaults, domes, tunnels, silos, pipes, and trusses. Externally bonded FRP systems have also been used to strengthen masonry, timber, steel, and cast-iron structures.

Fiber-reinforced polymers (FRP) have been proposed for use instead of steel prestressing tendons in concrete structures. The promise of FRP materials lies in their high-strength, lightweight, noncorrosive, nonconducting, and nonmagnetic properties. This paper provides an overview of the ACI 440.4R-04 document on "Prestressing Concrete with FRP Tendons" reported by ACI Committee 440 on "Fiber-Reinforced Polymer Reinforcements". The document is one of the Emerging Technology Series published the American Concrete Institute. The paper outlines the content of the document and the philosophy of applying FRP technology as opposed to conventional steel for prestressing. The document offers general information on the history and use of FRP for prestressing applications, and a description of the unique material properties of FRP. It also focuses on the current state of design, development, and research needed to characterize and ensure the performance of FRP as prestressing reinforcement in concrete structures. The proposed guidelines are based on knowledge gained from worldwide experimental research, analytical work, and field applications of FRPs used as prestressed reinforcement. The current development includes a basic understanding of flexure and axial prestressed members, FRP shear reinforcement, bond of FRP tendons, and unbonded or external FRP tendons for prestressing applications. The document concludes with a list of research needs.

The steel industry works hard to improve and maintain the art and science of steel design and construction for the structural engineering profession. This is accomplished through the support and tracking of research in steel design, committee deliberations based on design practice and performance, and the ongoing development of design aids and design procedures that appropriately predict the strength and serviceability of modern steel construction. However, since 1989, many designers have been resistant to the idea of giving up their green, 9^th edition ASD Manual of Steel Construction. This has been done for a variety of reasons: a perception that little has changed in steel design, a desire to consider steel design in the context of stress rather than strength, or a general comfort level with the book that has been so familiar for so long.

The American Institute of Steel Construction is introducing a new standard in 2005 entitled, Specification for Structural Steel Buildings. This document will unify the two design methods presently used for steel design in the United States, allowable stress design (ASD) and load and resistance factor design (LRFD), into one standard which will restore the uniformity and clarity originally intended for the AISC specification. In addition to this unification, the entire document has been reorganized and updated where necessary.

The provisions of the 2005 AISC Specification for Steel Buildings provide for the design of members according to building code provisions for both ASD and LRFD load combinations. The requirements for design to resist tension, compression, flexure, and shear will be discussed along with the provision for force interaction. Comparisons will be made between the 1989 ASD, 1999 LRFD Specifications and the new 2005 "unified" Specification. Emphasis will be given to the unifying philosophy of the Specification and the specific nominal strength provisions.

The provisions regarding analysis and, especially, stability in the 2005 AISC Specification for Structural Steel Buildings represent a significant departure from previous editions. The changes were intended to reflect the current state of knowledge and also to make the logical basis of the stability provisions more apparent to the user. The new provisions spell out the general safety- and reliability-based requirements that must be satisfied in all structural designs and give designers the freedom to select or devise their own methods of analysis within these constraints. The Specification also offers designers specific procedures applicable to many typical building structures.

Chapter I of the new 2005 AISC Specification contains design provisions for composite columns composed of rolled or built-up structural steel shapes, HSS or pipe and structural concrete acting together as a composite member. It also covers steel beams supporting a reinforced concrete slab where the steel beam and the slab act in a composite manner using shear connectors. Simple and continuous composite beams with shear connectors and concrete encased beams are also included. The new Chapter I includes extensive technical as well as format changes. The major technical changes consist of new design provisions for composite columns in Section I2, which now includes new strength models, provisions for tension and shear design and a liberalization of the slenderness limits for HSS tubes and pipes. Other technical changes include new more rational shear stud strength provisions in Section I3.2d, the use of an ultimate strength model for ASD design of composite beams in Section I3.2 and new material limitations in Section I1.2. New material has been added that covers the ASD provisions for composite columns and beams — something that was not covered at all in previous ASD Specifications. Major format changes include the elimination of the former Section I1, Design Assumptions and Definitions. The contents of that section have been moved to the Glossary, Notation and other sections within the chapter or specification. This section has been replaced by a section entitled General Provisions. Other format changes include the separation of composite column design into distinct provisions for concrete-encased and concrete-filled sections, the incorporation of the former I5 Shear Connectors section into the current Section I3 Flexural Members and elimination in the new commentary of all the historical development notes on composite design provisions as that material is now considered to be widely known.

The role of the structural engineer in the Special Inspection process varies considerably across the country. In Connecticut the structural engineer typically plays a very major role in not just managing special inspections, but also performing the inspections. This has resulted in a significant improvement in the quality of building structures. Special Inspections have also proven to be an extremely lucrative business opportunity. In contrast, on the west coast the structural engineer plays almost no role in Special Inspections and the process is almost completely controlled by Testing Laboratories. The structural engineering profession would benefit from a more consistent and more active role in managing Special Inspections.

Self-consolidating concrete (SCC) has become of high interest to the precast concrete industry due to the benefits it offers in enhancing construction productivity. In spite of this interest and rapid developments on SCC technology, its acceptance in the U.S. is lagging due to material and structural performance concerns; among these is the issue of bond. An on-going study is thus evaluating the bond of prestressing strands on precast/prestressed girders. The study is based on experimental evaluation of transfer and development lengths of 13mm (0.5 in.) diameter seven-wire strands on small-scale T-beams. Three different SCC mix designs, which bound the approaches to achieve SCC behavior, and a reference normally consolidated concrete mix were considered. Preliminary results indicate that transfer lengths for SCC deviate from those for normal concrete depending on its composition. In all cases, however, the experimentally determined transfer lengths are within code recommendations. Flexural tests for determining development length are on-going.

Spatial truss models are developed for the design of reinforced concrete beams subject to biaxial shear forces. They are derived from a resolution of the vector of the resultant shear force and applied to beams with rectangular cross sections. The models consist of tensile struts of stirrups and longitudinal bars, spatially inclined shear struts as well as compressive struts stiffening the stirrup corners. They are further elaborated to derive charts for the stirrup design. An example shows the practical application.

The paper presents measures by which significant changes in the damping of concrete may be achieved. A variety of alterations in mix design were examined experimentally, and the most effective treatments involved the use of polymer admixtures. Significant changes in damping were associated with the use of several types of styrene-butadiene rubber (SBR) latex, ethyl-vinyl-acetate (EVA) latex, and styrene-acrylic ester (SAE), as well as a blend of SBR and partially-solidified vegetable gum. The polymer treatments resulted in frequency- and temperature-dependent damping properties.

Shear lag reduces the fracture capacity of steel tension members if some, but not all, elements of the cross section transfer force at the connection. Axially loaded rectangular hollow structural sections (HSS) are commonly connected by slotting two walls and inserting a plate into the slot. The AISC Load and Resistance Factor Design Specification for Steel Hollow Structural Sections has equations to account for shear lag in slotted HSS connections. The equations are based on tests on open structural shapes. This study compares the AISC equations to the available data from previous tests and finite element models to determine whether the equations are valid for rectangular HSS members. The results of four research projects were compared to the AISC equations. The AISC equations were found reasonably accurate and generally conservative.

A numerical study is carried out to predict the behavior of slotted beam-column welded connection by finite element method. Computational results on deformation pattern and stress distribution are obtained and discussed.

This paper describes the analytical method of predicting the yield capacity of steel pipe section subjected to axial and bending moment loads. It is based on the linear stress/strain plot of the steel material when subjected to these loads. The resulting expressions for steel forces and moments will produce the yield capacity curve of the steel pipe section. The analytical method uses basic mathematics in determining the forces and moments developed in a steel pipe up to the its yield stress. Variables considered are the radii of the steel pipe, thickness of the shell, compressive depth and yield stress f_y of the steel material. Yield capacity is measured by sets of axial and moment capacity of the steel pipe section. The equations are programmed in a Microsoft Excel worksheet to plot the yield capacity curve of a steel pipe section. Example of yield capacity curve for a typical steel pipe is shown as proof that the analytical method is feasible.

In some courts, technical expert testimony can be ruled inadmissible at the sound judicial discretion of the judge. The Daubert and Kumho cases (among others) establish standards under which technical evidence can be admitted, and based on these cases, a judge may deem inadmissible any expert testimony that is not developed through valid, reliable means. However, there is no published valid, reliable method to determine the standard of care of structural engineers. This paper presents a valid, reliable method for expert structural engineers to come to their opinions regarding the standard of care, and regarding a defendant's fulfilling that duty. The method presented here is based on the deductive logic of case-based reasoning, with the support, if necessary, of the Delphi Method of determining a consensus of expert opinion. An example of the use of the method as the basis for an expert's opinion in a test case is presented.

This paper is written to introduce the Pennsylvania Department of Transportation's (PENNDOT) Bridge Automated Design and Drafting Software (BRADD Version 3.1.0) to the bridge design community. BRADD is a computer software program developed to automate the highway bridge design process from problem definition through contract drawing development for typical single span bridges in Pennsylvania. The purpose and reasons for developing this software will be stated and explained. User input and guidance during both development and maintenance will be addressed. The description of the capabilities, functionality, assumptions, and limitations will be presented along with an overview of the Windows-based Graphical User Interface (GUI). Upon review of this paper, the bridge design community, especially in Pennsylvania, should have the necessary information and understanding to determine if BRADD is a design tool that can significantly assist them in their design of single span highway bridges.

