Structures Congress 2013: Bridging Your Passion with Your Profession

The collapse resistance of a seismically-designed steel moment frame building is investigated using 3-D nonlinear models. Finite element analyses are carried out using validated models to investigate building response to loss of internal and external columns at various floor levels. Contributing mechanisms to collapse resistance that were studied include composite action between the slab and underlying steel beams and slab membrane action. The results of the simulations suggest that the building is more vulnerable to loss of columns in the upper stories than in the lower ones. It is also vulnerable to loss of interior gravity columns at all floor levels. Although the slab can contribute significantly to the robustness of the structure, it can become detrimental in the final stages of collapse.

The progressive collapse of structures under accidental loading is a serious threat to public safety. In this paper, the results of recent research on the dynamic response, failure mechanism and changes in the load transfer paths of a 3-bay x 3-bay, 3-story, half-scale reinforced concrete frame structure subjected to a series of sudden column removals are presented. A corner column and an adjacent column along the short span direction, two middle columns along the long span direction and one interior column were suddenly removed by impact load. After the imposed failure of the respective columns, the forces resisted by the removed columns were transferred to the surrounding components. The structure was on the verge of local collapse following the removal of the two exterior columns in the long span direction but performed well in all other column removal scenarios.

Since the collapse of the Ronan Point apartment, extensive efforts have been devoted to developing numerical models for assessing the vulnerability of buildings against progressive collapse. The existing numerical models are largely deterministic, where the uncertainties in structural behavior are not considered. This study presents a stochastic numerical model of progressive collapse of reinforced concrete (RC) buildings. In the proposed model, each structural component, such as beam, column, beam-column joint, wall, etc., is modeled as several elastic or rigid blocks connected by nonlinear cohesive elements, which represent the potential damage zones. The constitutive relation of the cohesive element is calibrated by the detailed finite element simulations of the potential damage zone. An Incremental Latin Hypercube Sampling method is further incorporated into the numerical model to perform stochastic simulations of progressive collapse of RC buildings under different column removal scenarios (i.e. different number of columns removed at different locations), in which the fracture energies of the macro-scale cohesive elements are randomized. A family of fragility curves for various possible collapse patterns is generated, which represents a reliability-based measure of the vulnerability of buildings against progressive collapse.

An ongoing collaborative research project is studying the structural integrity of steel gravity framing systems composed of steel beams and girders with composite slab on metal deck. A major contribution of this project is large-scale testing of critical components in gravity systems subjected to severe demands consistent with column loss scenarios. Detailed component tests of the steel-concrete composite floor slab and simple beam-column connection are complemented by complete floor system tests. Component tests on the concrete slab on metal deck include uniaxial tests to evaluate behavior in both orthogonal directions with respect to the deck corrugations, and shear and tension tests of the sidelap splices. Parameters include metal deck thickness and type of sidelap (e.g., button punch, screw, weld). The full-scale beam-to-column connection tests consider a wide range of parameters, including connection type, number of bolts, edge and gap distances and slab effects. The floor system tests are being conducted at half scale on a three-bay square configuration that considers interior, exterior and corner column loss scenarios.

Modeling approaches for analyzing the robustness of steel moment-frame and braced frame buildings against column loss are presented, including the nonlinear behavior and failure of the shear and moment connections and the effect of the composite floor slab. For the braced-frame buildings, modeling of the buckling behavior of braces incorporates nonlinear rotational springs representing the out-of-plane behavior of the gusset-plate connections. These approaches are used to analyze the nonlinear system behavior of 10-story prototype steel moment frame and braced frame buildings under column loss scenarios. Analysis results are presented, and the behavior of braced frame and moment frame buildings under column loss is compared.

An important aspect of progressive collapse evaluation of steel gravity frame structures is to consider the mobilization of composite action between the slab and the steel frame. After initial column failure and experiencing large deformation, catenary action develops in the floor slab and the overall response of the system changes the load transfer mechanism to the connections. As a consequence, shear connections experience extreme rotation and demands that are significantly different from those imposed by an equivalent bare frame system. This research investigates the structural integrity of steel gravity frames in conjunction with the concrete floor slab and the induced demands on the shear connections. A high-fidelity model including the floor slab components is developed to examine the performance and behaviour of the shear connections, in addition to the overall load carrying capacity of the system. Explicit quasi-static analysis is employed to overcome the convergence difficulties related to contact, geometric and material nonlinearities, large deformation, and fracture simulation. Challenges involved in developing numerical simulations of conventional composite floor framing systems when a column has been compromised are discussed. The component finite element model is validated against the results of physical tests in terms of failure mode. It is shown that the finite element analyses give reasonable accuracy and agree well with the experimental results. The research results show that while the presence of the concrete floor slab contributes to the capacity of connections, it also amplifies the demand on the connections and alters the load transfer mechanism as compared to a bare frame.

This research tested a series of three one-quarter scale, two-bay by two-story frames under static and dynamic loads. The research considered the ability of frames with different reinforcement designs to develop resistance mechanisms, and evaluated the effective dynamic increase in response when a frame was loaded dynamically. The static tests were slowly loaded through a hydraulic actuator at the location of the removed column. The dynamic test was loaded with additional dead weight and the center supporting column suddenly removed to simulate a collapse scenario. The dynamic loads and strains in the horizontal direction were 1.3 to 3.4 times greater than the corresponding static loads. Subsequent analysis of the test data and results from static simplified analyses and dynamic single degree of freedom (SDOF) models showed that there is a very fine tipping point at which the structure is pushed past the compressive arch or flexural range of response into the catenary action range of response. At this point the dynamic effects increase significantly due to a snap through type of effect.

Blast mitigation became of a global interest for architects and civil engineers with the increase of terror threats that targets key structures worldwide. The current work, which is largely exploratory in nature, involves blast wave traveling within a matrix of columns. The fluid structure interactions reach a maximum peak of vibration at a particular set of input parameters. Therefore, possible damage which affects structural integrity and reliability may occur. The presented numerical blast tests will accompany the study of different column cross-sections and orientation taking into account the charge weight and the stand-off distance variations. The effects of these variables have been quantified and compared to find a safety criterion for reinforced concrete buildings that might be subjected to blasting. Reliable and robust analysis (ANSYS and LS-DYNA) software will be used to complete a three-dimensional finite element model and conduct numerical testing. Analyst of this study will be able to find blast wave mitigation strategies to minimize the structural damage and/or progressive collapse and recommendation for safe design against blast loading. With the help of the guidelines mentioned in this study, the designer will be able to apply one or more of the mitigation techniques to build up a safer and more reliable model which if subjected to blast will have the minimum degree of collapse. This academic work also inspires researchers to develop more blast wave mitigation strategies using economical and practical solutions similar to those discussed in this study.

One commonly used method for designing elements to resist dynamic blast loads is single-degree-of-freedom (SDOF) analysis. This method uses an equivalent mass and stiffness and provides accurate displacement estimates for the center of the beam. This procedure allows for the consideration of different support conditions as well as various loading configurations. Although very practical for the calculation of displacements, the SDOF method does not provide member end reactions which are required for the design of the members' connections to the supporting structure. Simplified equations have been formulated to obtain these reactions based on force and moment equilibrium. These equations assume that the inertia forces always oppose the blast loads and that their distribution follows the first mode of the beam. This paper presents the results of analytical finite element (FE) analyses of a simply supported beam with a uniform blast load. These analyses show that the inertia forces do not always resist the blast load; in some instances they add load to the beam while the blast loads are still being applied. Additionally, it is shown that the inertia forces' distribution changes with time and that it is influenced by the first few modes of the beam. As a result of the behavior of the inertia forces, for beams undergoing high levels of ductility, the FE peak reactions can be significantly higher than reactions calculated with the simplified equations. The parameters that influence this response are presented.

Explosions in air create both an initial overpressure load (i.e., positive phase) and a subsequent underpressure load (i.e., negative phase), before conditions ultimately return to equilibrium. The positive phase load has traditionally been the focus of blast design efforts. While the magnitude of the exterior peak positive pressure is always higher than the subsequent negative pressure, the effect on loaded structural and façade components may not be fully represented by considering only positive phase blast loads. In addition, there are cases when negative phase infill loads may be significantly higher than positive phase infill loads. Design professionals need to understand negative phase blast loads and their corresponding impact on structural response. Herein we provide a brief review of airblast wave characteristics for high explosives (HE), discuss the impact of negative phase both analytically and based on explosive testing, and review typical design practices. Based on this review, we present key recommendations for design professionals and researchers in consideration of negative phase in testing, analysis, and design.

Design of concrete structural elements under near-contact explosive charges is commonly outside the scope of simplified single-degree-of-freedom methods. While the overall bending/shear failure of a structural member can be approached, the complex response such as breach or spall can only be predicted using discrete multi-degree-of-freedom approaches or experimental methods. Spall is a failure mechanism of a concrete element caused by the compression/tension wave traveling within the concrete material when the tension forces exceed the tension strength of the concrete. Breach is achieved when the extension of the spall exceeds the thickness of the concrete element. The experimental approach to predicting spall and breach was developed by the Department of Defense and presented in several papers and manuals such as the UFC 3-340-02 Structures to Resist the Effects of Accidental Explosions dated 5 December 2008. This approach defines the occurrence of spall/breach based on geometric parameters, and charge size and encasement. Thresholds for spall and breach are presented based on a suite of experimental data that documents blast tests performed on concrete elements with different concrete strengths and reinforcement ratios. While this approach is useful to predict the spall/breach failure of simple concrete elements, it has limitations when used as a design tool, as it does not account for the interaction between spall/breach and the overall bending/shear response of the element or the effects of reinforcement/steel plate protection to minimize breach/spall effects. This presentation introduces the spall/breach evaluation of concrete elements using the Applied Element Method, implemented in the non-linear dynamic structural code Extreme Loading® for Structures (ELS), and compares it with the predictions based on the experimental approach.

When subjected to an air blast, conventional window glasses of different sizes break into numerous flying shards which account for most injuries ranging from minor cuts to severe wounds and, in many cases, to death. A successful blast-resistant glazing design considers the principles of reasonable degree of protection against explosive threats based on the proposed level of security and previous lessons learned. It also requires balancing of the safety and security of the window panels with physical appearance and cost-effectiveness. In an air blast event, reducing the injuries from flying glass and protecting the occupants shall always remain a top design priority. For such, having more accurate glazing models is inevitable and thus, it is necessary to have a better understanding of the glass panel responses to different window parameters and blast load characteristics. For this purpose, we have modeled conventional annealed glass window panels of different sizes using explicit finite element analysis. The results show that the response of the window panels to different blast loadings is highly dependent upon the window pane sizes. Therefore, we have studied the window size effects and panel aspect ratios on window fragmentation patterns using brittle failure theories and crack propagation micromechanics of structural glazing subjected to air blast. These results will help developing a more comprehensive flying glass injury model that eventually enables the decision makers to bring the appropriate structural, architectural and building perimeter choices to better address the threats facing the safety of the residents during an air blast.

To advance blast mitigation technologies and their widespread use by the engineering community, it is necessary to develop engineering design guides that are based on sound analytical models of the static resistance function. Such resistance function models need to be verified in the lab using full-scale quasi-static testing procedures. The University of Missouri has lead the research effort for the past 14 years for experimentally evaluating the static resistance functions of various blast-mitigation wall systems using full-scale loading trees and a vacuum chamber. The focus of this paper is on the full-scale vacuum chamber system to demonstrate through examples how it was used to investigate various wall systems for developing engineering analysis and design models under blast loads. The results of various walls tested using the vacuum chamber will be presented and some will be compared to analytical models.

In this paper, the behavior of spliced steel girders under impact loading is investigated. A group of seven steel I-girders (one reference and six spliced) are tested experimentally. Each girder is subjected to impact at mid-span through a specially designed test rig. The test variables are: the connection type for the splice (bearing type or friction-grip type connection), the location and the number of splices along the span, and the falling mass and its height of drop. The impact force-time history applied to the girder, the dynamic deflection (deflection-time history) at different locations, the bending stress-time history at different locations, and reaction-time history are all recorded for each impact test. It is observed from the experimental results that the increase in maximum dynamic deflection is in the range of (6%) to (48%) of that for the reference girder.

Mitigation techniques are continuously being developed to ensure public safety in the event of intentional or accidental explosions. Building material fragmentation is a major cause of human injury during such events. Glass products are commonly incorporated into the façade system of modern buildings. However, glazing is extremely susceptible to producing airborne debris that might cause severe injury to occupants or damage sensitive equipment housed within the building. An ultra-high performance glazing material is being developed to provide superior protection during a blast event. The glazing material consists of laminated, transparent polymer panels, hence, the system is inherently transparent, which lends itself to an open architectural appearance, but also can be decorated as desired. Shock-tube and open-range blast tests indicate that the glazing system is able to withstand a peak blast pressure of 80 psi with an impulse of 380 psi-msec. Tests also show that this innovative glazing material possess large ductility which enable it to absorb blast energy without generating any airborne debris. Windows, once designed solely by architects, become a structural issue when explosive effects are taken into consideration. In designing windows to mitigate the effects of explosions they should first be designed to resist conventional loads and then be checked for explosive load effects utilizing a balanced design approach. Balanced design philosophy requires that the glass is designed to be no stronger than the weakest part of the overall window system. The glass should fail at pressure levels that do not exceed those of the frame, anchorage, and supporting wall system. If the glass is stronger than the supporting members, then the window is likely to fail with the whole panel entering into the building as a single unit. This failure mode may be more hazardous than if the glass fragments enter the protected space. Blast tests were carried out and finite element models were developed to predict the blast performance of this proposed glazing system. This paper will present the analytical simulations, experimental testing, detailing, and practical applications of this innovative glazing system.

This paper focuses on the analytical modeling performed in conjunction with high explosive shock tube testing performed at the University of Kentucky (U.K.). Two 5 ft × 10 ft curtain wall systems were modeled, of which one was tested by U.K. using a shock tube. The modeling efforts were performed using an existing commercial code, LS-DYNA. The results indicate that the dynamic response in terms of the center deflection of the glazing and the mullion were closely predicted using the numerical models. Additional investigations and comparison with field tests are recommended to improve the design of the curtain wall system and the predictions of the numerical model.

Double-façade curtain wall systems, where two layers of façade exist on the exterior of a building, have gained popularity in new and retrofit construction due to advantages in thermal performance, sound transmission, and exterior aesthetics. However, when the systems are used on government or high-profile buildings, they are often required to achieve a certain level of blast performance. Common blast engineering practice does not currently have a validated method to determine the pressure wave which may be transmitted due to "piston effects" short of computational fluid dynamics. An analytical study was carried out using the Arbitrary Lagrangian-Eulerian (ALE) formulations of the LS-DYNA explicit finite element code. Considering the example of an existing building undergoing an over-clad retrofit, the pressure-time history on the existing building face was calculated for varying blast loads, gap spaces, and window pane sizes. The exterior layer of curtain wall was designed to resist the blast loads without failure and so no direct venting of blast pressures occurs in the gap space, but the interior façade layer is loaded due to the "piston effect". The analytical modeling results were validated with test data from a series of previous full-scale shock tube tests. Previously developed design tools were evaluated and recommended modifications to these tools are presented. The results of this study are intended to aid designers with preliminary design of double curtain wall systems without need for upfront finite element analysis or physical testing.

In this paper we present structural analysis of an aluminum-backed curtain wall system to inform blast-resistant structural design practice. This analysis focuses on the expected material response of an aluminum curtain wall support to a set of typical blast loadings used in U.S. government criteria for domestic projects. Dynamic experiment test results of aluminum samples are used with a three-dimensional finite element hydrocode to account for over-strength, dynamic strengthening, and post-failure response of aluminum. The analysis shows that a curtain wall member under high-rate (blast) loading that is prescribed to fail using the recommended values of yield strength in the criteria documents, actually has very little evolved plastic strain. Further analysis shows that the combination of both the measured quasi-static yield over-strength and dynamic yield strength of specific aluminum evaluated exceeds the design recommendations prescribed in government design criteria in excess of 30% for the 6063 aluminum evaluated. The additional fidelity provided by the more accurate material properties improved the calculated performance of the structural member to the point that a member upgrade would not be required.

The Glass Failure Prediction Model (GFPM) is the basis for ASTM E1300 which is used across the nation for window glass design. ASTM E1300 states that the model is valid only for annealed (AN) glass but, ASTM E1300 does incorporate heat strengthened (HS) or fully tempered (FT) glass through the use of multiplication factors. Rather than multiplication factors, this paper will address modifying the GFPM to incorporate these types of higher strength glass. Designers can then incorporate the model into common design methods such as single degree of freedom (SDOF) and finite element analysis (FEA) to analyze dynamic performance of glazing layups. To validate the modified GFPM, both static and dynamic testing of high strength glass was completed and comparisons to analysis predictions will be provided in the paper. The paper also illustrates how to use the modified GFPM in window design for both static and dynamic applications through the use of Single-degree-of-freedom Blast Effects Design Spreadsheet for Windows (SBEDS-W), a SDOF design tool to be released by the U.S. Army Corps of Engineers Protective Design Center (USACE PDC). Dynamic window analysis tools like SBEDS-W will be utilized more frequently due to the new threat-based requirements for windows and doors in the Unified Facilities Criteria (UFC) 4-010-01 DoD Minimum Antiterrorism Standards for Building (9 February 2012). Also, an understanding of the failure prediction capabilities of SBEDS-W is important for engineers performing the dynamic glazing analyses.

This paper summarizes a research program undertaken to develop cost-effective solutions aimed at protecting new and existing bridge column bents against multiple natural and manmade treats such as earthquakes and blast. Concrete-Filled Double Skin Tube (CFDST) column and Modified Steel Jacketed Column (MSJC) designs were proposed and tested for new bent construction and retrofit, respectively. While CFDSTs maintained their cross-section geometry in dissipating energy under cyclic loading, under severe blast loading, they deformed locally to dissipate the applied impulse and have enough residual strength to prevent collapse. MSJC, on the other hand, were developed to eliminate a deficiency observed in the form of direct shear failure near the top and/or the bottom of the jacket for steel jacketed column (SJC) subjected to blast loading. The proposed modification increases direct shear resistance in the shear-deficient areas and does not interfere with the ability of the jacketed columns to resist earthquake loading.