I am a Registered Professional Structural Engineer and ASCE member. I worked as a consulting engineer for numerous firms for 15 years, as a staff engineer, senior engineer, and finally as a Project Manager. My clients have included owners and architects. Six years ago, I joined the Real Estate Division at a major U.S. Corporation and became an "owner." I quickly learned the important issues to an owner regarding the hiring and managing of engineering consultants. What I knew for 15 years about what is important to an owner was off the mark. The important issues to an engineer and to an owner are clearly different, and sometimes conflict. For example, engineers need to focus on their own profitability. However, most owners have very little regard for this and instead focus on minimizing their consultant costs thru leveraging of services, on-call contracts, negotiating and other methods. Owners instead focus on minimizing their construction costs, while engineers at times, in order to meet aggressive deadlines or for other reasons, will be overly conservative in their designs. Most owners do not fully understand the profession of engineering design. Indeed, most of the owners that hire you will have a non-engineering background, and increasingly will have a purely financial background. I am a rare "owner" in that I am able to understand the language and needs of engineers, having been one. Therefore, it is important for engineers to educate the owner, without going into technical detail, exactly what it is that an engineer does. Owners may have expectations that are not in line with the engineers legal responsibilities. Setting proper expectations, and then meeting those expectations, can make for a satisfied client. There are a few methods of presenting this, which include marketing brochures, proposals that include specific work efforts, and so forth. One of the most important issues to an owner is clear communication. One of the most frustrating things to an owner is not hearing from a consultant for weeks and not knowing the status of work. Engineers need to clearly communicate their schedule of work and then provide unprompted status updates.

As chairman of the Council of American Structural Engineers (CASE), I would be remiss and probably impeached if I do not start by highlighting the need for an agreement between the Architect or the Engineer and the client to establish a scope of services to be provided by the design team and an understanding of the relationship among parties. CASE Document 2-2003 — An Agreement Between Client and Structural Engineer of Record for Professional Services is available on the ACEC — CASE Website.

A seismic rehabilitation design project is presented to demonstrate how buckling-restrained braces were used in the voluntary seismic rehabilitation of two office buildings in the San Francisco Bay Area in accordance with FEMA 356. A seismic evaluation concluded that the existing braced frames were likely to experience connection failures and the diaphragm deflections under seismic loads would cause an undesirable splice failure in the gravity columns. The rehabilitation design for the two buildings required careful consideration of the owner's objectives including costs, aesthetics, and continued building operation by minimizing disruptions to building occupants. An external splayed brace design was selected using buckling-restrained braces, which have such desirable characteristics as stable hysteresis behavior with high energy dissipation and a lower yield capacity than conventional steel braced frames. Three-dimensional modeling and analyses of the buildings were performed in accordance with the FEMA 356 methodologies for linear static, linear dynamic response spectrum, nonlinear pushover, and nonlinear rime history analysis procedures. The analysis confirmed that the external braced frame design using buckling-restrained braces would efficiently enhance the building performance to achieve the owner's established rehabilitation objective.

Recent research efforts for FEMA 356 have developed performance based design procedures for structural components of buildings. However, little work has been focused on development of comparable procedures for nonstructural components. FEMA 356 includes provisions for evaluating nonstructural components, but these provisions are based on building code procedures rather than focusing on a performance based approach. During the process of developing FEMA 356 into an ASCE Standard document, revisions have been proposed to the provisions for nonstructural components to provide more of a performance based approach. Changes to nonstructural component demands and capacities have been considered along with changes in the acceptance criteria for various performance levels. Some effort have also been made to simplify some nonstructural component designations so that similar components are treated the same rather than including unnecessary component distinctions. Due to the large variety of nonstructural component types and configurations, design and evaluation procedures, such as FEMA 356, cannot completely address all of the possible conditions of nonstructural components. Future research and development efforts will be needed to provide additional data to be used to further refine the provisions.

This paper will briefly discuss some of the research that will likely be incorporated into US design codes for the use of steel plate shear wall as seismic load resisting systems. References to numerous research projects carried in the United States, Canada, and other countries will be given and their major findings highlighted. The strip model, developed by others for the representation of SPSW, is described and plastic analysis results for that model and their use in design is discussed. Finally, new directions for SPSW research, including the use of light-gauge and low yield-point steels, strategic hole placement, and reduced beam sections are described in the context of some recently completed and ongoing research.

A systematic program to develop innovative seismic-force-resisting precast structural systems was initiated by the Precast/Prestressed Concrete Institute (PCI) and the National Science Foundation (NSF) in 1990 and culminated with the testing of a five-story 60%-scale building in 1998. In 2003 the American Concrete Institute (ACI) completed the codification of one of the precast systems used in that building, the hybrid frame. This paper summarizes the research, analytical and experimental work that went into developing ACI's hybrid frame provisions and the progress towards codification of the other primary seismic-force-resisting system used in the building—one consisting of unbonded post-tensioned vertically coupled shear walls.

The paper illustrates a design procedure for Buckling-Restrained Braced Frames using the current draft versions of the 2005 ASCE 7 and AISC 341 standards. The system is new to this cycle of codes, and presents many performance and design advantages over conventional braced frames. The illustration of a design procedure is intended to aid engineers in understanding the requirements of the provisions as they pertain to buckling-restrained braced frames, and to convey the intended performance of the system under severe seismic loading. The procedure illustrated optimizes the design, and is consistent with the required capacity-design methodology without requiring numerous calculations to supplement an elastic analysis. A five-story building in San Francisco is designed to meet the provisions, and the final design information is described.

Research and studies on seismic damping devices and systems started in earnest in the United States in the 1980s. In less than 15 years, damping devices moved from the university laboratory into mainstream design practice. Damping devices, damping systems, analytical tools, design procedures and codes and guidelines have been developed and deployed in building structures. The paper summarizes briefly the research work and describes in some detail the guidelines and codes of practice now available for implementing damping devices in buildings.

Three Virginia Tech research projects that made extensive use of Incremental Dynamic Analysis (IDA) are described in this paper. Special IDA features that have been incorporated into the NONLIN computer program are also described.

Probabilistic seismic demand models (PSDMs) are mathematical relations between measures of earthquake intensity and measures of structural response. The models include best estimates as well as considerations of uncertainty. Three methods for obtaining the demand models are compared in this paper: cloud, incremental dynamic, and stripe. A PSDM is derived using each method for a sample reinforced concrete bridge using a single IM (first-mode spectral acceleration) and EDP (column drift ratio). Merits and drawbacks of each method are discussed. For the structure considered, cloud and incremental dynamic analyses yield interchangeable PSDMs for the same amount of computational effort. Stripe analysis should be used only for individual intensity levels, not as a means of generating complete PSDMs.

The modal pushover analysis (MPA) procedure for estimating seismic demands is extended to unsymmetric-plan buildings. In the MPA procedure, the seismic demand due to individual terms in the modal expansion of the effective earthquake forces is determined by nonlinear static analysis using the inertia force distribution for each mode, which for unsymmetric buildings includes two lateral forces and torque at each floor level. These "modal" demands due to the first few terms of the modal expansion are then combined by the CQC rule to obtain an estimate of the total seismic demand for inelastic systems. The MPA estimates of seismic demand for torsionally-stiff and torsionally-flexible unsymmetric systems are shown to be similarly accurate as they are for the symmetric building; however, the results deteriorate for a torsionally-similarly-stiff unsymmetric-plan system.

Performance-based seismic evaluation of existing structures involves an estimation of demand on structural elements imposed by earthquake ground motions. Such demand evaluation involves linear or non-linear structural modeling and static or dynamic analysis. Structural irregularities, such as gaps, present a modeling and analysis problem: which type of analysis should be used to accurately evaluate the effect of such structural irregularities on they dynamic response and seismic demand on the structure? In this paper, we use two demand analysis procedures, the non-linear static pushover (SPO) and the non-linear incremental dynamic analysis (IDA), to compute the seismic demand on a structure comprising a stiff interior and a flexible exterior sub-structure separated by a gap. We use ratios of the engineering demand parameters computed using structural models with and without the gap to compare the demand estimates. We found that incremental dynamic analysis should be used to accurately capture the dynamic response amplification caused by the gap irregularity.

Incremental dynamic analysis (IDA) has recently emerged as an alternative approach to investigate inelastic limit states in performance-based seismic engineering. IDA requires a series of nonlinear-time history analyses at increasing intensity levels of the same ground motion. The intensity of the earthquake load is typically expressed in terms of the first mode spectral acceleration. In this study the validity of such an intensity measure is investigated. IDA is applied to an existing six-story steel moment-frame building. Two sets of ground motions representing far-fault and near-fault records are considered separately to study the influence of different seismic source characteristics on the outcomes of the IDA. The study reveals that first mode spectral acceleration, S_a(T₁,5%), is an inadequate and incomplete measure of seismic intensity in the context of generating IDA capacity curves. It is concluded that alternative intensity measures are needed to make IDA more meaningful in seismic assessment. A preliminary investigation utilizing spectral magnitudes from constant-ductility inelastic spectra as a measure of seismic intensity is reported in this paper.