In this paper we present design and detailing concepts of metal stud wall systems for achieving a predictable performance when subject to air-blast loading effects. These systems are commonly used for exterior cladding and interior partition systems and while they can provide both strength and ductility their performance is highly dependent on proper detailing to minimize buckling effects and connection failure. As a result, current practice assumes more conventional design and detailing methods and penalizes the designer with restrictive deformation limits that leads to labor intensive and costly designs. Due to lack of available testing data, current practice either restricts metal stud performance to low deformation limits or attempts to yield the section under full tension membrane action. The first approach is limiting in its application under even moderate blast loading without the need for heavy robust sections due to the minimal extent of allowable deformation, typically in the range of 1- to 2-deg. The second approach, while effective in providing high resistance to blast loading due to the absorption of energy through excessive deformation, requires specialized connections that are both costly and difficult to install. We focus on the transition region between these two responses, where the metal stud section has yielded and the performance is governed by the behavior of its connections and its ability to maintain stability under buckling effects such as web crippling and lateral torsional buckling. Using a combination of available test data and design examples, this paper examines the performance of metal studs within the plastic region. It discusses conventional construction methods and their ability to provide blast resistance, as well as provide design and detailing guidance for architects, engineers, and light gage framing sub-contractors to achieve a high level of performance, while minimizing cost and constructability issues.

This research seeks to determine realistic mitigation strategies to improve the performance of reinforced concrete slabs under close-in blast events. These extreme events can create pressures high enough to powderize concrete and leave an empty rebar cage. This paper will cover the design, construction, and testing of a reinforced concrete beam specimen. The specimen was strengthened with steel plating on the compression face and carbon fiber reinforced polymer pultruded plates on the tension face. Bolts were placed through the specimen to anchor each face. The static test results show that the mechanical anchorage system used was able to fully develop the CFRP plates. However, there was significant slipping of the CFRP leading to increased deflection.

This study focuses on the robustness assessment of a steel truss bridge. In the first part, a brief overview of several robustness indexes found in literature is provided, together with the principal approaches on the topics of structural robustness, collapse resistance and progressive collapse. In the second part, the extensively studied I-35W Minneapolis steel truss bridge is used as a case study for the application of a consequence-based robustness assessment. In particular, focus is given on the influence that the loss of primary elements has on the structural load bearing capacity.

Due to the use of discrete connections, traditional precast and prestressed concrete structures are often susceptible to disproportionate collapse. This perceived weakness discourages the use of precast systems for high-risk facilities such as those used by the United States Federal, State, and Military groups. This paper examines methods that can be utilized to provide resistance to disproportionate collapse using precast components. The prevailing approaches will be summarized and the implications for precast concrete building systems will be discussed. A background on the mechanism of collapse, examples of past failures and successes, comparisons of the recommended approaches, and examination of the applicability of these approaches as they apply to precast concrete construction will be covered. A summary of the Precast/Prestressed Concrete Institute recommendations currently under development for the 8^th Edition of the PCI handbook will be provided.

Long span bridges are complex structural systems, often having strategic roles in the network infrastructures; consequently, their susceptibility to a disproportionate response in case of local failures needs to be assessed. In particular, current regulations prescribe that the structural robustness should be maintained in case of an accidental hanger detachment. Local damages in bridges, which are characterized by an horizontal load transfer system, may progress along the deck or along the suspension system, as the dynamic overloading of the structural elements immediately adjacent to the failed ones may lead to subsequent failures. In suspension bridges, which are characterized by a relatively low continuity of the system, the damage of the deck may favor a collapse standstill, in case of an early detachment of the deck collapsing section. In the paper, a long span suspension bridge is taken as case study and the robustness of the system is investigated with respect to the entity and the location of an initial damage of the hangers. The analyses are carried out with the avail of a finite element model (FEM) of the bridge, which accounts for mechanical and geometrical nonlinearities as well as for the dynamic effects of the abrupt failures. Critical aspects of the design are highlighted and possible countermeasures for enhancing the structural response are suggested.

Since the World Trade Center disaster in 2001, various theories have been proposed as to what caused the observed rapid collapse progression ending in the total collapse of both towers. According to the theory now widely accepted, the columns in the aircraft impact zone lost their load-bearing capacity due to the effects of fire. As a result, the upper part of the building fell over the height of at least one story. The resulting impact forces greatly exceeded the buckling load of the columns near the impact zone, which led to the release of a new portion of potential energy. It was also shown that the energy absorbed by the columns during buckling was significantly less than the potential energy released during this process. This led to the conclusion that the total collapse was inevitable once initial failure occurred. However, there are some examples of buildings, in which the columns of an entire story failed and no collapse progression occurred. Such cases were observed in the 1995 earthquake in Kobe, Japan. There were also some failed controlled demolition attempts, in which a progressive collapse to be triggered by the destruction of vertical load bearing elements in lower levels did not occur. No significant deformations of vertical elements were observed in these cases. This raises the question if there are energy absorption mechanisms other than column buckling that may play a role in the structural response to initial failure. Numerical simulations of simple systems are presented showing that there are such mechanisms indeed and that column buckling is not the only possible way to absorb the energy at impact. An analytical approach based on a simplified model is also described.

Engineered trusses, made from a variety of materials, have a wide range of applications in civil engineering. A large part of them is used to realize entertainment structures; their structural safety gains, therefore, much more importance due to the people that can be involved in the collapse, (Giuliani, 2012). The latest collapses of those particular structures in USA and in Europe brought the attention of the society on the study of their safety. In this paper a metal truss structure is taken under consideration in order to identify the causes that could lead the structure through a collapse. In this specific case the errors in the structural design and the human errors during the erection of the structure are taken under consideration. By means of nonlinear analyses on a Finite Element Model (FEM) the failure sequence of this particular structure will be shown.

Movable bridges are one of the least studied bridge types. In this paper, examples from a movable bridge evaluation study are presented based on the research conducted on a movable bridge in Florida over the last several years. Movable bridges face operational and maintenance challenges mainly due to complex structural, mechanical and electrical systems which, at the same time, provide their versatility. Although there are a few studies focusing on movable bridges, none of these studies provide a complete list of the problems related to the condition of movable bridge populations in conjunction with possible monitoring applications specific to these bridges. This study summarizes these issues related to movable bridges considering both the structural and mechanical components. After presenting the design and implementation of a monitoring system to a representative bascule bridge, analysis of image data for evaluating the lubrication levels in an open gear is presented. The findings from this analysis are compared with the maintenance logs. It is shown that continuous monitoring may provide invaluable information about safe, reliable and cost-effective operation and maintenance of movable bridges.

Results of two doctoral theses, one using a model-based approach and the other using model-free methods, are summarized. Both included case studies of full-scale bridges. While model-based approaches are appropriate for individual infrastructure-management cases where the cost of modeling can be justified, model-free methods provide inexpensive anomaly detection for long-term monitoring of many structures. Both are compatible with civil-engineering contexts of modeling simplifications, incomplete knowledge, noise, outliers and missing data. They provide useful additions to an engineer's data-interpretation toolbox.

Vibration of a steel truss bridge is monitored throughout a year in different environmental and operational conditions in order to identify the dynamic characteristics of the bridge through modal testing, and also to observe the variability of results due to the changes in environmental and operating conditions of the bridge. For this purpose, the vibration of the bridge is measured multiple times in different seasons while the temperature and the number of vehicles passing the bridge were recorded at the time of each test. Having the modal parameters identified from separate data sets, a statistical study could be performed to provide the mean and standard deviation of the identified parameters and also determine the possible dependency of the identified parameters on the mentioned variables. Furthermore, to observe the dependency of the estimated modal parameters on the selection of identification algorithm, one set of measured response is passed through different time- and frequency-domain algorithms and the obtained results are compared and their variation is extracted.

Fiber research in concrete construction is an ongoing field and the use of carbon nanofibers (CNF) will be examined in this study. Fibers improve brittle materials such as concrete by enhancing tensile strength, ductility, toughness and conductivity. Short-fiber composites are a class of strain sensor based on the concept of short electrically conducting fiber pull-out that accompanies slight and reversible crack opening. For a fiber composite to have strain sensing ability, the fibers must be more conducting than the matrix in which they are embedded, of diameter smaller than the crack length, and well dispersed. Their orientations can be random and they do not have to touch one another. The electrical conductivity of the fibers enables the DC electrical resistivity of the composites to change in response to strain damage or temperature, allowing sensing. Because of the high cost associated with CNF, a CNF aggregate (CNFA) was developed. The CNFA is 2.54 cm x 2.54 cm x 2.54 cm (1.00 in. x 1.00 in. x 1.00 in.) so that it is roughly the same size as a normal aggregate found in the concrete matrix. The CNFA consists of a mortar containing dispersed CNF, is self-sensing, and can be used to determine the damage in the CNFA. The CNFA can be embedded in reinforced or prestressed concrete structures and used to determine the localized strain in a structure for implementation of structural health monitoring in full-scale bridges and buildings.

This study is focused on detection of damage as added mass on a pedestrian bridge using a finite element model updating strategy. The Dowling Hall Footbridge, located on the Tufts University Medford, Massachusetts campus, is equipped with a continuous monitoring system that measures the ambient vibration response of the footbridge. Structural damage was physically simulated to the bridge through the addition of mass in the form of concrete blocks placed on the deck of the bridge. Changes in physical properties due to the simulated physical structural damage are detected through changes in the identified modal parameters. The location and extent of simulated damage (added mass) on the bridge is successfully estimated using a finite element model updating method. The variability of the "damage" identification results due to different sources of uncertainty is discussed.

A bridge's performance depends on different factors including superstructure type, support conditions, construction materials, environmental conditions and external loading. Identifying and evaluating the loads applied to a bridge, during its effective life, are necessary for assessing the condition and remaining life of that bridge. Furthermore, the influence of dynamic loading on the structural response of the bridge is important to be considered for the safety of the structure. Although ambient loads can be helpful to assess the changes in bridge condition as monitored over time, applying identified loads allows for a more clear understanding as to how the bridge responds to different types of loads. As a part of the Long Term Bridge Performance (LTBP) program, a bridge located in Perry, Utah was subjected to four sets of high speed, live-load tests. For evaluating the bridge response to different types of loads, these live load tests were conducted with four different truck configurations that traversed the bridge in both right and left lanes. In addition, the different trucks traversed the bridge at three different speeds to make it possible to assess the effect of dynamic response of the bridge. Moreover, these different trucks varied in weight and axle spacing. The data were collected through multiple types of sensors, located on different points and some co-located on same points. The sensors used in this study are strain gauges and the response of the co-located sensors was compared to evaluate the reliability of the results. Two different B-WIM methods were used to calculate the speed of the trucks. These methods include the correlation of signals and the derivative of the strains. The performance of these methods for evaluating the accurate speed was studied. The axle distances of the trucks were also calculated through the derivative of the strains. Through these B-WIM algorithms and the long-term monitoring system installed on the bridge, the traffic on the bridge and the bridge dynamic response to the truck loadings can be quantified with very simple and inexpensive calculations.

With advances in application programming interfaces (API) for modern computing, the traditional boundaries between different standalone software packages hardly exist. Now complex problems can be distributed between different software platforms with advanced and specialized capabilities. To address the evolving needs for structural health monitoring, an open source framework, called PARIS (PARameter Identification System) has been developed for automated finite element model updating of full-scale structures. The model updating capabilities of PARIS are validated using simulated nondestructive test data of two full-scale structures with typical structural damage scenarios. Substantial new insights into such computing with realistic structural models are presented.

Three Coldwater River crossings located on federal highways in north Mississippi are selected for study of seismic vulnerability. Regional risk analysis has highlighted the critical nature of the highway bridges in the transportation network. Finite element models of varying complexity are developed for these structures to enhance the results of the fragility based risk study. The results are used to establish potential vulnerability incorporating site specific soil and design data based on seismic response analysis. Time history and pushover analyses are performed to estimate axial forces, shear forces, and bending moments in piles and columns of typical bent substructures. The results are compared to substructure design and test pile data. Complex interaction of axial force and bending is observed in the batter piles of typical intermediate bents during lateral shaking at levels far exceeding the design capacities which did not account for seismic demand. Uncertainties in the model assumptions and soil data indicate a more comprehensive study considering the influence of liquefaction is recommended.

The knowledge about the deteriorating characteristics and future operation conditions in civil infrastructure systems is never complete. Any relevant decision-making framework must include a rational approach for quantifying these uncertainties and their bearing on the decision making process, through the entire life-cycle of operation. Health monitoring data, when available, may be used to update the probabilistic quantification related to these uncertainties. This work presents a Bayesian framework for updating the assessment of bridge infrastructure systems through use of monitoring data with focus on the deteriorating characteristics of bridges. Stochastic simulation techniques are proposed for the Bayesian updating and various model classes are examined for the bridge system, describing different possible deteriorating models. The updating of the relative likelihood for each of the models through monitoring data is also investigated.

Civil inspection images are notoriously difficult to interpret for those who were not present during an inspection, and so they have historically had limited value in ongoing condition monitoring applications. Within the last decade, the advent of inexpensive digital imaging has compounded this problem by making it possible and common to capture and report numerous images, creating information overload that can easily overwhelm engineers and structure owners. More recently, developments in electronics manufacturing and robotics have made automated structural inspection an emergent technology for civil engineers, which inevitably is leading to even more digital image data that must be handled. This paper presents a systematic view of capturing, processing, and representing inspection image data such that the inherent value of these increasingly large data sets can be realized. Using a data pipeline combining automated image capture, contextualized 3D visualization, and robust computational imaging and data fusion techniques, the goal is to allow engineers to view inspection images in their 3D spatial context, while aiding them through enhanced damage detection routines, all while helping to minimize field inspection disruptions through the use of robotic imaging. In the development of this pipeline and an associated prototype implementation, several key issues have been addressed: (i) automated systems capable of comprehensive field imaging; (ii) 3D reconstruction algorithms which provide accurate, photorealistic image interpretations; (iii) robust computer vision algorithms suitable for field applications; and (iv) data fusion models which correlate the relationships between extracted image information and structural performance. The results of prototype testing show that, given due consideration to the inherently large data sets that robots produce, automated inspection can enable entirely new ways of visualizing and interacting with inspection information.

Dynamic testing is an effective approach for quantitatively characterizing in-service bridges. The two main approaches used for the dynamic characterization include experimental modal analysis and operational modal analysis. In the former, the excitation is controlled and measured and both the inputs and outputs are used to identify modal properties while in the latter, the excitation is uncontrolled and only the measured outputs are used. Operational modal analysis is by far the most commonly used approach for in-service bridges due to cost and logistical advantages. The writers have developed a new dynamic testing approach that utilizes a network of low cost dynamic exciters that may be suitable for controlled dynamic testing of bridges. The performance and capabilities of this system were evaluated through a series of laboratory studies and from a dynamic test of an in-service bridge. The small-scale dynamic exciters were found to have operational characteristics that are compatible for dynamic testing of short to medium span bridges, and the bridge test demonstrated that a system of these devices could dynamically excite a full-scale structure in a controlled manner to produce larger and more uniform vibration responses than those produced from natural sources and traffic.

This paper presents a novel method for interpreting data to improve the indirect structural health monitoring (SHM) of bridges. The research presented in the study is part of an ongoing study aimed at developing a novel SHM paradigm for the health assessment of bridges. In this paradigm, we envision the use of an instrumented vehicle that assesses a bridge's dynamic characteristics while traveling across the bridge. These characteristics are then correlated to the health of the structure by means of advanced signal processing and pattern recognition approaches. In this paper, we present and compare two classification algorithms that locate the presence of damages at well-defined locations on the structure: sparse representation and the Fourier discriminant methods, and find that the sparse representation method provides superior classification accuracy.

The longevity of a large number of reinforced concrete bridge piers has been compromised as a result of Alkali-Silica Reaction (ASR) and Delayed Ettringite Formation (DEF). These structures need to be assessed for strength and performance to ensure their general load capacity and safety. It is shown that present code-based methods of analysis, whether they be the sectional method or strut-and-tie methods, are unable to discriminate between these concrete elements without and with the effects of ASR/DEF. A Compatibility Strut-and-Tie Modeling (C-STM) technique is therefore used. Prestress forces are applied at certain truss nodes to mimic the swelling effects of ASR/DEF damage to the concrete. The computational C-STM approach is verified against several large-scale experimental tests. It is observed that the C-STM predicts the overall force-deformation behavior of the structure without and with ASR/DEF effects with good accuracy.

Scour has been reported as the cause of approximately half of bridge failures in the United States. It alters static and dynamic characteristics of bridges, which can lead to excessive deflections, and subsequent bridge failures. To estimate the response of bridges under scoured conditions, comprehensive models of representative bridges that overpass waterways are developed in this study. The bridge models account for i) the nonlinear soil-pile-structure interactions, ii) the nonlinearity inherent in abutments and columns, and iii) the contribution of superstructure. Different types of scour, including local scour, general scour, and aggradation and degradation are investigated and their effects on the bridge structural system are quantified. In addition, a set of scour scenarios are introduced to represent various scour intensities that a bridge may experience during its life cycle. The outcome of this study is an efficient approach that can be used for the design and assessment of bridges subjected to extreme flooding conditions.