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 the 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 an earthquake motion recorded at a nearby site. In this paper, the results of a response spectrum analysis of the Tüpras stack and a generic U.S. stack are summarized. Then, a nonlinear static analysis of the collapsed stack is presented using a demand-collapse comparison. The demand is represented by an acceleration-displacement response spectrum based on the recorded motion as well as some smoothed adaptations typical of design spectra based on a statistically simulated time history data set, while the capacities are calculated from pushover curves using a nonlinear reinforced concrete finite element analysis. Results are presented that show the effects of the hole and the orientation of the motion with respect to the hole. Also higher-mode contributions to the pushover pattern are considered. The results confirm that the stack could readily fail under the considered earthquake and are also consistent with the debris pattern.

With the ultimate goal to benchmark the seismic performance of new (building-code compliant) reinforced concrete frame structures, we have applied a probabilistic assessment methodology, developed by the Pacific Earthquake Engineering Research Center, to a four-story office building. A key component of the assessment is characterization of the structural response through nonlinear time history analyses organized using an Incremental Dynamic Analysis approach. The results show that the seismic performance of the building is generally consistent with the expectations of current building codes, having a life-safety performance level (ASCE 356) for the 2% in 50 year event. A sensitivity study shows that the record-to-record variability is the single most important contributor to the total variability in structural response; however, the cumulative affect of other modeling and design uncertainties is also shown to be significant — resulting in the total dispersion being roughly double the dispersion due to record-to-record effects. To further investigate modeling uncertainties, we have compared the responses predicted by using a force-based fiber model and a lumped plasticity model. These comparisons show that the concrete tensile strength and tension stiffening effect significantly impact the response predictions, especially for structures with highly under-reinforced members subjected to lower ground motion intensity levels.

The nonlinear response of low and medium-rise special moment-resisting steel frames to simulated ground motion acceleration is compared with the response to actual ground motion records. A spectral-representation-based simulation algorithm was used to generate the simulated time histories. It is shown that in most cases the simulated time histories yield acceptable mean values for the response parameters with less dispersion. This means that only a few simulated accelerograms may be used to estimate the mean response parameters for a wide range of structures with different frequencies. The use of artificial acceleration time histories generated by this method is an efficient alternative to using scaled actual ground motion records.

This paper describes an analytical investigation on the nonlinear behavior of post-tensioned precast concrete beams to couple reinforced concrete walls. Different from conventional systems that use monolithic cast-in-place reinforced concrete coupling beams or embedded steel coupling beams, the nonlinear behavior of post-tensioned precast concrete coupling beams is governed by the opening of gaps at the beam ends. Steel top and seat angles are used at the beam-to-wall connections to yield and dissipate energy in the event of a large earthquake. Two types of analytical models are developed using the DRAIN-2DX and ABAQUS programs for verification of the results and more detailed evaluation of the expected behavior. Nonlinear reversed cyclic lateral load analyses of four coupling beam subassemblies are conducted. The main parameters investigated are the beam depth, the area of the post-tensioning steel, and the size of the top and seat angles. The results indicate that post-tensioned precast concrete coupling beams possess stable behavior through large rotations, significant self-centering capability due to the post-tensioning force, and significant energy dissipation provided by the yielding of the top and seat angles.

Failure due to fracture or low-cycle-fatigue in steel moment connections is an important limit-state in earthquake-resistant design. Current standards and codes deal with this problem based on semi-empirical, experimental approaches. In reality, such failures involve the complex interactions of material microstructure, irregular loading histories, and uncertainties in crack initiation and subsequent propagation processes. Traditional fracture and fatigue mechanics cannot capture many of these interactions and uncertainties, presenting a significant obstacle to accurate simulation of structural performance. Modern computational technologies coupled with novel micromechanical fracture and fatigue models enable a detailed and realistic simulation of the processes leading to crack initiation and propagation. A recent study by the authors postulated some new mechanisms and proposed micromechanics-based models to predict ductile crack initiation under ultra-low cycle fatigue (ULCF) experienced by structures in earthquakes, which is very different from high-cycle fatigue experienced by bridges. The paper discusses these models and a methodology to apply them by combining basic material microstructural data with complex loading histories to accurately predict seismic performance and failure.

Accurate prediction of peak floor absolute accelerations is a crucial step in defining simplified seismic design procedures for operational and functional components (OFCs) attached to common buildings. A particular characteristic of seismic accelerations is that they increase along the building elevation. The amount of this increase depends on many factors, such as the type of lateral load resisting system, the height of the structure and the input ground motion. The suggested linearly increasing height factor in current design codes is empirical and based on limited observation of recorded data. Some research and observations have suggested that this factor needs revision in order to include the characteristics of the building and input ground motion. This has motivated several countries concerned with high seismic hazards to embark in building monitoring programs. As a part of a project sponsored by the Taiwan Strong Motion Instrument Program (TSMIP), data from 11 extensively instrumented buildings in Taiwan during the 1999 Chi Chi earthquake were compared to modern code provisions. The studied buildings have different lateral load resisting systems and different heights, and they did not suffer any structural damage during the earthquake. Based on acceleration measurements, the article discusses the correlation in the elastic range between the height amplification factor and some of the building characteristics.

This paper explores the current building code seismic design requirements for typical buildings in regions of moderate seismic hazard. In particular, the costs and benefits of various levels of ductile connection detailing requirements are reviewed for steel buildings constructed in the northeastern United States. The design of lateral force resisting systems for seismic forces has been required in the northeast states for over 20 years. However, recent building codes have introduced special ductile detailing requirements that substantially increase the cost of building connections and lateral force resisting system framing. In general, these increased costs for improved ductility allow for the proportioning of members based on assumptions of higher levels of inelasticity, and thus lower member forces, in the response of the structure. In regions of high seismicity, the benefits of such ductile detailing are clear as they improve building performance and allow for economical design of structures for more severe earthquake effects. However, in regions of moderate seismicity, the cost of these details can be very high in comparison to conventional details. Consequently, many engineers in the northeastern United States have questioned the value of design for inelastic response in non-essential facilities. The purpose of this paper is to explore these requirements and provide a qualitative and quantitative basis for comparison of seismic force resisting systems of varying levels of inelasticity for non-essential buildings in the northeastern United States.

A new method of complex complete quadratic combination with three components (CCQC3) for non-classically damped irregular structures is proposed in this paper, in which both modal displacement-velocity and velocity correlation coefficients are involved along with the modal displacement correlation coefficients in the normal CQC3 formula. Moreover, the effect of the seismic incident angle on the structural response is considered; and formulas for the critical angle and the corresponding critical response are given. The new CCQC3 formula is as concise as that of the CQC3 method and has explicitly physical meaning. If the correlations among modal responses are ignored, the CCQC3 method is reduced to the method of complex square root of the sum of squares with three components (CSRSS3). Analytical examples are presented in the paper to compare the validity between CCQC3 and CSRSS3 methods. Meanwhile, time history analyses of structures subjected to artificial earthquake waves which are synthesized from design spectra are carried out to verify the complex mode superposition method. The results illustrate that the CCQC3 method is more accurate and suitable for seismic response analysis of important structures; however, the CSRSS3 method may be adopted for general structures in order to simplify the calculation.

The reliability of the semiactive controlled SAC Los Angeles 3-story structure is examined accounting for the variability of the seismic excitation. An evaluation model is developed, a semiactive control strategy proposed, and results on the improved reliability of the structural performance presented. The semiactive control strategy is able to reduce the maximum damage measures as compared to the uncontrolled system. The median and dispersion of the damage measures are similarly reduced for the semiactive controlled structure. The reliability of the semiactive controlled structure is increased with respect to the uncontrolled structure. Considering the reliability provides a useful perspective on the overall performance of a semiactive control strategy.

This paper presents a quantitative comparison of results from a number of studies that have focused on the performance of smart base-isolated buildings. Many strategies have been proposed in recent years to improve upon the performance of passive base-isolated buildings; the need for improved performance being motivated in part by the high isolation system deformation demand that can occur in the case of near-field ground excitation. One approach to improving the performance of base-isolated buildings is to incorporate elements within the isolation system whose mechanical properties can be modified in real-time. For such smart seismic protection systems, performance indices, which are often defined as the ratio of controlled response over uncontrolled response, have become a common tool for comparing performance in much of the literature. However, the use of such performance indices can result in a loss in the ability to identify the significance of the measured quantities. Thus, a systematic evaluation of the performance that has been achieved in prior studies is examined in this paper in an attempt to improve the understanding of the potential benefits of smart seismic isolation systems. Furthermore, the results from prior studies are compared with the recent Phase I results of the First Generation Smart Base Isolation Benchmark Building, which incorporates one of the most realistic isolated building models used in such studies to date.

Control devices can be used in civil structures to dissipate energy from earthquakes, reduce structural damage and prevent failure. Semi-active control devices have been shown to be more energy-efficient than active devices and more effective in reducing seismic structural vibrations than passive devices. A type of semi-active control device, the magnetorheological (MR) damper, consists of a hydraulic cylinder containing micron-sized, magnetically polarizable particles suspended in a liquid such as water, glycol, mineral or synthetic oil. The damping capabilities of this device can be quickly varied by changing the viscosity of the MR fluid from viscous to semi-solid through the introduction of a magnetic field. The objective of this research is to develop a fuzzy controller to regulate the damping properties of the MR damper. Because fuzzy control uses expert knowledge instead of differential equations, it allows for the development of simple algorithms. It does not require accurate information on structural and vibration characteristics of the system and is therefore an attractive alternative for complex and/or nonlinear systems.