This paper investigates the implementation of multi-scale numerical method for the analysis of the complex mechanical behavior of the steel cable anchorage system in self-anchored suspension bridges. A multi-scale model of the Taohuayu Bridge (THY Bridge in brief), which is the largest self-anchored suspension bridge, is developed based on the design drawings of the actual bridge. The software ABAQUS was selected to realize the multi-scale finite element modeling of the THY Bridge for the purpose of wide applications in civil structures. An experimental scale model of the steel cable anchorage system was manipulated and tested based on the prototype of the THY Bridge to verify the multi-scale numerical model. Two elaborate FE models, including a scale model and a full-scale model, are established for comparison based on conventional elaborate modeling methods. Extensive comparison study of the test and various FE models demonstrate that fuzzy region exists in the results of the test model, scale model and full-scale model, and the fuzzy region cannot be quantitatively determined, leading to unreliable analysis results and enormous try and error works. By employing the flexible BCs, which is a kind of generalized boundary condition proposed in the paper, the multi-scale model can accurately predict the complex mechanical behavior of the cable anchorage system. The multi-scale modeling method may become a popular approach to investigate the cable anchorage system by modeling the entire self-anchored suspension bridge in the future.

Temperature effects significantly reduce load ratings of reinforced concrete open-spandrel arch bridges, particularly when temperature drop is included in the analyses. Because of the non-linear axial force-moment (P-M) capacity interaction behavior, lower dead load factor results in a lower rating factor. For a symmetrical structure, the eccentricity (moment to axial force ratio) at every location in an arch rib due to uniform temperature changes is geometry dependent and it is an important factor affecting load ratings. The uniform temperature effect is more significant for bridges with lower rise to span ratios (flatter geometry) because the eccentricity is higher. Based on limited field measurements uniform temperature is observed only in the early morning after the heat has dispatched overnight and before the structure is subjected to solar heat. Vertical temperature gradient in the arch rib significantly depends on the exposure to the sun. The vertical temperature gradient in each rib does not appear to follow any pattern. Additional data will be needed to develop an applicable procedure to include vertical temperature gradient for open-spandrel arch bridge rating analyses.

Two steel I-girder bridges were field tested with a primary objective of recording cross-frame forces under a weighed vehicle. Because previous research has suggested that skew angle is a significant variable affecting cross-frame forces and the role of cross-frames becomes more significant in skewed bridges, one bridge has a skew angle of 32 degrees while the other has a skew angle of 65 degrees. The bridges were instrumented to record cross-frame stresses in approximately 50 locations per bridge and girder stresses in 12 locations per bridge while the bridge was loaded by a weighed vehicle in various positions. The results show that cross-frame forces increase with skew and can be significantly larger than girder stresses. The field testing is also reproduced using a finite element model for the purpose of performing a parametric study to further explore cross-frame forces in steel bridges in future work and discussion of this modeling is also presented.

Seismic issues are international interests. Non-linear Time History Analysis simulates the structure behavior under severe Earthquake movement more properly than other methods. In this paper, Non-linear time history analysis will be presented with one of the world famous project. Yerba Buena Island (YBI) West-Bound (WB) Ramps are portion of San Francisco-Oakland Bay Bridge Project. The Ramps are touched down to Yerba Buena Island (YBI) from YBI WB Widening by hinges. This project is located at a 0.627g Site Specific Response for Safety Evaluation Earthquake (SEE). YBI WB On-ramp is a highly horizontally Curved Bridge with radius of 127.3 feet (38.8 meters). Seismic behavior is very important for the project. In order to understand structural non-linear behavior, especially highly horizontally curved bridge behavior under severe earthquake events, YBI WB On-ramp stand-alone bridge is analyzed by non-linear time history analysis method. SAP 2000 with Hilber-Hughes-Taylor α direct integration method is used for Non-linear Time History Analysis (NL-THA). Seismic Modeling is discussed in this paper. Then six sets of acceleration time histories are used for NL-THA by SAP 2000. Open System for Earthquake Engineering Simulation (Open-SEEs) program developed by University of California-Berkeley is used to verify the SAP NL-THA accuracy in this paper. Site Specific Response Spectra ARS is also used for liner analysis and compared with results of Non-linear Time History Analysis and Open-SEEs Analysis. Non-linear push-over analysis is performed to determine the structural capacity and ductility under severe earthquake events. Finally, discrepancies between different program analyses are discussed and recommendations are presented.

This study pertains to the vehicle-bridge interaction analysis of Homer M. Hadley Memorial Bridge located in Seattle, Washington. The bridge carries the westbound and reversible lanes of Interstate 90 across Lake Washington and is the widest and fifth-longest floating bridge in the world. Based on the proposed plan to design and construct a light rail transit system for the bridge, a detailed three dimensional finite element model of the west spans of the bridge was developed for dynamic analysis. The bridge model includes a fixed span, floating span and transition span connecting the fixed span and floating span. Two track bridge structures (one at each side of the transition span), anchors and a set of nonlinear springs to simulate variable buoyancy forces are presented. An innovative modeling approach based on bridge-vehicle coupled system was proposed to evaluate the bridge behavior under moving train loads. The light rail vehicle was separately modeled as a coupled system with rail, which includes the flexibility of the rail fasteners as well as dynamic characteristics of the vehicle. The train is a four-car consist composed of a three-section passenger vehicle with articulation joints between the bodies. The suspension system of the vehicle was modeled by nonlinear springs and dashpots. The vehicle was moved over the rail profile dynamically. Wheel reactions were extracted and applied to the rails in the bridge model. The floating bridge model was analyzed for a set of loading scenarios including different storm conditions and vehicle speeds. The vehicle accelerations, rail stresses, track bridge displacements and stresses were extracted.

Two different support configurations have been used in the seismic design of the State Route 21 over Interstate 69 Bridge and the Forrester Road over Interstate 69 Bridge, one employing seat-type abutments with rigid bent foundations (Basic support configuration) and the second employing stub wall abutment with flexible bent foundations (Nonlinear Spring support configuration). The accuracy and efficiency of each support configuration in terms of maximum seismic displacement demand and the displacement ductility has been evaluated using the SAP2000 computer program. The results indicate that using the Basic support configuration could provide more conservative results, while needing less modeling effort.

As bridges in the Central and Eastern United States approach their design life, deterioration due to corrosion becomes a significant concern. Steel components such as bearings are susceptible to changes in performance as a result of corrosion, particularly under seismic loads, putting the region's transportation network in jeopardy. To better assess the seismic performance of older highway bridges, an understanding of the behavior of corroded components, such as steel bearings, under seismic loads is needed. This paper examines the level of corrosion observed with a group of salvaged steel bearings, including rocker and bolster bearings, that were in use for over 50 years prior to replacement. Data from the measurement of corrosion-induced mass loss for the rocker bearings is provided to quantify corrosion level. A detailed account of the experimental setup to be used to correlate corrosion level with seismic performance also is provided. Preliminary findings from finite element simulations are briefly summarized with an explanation on modeling contact and friction inherent in the mechanical behavior of steel bearings.

This paper presents the structural behavior of composite girders composed of glass/carbon fiber reinforced polymers (GFRP/CFRP) I-profiles and precast Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) slabs. Three full-scale composite girders with varying UHPFRC slab widths were tested under four point loading to examine the flexural behavior of the composite girders. Shear connectors with/without epoxy bonding were used. The bolt shear connectors and epoxy were used to resist the horizontal shear flow at the interface between the hybrid CFRP/GFRP (HFRP) I-profile and the UHPFRC slab. The composite action between the HFRP I-profile and UHPFRC slab was investigated. The test results showed that the composite girders exhibited significant improvements in stiffness and strength, above those of single HFRP girders. Three-dimensional nonlinear finite element (FE) analysis was carried out and compared against the experimental results of the tested girders. A fairly good agreement among the experimental and FE analysis results was found. The study revealed that the HFRP-UHPFRC composite girders are efficient and can provide a competitive, cost-effective and sustainable solution to bridge structures.

A major application of composite materials in the reinforced concrete industry is to wrap concrete columns with Fiber Reinforced Polymer (FRP) composite fabric which results in increase in the columns' axial compressive strength and ductility. Presence of debonds between the FRP wrap and the underlying column surface inhibits the proper confinement of the column by the composite wrap. This research studies the effect of debonds on stress-strain behavior of FRP wrapped concrete cylindrical specimens. All laboratory tests were conducted on concrete cylinders of size 6'' x 12'' (152.4 mm x 304.8 mm). Simulated air-filled debonds, water-filled debonds and debonds made from Teflon sheets of various sizes were placed on the concrete cylinder's surface prior to wrapping with carbon or glass FRP fabrics. The FRP wrapped cylinders were tested to failure in compression, and the effect of debond area in terms of percentage of total area on the stresses and strains of the concrete cylinders was established. The results showed a significant decrease in axial compressive strength with an increase in area of debonds, and appropriate reduction factors have been proposed for use in strength computations. Both GFRP and CFRP composites were found to be effective in arresting the lateral dilation of the underlying concrete. Also, considerable ductility enhancement was evident in the FRP wrapped cylinders, particularly with CFRP composites.

Great manmade structures and spaces exhibit a universal truth. They incorporate a unified form and a creative vision based on shape, scale and geometric rules. Aesthetics and why certain manmade structures, forms, and art appeal to most people has been pondered and studied for over 2000 years. The ancient Roman architect Vitruvius begins the second chapter of the first book of his The Ten Books on Architecture with the statement that architecture depends on order, arrangement, eurythmy, symmetry, propriety, and economy. There is every reason to believe that Vitruvius was stating a commonly and widely accepted view in his emphasis on order and eurythmy, which is proportion and symmetry. A theory is that the appeal of certain architecture is grounded in nature. It appears that the architectural rules for the "perfect" buildings, the different members and forms must be in a comfortable scale, relation and sometimes symmetrical relation to the whole.

The seismically vulnerable East Span of the San Francisco-Oakland Bay Bridge will be replaced with a dual east bound and west bound 3.6 km long parallel structure. The iconic single tower asymmetric self-anchored suspension bridge was selected by an international panel of experts from a total of four design alternatives that were developed for the signature main span. Analytically, geometrically, and in the design of practical details for fabrication and durability, the Self Anchored Suspension Bridge is a unique structure requiring innovative solutions and departures from conventional design. Whether a similar bridge of this scale will ever again be proposed is unknown. However, the set of methods and technologies developed to meet these challenges offer an illustrative study of designing a world class structure, an architectural icon, and a seismic innovation all at one time in one of the most seismically challenged areas in the world. The bridge will be open to traffic by the summer of 2013.

Topic/subtopic: Transportation (Bridges)/Rehabilitation Target Audience: Bridge engineers/design practitioners, builders, educators and officials; professional and amateur historians; those interested in bridge design aesthetics. Take-Away Lessons from this Presentation: Two contrasting design approaches for arch bridges; an aesthetic program for a bridge rehabilitation design that is responsive to its historic context; and an interesting story about a historically significant bridge. The purpose of this project was to improve the Main Street crossing of the Dan River in the City of Danville. The existing seven-span, 823-foot long, four-lane concrete arch bridge carrying Main Street was designed by Daniel Luten and constructed during the late 1920s. It had deteriorated significantly and was in need of rehabilitation or replacement. Major traffic capacity improvements were needed on the crossing as well as the intersection at the north end of the bridge. Additional objectives were to maintain the historic integrity of the existing bridge and to affect as few businesses and residences as possible. The bridge is a contributing element to four historic districts that surround it. The Main Street/Riverside Drive intersection immediately north of the bridge was widened to accommodate future year traffic. Additional southbound lanes were constructed on a new two-lane concrete arch bridge located west of the existing bridge. Each span includes four parallel arches. The floor system is framed with AASHTO girders spanning from arch crowns to the adjoining substructure. The historic identity of the existing Main Street bridge was maintained by rehabilitating it in place. Existing concrete arches and piers were repaired while the spandrel columns , floor system and deck were replaced, in kind, with cast-in-place concrete. Its appearance will remain similar to the existing bridge by virtue of the arches and piers remaining. Architecturally designed form liners and pigmented concrete were also utilized. The intent of the aesthetic design is for both bridges to share common elements while having their own identity. A custom designed, open handrail was constructed on both bridges. The project was completed during 2006.

In the fall of 1949, a new bridge near Smithville, TN, carrying relocated SR56 over Caney Fork River was opened to traffic. Begun in fall 1947, the 1,786-foot-long structure rose 274 feet above the riverbed. The bridge construction and road relocation was necessary to accommodate closure of the newly constructed Center Hill Dam and subsequent 64-mile-long, 190-foot-deep impoundment of the river. The bridge, designed for AASHO H15-44 live load, was constructed by the Nashville District of the U. S. Army Corps of Engineers. It consists of two 18-panel, 771-foot-long, cantilevered deck trusses, each with 8-panel suspended deck truss spans and two 61-foot-long deck girder spans at each approach. The Warren type trusses are 24 feet curb-to-curb (c/)c and are 60 feet deep above the 215-foot-tall main piers. The bridge was originally constructed with a 24-foot, curb-to-curb roadway and 18-inch-wide safety curbs. In 1977, the bridge was widened to support a 44-foot, curb-to-curb roadway to improve sight distance at the entrance to the newly constructed Edgar Evins Appalachian Craft Center located near the east end of the bridge. Widening was accomplished by adding truss floorbeam cantilevers to support one new stringer line each side and by widening the approach piers and abutments to support one new line of beams each side. Analysis of the 1977 modification revealed the bridge could accommodate AASHTO H-15 at Inventory level and AASHTO HS-20 at Operating level. These live load capacities were acceptable considering the low volume of commercial traffic on the route at that time. In 2010, due to the Federal mandate to re-rate all deck truss bridges including gusset plate connections, the ravages of time on the condition of the bridge, and the 15 ton posting causing lengthy detours for heavy trucks and busses, Tennessee Department of Transportation (TDOT) decided to engage the firm of Modjeski and Masters, Inc. to re-analyze the bridge and provide strengthening recommendations to upgrade the capacity for AASHTO LRFD HL-93 live load. Extensive three-dimensional truss analyses were completed for design of strengthening to support the increased loading and to validate construction staging for minimal through traffic interruption. A thorough review of truss gusset plate capacities was also completed. Temporary support of several truss member webs was required during removal of lacing bars and attachment of new cover plates. Deficient gusset plate capacity at multiple locations required temporary support of truss panels while truss members were cut and spliced during installation of gusset "doubler" plates. Specific construction staging was detailed for removal and replacement of existing rivets and/or bolts with new high strength bolts prior to strengthening of the trusses. A supplemental support system was designed for installation on fracture critical pin-ended truss suspenders. External post tensioning was installed to supplement the capacity of the top strut of an approach span four-column pier bent. Using lightweight concrete for the deck slab and barriers, providing new stringers, reinforcing floorbeams, and with truss member and gusset plate strengthening the desired HL-93 live load capacity was achieved.

The reinforced concrete bridges located in coastal areas are exposed to deteriorating processes during their service lifetime. The performance of such bridges is highly dependent on the extent of change in properties of concrete and reinforcing bars. Therefore, it is necessary to employ reliable approaches to predict the capacity losses due to the deterioration mechanisms. Towards this goal, the current paper studies the effects of chloride-induced corrosion on the capacity of bridges. Considering the fact that the structural degradation occurs after corrosion initiation, this paper develops a computational framework to estimate corrosion initiation time through deterministic and probabilistic approaches. Although the deterministic approach significantly reduces the uncertainties associated with the simplifying assumptions, it does not consider the uncertainties corresponding to the model parameters. Hence, the effects of parameter variability are studied by the probabilistic approach. The probabilistic estimation of corrosion initiation time is used to predict the remained capacity of a set of case-study bridges.

Using state-of-the-art design methods and employing "lessons learned" from other projects, the $48M rehabilitation of the Rankin Bridge was complicated and complex. A four-lane, twelve-span, 2400' deck truss spanning the Monongahela River in Allegheny County, PA, originally constructed in 1949, the structure offers a notable example of steel bridge technology from that era - riveted construction, deck trusses, and girder-type approach spans. The rehabilitation project began in late 2008 and was substantially completed in December 2010. It contained elements typically associated with repairing and upgrading a 60-year-old bridge; i.e., deck replacement and widening, expansion dam replacement, repair of deteriorated steel and concrete components, and repainting. It also presented design and construction challenges that included the following: ≤ Truss Gusset Plates - inspection, load rating, retrofit. ≤ Truss Strengthening (49 members) - use of several analytical methods (ASD vs. LFD); design; installation. ≤ Dead Load Reduction - modified typical section (lane and sidewalk configurations); use of lightweight concrete. ≤ Global Redundancy - addition of supplemental devices at tie-downs of anchor spans and pin/hanger ends of suspended truss span. ≤ Truss Rocker Bearing Retrofit - addition of seismic backup devices. ≤ Phased (half-width) replacement of a two-girder approach span using a temporary girder. ≤ Widening of an abutment and wingwall with tangent caissons to accommodate the wider deck. ≤ Stabilizing both abutments and one pier with the installation of passive tie-back anchors.

The widening and rehabilitation of the Homestead Grays Bridge required a number of unique design solutions and the creation of several innovative construction techniques. The 3,10-foot-long, four-lane deck bridge, comprised of 17 spans and two intersecting ramps, spans the Monongahela River, railroad tracks, two streets, and The Waterfront, a 260-acre, mixed-use development located beneath the bridge. The structure is the first and one of the few bridges in the world constructed with Wichert Trusses. One of the major challenges was that all construction took place while maintaining two lanes of traffic on the main bridge. Some of the other issues were replacement of through girders, floorbeams and seismic vulnerable bearings while maintaining traffic which required innovative temporary support and jacking system, retrofitting of the existing foundation using lightweight flowable backfill, reinforcing of existing failing retaining without removing the exsiting wall and reuse of the existing 70-year-old concrete filled grid deck and pedestrian railing. This paper will present the unique design solutions and innovative construction techniques required to widen and rehabilitate this 3100 foot-long Wichert Truss bridge with two intersecting ramps. While maintaining two lanes of traffic at all time through girders, floorbeams, expansion dams and bearings were replaced. The reuse of the existing 70-year-old concrete filled grid deck and pedestrian railing will also be discussed.