This paper is on the seismic design of unbonded post-tensioned precast concrete frame structures that use friction dampers for supplemental energy dissipation. A procedure is described to determine the friction damper slip force and the area of post-tensioning steel needed in a frame to satisfy prescribed lateral strength and energy dissipation requirements. The proposed design procedure assumes that the lateral strength requirements for the frame have been obtained from a linear elastic analysis of the structure under equivalent lateral forces. Nonlinear reversed cyclic analyses of friction-damped precast concrete beam-column subassemblies and multi-story frame structures under lateral loads are conducted to evaluate the design procedure and to identify areas where improvement may be needed. The analytical results show that friction-damped precast concrete frames can achieve significant energy dissipation levels while maintaining a large level of self-centering capability due to the post-tensioning force.

The SR 99 Alaskan Way Viaduct was built in the 1950's. It runs along the downtown Seattle waterfront. The single level structure routes up a steep side slope, into a tunnel. The section of the viaduct under consideration varies from 90 to 78 feet wide and has 39'-9" span lengths. This paper will show how a typical existing transverse bent is analyzed. The material properties used in the 1950's is estimated. The bridge element plastic capacities are obtained using Response-2000 software. The plastic hinge capacity for shear and moment are compared with demand at the various displacements. A lateral force verses displacement curve is then generated to failure. Also shown are the steps involved in a 3-D dynamic response spectrum analysis of the structure retrofitted with new substructure elements and 1/2"; steel jacketing of the existing columns. The seismic design criteria followed the MCEER/ATC-49 "Recommended LRFD Guidelines for Seismic Design of Highways Bridges". It is found that the additional substructure successfully limits the displacements of the existing retrofitted members in the MCE below their plastic capacity.

Societies have limited resources with which to meet the needs of their members and improve the quality of life. The trade-off decisions of how to invest this money should reflect short-term needs and welfare of citizens as well as the long-term stability of the constructed environment. The professional engineering community has much to offer in these decisions, but sometimes reflects its bias of increased structural lifetimes at the expense of immediate societal needs. This is especially true in the case of seismic risks, where the use of expected consequences can easily mask the multi-attribute issues associated with low probability, high consequence events. Risk issues of trust, dread, degree of voluntariness and epistemic versus aleatoric uncertainty, for instance, are rarely discussed in engineering decision-making committees. Communities are at their essence social constructs, supported by the built environment. This means that community resiliency is very different from structural robustness, and there are even cases where structural failures have afforded the opportunity for reconstruction strategies focused on community enhancement. This paper raises issues of societal trade-offs and risks.

NCHRP Project 12-49 is intended to develop comprehensive specifications for seismic design of bridges considering all aspects of the design process including: (1) design philosophy and performance criteria, (2) seismic loads and site effects, (3) analysis and modeling, (4) design requirements, and (5) detailing. These new specifications will be nationally applicable with provisions for all seismic zones. Performance levels for life safety and operational are defined for the rare earthquake event (MCE) and for the Expected earthquake event. While the ratio of the accelerations from the two events is about 1.5 to 2 in the Western United States, this ratio is about 3 to 4 in the Eastern United States. The maximum considered earthquakes (MCE) in New Jersey based on the proposed NCHRP 12-49 provisions are significantly higher than current design level earthquakes based on current AASHTO LRFD criteria. Maximum accelerations for relatively stiff bridges in soft soils located in North New Jersey from NCHRP 12-49 are approximately twice those from current LRFD accelerations. The current guidelines for seismic design of bridges in New Jersey are based on the 1998 LRFD Specifications (w/interims 1999–2003). The specs also require Safety Evaluation performance level as well as Functional Evaluation level for "critical" bridges including bridges carrying Turnpike traffic. Because of the several significant changes in the design earthquake and design criteria provided in the new provisions, there are questions on how these new provisions will affect the design and performance of bridge in states nationwide as well as the retrofit of existing bridges and the associated costs. In particular, the return period and soil factors. The current recommended return period for the MCE in NCHRP 12-49 is 2500 years. This return period seems conservative for the Northeastern US especially for "non critical" bridges. The soil factors were based on seismic records from the Western US and were higher for soft soils. Hence, there is a need to evaluate the impact of the maximum accelerations in new seismic design provisions proposed in NCHRP Report 12-49 on the seismic design and detailing of bridges in New Jersey and the Northeastern US and whether the use of such high accelerations and high soil factors in areas of low seismic risk such as New Jersey is justified.

Selection of acceptable seismic risk revolves around significant societal and engineering concerns. One approach relies on cost-effective quantitative enhancement of seismic resilience for critical structures and systems. A Center perspective can provide a coordinated research framework to develop the advanced knowledge and technologies needed to develop the integrated engineering tools, decision-support systems, and related techniques, and procedures needed for that purpose.

Efforts to develop performance-based earthquake engineering tools have revealed some important differences between how clients and engineers tend to relate to low-probability, high-consequence earthquake risks. These differences can be significant for potential adoption and diffusion of performance-based approaches to engineering practice. This paper draws upon research on individual and organizational decision making to explore and explain some of the major disconnects. Implications for engineering practice are developed.

This paper describes the methods used to predict column failure, progressive collapse, and building occupant injuries from progressive collapse in the BICADS (Building Injury Calculator And DatabaseS) computer program. The BICADS program, which was developed recently by Baker Engineering and Risk Consultants, Inc. (BakerRisk) for the U.S. Department of Defense, is an approximate methodology that estimates injuries to occupants from structural and nonstructural components in a wide variety of blast-loaded buildings. The percentages of occupants with each of four different injury levels are calculated from blast-damaged walls, columns, roofs, floor systems, windows, and interior nonstructural components that are part of sixteen different conventional building types or a user input building type. Simplified or detailed input options defining the building structural components are available with photographs, explanatory text boxes, and several help files to aid user input. This paper describes the methodology used in BICADS to predict column failure and progressive collapse for typical concrete and steel framed buildings. The simplified, fast-running methodology is based on pressure-impulse diagrams developed from theoretical equations for single-degree-of-freedom response and on limited data from U.S. government tests and terrorist bombs on full-scale columns with close-in explosive loads. Shear response is assumed to control concrete column failure. Anchor bolt response is assumed to control steel column failure if there is a shear plane through the bolts. Flexural response is assumed to control if the column baseplate is buried in the floor slab or the column is continuous into a basement. Column failure causes progressive collapse if the column spacing is large enough or if the column is a corner column. All building occupants within the tributary building area subject to progressive collapse are assumed to have the highest BICADS injury level, which primarily includes fatalities. This is consistent with injury data from the Oklahoma City bombing.

Blast resistant curtainwall systems require the use of laminated glass that is adequately adhered within the frames to develop the membrane capacity of the glazing and mullions that are capable of transferring the collected forces to the structural slabs. Dynamic inelastic analyses are required to determine the response of these curtainwall systems to highly impulsive blast loading. The most economical design of these systems to minimize the hazard to the occupants allows the components to undergo large deformations prior to dismemberment and fragmentation. Often, the performance of these systems are evaluated using a sequence of single-degree-of-freedom (SDOF) analyses; the glass is represented as a SDOF to determine the reaction forces which are subsequently applied to the SDOF analyses of the mullions. While these individual calculations attempt to represent the dynamic inelastic response of the components, it often neglects interaction and phasing of the components. As a result, the SDOF methods very often over predict the response of the individual members and produce an uneconomical design. Dynamic inelastic finite element analyses that account for the interaction of the various components and accurately represents the potential failure mechanisms provide the most accurate and economical design for blast resistant curtainwall systems. In addition to material nonlinearities, the finite element approach also accounts for geometric stiffening of the different components and therefore produces more accurate anchorage forces that must be developed at the slabs.

Concrete Masonry Unit (CMU) walls represent one of the more prevalent exterior construction materials for conventional buildings worldwide. Due to their weight and low blast capacity, CMU walls present a significant hazard to building occupants when subjected to intentional or accidental blast loads. A recent study showed that more than half of the buildings within the US Armed Forces are constructed using reinforced or unreinforced masonry walls. CMU construction is also common within petroleum and chemical plants where the consequences of accidental explosions on occupied buildings must be addressed as required by OSHA 29 CFR 1910.119. The Office of Special Technology, Technical Support Working Group with technical oversight from the Defense Threat Reduction Agency sponsored a study to compile all known blast tests on masonry wall panels. The data was scaled to facilitate empirical derived pressure-impulse (P-i) damage curves, which removed a significant degree of conservatism associated with previous P-i curves developed in the 1980's. These curves were updated in a study sponsored by the Army Corps of Engineers, Protective Design Center. These updated response curves have been embedded into fast running blast vulnerability codes to quickly assess masonry hazards to building occupants. There is an interest that these assessment codes include similar validated P-i diagrams for various building retrofit options so retrofit costs can also be estimated expeditiously. This paper discusses the development of P-i response curves for common masonry wall construction upgraded with E-glass.

Rapid assessment of structures for vulnerability to progressive collapse has become a major concern in today's world environment. Homeland Security and Overseas U. S. government agencies need the ability to quickly, and realistically, evaluate buildings to determine the risk of progressive collapse and understand how proposed retrofits would improve the building risk. To support the development of a fast running assessment tool, a series of simplified finite element simulations were conducted to generate a response data base. Models of simplified concrete and steel frames were run under gravity collapse conditions to determine displacements, forces and moments transmitted to adjacent structural members. The simulations were performed using Weidlinger Associates' FLEX finite element code. This paper will discuss the salient issues involved in the nonlinear, dynamic modeling of progressive collapse. The FLEX modeling used to develop the data base and confirm expected response is also described in detail. Finally, the simplified analytical tool that was developed from the simulation data base development effort will be described.

In March 2003, the Department of Defense Explosives Safety Board (DDESB) established a technical working group to update the tri-service blast design manual, Structures to Resist the Effects of Accidental Explosions, Army Technical Manual 5-1300/Navy Publication NAVFAC P-397/Air Force Manual (AFM) 88-22 (TM 5-1300) (Department of the Air Force, 1990). The revision will be written to retain its open distribution, thereby providing an invaluable source of information to both government and private sector engineers. When first published in 1969, TM 5-1300 (Department of the Air Force, 1969) represented the state-of-the-art in the analysis and design of blast resistant structures. Based primarily on explosive tests of reinforced concrete walls, the manual provided a comprehensive introduction to the blast design process including load calculation, dynamic analysis, structural design, and detailing. Revision 1, published in 1990, updated and expanded the manual to include structural steel, masonry, and other materials. Since 1990, the increased computational capacity of mainframe and personal computers has revolutionized both conventional and blast resistant design. Armed with this powerful research tool, knowledge has advanced rapidly, necessitating not only update of TM 5-1300's present content but also added coverage. New sections are planned on innovative, cost effective materials; applicability and limitations of blast design software; and retrofit of existing structures. Constructability guidance will be expanded to aid engineers in reducing the cost of often complex, blast resistant structures. This paper summarizes the working group's activities to date and future plans. Emphasis is given to anticipated revisions and additions to the manual.

This paper summarizes the background and the planned development of a Design Guide for Physical Security by the ASCE Task Committee on Design for Physical Security.

Prior to September 11, 2001, the Department of Defense and other agencies of the U.S. Government had developed a number of engineering design documents that provided guidance for protection of government assets against terrorist and criminal acts. Almost all of those manuals were exempt from public disclosure under the Freedom of Information Act. Historically, that was not a serious concern because there was relatively little interest in such design guidance outside the government. In a new initiative, DoD is updating and expanding a number of its manuals as Unified Facilities Criteria, combining the design guidance used by the Army, Air Force, Navy, and Marines into a coordinated series of Security Engineering Manuals. Those manuals are being developed through the inter-service DoD Security Engineering Working Group. The current plan is to exclude distribution limitations from those manuals so they can be made available to the public. This paper will describe the Security Engineering series of Unified Facilities Criteria manuals and the DoD Minimum Antiterrorism Standards for Buildings, which the manuals are being designed to support and supplement. This paper will also describe other support publications such as Unified Facilities Guide Specifications and related initiatives to develop state of the art tools for designing to protect people and property against terrorist and criminal acts.

In July 2002 the US Department of Defense (DoD) published the Unified Facilities Criteria (DoD UFC 4-010-01). "This document represents a significant commitment by DoD to seek effective ways to minimize the likelihood of mass casualties from terrorist attacks against DoD personnel in the buildings in which they work and live (UFC 4-010-01, Page 1-1)." In August 2003 these standards were applied in a detailed assessment of the Ishtar Sheraton and Palestine Meridian Hotels in Baghdad, Iraq. Following combat operations, the US military cordoned off access to the hotels to create a single defendable complex. A large contingency of DoD and contractor personnel were planned to occupy these hotels and a detailed assessment was immediately required. UFC 4-010-01 provided a checklist of 23 individual standards and 16 additional recommendations to reduce the risk of mass casualties from a terrorist attack. A five-step assessment process outlined in this paper ensured a comprehensive anti-terrorist system based on the UFC 4-010-01 individual standards and additional recommendations. Comprehensive anti-terrorist systems are essential to meeting the desired defensive posture to protect innocent civilians from terrorist attacks. Specific techniques and recommendations were used that could benefit the development of similar systems for other buildings to form an effective antiterrorist barrier system.

Since the September 11, 2001 terrorist attacks in the United States (U.S.), there has been a marked increase in both public and private sector demand for increased blast-mitigation for existing buildings. To adequately address these new performance requirements, a rational and cost-oriented policy is needed to help building managers and owners make fiscally intelligent decisions regarding the retrofitting of existing structures. This paper presents a cost-rationalized approach to blast-mitigation retrofitting decision making, with respect to a building's criticality to the surrounding community. The proposed method addresses current approaches to blast assessment, typical blast-related vulnerabilities (structural and non-structural), and possible solutions for vulnerability mitigation. Special emphasis is given to non-gravity based loadings, post-incident functionality requirements, and transient versus permanent/semi-permanent loading.

The paper includes the description of the extensive Israeli practical experience in developing and implementing protective measures to strengthen existing structures. Much has been done recently to devise and implement protective hardening measures in new structures, such as preventing progressive collapse, minimizing the debris hazards, etc. However, much less has been done to strengthen existing structures, as this is considerably more difficult and more expensive, if at all feasible. As Israel has been constantly subjected to terrorist attacks for decades, a vast practical experience has been gathered from the observed damages to structures from real-life terrorist attacks. Learning from the effects of terrorist attacks on actual buildings, considerable efforts were made in Israel to develop cost effective and easy-to-implement retrofit protective hardening measures. Examples of various types of protective hardening measures implemented in existing structures are presented, as well as the SEPHRA (SEcurity Protection and Hardening Risk Analysis), developed by the author, resulting in the definition of the optimal cost effective protective retrofit measures.

Progressive Collapse Prevention of a high-rise building at the design stage under abnormal loading conditions is a very important subject in structural design. This article contains three major parts: (1) Code requirements regarding Progressive Collapse Prevention; (2) Design criteria for progressive collapse prevention under abnormal loading conditions, including local stability of some elements and global stability of a compromised structure. (3) Design loads from possible explosions inside of a high-rise building with a deflagration venting system. This article has an example to illustrate practical approximate approach.

This paper reviews the principles of progressive collapse analysis for the Alternate Path method, compares static and dynamic analysis methods, and reviews two case studies using nonlinear dynamic analysis.

A series of analyses were conducted on some of the most common steel connections used in the UK such as flexible end plates, web cleats, flush and extended endplates, using LS-DYNA (Hallquist 1998),, an explicit finite element analysis code, to assess their ability to resist rapidly applied tensile loads. Characteristics such as strength, energy absorption and response to different strain rates were studied. The effect of material strain rate sensitivity was accounted for using the Cowper and Symonds formulation. This paper reports this work and comments on the applicability of the British Standard recommendations for the avoidance of progressive collapse to situations in which tying forces are applied dynamically.

Although progressive collapse is not a new topic in the structural field, recent collapses have brought this issue to the fore. Following the partial collapse of the Ronan Point residential tower block in London, the UK building regulations provided recommendations to guard against disproportionate collapse. These UK guidelines aimed to prevent progressive collapse by ensuring adequate tying of a structure (in concrete or steel) so that catenary action could develop, and this approach has often been cited as good practice. It is noteworthy that little research has been conducted into these recommendations, particularly whether they are adequate to protect a damaged structure from progressive collapse. This paper reports an analytical study of the forces generated in steel framed structures during a progressive collapse. The analyses were conducted using LS-DYNA [Hallquist, 1998], a non-linear, explicit finite element code. A hybrid design method that can improve structural performance during progressive collapse is proposed.

Flat slab structures are widespread all over the world and have a high rate of occupancy. Therefore, an investigation of their vulnerability is an important task because the collapse of such a building can lead to fatal consequences. One of the hazard scenarios which could end in a progressive collapse is column failure. Hence, the consequences of a column failure for the remaining structure are investigated, and the key parameters are identified with the aid of simple mechanical models. As a result the failure time of the column should be assumed to be chosen close to zero. Since linear elastic analyses do lead to bad results, a non-linear structural behavior should be considered. For the calculation of forces damping and strain-rate effects can be neglected. To determinate the maximum deformations of the remaining structure, however, those two parameters are worthwhile to be considered.

Progressive collapse analysis of seismically designed, special moment resisting steel frame buildings is presented. The analysis model and the assumptions needed to create it are described and the results of a simulation in which a critical structural member is removed are shown. The limitations of the simulation are enumerated and the practical implications of the analysis results are discussed.

A concise methodology for evaluating the predisposition of a structure to progressive collapse is presented in this paper. A graphical modeling aid may be developed and used to investigate the potential for progressive collapse by evaluating the maximum ductility and corresponding rotations of critical structural load-bearing elements. Thus, a conventional dynamic computation with initial conditions can give an indication of the potential for progressive collapse of a structure.