Rapidly deployable mobile bridges are of direct relevance to military and civilian agencies. The US Army uses such bridges in a broad range of applications including assault, tactical, and general communication. Civilian use extends from temporary infrastructure improvements during natural disasters to equally critical planned situations. This paper describes a modular bridge system developed primarily for use in assault missions where rapid deployment is vital, but this design concept is also relevant in civil applications. The robust metallic design of the bridge is based on an aluminum trussed arch reinforced by steel cables. The segmental form of the structure allows it to be folded into a compact package for transportation in a standard 40-ft. container which facilitates shipping by road, rail, sea, or air (C-130 transport aircraft). The bridge segments also facilitate adaptation of the bridge to different spans ranging from 8 m to 32 m. The load rating for these spans varies, respectively, between 100 T and 40 T. The preferred deployment approach for the assault configuration uses a scissor method in conjunction with a winching mechanism mounted on an auxiliary bridge guide. Compared to more conventional techniques, this method reduces the demands on the bridge launcher. The complete system provides an integrated solution to cover a broad spectrum of bridging applications. Extensive computer analysis was used to refine and optimize the components in the bridge. Structural testing of components during the design stage also helped guide the design process, and operational and load testing of a prototype system using strain gaging confirmed the integrity of the final design.

This project is a good case study of an oversize vehicle rating of bridges that is beyond the usual scope of most, if not all, state departments of transportation. This presentation/paper will address the challenges faced, while presenting the methods and procedures used to handle the project. The project involve rating over 50 bridges in total for a specially fabricated super overload vehicle weighing over 715 tons transporting a steam generator from California to Utah. The vehicle consists of more than 33 axles, with each axle weighing 51,000 lbs. AASHTO Load and Resistance Factor Rating (AASHTO, 2008) procedure was utilized, with slight modification to several factors in accordance to California Department of Transportation. The unique challenge for the project was safely and efficiently moves the oversized vehicle. The route chosen for the move consists of some old bridges build between 1940's to 1980's, as well as some new bridges recently builds. With the mixture of new and older bridges, creative analytical methods were employed in order to account for the variation in steel used (Grade 40 versus Grade 60), concrete curing differential, and other factors to be discussed in detail.

As the transportation infrastructure in the United States is aging and expanding rapidly, demolition and replacement of the existing systems has increasing importance. Highway bridge demolition has become increasingly significant as the bridges reaching their service design life require rehabilitation or replacement. Highway widening to meet capacity increase also involves partial or total removal of highway bridges. This paper presents the fundamentals of highway bridge demolition by explaining the systematic deconstruction, crane usage, and the engineering involved with this process. Systematic deconstruction is the art and science of preventing unplanned potential energy release during a demolition process. Engineering is needed for the deconstruction process in order to protect the lives of the workers and in order not to damage any other structure in the close proximity of the structure to be demolished. In addition, the expensive machinery used for the deconstruction process needs to be protected. As a result, nearly as much engineering analysis is needed to demolish a structure than to initially build it. While the stereotypical image of demolition is a wrecking ball or an explosive event, these are not typically used in highway bridge demolition. Explosive demolition is rare, and is used occasionally for very large bridges (as well as other large structures outside of this scope). For a typical highway overpass bridge hydraulic excavators sit on the deck removing concrete with a hydraulic hammer or shear, and then excavators or cranes placed on grade remove the girders. The bridge structure is analyzed for different excavator positions and different stages of the demolition. The excavator body weight and the hammer tip weights are considered separately for different cases of demolition sequence. The engineering work required for cranes involves support design for crane bases (tracks or outriggers), pick plan and pick weights (pick sequence, radii, clearances, center of mass, lay-down area), and rigging design (wire rope slings, stability of structure while being picked, local failures of the object being picked or the rigging components).

The purpose of this research is to interpret the behavior of highway bridge decks due to the impact of moving vehicles. First, the analytical theory is briefly presented, including the vehicle model, bridge model, road surface model, and numerical methods. Both the vehicle and the bridge are simulated as thre- dimensional constructs. The road profile is modeled using a stationary Gaussian random process that is described by a power spectral density (PSD) function. Newmark's direct integration method is used in the bridge dynamic analysis. The bridge deck free and forced vibration characteristics, including the impact factors of moment and shear with both single and multiple trucks traveling over a rough deck, are investigated. Then, the stiffness effects of the longitudinal girders are studied. Analytical results show that the dynamic loading of the bridge deck is significantly smaller than that of the longitudinal girders as the direction of the deck main internal forces is perpendicular to the vehicle travel direction. The research results are applicable to bridge deck design.

Fatigue cracks are one of the primary factors affecting the safety of structural systems. Although the cracks are typically characterized by stable propagation rate, the scatter in fatigue performance is difficult to quantify and could be on the order of thousands, or even millions, of cycles. The development of maintenance and management programs for civil structures should, therefore, account for the inherent scatter in performance through probabilistic fatigue assessment. Realizing this objective through large number of experimental testing can be very costly. Probabilistic assessment can, therefore, be performed through conducting large number of finite element simulations. This paper presents a framework for probabilistic assessment of the propagation rate of cracks using finite element Monte Carlo simulations. The parameters influencing the propagation rate are treated as random variables with predefined statistical distributions. The results provide very useful insight on the most sensitive parameters affecting crack growth in the investigated plates.

The accurate prediction of the time-dependent damage level under uncertainty is an essential task in the management of fatigue critical steel structures. Structural health monitoring (SHM) and inspection actions can greatly improve the reliability of the prediction process. This is partly achieved by reducing the uncertainties associated with load estimates and actual structural responses. Additionally, SHM and inspections provide a deeper insight into the damage level at the time of application of such actions. This paper proposes a probabilistic approach for quantifying the reliability and the remaining service life of fatigue deteriorating steel bridges based on the information form SHM and inspection actions. The approach utilizes a probabilistic crack growth model which considers uncertainties associated with the damage propagation process and monitoring outcomes to find the remaining fatigue life and predict the lifetime reliability of the analyzed location. Future inspection actions are scheduled based on the predicted lifetime performance profiles. An updating process is employed to find the posterior model parameters based on the damage level quantified during inspections. Updated damage propagation and lifetime reliability profiles are established. The proposed method can support the decision making process and the life-cycle management under uncertainty. An existing fatigue critical detail of a steel bridge is used to illustrate the proposed approach.

Curved bridges are subject to combined loading conditions resulting from complex earthquake ground motions coupled with irregular geometry and asymmetry of the bridge structure. This paper describes a part of the CABER project that compares the experimental results of a curved bridge under multidirectional earthquake loading with its un-calibrated numerical predictions. The numerical predictions are obtained from an analytical model established using the Zeus-NL analysis platform. The experimental results are obtained from a hybrid experimental/analytical test performed at the NEES MUST-SIM Facility at the University of Illinois at Urbana-Champaign. In the hybrid test, the bridge is substructured into three analytical (one) and experimental (two) modules. Disagreements shown between the experimental and numerical results highly indicate the inadequacy of the existing analytical solutions. Numerical model assumptions and inaccuracy are identified and methods of numerical model calibration are proposed.

Investigation of the dynamic train-structure interaction is a key component of the design of high speed railway (HSR) bridges. Train-induced resonant vibrations of simple spans may lead to excessive deck accelerations at speeds lower than the maximum line design speed, with consequent partial wheel-rail contact, reduced passenger comfort, ballast destabilization, settlement of sleepers, and rapid deterioration of track quality with need for frequent maintenance.

Exposure of bridge structures to fire is a seldom-investigated area of bridge analysis and design. It has yet to be addressed on a national or international design code. The spectacle of these incidents and their potential for damage to bridge structures has lead to a call to further investigate the sources, frequency and effects of fires that threaten bridge structures. The results of a statistics investigation, case study analysis and literature review will be presented. The statistic study used data available in several national database maintained by agencies such as the NHTSA and FEMA. The three sources of statistics on bridge fire events returned similar results of fire occurrences in a year.

The goal of this research is to adapt a direct test to estimate the residual flexural properties of individual framing members for post-fire evaluations. The adaptation incorporates partial-composite action with an aluminum stock and a cycle of increasing load tests that are applied gradually. The evaluations of the methodology were based on eight, glued-laminated beams of which four were fire-damaged. The fire-damaged specimens were exposed to a time-temperature curve intended to produce a significant zone of damaged wood due to elevated temperatures within the residual wood section. Very good agreements were obtained between the estimated and actual apparent modulus of elasticity and modulus of rupture. The proposed test applies when flexural properties are to be refined beyond estimates that rely solely on reductions in cross section and tabulated values for fire damaged members and members that can otherwise be expected to remain in service.

Research on the effect of fire on bridge structures is still an area that has received little attention by the structural engineering community. In cases where bridges have been exposed to fire, the need to quickly ascertain, re-open or replace the afflicted structure has limited detailed investigation of the effects of fire exposure. A modeling investigation on the behavior of bridges when exposed to simulated fires will be presented in this work. A modeling methodology was developed to integrate the fire dynamics simulation with a finite element model of the structural response. The discussion will cover the use of the Fire Dynamics Simulator in conjunction with the finite element software ABAQUS. The modeling methodology developed was utilized to simulate several fire scenarios with different parameters such as fire size and location altered. It was found that tanker fuel trucks were the only vehicle large enough to cause permanent deflections in the simulated events.

Design/Build delivery is underway for the replacement Gerald Desmond Cable-Stayed Bridge for the Port of Long Beach and the California Department of Transportation. This will be the first cable-stayed bridge built in California for highway or transit traffic. The replacement bridge is located adjacent to active seismic faults that can produce near-field ground motion effects that the bridge must accommodate. This will be the first large cable-stayed bridge built in the United States in a highly active seismic zone. Therefore, it will advance the state-of-practice for seismic design of cable-stayed bridges. In consideration of the large seismic threat and to provide a structure with superior seismic resistance, performance-based seismic design criteria were developed. For each evaluation earthquake, criteria specified the level of acceptable damage and maximum allowable material strains and residual drifts. Compliance with the performance-based design criteria is verified through detailed non-linear time-history analysis, which includes extensive soil-structure interaction modeling.

In the past decade, the performance-based method has been applied to the seismic evaluation and retrofit of bridges. Suspension bridges are not covered by AASHTO specifications. Therefore, project-specific performance criteria are required. Suspension bridges are mostly critical bridges and generally need to be analyzed for two levels of seismic hazards: function (e.g. 500-year return period); and safety (e.g. 2500-year return period). In addition, the analysis methodology including site-response analysis and soil-structure interaction analysis will generally be required for suspension bridges. Seismic retrofits following the performance-based approach are usually more cost-effective than those derived from the force-based approach.

New York City (NYC) has a total of 2115 bridges. New York City Department of Bridges (NYCDOT) owns 787 of them. Of these, about 20 fall in the bridge importance category of "Critical", 245 "Essential", and 522 "Other" from the seismic point of view. Remaining bridges are owned by local agencies—NYSDOT, PANYNJ, TBTA, NYCTA, SIRT, LIRR, O-RR, and miscellaneous owners. The paper will discuss recent seismic evaluations of such bridges under the new seismic design criteria guidelines and the new hard rock ground motions recommended by the consensus of a panel of national experts specializing in NYC region's geology and seismicity. The history of how these guidelines came into the final form and how the performance based seismic criteria was developed and implemented will also be discussed.

The effect of fault crossing on the response of the seismically isolated Bolu Viaduct is investigated. The seismic ground motions at the site of the viaduct are generated following a rigorous methodology which is based on seismological data available in the literature and finite-fault modelling techniques of the extended source. The generated motions are used to study the effect of fault crossing and fault crossing angle on the nonlinear behaviour of the seismically isolated viaduct. It is shown that fault crossing is an important factor in the earthquake response of seismically isolated bridge, and that this effect needs to be considered in the design and detailing of the isolation system.

Seat-type bridge abutments consisting of expansion gaps and sacrificial shear keys significantly affect the response of bridge during strong earthquakes. Seismic pounding between the bridge deck and the abutment backwall caused by expansion gap closure may alter the dynamic characteristics, consequently affecting the demands on various bridge components. Additionally, the loss of sacrificial abutment shear keys changes the support boundary condition and may significantly increase the demands on the bridge piers. A small scale parameter study investigates the influence of ground motion and bridge curvature on the performance of seat-type bridge abutments. Straight and highly curved configurations of a benchmark bridge are subjected to two suites of motions, one set with and the other without large velocity pulses. The vulnerabilities of the bridge configurations are compared using component fragility analysis. It is shown that bridges are more vulnerable to ground motions with large velocity pulses and that curved bridges are more likely to experience column damage and abutment unseating compared to straight bridges.

This paper discusses the implementation and validation of an approximate analysis method for determining the seismic displacement demands of curved bridges crossing fault rupture zones. The considered method is the Fault Rupture-Response Spectrum Analysis (FR-RSA) procedure, which was initially developed for ordinary straight bridges crossing fault ruptures and has been recently shown to be adequate for curved bridges crossing fault ruptures. In the FR-RSA method, the bridge displacement demands are estimated by superposing the peak values of quasi-static and dynamic bridge responses. These responses are respectively calculated by combining nonlinear static analysis of the bridge under the ground displacement offset associated with fault rupture and peak response from linear response spectrum analysis. An actual three-span curved bridge in California was selected and analyzed in this paper. The bridge displacement demands were first estimated by implementing the FR-RSA procedure in the Open System for Earthquake Engineering Simulation (OpenSees), which captures the nonlinear bridge pier response using fiber elements with distributed plasticity. Secondly, the procedure was implemented in CSiBridgeTMwhich has a user friendly interface and considers the nonlinear bridge pier response through beam-column elements with lumped plasticity. The paper compares the results from OpenSees and CSiBridgeTM. It is shown that the FR-RSA implemented in CSiBridge^TM provides results similar to those from OpenSees.

At one point in the not-too-distant past, Building Information Modeling (BIM) was known merely as a "buzz" term in the building industry. Today, BIM has evolved into a necessity for the Architecture, Engineering, and Construction (AEC) community, particularly among those who practice structural engineering. Of all the engineering disciplines, structural engineering is one of the most advanced users of BIM from an application and implementation standpoint. Much of their expertise was developed from the use of computer modeling in various forms for the past 20 years. Promoters of BIM frequently mention its known advantages. However, a numbers of challenges arise when using BIM in structural engineering. Regardless of the exact state of the industry at the present time, most concur that given continued advances in software combined with creativity of structural engineers, the ultimate potential for BIM in structural engineering is far from its maximum.

The Vegas High Roller will set the new standard for large observation wheel design. At 550' in height the Vegas High Roller will become the largest of these impressive structures, surpassing the likes of the London Eye and the Singapore Flyer (443' and 541', respectively). This project falls within the niche market of atypical moving structures, resulting in challenges not seen in everyday construction. One such challenge is modeling and coordinating a complex structure that falls outside the norms of modern day building information modeling (BIM). 3D modeling is the new standard in the building industry. The complexities and precision required to build the Las Vegas Wheel were developed, designed, and portrayed using the most advanced BIM practices available. On a project where everything is custom made, the need to successfully model and coordinate in 3D is vital.

With the increasing complexity in construction projects, monitoring project schedule and managing projects effectively are becoming increasingly important. Most projects being deadline-oriented, timely completion becomes a must. A schedule overrun is never predicted at the start of the project. As per the current scheduling practices, float is considered free. It is an expiring resource. To handle the issue of total float management more efficiently, a new concept of total float management is proposed as an effort towards a more fair and equitable system. This concept respects the dynamic nature of construction projects and recognizes float to be an asset for both parties. The new concept proposes to allocate float in the ratio 50:50 between the parties at the start of the project. Monitoring procedures were developed to facilitate monitoring a project schedule closely as well as help in keeping track of the allowable total float for the owner and the contractor.

This paper summarizes data that examines the gender and racial diversity in the structural engineering profession. The first section describes the representation of women and racial/ethnic minorities among university faculty and students in the area of structural/civil engineering, among licensed professional engineers, members of structural engineering related professional organizations and companies employing structural engineers. The second section describes results of a survey filled out by 741 current and former structural engineers. Survey responses shed light on differences in career satisfaction, compensation and other experiences between engineers of different genders or races. The paper was conceived by the ASCE/SEI Young Professionals Committee; a complete report will be available from SEI in 2012.

Engineers often complain that non-engineers do not really understand or appreciate what engineers do for a living. At least some of this can be attributed to the persistent failure of engineers to reflect thoughtfully on what they do, how they do it, and why it matters, so that they can then articulate such information clearly to others. A more systematic approach grounded in philosophical principles and organized around the traditional branches of philosophy would enable engineers to develop a more refined self-image, clarify and perhaps improve their public image, and develop potential solutions to at least some of the other non-technical issues that the profession is currently facing.

Engineers, being human, have the same innate characteristics as all humans. One of these is that we have two thinking modes, intuition and analysis. The intuition mode, system 1, is automatic and acts with great confidence, while the analysis mode, system 2, requires conscious activation and would typically rather not be activated because it is lazy. System 1 uses a range of heuristics that leads to certain biases in our thinking. Sometimes these heuristics and biases produce glaring errors. As engineers, we must be vigilant so that system 1 does not act alone in inappropriate situations since the mistakes that it tends to make could produce serious errors in environments where we do not have significant experience. Encouraging an awareness of these dangers is the purpose of this paper.

Engineers routinely use conceptual, mathematical, and computational models in their work, and sometimes physical models as well. These models serve as heuristic tools which abstract from objectively real or ‘imagined' (yet to be built) engineering systems to represent them incompletely and inaccurately, with the extent of inaccuracy often being substantially uncertain. The modeling process is also heavily shaped by the background knowledge, creativity, goals, and preferences of engineers, thus incorporating a subjective aspect which involves a variety of unconscious biases that can adversely affect models. This paper explores the nature and implications of modeling in engineering, mainly at a philosophical level, and also offers practical modeling suggestions derived from this exploration.