In light of current world developments, engineers are increasingly being required to consider progressive collapse mitigation as a basic design criterion. Progressive collapse requirements, however, must typically be incorporated into a structure without substantial increases in the cost of the structural system. One method of achieving this goal efficiently is to utilize a multi hazard approach to structural system selection. This paper illustrates how progressive collapse criteria can be economically incorporated into typical structural systems. Although three very different examples are presented, the overall design approach is basically the same. Structural Steel Office Building — The use of some types of moment frames to resist lateral wind and seismic loads can also provide the additional ductility and redundancy required for progressive collapse resistance. Reinforced Concrete Office Building — Analysis of an example existing building indicated that seismically designed reinforced concrete moment frames have an inherent capability of providing progressive collapse resistance. Reinforced Concrete Apartment Building — One cost effective construction method for apartment building construction is the tunnel-form concept. Analysis methods indicate that with minor modifications these structures can be designed to withstand the loss of a supporting wall without experiencing progressive collapse.

The paper presents research program on retrofitting reinforced concrete slab-column connections to increase their punching shear strength. The proposed technique using shear bolt reinforcement allows increasing strength, ductility and rotation capacity to reinforced concrete slab-column connections. The results of the experimental work include both interior and exterior connections. The load-deflection curves are presented which show how transverse reinforcement increases punching shear capacity and post failure ductility of slab-column connections.

A progressive collapse is a catastrophic partial or total collapse that initiates from local structural damage and propagates, by a chain reaction mechanism, into a failure that is disproportionate to the local damage caused by the initiating event. Procedures for mitigating the risk of progressive collapse can be developed using concepts of structural reliability analysis and probabilistic risk assessment, supported by advanced computational tools. This paper summarizes methods for assessing the risk of progressive collapse and implementation of strategies for its mitigation in national standards such as ASCE Standard 7, Minimum design loads for buildings and other structures.

The provision of structural integrity to avoid major failure and progressive collapse of buildings under abnormal loading is a major concern for design engineers. Current codes and standards that deal with this issue often propose only qualitative criteria. As well, there is generally no consensus among researchers and engineers concerning performance-based requirements for design against progressive collapse. One of the major reasons seems to be the difficulty and complexity of the collapse analysis of buildings. This paper proposes some simplified methods for the analysis of building structures subjected to abnormal loads that result in structural collapse. The computational techniques can be used to track possible failure mechanisms and give useful information to the designer about the integrity of the structure. The proposed methods are based on simple linear analysis and can easily be implemented in conventional commercially-available engineering analysis and design software for building structural systems. The results of the simplified methods are verified through comparison with those found using rigorous nonlinear failure analysis.

This paper was written to identify and discuss two issues that need to be considered in the development of a comprehensive R and D strategy to address the structural integrity of built facilities. The first issue concerns the need to obtain better information and develop a better understanding of the incidence and nature of abnormal and exceptional loads. The second issue involves the use of spiral or helical reinforcement and the potential for stable ductility that is available, especially for single pass events such as a terrorist bomb blast.

Often architects and designers look to the past for historic precedents and inspiration on current projects, particularly that involving masonry construction. Although this practice can offer an improved aesthetic and a greater variety of interest in otherwise bland exteriors, they add construction complexity and do not always perform well. Traditional masonry wall systems were generally designed and constructed as mass walls, with little or no thought to water penetration, lateral load distribution, or other factors that impact the performance of modern cavity wall construction. Drainage provisions have taken the place of collar joints, steel structure has taken the place of thickened walls, flexural stresses control more than compressive stresses, and modern ties have replaced true headers. Lighter, thinner, taller...better? Maybe, but most definitely these changes have lead to increased consideration of performance and greater design complexity. With this transition to modern masonry wall construction, the integration of traditional details sometimes conflict with the realities of modern cavity wall construction. Architectural elements such as steep sloped roofs, roof base flashing, parapets, flat arches, conventional arches, dentils, water tables, and cornices do not always coordinate with flashing requirements, supporting structure, and expansion control requirements. This paper attempts to identify some critical considerations when traditional aesthetics are applied to modern cavity wall construction. Through exploration of these challenges and their eventual solutions, it is hoped that designers will consider the constructability of desired masonry detailing to help avoid serviceability problems in the future.

The Chrysler Building, an icon of Modern American Architecture and a personal tribute to automaker Walter P. Chrysler, the building's owner, Architect William Van Alen and the machine age, was opened in May 1930. At 1,045-feet tall, it was the world's tallest building when completed and is still in the top 20. (This distinction continues to change as taller and taller buildings are completed.) The building was constructed similar to most large commercial structures of the time: structural steel frame encased with concrete and terra cotta fireproofing; brick facades with granite and marble decoration on the lower floors; backup brick and terra cotta block infill; cast-in-place draped mesh reinforced concrete floor and roof slabs.

To effectively define and repair problems that develop in facade cladding systems, the design professional must understand the original construction sequencing, and how the repair construction sequencing will influence the cost, performance, and risk of damage to the facade cladding system. Repair details that do not consider the execution of the construction may lead to short-term difficulties including damage and cost concerns during the assembly and long term durability concerns including increased distress that may cause increased maintenance requirements, localized failures or even catastrophic failures. All too often designers detail repairs without thought as to how and when the work will actually be performed. As a result, difficulties arise during construction because of lack of forethought on critical issues such as tolerances, access requirements, temperature restrictions, owner requirements, interface between trades, and material and system compatibility issues. Anticipating potential constructability conflicts during the design of the repairs reduces the number of issues that develop, and minimize delays and extra costs.

Water intrusion through building envelopes, and resultant damage it causes, including mold, has become a well-publicized tragedy and substantial cost burden for building owners. So why wait for moisture intrusion problems to occur when there are techniques available to catch problems before they occur? Based on its extensive forensic experience (more than 15 years) in evaluating hundreds of buildings with moisture intrusion and mold problems, CH2M HILL has found that nearly all of these problems could have been prevented. The critical role of the building envelope in building performance is now being recognized by the industry. In fact, more and more owners are asking how to commission the building envelope. Unfortunately, little information currently exists on building envelope commissioning. Taking a proactive approach, CH2M HILL has developed a complete strategy for problem prevention, which builds on the construction industry's use of commissioning and established testing protocols. Our goal is to minimize the potential for water intrusion problems. For each project phase, CH2M HILL has established peer review strategies, specifications, and construction phase testing and inspection protocols for commissioning of new building construction. A number of methods for both qualitative and quantitative testing of building envelopes can be applied as part of commissioning activities during construction. These tests are generally based on accepted industry standards. Using case studies of buildings recently commissioned by CH2M HILL, this paper will present an overview of the techniques CH2M HILL has developed for commissioning of building envelopes to minimize water intrusion.

The Empire State Building was completed in 1931, after only one year and 45 days of construction. At 1250 ft. to the top of the dirigible mooring mast (plus a two hundred foot tall T.V. and radio antenna mast) the Empire State Building was the world's tallest building for over 40 years. A New York City Landmark, the Empire State building has also been cited by the ASCE as a National Civil Engineering Landmark. One of the many innovative characteristics of the building is the unique early curtain wall design for this building's facade. This wall system bridged the technology of the traditional rigid masonry facades that predated it, and pioneered the concepts of flexibility that are now commonly incorporated into high rise building curtain walls. This article examines the challenges faced by the designers of the building and by the restoration engineers in its repair after 60 years of service. It should be noted here that 60 years without major restoration represents a very respectable service life for the exterior wall system of a high-rise building.

The Houston Chronicle Building is a 10-story office and administration tower representing a landmark building of the city of Houston, Texas. The building is sited on a city block bounded by Texas Avenue, Travis Street, Prairie Avenue and Milam Street. The current façade of this building consists of 7/8" thick marble panels that are currently experiencing bowing and dishing as a result of the "hysteresis" of the marble panels. In addition, seasonal volumetric changes have created dysfunctional conditions with respect to the behavior of the expansion joints. Once the owner became aware of the problem, a proactive initiative was taken, and a decision to re-clad the building was made. A design team composed of engineers, architects, and contractors was put together with the purpose of generating a new façade meeting aesthetics and functional requirements. One of the biggest challenges in this re-cladding project was the unknown substrates of the building. The Houston Chronicle Building is a composite of four structures — the original Houston Chronicle Building built in 1909, the Palace Theater built in 1910, the Milam Building built in 1922, and the four-story addition built north of the original Chronicle building in 1937. Each building has its own distinctive architecture and design. In 1967, the Chronicle owners initiated a major step in the history of the building by attempting to create one single building; the culmination of this effort is the current façade. Unfortunately not enough records exist that document the critical changes during the early additions between 1910 and 1939, including the major renovation that occurred in 1967. This paper presents the methodology utilized by the design team to determine strategies to investigate a building, containing different substrates which is intended to receive a new façade that meets current building code demands (i.e. wind loads), state-of-the-art anchorages, and other miscellaneous requirements under very stringent deadlines imposed by the demands of this project.

Empire State Plaza, located in Albany, New York, represents one of the most ambitious stone cladding projects in modern times. A complex of 11 buildings encompassing nearly 100 city blocks, the buildings are clad with 125,000 marble panels and nearly five acres of a sedimentary stone called Llenroc, which is indigenous to Upstate New York. This paper discusses performance problems that developed due to original detailing problems with the Llenroc, and the survey and stabilization program that was implemented to stabilize the cladding while long-term solutions can be developed.

Among other pursuits, the late Dr. Robert H. Scanlan worked for nearly 40 years to understand the problem of bridge-deck flutter. This paper presents an overview of his work, with particular attention to the original investigative work that dealt with the most-famous bridge flutter problem of all; Galloping Gertie—the Tacoma Narrows Bridge. Dr. Scanlan worked to develop the theory underlying the phenomenon, he advised and worked closely with wind-tunnel investigators to physically model deck sections, to measure the flutter coefficients so important to his equations, and he advised and guided practitioners in developing real solutions to the problem of wind-induced torsional flutter. The results of his work are reflected in the striking similarities among the geometries encountered in modern, long-span-bridge designs, from Christian Menn to Santiago Calatrava. Among other goals, this paper and its accompanying presentation hope to correct a common misconception that the failure of the Tacoma Narrows Bridge serves as an example of forced resonance.

The course described in this paper is a failure class for civil engineering seniors, that has been taught at the University of Kentucky for the past six years. It is an internet-based and student-based learning class that requires independent study outside of lecture. In addition to classical failure cases from every civil engineering sub-discipline, many current failure cases and issues are discussed in class. Student driven group discussions and written and oral presentations are additional components of the course. Through failure case studies, students learn the technical issues, the causes, and modes of failure of various civil engineering constructed facilities. Students also learn the basic procedures of conducting forensic engineering investigation. Ultimately, based on the knowledge gained in the course students become well informed about past experiences and better prepared technically to enter the profession with the capability of successfully facing the challenges that will arise in the 21^st Century.

The study of engineering failures can offer students valuable insights into associated technical, ethical, and professional issues. Lessons learned from failures have substantially affected civil engineering practice. For the student, study of these cases can help place design and analysis procedures into historical context and reinforce the necessity of lifelong learning. Three approaches for bringing forensics and failure case studies into the civil engineering curriculum are possible. These are stand-alone forensic engineering or failure case study courses, capstone design projects, and integration of case studies into the curriculum. The ASCE TCFE Education Committee held one-day workshops in Birmingham, Alabama on July 12, 2003 and in Cleveland, Ohio on September 17, 2004 for approximately two dozen engineering educators per workshop. The participants estimated that over 75 courses and 2,000 students would be affected by the project over the next two years. The participant workbook had case studies in engineering mechanics, structural engineering, other civil engineering courses, ethics/professional issues/capstone design courses, and forensic engineering/failure analysis courses. Presentations were provided on a CD. The materials have also been disseminated on a web site.

This paper discusses two case studies of structural steel framing failures including what went wrong and lessons learned. One study involves a department store collapse in New York. The other involves freezer warehouses in Pennsylvania.

The connections of structural steel framing members are a critical component of building safety and stability. Connection failures occur when the connection is no longer able to perform its intended design function of transferring the reactions from one element of the structure to another. In some circumstances the failure of a connection in structural systems which have no redundancy may result in an overall catastrophic collapse of the structure itself. In the event of a weld failure, it is imperative to follow proper forensic engineering procedures. The implementation of repair procedures, without investigating and determining the actual cause, simply masks the problem and will not reduce the risk of repeated failures in the future. This could be detrimental to the safety and stability of the whole structure. In a recent construction project of a thirty story high rise building, weld cracks were observed on both shop and field welded connections and members. Some cracks appeared months after the testing and inspection agency had certified these welds as acceptable. In addition, the failures occurred in connections and elements that were significantly dissimilar from one another. This paper examines the forensic engineering analysis procedures used in determining the causes of the various weld failures, the procedures to repair them, and how to avoid the recurrence of additional failures.

In February of winter 2003, a multi-storey building of ten floors, six of which are basement floors located at a lower level than the adjacent street, totally collapsed in Amman. The event attracted engineering attention since it was the second failure of multi-storey buildings in less than one year. Both buildings were constructed beside a street at a deep cut made in soil. Retaining walls are constructed adjacent to the cut to retain soil and connected by tie beams to the floors of the buildings, the purpose of such connection is to give additional structural support to the retaining walls which are usually of considerable height supporting large amounts of fill behind. Construction works in the building under consideration were completed in May 1997, a retaining wall adjacent to the building of 20m height was also constructed, however, the wall was left without any fill behind. In the following years, the space behind the wall was used as a dumping area for public. By the mid of 2002, the wall had been totally backfilled with bad fill materials. Cracks started to propagate within the wall starting from the footing and going diagonally up through the wall. The designer tied the retaining wall to the main building at the slabs level of the basement floors by tie beams in an attempt to support the retaining wall. A snowy storm affected the city in February 2003, and due to the accumulation of rain water with no drainage behind the wall, a catastrophic failure occurred which led to a complete failure of the retaining wall and the whole building tied to it. A complete structural evaluation has been carried out to focus the light on the causes of failure. The first step was a complete review of the structural drawings and all available documents, followed by field investigations in which several tests of the backfill, cores of concrete, and steel reinforcement have been conducted. Finally, a comprehensive structural analysis was carried out. Investigations were carried out upon the request of Ministry of Interior Affairs and Police departments to identify liabilities of the different parties of the contract. This paper presents the steps followed in the evaluation process, highlights the causes of failure, addresses the lessons learned from such a case, and sets recommendations for tied retaining wall multi-storey buildings. The paper also intends to show the typical aspects of physically carrying out a forensic investigation, such as the handling of evidence for courtroom presentation.

This paper summarizes a structural failure evaluation and a repair procedure for a precast double-tee system that is part of a reinforced concrete hotel building. The ballroom floor supported on the precast double-tee system exhibited signs of distress and was fortunately noted by an observant maître d'. Noting this prominent sag on the ballroom floor, prior to a major event to held that same evening, the hotel owner contacted the structural engineers to get an immediate assessment of the observed distress. In order to determine the cause/s of the movement in the ballroom floor, a field investigation, including a review of existing drawings and a detailed analysis was carried out. Our review indicated that performance of field modifications of the precast double tee beam bearing during original construction (approximately 30 years prior the subject incident) under normal service loads had resulted in a structural failure. Various repair schemes were evaluated and the selected retrofit option was one that was easy to construct and did not encroach on the current usable space.

The first steps following a collapse are critical. The structural forensic engineer who is called in following a collapse plays a crucial role in determining what those first steps should be. He is the most qualified to recognize perishable evidence and its potential value. The decisions made will directly affect the evidence upon which investigations will depend. This paper deals with immediate issues that a structural forensic engineer may be faced with when arriving at a collapse site and in the ensuing days. It includes issues such as safety, preserving perishable evidence, reserving samples, documentation, interviews, document gathering, and preliminary evaluation. For fuller explanations and illustrations, the reader is referred to Chapter 4 in the Handbook of Forensic Engineering.

This paper presents the results of the investigation of the collapse of the falsework of a bridge in California. The bridge runs in the north-south direction and consists of two box girder bridges, each supporting three lanes, one in each direction of flow of traffic. The structures have three spans, with two piers in the middle and two abutments at the ends. The overall length of the bridge is 387 ft long, with piers as high as 12 ft and the box girder was 4.9 ft deep. The incident occurred during the placement of concrete in the second span of the right bridge. At the time of the incident, only 20% of the concrete for the bridge deck had been poured and the vertical supports in the falsework were only loaded up to 50% of their design loads.

On May 20, 2000, shortly after the finish of a racing event at the Motor Speedway in Charlotte, NC, one span of a four-span, simply-supported, precast, pretensioned concrete pedestrian bridge collapsed injuring 107 people. The bridge was approximately five years of age at the time of collapse. Although this was a private structure used to facilitate crowd movement from the speedway to parking lots, it spanned a major US highway. The litigation that resulted from this collapse was settled over the following years. WDP & Associates, Inc. was retained by one of the principal parties in the various lawsuits and given the task to determine the probable cause of the collapse. The investigation included an extensive review of information from public and private sources; field investigation of the debris from the double-T beams that collapsed and of those that were dismantled from the other three spans did not collapse; and, an extensive laboratory investigation. This paper summarizes the investigation into the cause of the collapse. Details of the various phases of the investigation are provided. An important question that became particularly relevant during the litigation, such as whether the bridge had ever been inspected, and if it had, what was observed, is discussed.

Wood-framed roof structures can experience significant damage associated with long-term loads as well as distress resulting from transient loads. In our experience, there often exists substantial evidence that points to the cause or causes of the distress or damage as well as evidence that points to when the damage likely occurred—if engineers are observant and diligently look for such evidence. However, many engineers overlook basic evidence that would shed substantial light on the cause(s) and nature of the damage. This paper presents five case studies where proper and methodical field work and analysis helped determine the actual cause(s) and/or timing of the damage.

This paper reviews past and current building code provisions for repairing damage to existing buildings. Shortfalls, ambiguities and contradictions in the code discovered during the course of designing repairs to buildings damaged during the March 2003 blizzard in Denver, Colorado are discussed. The paper discusses the significant challenges involved in providing responsible engineering services for the repair of damaged structures considering current code and jurisdictional requirements and attempts to clarify the requirements with authoritative interpretations from code officials. Recommendations for improving repair provisions in the code are presented.