The process of detailed design and construction of building systems made of a large number of components is quite involved. Further, since design is an open-ended problem (i.e., in general, there are a large number of design alternatives satisfying code provisions) the selection of the optimum design becomes an extremely challenging activity. The key approach to this problem is harnessing automation technologies beyond the traditional application of Building Information Modeling (BIM). The main focus of this paper is to discuss the development and interplay between Smart Codes and BIM and their role in simplifying and enhancing the design process. This includes studying the Smart Codes implementation in building design, BIM model requirements and their interaction with automated code checking systems.

This paper argues for the adoption of a single, detailed, component-based, Building Information Model as a strategy for handling all design and construction information about a building at any stage of design. Using the recladding efforts for the Aronoff Center for Design and Art at the University of Cincinnati as a backdrop, the paper frames current issues that plague the building design and construction industry. It then illustrates how a single, parametricall-driven, component-based model can reunite the fragmented industry and deal with a vast amount of information that accretes during the design-construction process. The paper also describes how the detailed, comprehensive, singl- model paradigm opens the possibility of more sophisticated structural or energy modeling and analysis while taking into account contributions of interior walls, mechanical systems, etc. to overall structural stability, thermal mass, etc. The paper concludes with implications to practice in the current splintered AEC industry and a candid discussion about obstacles and limitations.

Analysis and design of cold-formed steel (CFS) structures subjected to seismic forces usually focuses on the behavior of systems such as strapped/sheathed shear walls. Experimental data from tests on these systems offers limited information concerning the seismic performance of the individual CFS components or other configurations of shear walls. Buckling and cross-sectional deformations (unique to thin-walled steel sections) highly influence the response under cyclic loading of CFS members and the associated systems. Therefore, accurate and computationally efficient hysteretic models are required to predict the seismic performance of individual CFS components and CFS buildings. Experimental data from twenty-four axial tests is utilized to calibrate a hysteretic model that represents the axial cyclic response of cold-formed steel C-section structural framing members. The model includes strength degradation, unloading stiffness degradation and pinching behavior of the observed experimental response. Model parameters and damage rules are calibrated for local, distortional and global buckling based on the hysteretic energy dissipated. The calibrated parameters can be utilized to develop a toolbox of nonlinear hysteretic springs to represent framing axial members in CFS structures for seismic analysis and facilitate performance based earthquake engineering of CFS structures.

Building construction with cold-formed steel stud load-bearing walls has shown cost and construction time competiveness in military barracks projects compared to other construction material. The UFC 4-023-03 (2010) requires all new and existing military facilities of three stories or higher to be designed to avoid progressive collapse. The UFC 4-023-03 (2010) offers two general approaches: direct and indirect design. The direct design approach includes the Alternate Path method, which is based on flexural performance of the floor slab and the wall panel, as the building must bridge across removed vertical supporting elements. The indirect design approach includes the Tie Forces method. Applying progressive collapse design to steel stud bearing wall systems may necessitate an increase in the size and thickness of wall and floor framing members. Three framing components may be utilized to enhance the resistance to progressive collapse in case of wall section removal from the structure; the wall lateral bracing, the wall studs, and added tension straps. A nonlinear 3D static analysis was performed on cold formed steel stud bearing wall system to investigate the effectiveness of these framing components. It was observed that the collapse was mainly due to the global buckling of the studs adjacent to the removed wall section. However, the addition of tension straps redistributed the load of the lost wall section to the system and hence improved the resistance to progressive collapse. Adding double studs at each wall panel end delayed the global buckling of some studs. Improving the compression stiffness of the wall lateral bracing also contributed to increasing the resistance to progressive collapse. The study recommends finding a way to delay the global buckling failure mode of the studs to achieve the desired wall system resistance.

In light of recent efforts by US government agencies in both federal and state jurisdictions to implement disaster response and hardening features into the design and construction of new facilities to improve the execution and response times to deliberate and natural disasters, the topic of structural hardening and demand critical construction has become more important than ever. Knowledge of how to properly execute such projects in an efficient and economical manner has become vital to the Design and Construction Professionals responsible for the implementation of the special code requirements and standards needed to achieve "near-absolute" protection to first responders and to the public in the event of such disasters. In turn, a basic understanding of the engineering and management behind the successful execution of such projects is a subject that should be learned and mastered by Architects and Engineers at all levels of our profession as well as Construction Managers in order to provide value engineered solutions compliant with the strictest Architectural, Structural, and Systems requirements. This paper outlines and dissects these solutions by providing by outlining a methodical approach and design process that can be tailored to each specific project.

Following a tornado, representatives of the National Weather Service (NWS) rated the event on the Enhanced Fujita (EF) scale based on observed damage, not measurements of wind speed or direction. Variations in building codes, code enforcement, construction techniques, and construction quality limit the accuracy of these rating, and may result in misleading interpretations., This paper will compare our observations of tornado damage to residential and commercial structures with the EF ratings assigned by the NWS. The scope will include residential and commercial structures of various building types, exposure, age and condition. The discussion will include case studies of detailed inspections in which construction defects and code variations may have contributed to an inaccurate EF rating. The paper will discuss inspection techniques that may improve the accuracy of future tornado ratings by the NWS.

Designing structures in northern Greenland, where the environment consists of extremely cold temperatures, permafrost subgrade, and high wind speeds, presents a number of technical and logistical challenges. In this specific case study, procurement of construction materials in the United States, delivery to a multinational installation in Greenland, limited availability of construction equipment and materials, and a restrictive three-month summer construction season all contributed to a complex engineering and construction pursuit. This case study discusses how U.S.-based engineers successfully overcame these challenges by working closely with a domestic concrete precaster, domestic and foreign steel fabricators, and a foreign general contractor. The authors highlight several approaches that the design and construction teams implemented on this international project to satisfy the technical and logistical project constraints, with a focus on the precast foundation design, fabrication, and installation.

The Whatcom Middle School Replacement project began abruptly after a fire damaged the historic school building and displaced students. To resume school as quickly and as cost effectively as possible, an innovative building replacement strategy was developed whereby the existing building was used as a mold for a new reinforced concrete (shotcrete) exterior wall (shell) that was constructed outside the original building. Once the new exterior wall was installed, the fire and water damaged interior structure was replaced with new concrete and steel structure from within. Carved on the lentil of the original entrance, "Waste Not Thy Hour" became the motto of the fast-track project team. The result was a new building that meets the programmatic needs of a modern school and yet, preserves the iconic main entry and architectural character of the original 100-year-old ornate historic building. Additionally, the existing historic auditorium and gymnasium buildings were seismically retrofitted and united with a new classroom wing. Through an uncommon level of collaboration between the owner, design team, and contractor, the fast-tracked project was phased to expedite construction. The project was completed in time for Whatcom Middle School to reopen in less than two years.

Gilsanz Murray Steficek, LLP investigated the removal of the tie rods in the floors of the landmarked Metropolitan Life Tower in New York City when the tower was converted for residential use. The typical floor is constructed of segmental concrete arches supported on steel beams with tie rods perpendicular to the beams below the arches. GMS evaluated the removal of the rods by performing linear and nonlinear 3D finite element analysis of the floor system. Load tests were also performed at 5 locations in the building in order to confirm that the floor would perform satisfactorily. Both the analysis and the load test confirmed that the tie-rods could be removed safely.

King Abdullah Financial District Conference Center, with a total gross area of approximately 125,000m², consists of auditorium, conference and banquet facilities, a monorail station, and parking. These functions are housed within four substructure and three superstructure levels and enclosed in a multifaceted roof with a unique shape and geometry. The primary feature of the building is this multifaceted, monolithic, independent Mega-Roof structure that covers all the program spaces housed within the building. The architectural, structural and environmental engineering teams collaborated closely to devise a parametric design and analysis methodology to integrate the multiple parameters controlling the design of the roof. Further, the extremely aggressive design and construction schedule was achieved by assimilating the work of the design, fabrication and construction teams into a seamless process incorporating digital exchange of information.

The Center for Character and Leadership Development (CCLD) on the campus of the United States Air Force Academy is a distinctive new landmark which remains true to the significant architectural history of the Academy. Upon completion, the CCLD will provide the Air Force Academy with new classrooms, offices, assembly space and an iconic 105- tall skylight. The design of the skylight is a true reflection of the integrated multi-disciplinary approach that resulted in a form that responds to the demands of its surrounding environment as well as optimizes the use of structural materials. The architecturally exposed structural steel skylight is devoid of any embellishments and its sleek connections are coherent with the dynamic aesthetics of the structure. The final design of the structure and its various components ensured that it could be easily constructed.

Viscous dampers were implemented in the seismic retrofit design of a building in a large industrial facility near Seattle, Washington. The retrofit of this late-1960's building was designed to meet the ASCE 41-06 Standard for the Seismic Rehabilitation of Existing Buildings' Basic Safety Objective (BSO), which consists of the dual performance goals of life safety in the BSE-1 event (seismic hazard with 10% probability of exceedence in 50 years) and collapse prevention in the BSE-2 event (seismic hazard with 2% probability of exceedence in 50 years). The four-story, 112' tall steel-framed building consists of three levels of nonductile concentrically braced frames above a thirty-seven foot tall unbraced (soft) story. Due to the manufacturing processes contained within the building, the retrofit design reduces but doesn't eliminate its soft-story configuration; viscous dampers will be located within the bottom story to reduce lateral displacement demands and minimize required foundation work. In addition to the installation of dampers and limited foundation work, connection strengthening was required within the braced frames and the roof trusses to meet the selected performance objective. Parametric studies verified superior performance of viscous dampers relative to that of similarly sized friction dampers. Building performance was validated through a non-linear response history analysis. The building site is located approximately 100 kilometers from the Cascadia Subduction Zone (9.0 Mw) and less than one kilometer from a 7.44 Mw crustal source. Probabilistic seismic hazard analysis confirmed the significant contribution from each source and a total of ten earthquake records were selected and scaled to capture the range of seismic ground motions expected at the site. Vertical acceleration components were included for each record to predict the response of the long-span roof trusses.

The Dr. Phillips Center for the Performing Arts in Orlando, Florida is a project that was many years in the making and the design of which was carefully planned and executed to provide an exciting facility. The design of a Center for the Performing Arts involves many different considerations from acoustic requirements, architectural expectations and tolerances, challenges of building a basement below the water table, concrete elements, main canopy requirements, and others. The paper discusses some of these challenges and the solutions used to design and construct this iconic facility.

Various lateral load-resisting structural systems for high-rise buildings are rigid frame, shear wall, wall-frame, braced tube system, outrigger system and tubular system. Recently diagrid structural system is adopted in tall buildings due to its structural efficiency and flexibility in architectural planning. In this paper analysis and design of a 36 story diagrid steel building is presented. A regular floor plan of 36 m × 36 m size is considered. ETABS software is used for modeling and analysis of structural members. Dynamic along wind and across wind are considered for design of the structure. Load distribution in diagrid system is also studied for 36-story building. All structural members are designed as per IS 800:2007 considering all load combinations. Optimum angle for diagonal column on periphery is derived based on stiffness approach. Similarly, analysis and design of 50-, 60-, 70- and 80-story diagrid structures is carried out considering optimum angle of diagrid columns. Comparison of analysis results in terms of time period, top story displacement and inter-story drift is presented. The sizes of diagonal columns and internal column for all buildings are also presented in this paper.

Structural design of tall buildings is always driven by forces of nature, such as wind and earthquakes. As buildings get taller, wind-induced dynamic response dictates the design of the lateral system to meet both strength and serviceability requirements. The wind-induced dynamic response depends on geometric properties, such as building shape and height, as well as the structural properties. Building geometry is usually beyond the control of the structural engineer. However, building's structural properties; mass, stiffness and damping, are selected by the structural engineer. This paper discusses the effect of mass, stiffness and damping on dynamic wind forces and floor accelerations in the across-wind direction. Simple charts that relate the change in natural frequency to the base moment are developed. Furthermore, another set of charts showing the relationship between the building's natural frequency, the mass and the top floor acceleration is also presented. The use of these charts is demonstrated through a number of example projects, which describe situations that can face the design engineer after wind loads and accelerations have already been determined by wind tunnel testing. These situations include change of mass, stiffness or performance objectives.

Performance-based engineering of tall buildings in regions of high seismicity is becoming an increasingly popular design methodology. However, the current state of knowledge and practice in nonlinear modeling and performance-based design of this type of buildings is not growing fast enough to meet the profession's needs. Many nonlinear modeling parameters in core wall tall buildings are controversial in the professional community and are still undergoing extensive academic research. This paper summarizes the current state of knowledge and best practices of core wall tall building seismic engineering from the perspective of the nonlinear, performance-based design of a 30-story, concrete core-wall-only office tower located in downtown San Francisco. The tower consists of a cast-in-place center concrete shear wall core and perimeter gravity columns connected with a 14-in.-thick, void-formed post-tensioned concrete diaphragm. The nonlinear model features a fiber element core wall along with shear displacement hinged link beams and explicitly models the ground floor and basement level slabs. Inelastic gravity columns and floor diaphragm components are included to detect any nonlinearity and incorporate its effects. A suite of 10 pairs of maximum considered earthquake (MCE) ground motion records is used to evaluate building performance. The nonlinear response of the 10 ground motion analysis is highlighted. The paper also presents results of parametric studies that highlight the effect of some of the critical nonlinear modeling parameters on the performance of core wall tall buildings undergoing inelastic deformations. These parameters include the combinations of modal and Rayleigh damping, link beam modeling properties, and podium diaphragm effects. Based on the results of the performed parametric studies, best modeling practices are suggested.

This paper discusses the out-of-plane axial-flexural behavior of two full-scale reinforced concrete (RC) bearing wall test structures subjected to fire loading. RC bearing walls are one of the most common primary lateral and gravity load-carrying structural systems in building practice. Severe exposure to fire can compromise the out-of-plane stability of these walls under service-level gravity loads, causing catastrophic overall or disproportionately large failure of the building. In addition to an array of thermocouples, strain gauges, and displacement/rotation sensors, full-field deformation and temperature response of selected faces of the test specimens were captured using three-dimensional digital image correlation (3D-DIC) and infrared thermography. The walls were fixed at the base and free to displace vertically and rotate at the top. A constant axial load representing tributary gravity loading was applied at the top. In the out-of-plane lateral direction, the top of the first specimen was restrained while the top of the second specimen was allowed to displace under increasing temperatures and a step-wise increasing external lateral load. A previous paper presented the global response and loading of these two specimens. This paper presents the response of the specimens based on the full field measurement techniques - the fundamental cross sectional behavior including through thickness temperatures, out-of-plane displaced shapes, axial strain, and curvature.

In recent years, fiber-reinforced polymers (FRP) are widely used in strengthening and rehabilitation of structural members in buildings. However, there is very little guidance in codes and standards for fire design of FRP-strengthened concrete beams. This paper presents guidelines for achieving optimum fire resistance in FRP-strengthened reinforced concrete beams. A macroscopic finite element based computer model is applied to quantify the influence of various parameters on the fire response of FRP-strengthened concrete beams. Results from these of numerical studies are utilized to develop guidelines on insulation thickness and insulation configuration for achieving optimum fire resistance in FRP-strengthened concrete beams.

The new paradigm of performance-based fire engineering (PBFE), with its systematic approach to identifying building performance objectives, quantitative structural analysis to verify that these objectives have been achieved, and management of uncertainties provides incentives for engineered alternatives to traditional prescriptive approaches keyed to the ASTM Standard E119 fire test. Recent advances in fire science and engineering have made it possible to consider realistic fire scenarios and fire effects on the building structural system as part of a risk-informed design process. This paper describes an approach to developing reliability-based resistance criteria for use in practical structural engineering approaches to fire safety assurance.

On May 13, 2008, a fire occurred at the Faculty of Architecture Building at the Delft University of Technology (TUD) in the Netherlands. The fire ultimately led to the collapse of a major portion of the building. Data was collected on this fire event by an international team that included researchers from the Netherlands and from the U.S. The data collection effort was followed by a preliminary structural analysis to identify possible contributing causes to the structural collapse. This paper will provide an overview of this fire event, a description of the structural system of the building, and results of preliminary analysis of structural response to the fire. The paper will conclude with a discussion of broader lessons learned on structural fire safety from this event.

Hybrid masonry structural systems are composed of steel frames that work in conjunction with reinforced concrete masonry wall panels to resist lateral loads. Current applications of this new system have been limited to areas of the United States where wind loads dominate and the seismic hazard is low. However, recent studies suggest that the application of hybrid masonry bracing systems may be extended to include higher seismic regions. This ongoing research is funded by the National Science Foundation under the NEESR program. As interest in hybrid masonry structural systems grows, the need increases for a comprehensive design procedure. Currently, a capacity-based design procedure is under development, and this paper describes aspects of the preliminary framework. Capacity-based design provides a tool for sizing individual components, including steel columns, beams, connector elements, and the masonry panels to force ductile limit states that lead to favorable global performance. Once completed, the full design procedure will serve as a beneficial tool for designers and further the development of the hybrid masonry system and the range of its application.

The National Science Foundation is funding hybrid masonry research to investigate the seismic strength, behavior, and performance of structural steel frames braced with hybrid masonry. One aspect of the research is a sub-study performed at the University of Hawaii at Manoa (UHM) to investigate the strength and behavior of steel connector plates that transfer story shears from the structural steel frame to the masonry panels. Results of this sub-study were then used in large-scale testing at the University of Illinois at Urbana-Champaign (UIUC). The primary goal of the study was to determine whether the connector plates can be developed as ductile fusible links for the overall system under seismic events. This paper presents the results of a series of experiments performed in the structures laboratory at UHM. Details of the experiments are summarized and sample results are presented that illustrate expected behavior of the connector plates, bolts and masonry for the large-scale experiments performed at UIUC. Some connectors were designed as link plates with sufficient strength to transfer full story shears to masonry panels without yielding, whereas other connectors were designed to behave as energy dissipating ductile fuses. These fuse plates will absorb most of the seismic deformation, thereby protecting the masonry and steel frame elements from damage. The fuses can then be replaced after the earthquake to restore the hybrid masonry system to its original condition.

In this manuscript, the authors present a combined experimental and numerical study that empirically links measured vibration response characteristics to the remaining load carrying capacity of a masonry dome as the structure is gradually damaged with discrete and distributed cracks. First, the three-dimensional, nonlinear finite element model of the dome is calibrated using both nondestructive vibration response measurements and destructive load displacement tests. The gradual development of major, discrete cracks is simulated by introducing a mesh discontinuity, while the development of minor, distributed cracks is incorporated by the inherent smeared cracking capability of the finite elements. The calibrated numerical model is used to estimate degradation in both the strength and stiffness of the dome, indicated by a reduction of the load carrying capacity, and by the reduction in natural frequencies, respectively. An empirical function is trained to link the reduction in natural frequencies (a quantity related to stiffness that is feasibly measurable), and the remaining load carrying capacity (a quantity related to strength that is not feasibly measurable) for spherical domes. This empirical relationship is generalized for spherical domes with different span-to-height ratios.

Unreinforced masonry buildings have shown to be prone to seismic forces due to their brittle behavior. Therefore, many experimental and analytical studies have been conducted on different retrofitting methods, including the use of Fiber Reinforced Polymers (FRP,) to improve the lateral strength and ductility of these structures. Time history dynamic analyses were conducted on an uncracked and cracked one story unreinforced masonry structure with window and door openings. Based on the analytical results, the effect of cracks and openings on the seismic response of the structure was investigated. Thereafter, the building was retrofitted with FRP strips to improve the post cracking behavior. It was found that masonry cracking increases the damping of the masonry structure. However, the integrity of the structure must be maintained to prevent total collapse, thus satisfying the life safety requirements of the occupants.

Confined masonry (CM) consists of unreinforced masonry walls confined with reinforced concrete tie columns and beams. Due to satisfactory performance during past earthquakes, CM has become the predominant construction style in several developing countries. However, poor construction practices and substandard materials may result in inadequate performance. This paper evaluates the feasibility and efficiency of an affordable and practical retrofit technology designed for CM constructions in developing areas. Corrosion-resistant aluminum strips are inserted into grooves and embedded in mortar along bed joints to enhance the in-plane shear resistance and deformability. Experimental evidence was obtained through in-plane cyclic load tests on full-scale CM walls. It is shown that a substandard CM wall can be transformed into a wall with acceptable strength and deformability, and the in-plane shear strength of a retrofitted wall can be estimated by means of a simple analytical model that accounts for the contribution of the horizontal reinforcement.

Light-frame wood buildings have performed well with regard to life safety during recent U.S. earthquakes with several exceptions being the Loma Prieta and Northridge earthquakes. In these earthquakes, particularly Northridge, there was substantial damage resulting from several and perhaps even a single large amplitude inter-story drift during seismic excitation. Designing buildings to be more resilient to earthquakes is the impetus for the study presented in this paper. Architectural features and the ever-increasing demand for larger (and more) windows reduces the strength and stiffness of light-frame wood structures necessitating alternative procedures to be considered. An alternative procedure is developed and presented in this paper. Superelastic shape memory alloys (SMA's) which have been shown to perform well as energy dissipation devices thereby improving the response of civil engineering systems to earthquakes, are applied to modify the response of light-frame wood building systems with large openings. Fragility curves and design charts for a range of seismic weights and desired performance levels are developed for a portfolio of SMA walls to be treated as replacement walls for wood shear walls in design or retrofit. The design of a large custom residential building using this technology is presented and performance verification is achieved using nonlinear time history analysis.

The seismic performance of multi-story buildings can be ascertained from structural deformations and accelerations. Direct displacement design (DDD) is a procedure that allows one to distribute the vertical stiffness to the levels of a multistory building such that all stories of the building meet the desired performance criteria. DDD, in its original form, provides a simple and effective procedure to ensure a multi-story building meets the desired story drift requirements when subjected to the specified seismic intensity by considering the stiffness and strength degradation at target story drift level believed or demonstrated to provide the desired building performance. However, this design methodology has not been able to be applied to buildings with severe in-plane torsional irregularities. This paper presents the details of a methodology to perform DDD on multi-story buildings with any level of translational and torsional response and with or without a soft story, thereby generalizing DDD for all buildings. The proposed method can be applied to any nonlinear system, and is illustrated here for woodframe buildings. The design method, which does not require time history analysis, was found to provide a design with the target performance and for building with torsion, was shown to provide significantly better accuracy than DDD in its original form.

The structural response of mid-rise hybrid light wood frame buildings, incorporating portal frame and shear wall, under either cyclic load or earthquake action were investigated via finite element analysis. Based on the structural behaviour of the one- and five-storey hybrid buildings under cyclic loading, a method to estimate the ductility ratio, μ, and ductility-related force modification factor, R_d, for the light wood frame building containing two types of lateral load resisting systems exhibit different ductility characteristics was proposed. Four- and six-storey buildings with layouts that rely on shear wall only or a combination of shear wall and portal frame were designed using the R_d factors estimated from the proposed method. Natural frequency extraction and non-linear time history dynamic analyses were performed on these multi-storey hybrid buildings using a macro element model to represent the lateral resistant element in ABAQUS. The influence of horizontal diaphragm flexibility was considered in these analyses. The results show that the use of a R_d value higher than the lowest R_d factor of the two lateral load resisting systems suggested by the National Building Code of Canada may be justified.

This paper presents a combined load sharing, nail-slip model to predict the failure of roof-to-wall toe-nail connections in residential wood-frame buildings. The roof elements, idealized as an equivalent 2-D beam in the model, are used in combination with bi-linear and non-linear load-displacement curves of the roof-to-wall-connections to describe the progressive nail withdrawal observed in full-scale experiments. In particular, elastic beam theory is used to quantify the load sharing between the adjacent roof-to-wall-connections, as well as to characterize the overall failure of such connections and the roof. The model demonstrates its capability in effectively capturing the damage progression and load sharing effects exhibited by a system of interconnected roof-to-wall-connections, subjected to fluctuating wind loads, by predicting the connection responses that reasonably match with the outcomes of controlled load sharing experiments. The validated model is further used to predict the response of the inter-connected roof-to-wall-connections for a gable roof building tested in full-scale.

Recent changes to the fall protection exemption for residential construction require that all workers at 6 feet above a lower level must have a net, guardrail system, or be attached to a personal fall arrest system. The purpose of this paper is to examine the capacity of anchorages for personal fall arrest systems on light-frame wood truss roofs. A novel lateral load testing device was used to load trusses out of plane. Both mono-slope queen post single trusses and two braced king post trusses were tested. Lateral loads of single trusses were approximately 10% of the capacity required for a personal fall arrest system. Variable displacement rates greater than 25.4 cm/min produced similar maximum load and failure results to impact testing. Workers should never tie off to a single truss element. Further research is underway to develop suitable truss assemblies that can be used as anchorages.

In 1994, a seismic event in Northridge, California caused between $16 billion and $20 billion worth of damage to multi-family wood apartment buildings and brought to question construction quality, framing practices, structural design methodology, and the role of local jurisdiction in adoption, enforcement and oversight of building codes. The majority of damage to wood-framed structures occurred in apartment buildings of 3 stories or less. Damaged structures showed evidence of inadequate or complete lack of structural designs and poor construction quality associated to building age and structural irregularities. Specifically, ‘tuck-under parking'1 apartment building presented some of the most significant failures due to the weak-story condition created by the lack of rigidity in the lower level space frames as well as building code allowances for the use of prescriptive path methods without the approval of a licensed Architect or Engineer, a construction method still available in most jurisdictions, but now subject to significant limitations. The 1997 edition of the Uniform Building Code (UBC) incorporated revised seismic hazard maps and regulated prescriptive path construction methods for wood commercial buildings resulting in trends towards the use of concrete or steel for mid-rise apartment buildings. Specifically, designs which incorporated known significant structural irregularities or reached the maximum allowable height limit of the code, a minimal level of partial design by a licensed professional became mandated by the code.

Hyperbolic cooling towers of ever greater height have been constructed since 1914. As the height of the shells increased, engineers became increasingly concerned about the effects of wind loads on these structures and likely adapted methodologies from tall building and chimney practice for the designs. The failure of three of a group of eight completed cooling towers with height 114 meters at Ferrybridge, UK, in 1965 exposed the inadequacy of the then prevailing procedures for wind design and served to initiate and accelerate research programs on a wide variety of subjects. Topics included wind pressure magnitude and distribution, stability, ring stiffeners, column supports, reinforcement quantities and placement, seismic resistance, imperfections, temperature and shrinkage, soil-structure interaction, concentrated surface loads, openings and construction loading. In this paper, several of these research topics are discussed briefly, with emphasis on those that have influenced the design, construction and damage assessment of cooling towers in the US. The evolution of tower design procedures is traced and a discussion of unanticipated damage and failures during construction is offered. The application of modern analytical methods and experimental research studies to understand the cause of the damages and failures and to propose corrective procedures is emphasized. This paper is based on an invited lecture presented at ISCM-2012 in Cologne Germany, June 2012. Permission for the use of the materials contained in the proceedings was obtained from the conference chairman Prof. Dr-Ing. Reinhard Harte. Due to page limitations, the paper has been slightly abridged here. The longer version is available from the Proceedings, ISCT 2012 6^th International Symposium on Cooling Towers, R. Harte and R. Meiswinkel, eds, Faculty D, Department of Civil Engineering, Bergische Universität Wuppertal,pp.3-16. The contact address is Pauluskirchstr.7, 42285 Wuppertal,Germany or www.bauing.uni-wuppertal.de.

The LRFD stress checking of steel members in the latest revision of STAAD.pro v.8i doesn't include the checking for Torsional Stresses. For steel members supporting pipes, which are very common in power plant projects, it is important to check the combined Flexural-Torsional stresses in order to qualify these steel members in accordance with AISC design guide #9. Thus, the engineer has to calculate the applied torsional stresses manually using the tables and charts available in this design guide and then has to combine these torsional stresses with the flexural stresses, obtained from STAAD.pro, using either hand calculations or some computer tool. In either case, the procedure will be time consuming especially if there are a large number of load combinations to be considered. In order to expedite the process of the LRFD Flexural-Torsional checking using the output results of STAAD.pro, an Excel workbook is developed and programmed to import the steel members' geometries and their "Enveloped" LRFD forces from STAAD.pro analysis and to calculate the Torsional Stresses at different places along any steel member of the structure, as well as the other member stresses, by just inputting their start and end member and joint numbers. The stress ratios for the Shear, Flexural and Torsional stresses as well as the combined interaction stresses are then automatically computed and displayed within few minutes. It is shown that performing the LRFD steel design by STAAD.pro without accounting for torsion, in such structures, can produce misleading results.

This paper focuses on the design of steel support structures within electrical power generating facilities (power plants) which burn fossil fuels to generate electricity. The design of power plant structures requires the determination of loads, some of which will already be familiar to any practicing structural engineer, and some of which are specific to power plant structures. This paper provides guidance to the structural engineer for developing the various loads for power plants, and for combining these loads to determine the overall design loads. The discussion will be based around the use of Minimum Design Loads for Buildings and Other Structures, ASCE 7-10. (Note: where specific text from ASCE 7-10 is quoted or cited in this paper, it is with permission from ASCE.) Many of the loads involved in the design of power plant structures are the same as those encountered in the design of any typical building or structure, such as dead load, live load, snow load, wind load, and earthquake load. However, for each of these loads there are characteristics which are specific to power plants, and these are discussed in this paper. In addition, the design of a power plant structure involves loads which are not specifically discussed in ASCE 7-10. Examples of these loads include ash load, unbalanced pressure forces, forces due to thermal expansion and contraction of high-temperature equipment and ductwork, and boiler loads. The characteristics of these loads are discussed. The basic allowable strength design (ASD) load combinations found in ASCE 7-10 are expanded to include the additional primary loads specific to power plants. Finally, considerations for developing factored load combinations with power plant loads, for use with load-and-resistance-factor design (LRFD) methods, are discussed.

The Vegas High Roller is a 550-foot-tall rotating observation wheel under construction in Las Vegas. It has 28 cabins, each capable of carrying 40 passengers. The efficient, lightweight structure and low natural frequencies render it susceptible to wind-induced vibrations. Spectral-based predictions show that many modes with natural frequencies between 0.3Hz and 2.5Hz could contribute to perceptible motion responses. The combined effect of these multi-mode responses on occupant comfort is difficult to assess using existing methods and criteria. In order to confirm that the performance was acceptable, purpose-developed testing was conducted allowing the client and design team to experience simulated motions in a cabin for various wind events. The wind-induced motions used for this test were obtained through a wind time-history response analysis which accounted for the spatial correlation of dynamic wind pressures over the entire wheel structure. Supplemental viscous dampers are being installed in the lateral restraining system between the wheel rim and the stationary boarding platform to improve occupant comfort to a level acceptable to the client. This enables the wheel to operate under winds up to the 1-year return period level, allowing the client to maximize ticket sales.

The design of structures deals in the laws of physics, principles of logic, and analytical problem-solving and focuses primarily on function in the support of buildings and bridges. The world of art, on the other hand, is often associated with the architectural side of the built industry, and focuses on form and aesthetics. Structures and art are not typically linked, and there is not always an appreciation for an artworks' structural requirements, especially of large public pieces. When structural support is integral to a piece, the detail and elegance of the structure are often overlooked, and the work involved can go unappreciated. This paper and presentation will feature some of the challenges of designing structural support for three-dimensional art that appears to defy the laws of physics. Projects featured include a sampling of free-standing structural art pieces as well as the design challenges they posed. This presentation will be of interest to all structural engineers, especially those with little or no experience designing atypical and unusual projects.

When complete, the Vegas High Roller will be the world's tallest observation wheel at 550 feet. Designing a moving structure on this scale is a case study in structural optimization and efficiency; the final superstructure, including cabins, will weigh less than 5000 tons. This efficiency is inherent in the design of tension wheels, the list of which includes the world famous London Eye and Singapore Flyer. Like a bicycle wheel, the Vegas High Roller will be constructed with tensioned spokes and a rim that is kept in compression. This design is not only structurally efficient, but also yields superior fatigue performance to fairground-style Ferris wheels. The High Roller is planned to operate 18 hours a day for 50 years, so every weld, penetration, and attachment is detailed with fatigue in mind. In addition to being the world's tallest observation wheel, the High Roller will also have the world's largest capacity at 1120 passengers per 30 minute rotation. This level of public exposure requires the wheel to be resilient to potential damage, whether it is environmental, accidental, or deliberate in nature. A detailed failure modes and effects analysis has been conducted to identify potential threats to the structure and ensure that there are redundant systems in place wherever possible to prevent operational and safety failures. Where redundancy is not feasible, the structural design goes above and beyond code requirements to ensure the wheel will be safe in a seismic event, comfortable in strong winds or extreme temperatures, and that potential damage from blast or fire is mitigated. The final product is a giant observation wheel that is well integrated with the site, and will be a world class tourist attraction for decades to come.

The across-wind loads on structures having a circular cross-section, such as chimneys or stacks, have the potential to exceed the along-wind loads due to the aerodynamic phenomenon of vortex shedding. Unlike sharp-edged bluff bodies, where wind tunnel testing can provide a direct evaluation of wind load representative of those on the prototype, the interpretation of measurements for circular or rounded structures is more involved due to the presence of Reynolds number effects. Wind tunnel testing was carried out to evaluate the responses of four steel stacks in a square arrangement. Four aeroelastic models were constructed, characterized by a length scale of 1:90 and a frequency scale of 10.7:1. Stacks in arrangements are known to be susceptible to aerodynamic instability, including increased vortex shedding response or interference galloping. A test on an isolated stack was first carried out, which provided baseline measurements of the mean and peak responses. Wind tunnel tests were then performed for the four stacks in square arrangement over a range of wind speed and direction; the response of each stack in the arrangement was measured simultaneously. Two levels of structural damping were investigated. Mean and peak resultant base moments for each stack are presented as a function of wind speed and direction, and as a ratio to that measured for the isolated stack. The effect of stack position within the arrangement is discussed for the mean and peak resultant base moments. It was found that the peak responses of the stacks in the arrangement exceed that for the isolated stack at many wind directions. The effects of aerodynamic instability can be mitigated by increasing the level of structural damping.

A pushover analysis of an existing hollow cylindrical pedestal of a concrete elevated water storage tank is presented. The analysis is based on the three-stress-invariant cap model incorporated in LS-DYNA. The computed results include stresses related to the failure surface of concrete at first damage and at maximum load and the lateral-load deflection curve. The pushover data is used to approximate the response modification factor in seismic response analysis of the pedestal.

The Equivalent Lateral Force Procedure (ELFP), as outlined in ASCE 7, is a method for developing and distributing equivalent static lateral loads on a structure to account for the horizontal seismic load effect. Other options, such as Modal Response Spectrum Analysis and Time History analysis, are more sophisticated and require more advanced analysis and more knowledge of structural dynamics. ELFP is meant for regular structures for which the fundamental period of vibration is dominant in the mass participation. Many typical nonbuilding structures are not regular and have higher participation of higher modes of vibration. This paper outlines the limitations of ELFP for certain nonbuilding petrochemical structure configurations. It provides recommendations for analysis of typical petrochemical structures that are subjected to seismic loading. In addition, it defines procedures used to identify typical petrochemical structure configurations that are outside the limitations of the ELFP This paper not only identifies the major considerations for using the ELFP for nonbuilding structures and makes recommendations for the mitigation of impacts due to these considerations.