Consider the following scenario: An engineering firm lands a design contract for a large commercial building project. The owner is pushing for a fast-track schedule, such that the final building design will not actually be complete before construction begins. From the start, the structural engineering discipline becomes the critical path for the project. There is tremendous pressure to complete the design of the structural system so that construction materials can be ordered and fabricated in step with the schedule. The structural engineer must perform analyses that consider code-required, minimum design loading conditions. The roof design incorporates live loads, snow loads, dead loads, wind loads, and seismic loads. What about rain loads? Will the roof drainage system be designed to prevent excessive rainwater accumulation on the roof? What assurances does the structural engineer have that the subsequent design contribution of other disciplines (i.e., mechanical engineering and architectural) will not result in excessive rain loads? Recent roof structure collapses in the north-central region of the USA (referred to as the Midwest) have raised these questions and related concerns about the building design process. Is reform needed? This paper will address proper roof drainage design, a rigorous analysis technique for estimating pre-deflection ponding depth on roof systems, the responsibility matrix for roof design, and suggested protocols to ensure that the roof drainage system is appropriate for the roof's structural design.

In this age of high technology, the expert consultant usually must state an opinion based on limited information. This is especially true in construction defect or delay claim cases, or when the damage to a structure is due to the occurrence of a natural phenomenon such as rainstorms, strong winds, or earthquakes. The ever expanding availability of tools for computer analysis of structures enable increasing insight and improved accuracy in helping to understand the cause of damage, but many of these analyses are very expensive. Often the client and the expert face the dilemma of balancing the expenditure of limited funds with the goal of achieving a desired level of confidence in the expert's opinion. This paper addresses how one can scientifically assess the confidence in an experts' opinion.

When properly designed as a scientific experiment, assessing the performance of a population of buildings following a natural disaster can lead to new insights and objective findings that cannot be obtained from traditional damage surveys. The main feature in a scientific damage assessment methodology pertains to the manner by which a representative sample of buildings is selected for study. This feature allows statistical analyses to be performed in relation to building characteristics, loading parameters, and observed performance. Robustness of cause-effect relationships and frequency of various types and levels of damage can be characterized by mean and variance as well as confidence limits in relation to the sampled building population. Because building performance data is collected following principles of scientific method, it has many useful applications for engineering sciences, risk analysis, and public policy.

Accidental explosions have resulted from numerous sources, including fuel/air mixtures, reactive chemicals, combustible dusts, bursting pressure vessels, and high explosives. These impulsive events call for specialized forensic analysis methodologies because the damaging loads exist for only short durations (see Figure 1), making conventional static analysis inappropriate for backing out possible root causes. An important part of an explosion forensic analysis is to use damage indicators from the surroundings to determine the strength of the blast wave. These damage indicators, such as deflections in metal panels, deformed structural members, debris throw, or broken windows are analyzed using methods that range from semi-empirical pressure-impulse based damage correlations, to single-degree-of-freedom analysis, to dynamic nonlinear finite element analysis. The final result from analyzing many indicators is a magnitude for the explosion source energy, which, in turn, often leads to an understanding of what caused the explosion.

This paper presents a method for the stability assessment of large-panel buildings with external and cross interior load bearing walls and for the examination whether a possible failure of such a building is expected to progress from an initial local failure to a global failure of a considerable part of the building. The method extends an approach that was originally developed by Y. M. Strugatzky (1936–2000). The original method is relatively unknown, it was hardly published and its existing resources are almost inaccessible [1,2]. Therefore, for completeness it is concisely presented (with some unavoidable changes in the original method that focused on exterior reinforced concrete walls only) and extended to include cases of initial local failure in the interior part of the structure, as well as to refer to buildings with transverse or longitudinal interior load bearing walls.

The University of Colorado at Denver (UCD) has been involved in cooperative research efforts with the City and County of Denver (CCD) since 1997. The CCD Department of Public Works owns, inspects, and maintains approximately 631 bridges in its inventory. The health condition of each of these bridges is fundamental to the overall transportation system for the City of Denver. This paper addresses the condition assessment of the westbound 6^th Avenue viaduct.

In recent decades, the governing ethos regarding repairs to existing structures has been that it is permissible to repair damage, with the repair itself required to comply with current code, but without requiring that either the element being repaired or the structure as a whole be re-designed to resist the vertical and lateral forces prescribed in the current code for the design of new structures. With the introduction of the 2003 International Existing Building Code (IEBC), however, this time-tested and proven philosophy has been discarded in that the 2003 IEBC sets forth specific loss-of-capacity-based "triggers" for partial and full upgrades of existing damaged or deteriorated structures. While these triggers may be well-intentioned, they seem quite likely to initiate widespread societal change due to the large scale adverse financial repercussions of the triggers that will ultimately be borne by the public. This paper discusses the historical development of the requirements of the "Existing Structures" provisions of various model building codes and how these requirements have long been enforced with respect to repair of existing buildings. The paper then examines the specific provisions of the 2003 IEBC to show how the provisions might be applied in communities that adopt it "as is." Using several case studies, the consequences of this proposed code are then discussed. Then, the ethical question is posed, "Is the IEBC good for society or is it good for engineers?"

Inspection by plant personnel revealed cracking in the concrete frame of a 326-ft-tall, 7-level preheater tower. On-site plant engineers deemed the cracking significant, especially since the structure supports critical manufacturing process equipment. A structural engineering consulting firm was retained to evaluate the extent of the problem and formulate a repair plan on a fast-track basis. The firm mobilized at the site in less than 24 hours and performed an initial structural safety assessment. A comprehensive structural evaluation indicated that the structure required strengthening. Restoration consultants were engaged to assist locally with engineering and construction administration. A specialty repair contractor also was engaged to review the constructibility of several alternate repair schemes and maintain the fast-track schedule. After considering structural capacity and serviceability requirements, durability issues, the high-temperature operating environment, constructibility, and an aggressive construction schedule, the team recommended a retrofit consisting of bonded post-tensioning within internal holes drilled in the beams. This solution was quite extraordinary, as it required precision-drilling horizontal holes up to 87 ft long in the beams of the elevated frame structure, without cutting existing embedded reinforcement. Nondestructive impulse radar testing was used to locate existing embedded reinforcing steel, as well as to monitor the drilled holes' trajectory. This process helped ensure proper hole alignment and prevent damage to embedded steel. The cored holes served as post-tensioning ducts. The repairs were executed on a fast track basis and under challenging circumstances, which included working high on the exposed structure through a cold winter with severe wind conditions. The unique retrofit resulted in a structure that is stronger, more serviceable, and more durable than the original tower. The project represented an exceptional team effort, and its success is attributable to the leadership of the owner and client, the ingenuity of the engineering team, and the resourcefullness of the contractor.

In March 2003, Denver Colorado and the surrounding area experienced code-level snow loads due to one of the most destructive blizzards in Colorado history. According to the Rocky Mountain Insurance Information Association, personal claims from the storm exceeded $93,000,000 due to damage across the region. This is in addition to large commercial building losses as well as untold dollar amounts in lost business revenue while buildings were unsafe to enter. Hundreds of structures collapsed or sustained severe structural damage, and thousands of buildings sustained repairable structural or cosmetic distress This paper discusses the types of structural systems commonly damaged as a result of the snow loads. Based on the lessons learned from this storm, recommendations are made for future design and structural condition assessment of snow loaded structures.

This paper summarizes the structural evaluation and repair procedure applied to a deep reinforced concrete transfer girder. The girder exhibited cracking immediately following construction. However, this first distress indication was not properly addressed and was followed by the formation of additional diagonal cracks indicative of shear-flexural problems. In order to determine the cause(s) of the cracks, field investigation, review of the construction process, design review and detailed structural analysis were carried out. The review found several design and construction deficiencies and concluded that the girder needed to be strengthen. Different repair schemes were investigated and the scheme that presented the best retrofit potential (defined by cost, constructability and structural reliability) was implemented.

The lessons learned and issues raised from 9/11 have influenced the investigation, vulnerability analysis and remedial protective design of existing buildings. This paper presents common vulnerabilities of buildings to blast and impact effects. Cost effective strategies of hardening and redundancy as an effective means of strengthening and protecting buildings are discussed.

New York City is commonly acknowledged as having one of the most stringent building codes in the country, if not the world. However, catastrophic events such as the WTC attacks were not generally considered in building design. Given the reality of the post-9/11 environment — when New York City faces unknown and elevated risks — our standards must be the highest that we can make them without compromising the growth of this great city. To that end, on March 19, 2002, the Department of Buildings convened the World Trade Center Building Code Task Force (Task Force) to review current building design, construction, and operating requirements. Lead by the Buildings Department, the Task Force comprised experts from local, state and federal governments — notably the New York City Fire Department (FDNY), representatives from the real estate industry, design professions, academia, the special needs community, and 9/11 survivors and relatives of the victims. The goal of the Task Force was to examine New York City's Building Code and related codes to target areas to enhance public safety in a practical way. A guiding principle to its mission was that safety and economic viability would be appropriately balanced, but neither would be compromised. Four major areas that affect building performance and occupant safety during emergency events were considered: structural strength, fire protection, emergency evacuation and mechanical systems.