Passive energy dissipation devices add supplemental damping to improve the dynamic response when subjected to earthquake motion. The challenge lies in effectively implementing such devices in existing slender and tall structures, such as those found in high-voltage electrical substations. One such device was considered in this research and utilized friction springs in a device that allowed for installation on existing anchor locations. The main objectives of the research were to quantify the performance of the device and to evaluate the effectiveness of the retrofit under simulated earthquake motion. The device exhibited a consistent and repeatable hysteretic behavior. Full-scale shake table tests were then conducted on 115 kV current transformer mounted on top of a steel tube extension that was retrofitted with the damping device. The input motion was incrementally increased in amplitude to simulate increasing levels of demand. The use of the device was found to be effective by reducing the transformer center of mass acceleration up to 50%. The benefit of the dampers was realized significantly more effectively when a gap was included between the base plate and the foundation.

Firs, the paper compares experimentally obtained values in the literature to those obtained from the time history analysis by the finite element method using ANSYS computer program for a 65 kW wind turbine when the turbine is subjected to the Landers earthquake. It is shown that the experimental and numerical values compare well at the damping ratio of 0.86, establishing the validity of the numerical method. The effect of wind turbine sizes (65 kW, 1 MW, 2 MW, and 5 MW turbines), damping ratios, base acceleration directions, and different earthquakes (Landers, Imperial Valley, and Northridge) on the wind turbine responses are studied. The responses studied are the peak accelerations and deformations on the top of the nacelle, and the maximum von Mises stresses at the base of the wind turbine towers.

Generally, foundations for large dynamic equipment are made of considerable amounts of concrete. Thus, they contribute to a significant percentage of a project's total concrete quantity. Designing these dynamic foundations requires a series of static and dynamic analyses to meet the vibration requirement of the equipment and any seismic requirements of the local building codes. This article researches and compares different modeling methods, and then proposes an efficient approach for their design in high seismic regions. For design optimization, two different types of foundations should be considered for these areas: mats and piles. Liquefaction, downdrag on piles, bearing pressure, spring constants, soil stratification, water table, soil types, bearing depth, and settlement, can all affect a foundation. Reviewing the geotech report and performing a cost/risk analysis with the contractor should make it clear which type of foundation should be considered at a given locale. Engineers should design these foundations using a finite element modeling software that can handle dynamic loading. Modal analysis can be used to provide frequencies and displacement amplitudes to verify vibration performance criteria based on ACI 351 and the manufacture's requirements. Modal analysis can also be used to analyze seismic response spectrum per SAP time history analysis. In addition, a model based on static and quasi-static loads as required by the applicable building code should be analyzed. The ability to combine these models into one can help to solve these problems more quickly and efficiently. A case study for a foundation of steam turbine generator is presented, in which, the analysis features and design procedures used are described in detail. This may help design engineers understand the different advantages and results of finite element models, to pick the best modeling option for any given situation.

Some industrial facilities, such as those associated with the energy industry, are designed for incidental high speed impact from airborne debris that might accidentally originate within the facility. High-speed rotor balancing bunkers are among those industrial facilities that require design for incidental high speed impact. Under such extreme loading scenarios, the design of reinforced concrete structures becomes a challenging task. When high speed projectiles impact the concrete, the structure might be breached, or a crater might be generated. An acceptable level of damage to the structure can be established with the facility operator. The level of damage can be identified in terms of the size of the crater, the projectile penetration depth and the minimum perforation thickness. This acceptable level of damage serves as the performance criteria to which the structure is designed. This paper will present a suggested analysis and design process of industrial facilities subjected to high speed impact loading. As an example, the paper will focus on a commercial high speed balancing facility. The suggested design procedure is a collaborative team work between the designers and the facility operators, and it requires the identification of the potential impact scenarios, the use of High-Fidelity Physics Based simulations in order to investigate the structure response under high speed impact. Three-dimensional finite elements models were used to simulate the impact of various projectiles with structure walls. The LSDYNA software was used for all simulations.

Automation has improved the precision capabilities of manufacturing large-scale machines. In order to achieve these increasingly tighter tolerances, however, the foundations supporting manufacturing operations must abide by serviceability criteria that seem excessively restrictive in comparison to objectives traditionally established for the design of most structures. Stringent limits on deformations and rotations of massive foundation plinths appear ridiculous relative to normal magnitudes of variability in the properties of foundation soils and concrete. Yet, such limits are critical because minute movements of the base translate into larger movements of the tooling equipment. Stiff soils and mass concrete offer the means to achieve a foundation block that is virtually motionless, but the design takes on tremendous risk. In large-scale construction projects, removal of defective materials or stiffening of the foundation plinth after initial construction is impractical. This paper presents the design and construction of a successfully completed mass concrete foundation that supports a giant moving gantry, while keeping deformations and rotations below the gantry producer's specified imperceptible limits. Actual deformation and rotation results from load tests are compared to the design predictions made by finite element analysis of concrete-soil interaction. In addition, actual temperature data of the concrete during curing and cylinder strengths are used to evaluate the project specifications for mass concrete and refine assumptions of material stiffness. Practical recommendations for design, specification, and detailing mass concrete machine foundations are based on these comparisons of design results, actual test data, and proven construction techniques.

Current codes and standards do not address all aspects of design of soil-supported vibratory machine foundations for Petrochemical and other industrial facilities. Many engineers and companies involved in the industry have developed procedures and techniques for calculating loads for analyses on such foundations. The criteria for performing a dynamic analysis can vary significantly from source to source and lead to inconsistent structural reliability. This paper provides a summary of design requirements found in selected current standards and common practices in the industry. A study is presented for small vibratory machinery foundations examples analyzed using commercially available software. Recommendations are provided for determining if it is necessary to perform a dynamic analysis for soil supported vibratory machine foundations based on parameters included in the study. Also included is a discussion of other related issues to vibratory machine foundation analysis. This paper identifies the major considerations for vibratory machine foundation design. It makes recommendations for when it is appropriate to use the rule-of-thumb methods for foundation systems and when to perform dynamic analysis.

A full-scale, five-story building was constructed on the NEES@UCSD large, high-performance shake table and subjected to a series of earthquake motions while supported at its base in two different configurations. The building was completely furnished with nonstructural components and systems (NCSs) including: a functioning passenger elevator, prefabricated stairs, façades, partition walls, piping, HVAC, ceiling, sprinklers, building contents, as well as passive and active fire systems. The program included testing the building in a base-isolated configuration -- prior to testing it fixed at its base-- with earthquakes of increasing severity imposed on the building in both configurations. In this paper, the performance of two major NCSs is of primary interest, namely the egress subsystems (elevator and stairs) and façades (balloon framing overlaid by stucco and precast concrete cladding). Physical observations and measured seismic response of these subsystems and the associated impact on post-earthquake fire performance are presented.

This paper is one of five within a session discussing the findings from a full-scale, five-story building test program conducted at the NEES-UCSD Large High Performance Outdoor Shake. This paper focuses on the pre-test simulation of the test building with the objective of predicting its seismic response and thereby; (i) guiding the selection of earthquake motions for the test program and (ii) providing guidance for the design of various nonstructural components and systems (NCSs) to be installed in the test building. Two nonlinea,r finite element models were developed independently in this effort: 1) a macro-element based model was implemented in the OpenSees platform, and 2) a detailed finite element model was prepared using the general finite element software package DIANA. In this paper, the description and assumptions adopted for each of these two models are first provided, and then the simulation results for selected test motions are presented and compared. Utilizing these models, a procedure is developed to define project-specific NCSs design criteria. The developed design criteria are compared with the current code provisions and their implications discussed.

A full scale five-story reinforced concrete building was built and tested on the NEES-UCSD shake table. The purpose of this test was to study the response of the structure and nonstructural components and systems (NCSs) and their dynamic interaction during seismic excitations of different intensities. The building specimen was tested under base-isolated and fixed-based conditions. To analyze the effects of the construction process, NCSs, the isolation system and structural/nonstructural damage on the dynamic properties of the building, a sequence of dynamic tests were performed on the test specimen, including ambient vibrations, impact/free vibration and forced vibration tests, the last including low amplitude white noise (WN) and seismic base excitation. In this paper, the measured response data from accelerometers are used to estimate the modal parameters (natural frequencies, damping ratios and mode shapes) of the building at different stages of construction, during the placement of NCSs and at different damage states of the structure and NCSs. Different system identification methods, including output-only and input-output, are used for this purpose. The results clearly show the effects of changing the state of the building on its equivalent linear dynamic properties, providing significant insight on the effects, absolute and relative, of the construction activities, NCSs and structural/nonstructural damage on the modal properties of the building.

A unique collaboration between academia, government, and industry, the Building Nonstructural Components and Systems [BNCS] project, was formed to contribute to the understanding of earthquake and post-earthquake fire resiliency of nonstructural components and systems. The centerpiece of this effort -- full-scale shake table and live fire testing of a five-story reinforced concrete building fully outfitted with nonstructural components and systems -- will contribute a wealth of high-resolution physical data to the earthquake and fire engineering communities and provide direct input to modeling tools, future design codes and construction practices. This paper is the first in a session devoted to this project, providing an overview of its scope, details of the experimental program, and preliminary summary of major results from the testing. Four other papers in the session focus on response of the major nonstructural components and systems (Pantoli et al., 2013a), system identification (Astroza et al., 2013), pre-test numerical modeling (Wang et al., 2013), and fire test phase results (Meacham et al., 2013).

Following the series of shake tests on the full-scale, five-story BNCS building, a set of six live burn tests was conducted on the third floor. The fires ranged in size from about 500kw to 2000kW, depending on compartment configuration, fuel and ventilation controls. Fire duration was limited to less than 15 minutes to avoid structural damage. The focus of the fire tests was to assess the potential for smoke and fire spread given seismic damage to such systems as wall and ceiling systems, doors, window openings, vertical shafts, firestop material and fire sprinkler systems. Tests were conducted with the sprinkler system off. Testing revealed the propensity for horizontal and vertical fire and smoke spread given damage to interior compartment barriers, the elevator system and the external balloon framing system. Data on the fires, temperature profiles, smoke and fire spread will be presented.

Floor motions in structures are substantially different than the ground motions exciting their foundations. Vibration amplifications of ground motions occur differently in the vertical and horizontal directions in these floors. Excessive motions influence adversely most of building contents and non-structural systems suspended on those floors. Shake table testing allows reproducing true effects of earthquake motions that can challenge a complex specimen, but their reproduction is usually imperfect because of the dynamics of the control system, the shake table controllers and the specimen dynamics, even when basic closed-loop control is used. This paper presents the analytical development and the experimental verification of an ‘open-loop' feed-forward iterative procedure for simulation of desired "floor motions" at any specific location of a test structure. As the first step, a well-known global feed-forward procedure using transfer functions is reformulated for uni-axial systems. Subsequently, the same concept is extended to multi-directional systems using a newly developed 6×6 transfer function matrix. The compensation procedure was implemented and experimentally demonstrated, assuming uncoupled system in each axis of a 20ft×20ft×10ft test frame mounted on a shake table at the Structural Engineering and Earthquake Simulation Laboratory at the University at Buffalo. The procedure developed for a coupled system using the transfer function matrix is verified analytically. The developed ‘open-loop' compensation procedure is limited to linear systems and is affected by the uncertainty and nonlinearity of the systems. Challenges of shake table-structure control observed in this study are also addressed.

In the past decade, manufacturers of suspended ceilings have conducted a series of earthquake simulator tests of suspended ceilings focused on obtaining data for certification of manufacturer's proprietary subassemblies and components. These tests have utilized comparative evaluation because no standard has been developed yet for seismic testing and qualification of suspended ceilings. Although adequate for their limited purpose, tests have used limited instrumentation and thus cannot be used in assessment of either prescriptive requirement in building codes or for vigorous performance evaluation. To address these issues, a comprehensive experimental and evaluation program is proposed utilizing both static and earthquake simulator tests. The primary objectives of this program are to develop a rigorous procedure for component evaluation, and to provide a methodology for correlating experimental data to code requirements and for assessing the efficacy of building code design and installation requirements. As a case study, a critical component of suspended ceiling system was then evaluated using the proposed protocol and it was shown that the proposed methodology effectively addressed the stated objectives and can be extended to evaluate various other components of suspended ceilings.

A numerical modeling method was developed for fire sprinkler piping systems with threaded/grooved joints and used to generate seismic fragility parameters. The model accounts for the inelastic response of grooved/threaded joints, braces, hangers, and wire restrainers. The model incorporates a nonlinear model recently developed for the cyclic moment-rotation relationship of grooved tee joints, as well as a numerical model formerly developed for threaded tee joints. The models are validated using the results of extensive experiments on tee subassemblies performed at University at Buffalo. The model is adaptable to different pipe diameter joints. A real fire sprinkler system of a hospital was simulated and its seismic response was obtained under a suite of ninety-six artificially generated tri-axial floor acceleration records. Three system damage states were defined and a combined probabilistic seismic demand model was utilized to obtain system-level fragility parameters.

The increased interest on renewable energy resources had a direct effect on the use of solar collectors and photovoltaics (PV) on residential and other structures. Current building codes and design standards offer little assistance to the designers regarding provisions for wind-induced loading. The scope of this paper is to address the wind-induced loads applied on particular rooftop energy structures. Findings from a study that was carried out in the boundary layer wind tunnel of Concordia University, Montreal are presented. For this study, a 1:200 geometric scale flat roof low-rise building was constructed and instrumented. The building model was equipped with three identical solar panels, which had six pressure taps on each surface. Several configurations were examined by changing the position of the panels on the roof as well as their inclination. The paper presents detailed results from the various configurations and discusses the effect of panel location and inclination.

The existing literature has limited aerodynamic data for the evaluation of design wind loads for solar panels. Furthermore, there are no provisions in building codes and standards to guide the design of these types of structures for wind. This paper presents a systematic wind tunnel study to evaluate wind loads on solar panels mounted on low-rise gable buildings. A preliminary geometric scale effect study using a simple isolated solar panel was carried out to permit design appropriate wind tunnel experiments. Following the scale effect study, wind loads on solar panels mounted on different critical zones of low-rise residential roof are systematically investigated. The results of the current paper provide useful information for the design of the solar panels.

The Pittsburgh Steelers, founded in 1933 by Art Rooney, is the fifth-oldest franchise in the NFL. The Steelers first home was Forbes Field, built in 1909, and was the third home for the Pirates. The University of Pittsburgh, "Pitt" Panthers played in Forbes Field (Figure 1) from 1909 until 1924. The Panthers began playing on campus at Pitt Stadium in 1925. Starting in 1958 the Steelers began mixing their home between Forbes Field and Pitt Stadium until 1964 when the Steelers moved to Pitt Stadium where they played until 1969.

Even before the Pittsburgh Penguins National Hockey League (NHL) franchise won the Stanley Cup in 2009, it was clear that the team needed a modern new home to match its growing success. The Penguins' previous facility, the dome-shaped Mellon Arena (previously known as the Civic Arena and owned by the Sports and Exhibition Authority of Pittsburgh and Allegheny County), was nearly 50 years old and was outdated with respect to fan accommodations, number and size of suites, and ability to accommodate various modern shows and concerts. Thornton Tomasetti was responsible for the structural design and construction support engineering services of the Penguins' new home, a $321-million, 720,000-sq. ft. arena. Populous (formerly HOK Sport Venue Event), of Kansas City, Missouri, was the architect of the new, larger venue, which is officially known as the Consol Energy Center. Designed to accommodate hockey and basketball games, as well as other events, the 19,000-seat arena is the first NHL arena to achieve gold certification in the Leadership in Energy and Environmental Design (LEED) Green Building Rating System, which is administered by the U.S. Green Building Council, of Washington, D.C. This paper will discuss the development, design and construction process for the new Consol Energy Center in Pittsburgh, PA. While technical topics will be addressed, the focus of this paper will be the bigger picture topics related to the CM at risk, fast-track design and construction process with the early procurement of critical subcontractors and the effective utilization of the design assist philosophy. We will discuss the close collaboration of the design team and the projects Construction Manager- a joint venture of Pittsburgh-based P.J. Dick and Indianapolis based Hunt Construction to not only select the most economical structural systems and components, but to also develop procurement strategies that supported the fast-track process that allowed the construction to be completed in 24 months, from the August 2008 ground breaking to the 2010 opening. The use of building information modeling systems and the close collaboration of key structural contractors in a design assist role were critical in keeping the project on its tight schedule

Based on test results made of normal-strength reinforced concrete, Joint ACI-ASCE Committee 352 has developed design recommendations for joints of special moment frames including requirements of moment strength ratio, shear strength, confinement, and development length. Due to the interests of the reinforced concrete industry in using high-strength reinforcement for special moment frames, ACI-ASCE Committee 352 appointed a task group to review available test data in the past 30 years and constructed an unified database of 357 beam-column joints made with normal-strength and high-strength concrete. Based on the database investigation and experimental verification, we concluded that current ACI 318 design provisions can be extended to include the use of high-strength reinforcement with modifications in the requirements of joint effective area, confinement, and development lengths. Provided information can be referred for further updating the design recommendations of beam-column joints for high-strength reinforcement.

This paper presents an analytical study that focuses on the development of a parametric equation to predict the joint shear strength. In this analytical study, a database containing geometric properties, material strengths, configuration details and test results of reinforced concrete beam-to-column connection subassemblies tested under cyclic loading is established. Then, based on the experimental results, parameters affecting joint shear capacity are determined. The most influential parameters are selected and an equation to predict the maximum joint shear strength is formed. The developed equation includes parameters that take into account the effect of eccentricity, wide beams, transverse beams and presence of slab on the seismic behavior of the connection region, besides the key parameters such as concrete compressive strength, effective joint width and joint transverse reinforcement ratio. The maximum joint shear strength vales obtained by the proposed equation are then compared with different code approaches and the comparison results are discussed.

Current seismic provisions for determining the amount of transverse reinforcement in beam-column joints vary from one code to another. Moreover, the seismic provisions in current codes were developed for the most severe earthquake effects. There is a lack of an accepted designed method for determining the amount of lateral steel in areas of low to moderate seismicity where the ductility demand may be lower than that in areas of higher seismic activity. This paper will provide an overview of simulated seismic loads tests conducted on reinforced concrete beam-column joint subassemblages at Nanyang Technological University (NTU) during recent years. Research conducted at NTU is aimed at examining the ductility and behaviour of moment-resisting frame with limit ductility in order to develop detailing guide lines for areas of low to moderate to seismicity.

Voided slabs have been used in construction of reinforced concrete floor systems for over 100 years. This article offers an historical perspective on this technology. A discussion of early applications through several case studies is followed by on overview of traditional systems still in use and concludes with a discussion on new technological advancements. The improvements on this "old" concept have taken the efficiency of voided slab systems to the next level with economical spans reaching 50 feet. In voided two-way slabs, a hard shell with struts is created through the use of appropriately located cavities formed by void formers. This is achieved by placing hollow spheres made of recycled plastic between the upper and lower static reinforcement of the concrete slab while displacing concrete where it has no structural benefit. The effect is to decrease the overall weight by as much as 35% when compared to a solid slab of the same bearing capacity. This paper describes traditional two-way slab systems as well as new technological improvements on the design and construction of two-way slab systems.

This paper presents the development of a 3D nonlinear finite element (FE) model to predict the punching shear of two-way reinforced concrete (RC) slabs. Six slabs casted with regular and steel fiber reinforced concrete (SFRC) and reinforced with normal and high strength steel reinforcement were modeled using ANSYS. The results were validated with published experimental data. The computational model accounts for the different nonlinear constitutive material laws by utilizing state-of-the-art modeling methodologies. Concrete cracking and softening, effects of the embedded steel fibers, steel yielding, and the bond-slip mechanism of the embedded steel reinforcement were all taken into account. A good correlation was obtained between the predicted and measured results at all stages of loading including failure of the specimens. It is concluded that correlating models could be used as a reliable tool to conduct parametric studies to evaluate the punching shear behavior of two-way slabs cast with normal and SFRC concrete, reinforced with ordinary and high-strength steel reinforcement.

This paper investigates the behavior of concrete columns under different loadings. The columns are confined with single conventional spiral or two cross spirals. The two opposing spirals (cross spirals) were developed to confine circular columns in order to enhance their strength and ductility. This new technique facilitates the passage of concrete while effectively increasing the volume of the confining reinforcement without violating the minimum spacing as specified by ACI. Twenty-one 1000-mm-long, circular, high-strength columns with a diameter of 350 mm with four different steel ratios (0.016, 0.020, 0.024 and 0.028), and various spiral spacings were built. The columns were tested under monotonic axial loading in order to study the influence of the new confinement technique on the axial compressive strength and ductility of bridge columns compared to columns confined with the conventional single spiral. Also, twelve reduced-scale, reinforced concrete, circular bridge columns with two different lengths and several spiral spacing and patterns were tested. The columns were subjected to constant axial load (bridge self weight) and reversed cyclic lateral displacement (seismic load) to study the influence of the new confinement technique on the lateral strength and ductility of reinforced concrete circular bridge columns compared to columns confined with conventional single spiral. Six of the columns (1000 mm high and 200 mm diameter) were designed to study the flexural behavior, and the other six columns (500 mm high and 200 mm diameter) were designed to study the shear behavior.

This paper presents an experimental investigation carried out on ten reinforced concrete (RC) columns with L-shaped sections, which were subjected to simulated seismic loadings. The primary objective of the study was to investigate the suitability of using L-shaped RC columns in regions of moderate-to-high seismic risk. The variables of interest included the axial load, loading direction and aspect ratio. The specimen performances were analyzed in terms of crack patterns, hysteretic response, initial stiffness, shear strength and drift capacity. This paper hopes to present some preliminary observations of the tests performed.

Seismic demands on steel building frames can result in concentrated inter-story drift and a "soft-story" mechanism in multistory buildings which has led to severe structural damage or collapse in past earthquakes. Such deficiencies in building structures can be found in seismic regions throughout the world and are especially problematic in developing countries. Furthermore, structures near an active fault may be subjected to near-fault ground motion pulses that have tendency to induce very large, concentrated ductility demands in the lower stories of buildings. Economic and sustainable solutions for rehabilitation of such structures are needed in order to reduce seismic risk. An approach for rehabilitation of seismically sub-standard buildings through implementation of stiff elastic rocking cores (SRCRehab) is currently being investigated by researchers. The stiff rocking core (SRC) has the potential to re-distribute seismic forces along the core height, creating a more uniform drift and ductility demand distribution. Due to the core's low rotational resistance at its base and high inter-story stiffness, it does not attract all of the seismic forces through the core to the foundation however re-distributes seismic ductility demands from floors that would otherwise have significant concentrated ductility demands, potentially preventing collapse. The fundamental behavior and interaction of rehabilitated steel frames using a stiff rocking core is presented along with the results of nonlinear, response history analyses which applies suites of ground motions typical of far-field and near-field sites. The results are used to identify the key parameters for effective rehabilitative design of sub-standard frames. The paper also describes the planned hybrid experimental testing program which will be performed at the UB-NEES site. The hybrid testing technique will allow for computational modeling of the existing sub-standard framing and experimental testing of the SRC.

This paper presents the results of a self-centering steel plate shear wall (SC-SPSW) experimental program conducted at the National Center for Research on Earthquake Engineering (NCREE) as part of a collaborative research endeavor. Two full-scale, two-story, SC-SPSW specimens were tested under pseudo-dynamic loading. The specimens investigated two different post-tensioned (PT), beam-to-column connection configurations—one using a PT connection detail where a gap forms in a connection as the beam rocks about its flanges, and one using a PT connection (called the NewZ-BREAKSS connection) where the beam in a connection always rocks about its top flanges, thus eliminating the problem of frame expansion. The test specimens also incorporated a post-tensioned column base connection that allowed the column to rock about its flanges, relying on vertical post-tensioned rods anchored along the column height. The PT column base provides additional recentering capabilities, as well as eliminates the damage and residual plastic deformations that occur in the moment resisting base connections of SC-SPSWs. The results from this project will be used to validate numerical models and inform construction and design recommendations.

Self-centering beam moment frames (SCB-MFs) have been developed with the capability of resilient performance during large earthquakes by virtually eliminating residual drifts and concentrating structural damage in replaceable fuses. The self-centering beams (SCBs) can be shop-fabricated with the self-centering mechanism built into the beam unit, thus eliminating deformation incompatibility with the surrounding gravity framing that is associated with some other self-centering systems. Furthermore, the SCB-MFs allow for conventional field construction methods, making them more accessible to the construction industry. A parametric study was performed on single self-centering beam units to investigate the sensitivity of the SCBs to a variety of design variables. The results of this study show that these units can be implemented in a wide variety of configurations for various beam depths and moment capacities. Additionally, the ability of the system to dissipate seismic energy while minimizing residual drift was demonstrated. Ultimately, this study has shown that SCB-MFs are a viable seismic force resisting system for reducing structural damage due to large earthquakes and have the potential to limit the cost and time required to make a building operational following a strong ground motion.

The Linked Column Frame (LCF) is proposed as a steel seismic load resisting system consisting of two components: a primary lateral system made up of dual columns interconnected with link beams (denoted linked columns); and a secondary moment frame. Previous research has developed a procedure that ensures the links of the linked columns yield at a significantly lower story drift than the beams of the moment frame, enabling design of this system for three distinct performance states: linearly elastic; rapid repair, where only link damage occurs and quick link replacement is possible; and collapse prevention. This paper discusses additional analyses that have been performed to establish recommended values for seismic design parameters, including the response modification coefficient, R, the system over- strength factor, Ω, and the deflection amplification factor (C_d). A suite of archetype LCF buildings were developed and incremental dynamic analysis was performed according the procedures in FEMA P695.

In recent years the evolution of construction techniques and design of pedestrian bridges led to structures more and more slender and light. This process entailed the development of unexpected interaction phenomena owing to pedestrian and wind loads. The impact of wind or pedestrians loads on low stiffness structures resulted in including, in the design phase, the concept of vibrations control and design of structural damping devices. These aspects are the one that we tackled on the Nomi-Calliano footbridge located in the northern part of Italy (near Trento). Semi-active control systems, coupled with smart materials such as magneto-rheological fluids, became more and more attractive. In this respect, the University of Trento, in cooperation with the Autonomous Province of Trento, decided to use a Magneto-Rheological Tuned Mass Damper (MR-TMD), in order to limit discomfort on the aforementioned footbridge. This paper presents the main dynamic characteristics of the footbridge and the preliminary experimental program devoted to the MR fluid characterization. Finally, the optimal clipped control strategy for the control of the MR-damper is outlined.

A negative stiffness device has been tested within a quarter-scale highway bridge model on the seismic shaking table at the University at Buffalo Network for Earthquake Engineering Simulation (NEES) site. Based on the experiments, numerical models have been developed, calibrated, and used to simulate the response of the bridge under a wide range of ground motions. In addition, performance indices have been developed to systematically and quantitatively evaluate the relative performance of different isolation system configurations that employ combinations of positive and negative stiffness as well as various levels of positive damping. Further, the influence of boundary conditions (rigid versus flexible bridge piers) on the effectiveness of employing negative stiffness devices has been evaluated. Finally, concepts for graphical interpretation of the performance indices are presented and used to demonstrate the degree to which employing negative stiffness may be beneficial in improving the seismic response of bridge structures.

Nationally, existing building owners constantly work to update existing building spaces to make them more appealing to potential lessees. These spaces are cleared of old, heavy furniture and tall, heavy partitions to make way for open plans with lightweight cubicle partitions and lightweight furniture. The change in office architecture along with the lighter paperless office lead to lower damping and reduced mass that often bring to light annoying floor vibrations that had never existed before. Structural engineers and architects are hired to assist in these updates, and often make the mistake of failing to review the adequacy of the floors for possible annoyances from walking induced vibrations. Once a problem has been identified, there is little guidance on the necessary standard of care of the renovation or the execution of a successful retrofit to mitigate the vibrations. Structural engineers need better guidance in the investigation and quantification of annoying floor vibrations of existing office space floors, as well as in estimating the likely success of possible retrofits. This paper explores these issues in a real-world office space with annoying walking induced vibrations. Field measurements quantified vibration characteristics and these were compared to established criteria to assist in determining the necessity of mitigation. Multiple mitigation measures were considered to reduce walking induced vibrations and these were explored through finite element modeling. Finally, the performance of a compact pendulum tuned mass damper (PTMD) to mitigate these vibrations was assessed through comparison of before and after field measurements.

It is sometimes necessary to provide retrofit solutions to objectionable floor vibrations due to human activity. Because such problems are usually detected after the design and construction phases are complete, and the owners have occupied the building, the evaluation of retrofit options must be fast and economical. This paper describes a framework for evaluating retrofit options and presents a case study.

Generation of artificial time histories is a promising solution in the absence of sufficient and appropriate (potentially damaging earthquake) recorded seismograms for the seismic time history analysis of structures. In this study, we propose a new method to generate seismograms appropriate for the Central and Eastern North America (CENA). Here, the broadband synthetic seismograms are generated for moment magnitudes 5.5, 6.5 and 7.5 for the CENA region. Utilizing the deterministic synthetics at low frequencies is critical to simulate the characteristics of the actual strong ground motion records like coherent ground motion pulses as well as the near fault effects like forward rupture directivity. In this study, we use the numerical technique of Spudich and Archuleta. The Green's functions which are considered as wave propagation are achieved using the discrete wavenumber finite element method. We consider a spatial random field model to characterize the complexity of the slip distribution on a heterogeneous fault. Also, uncertainties are considered in the method by applying random components in the generation process. Some results for moment magnitude of Mw=6.5 are presented. As it is expected, the effect of forward directivity in the long period velocity time histories is observed in the stations along the strike of the fault.

A simulation-based, probabilistic framework is discussed in this paper for design of viscous dampers based on life-cycle cost criteria. This framework allows for explicit consideration in the design process of all important nonlinearities for both the dampers and the structural behavior, as well as of all sources of uncertainty related to the seismic hazard. It is based on an assembly-based vulnerability approach for estimating earthquake losses, and on description of the earthquake hazard through stochastic ground motion models. In this setting, the life-cycle cost is quantified by its expected value over the space of the uncertain parameters for the structural and excitation models, and is estimated through stochastic simulation. For the design-optimization an algorithm appropriate for costly global optimization problems is adopted. An illustrative example is presented that considers the optimal life-cycle based design of fluid viscous dampers for retrofitting a three-story concrete structure.

Resilience index is used to analyze complex systems, such as communities, when they are subjected to disasters like earthquakes, hurricanes, floods, etc. in order to quantify preventive measures, emergency measures and restoration measures. Physical infrastructures within a community have a certain degree of interdependency. Interdependencies can generate cascading failures or amplification effects which can eventually also affectthe restoration measures right after an extreme event. These effects can be described by a reduction of the resilience index within a given region. In this article, starting from the restoration curves of the March 11, 2011, Tohoku Earthquake in Japan, a method to evaluate the interdependency index and to calculate the community resilience index is proposed. The weights of each infrastructure which are used to evaluate resilience are evaluated starting from the degree of interdependency indices which are evaluated using time series analysis.

The aggregate damage and loss to a portfolio of buildings is often of interest to parties such as insurance companies, developers, political organizations and community planners. There is an additional level of complexity involved with assessing risk on a regional basis because of the correlation present between the structural components within a region. This research proposes a new reliability-based approach to quantify seismic risk for a portfolio of buildings, while incorporating this correlation. The proposed framework characterizes a suite of structures as a system composed of individual building sites and the correlation that exists between them. In the proposed method, an empirically modified First Order Reliability Method (FORM) is used to evaluate damage to various building combinations as well as their influence on overall system damage. Using probability set theory concepts, the damage to each building combination is combined to compute system damage to the portfolio.

Three well-known examples -- I-35W bridge failure, London Hammersmith Flyover closure, and the UK M1 motorway under-bridge fire -- highlight the need for a reliable decision support methodology to enable better informed decisions on timely intervention and/or resilient recovery from a damaging event. It seems that, quite apart from extreme man-made or natural hazards, our transportation infrastructure is not resilient under man-made or natural loads, and we need to leverage technology to better understand and respond to societal risks due to a lack of resiliency. The challenge to improve infrastructure resilience has led to major infrastructure research initiatives that are relevant to the case of bridges. FHWA created the Long Term Bridge Performance Program, while in the UK, EPSRC recently promoted the two themes of resilient infrastructure and monitoring and field investigation of existing infrastructure. The paper will describe these initiatives and how they aim to improve the resilience of bridges, which are key components in our transport infrastructure. It will also suggest some specific activities for developing closer interactions between a wide range of academic and industry stakeholders leading to development effective decision support methodologies.

The objective of this paper is to review and evaluate current methods for communicating risk within the context of infrastructure management decision support systems. A generic model of a risk-based infrastructure management system is presented to illustrate important relationships of key decision support functions and the critical flow of risk communication within this framework. The model includes three primary functions that are deemed necessary to effectively manage risk, namely (i) risk assessment, (ii) decision analysis, and (iii) executive decision/policy making, which typically involve engineers, statisticians, decision analysts, and owner/stakeholders. Another important function of risk-based management systems discussed in the paper includes the communication of risk to system users. The paper will present key metrics and performance measures used to communicate risk by and among various agencies and owners of critical infrastructure systems. The study concludes with a general assessment of the current means and methods of communicating risk within the framework of risk-based civil infrastructure management systems and identifies important needs for future research related to developing more effective and efficient communications.

This paper compares the different reliability-based criteria adopted by different structural design and safety evaluation codes. Ongoing evolution from point-in time, member-oriented, reliability-based criteria to risk-based structural system criteria is described including recent proposals to establish quantifiable measures of structural redundancy and robustness. Issues related to the implementation of reliability-based serviceability and durability criteria are also summarized.

The design and preservation of civil infrastructure systems have been driven, for a long time, by cost minimization while maintaining system reliability at an acceptable level. The growing concerns with aging and deteriorating infrastructures and the need to ensure resilient and sustainable infrastructures and communities require the development and use of innovative construction materials and structural systems and management practices that yield infrastructure resiliency and achieve an adequate balance between social, economic and environmental sustainability. and the emerging needs for sustainable and resilient infrastructure and communities. This paper discusses some key performance measures and approaches that can be used to assess resilience and sustainability and presents a risk-based decision-based approach to help decision-makers optimize the design, evaluation and management of infrastructures that considers all possible hazards and provides alternative risk mitigation strategies that can be evaluated using a cost-benefit analysis, and rational criteria are presented to support the selection of the most sustainable and resilient risk mitigation strategy indicators, such as safety, serviceability, costs, traffic disruption, greenhouse gas emissions, which can be used for life cycle design of highway bridges. An example, taken from the North American context, illustrates how different design and rehabilitation approaches can contribute to achieve the sustainability of a highway bridge.

Accelerated ageing tests in lab indicated that tensile properties of GFRP bar reduced with exposure to simulated alkaline solution at elevated temperatures, which resulted in concerns of using GFRP bar in concrete having high pH value pore water. Developing a convincible Environmental Reduction Factor (ERF) to account for GFRP long-term durability is of significant importance. In this paper, a newly developed GFRP long-term performance prediction approach that incorporates effects of time, temperature, and moisture, is adopted to develop ERFs for GFRP bar as to be applied in the United States. Average monthly temperature and relative humidity across the US are used to simulate real application environments. ERFs obtained from this research are compared to that in ACI 440.1R-06 to justify the current code-defined ERFs, and corresponding modifications to current guideline are suggested.

Aftershocks are known to cause collapse of structures after acquiring damage from main shock. When strong main shock strikes, reinforced concrete (RC) structures are often vulnerable to impact of aftershocks as they already are weakened due to damage accumulation. This research presents a new type of shape memory alloy (SMA)-based composite reinforcement with ability to withstand seismic forces from main shock and aftershocks while exhibiting pseudo-elastic behavior. In this study, small diameter SMA wires are embedded in thermoset resin matrix with or without additional glass fibers to develop composite reinforcement. These SMA composite specimens are first tested under quasi-static cyclic tensile loading to achieve constitutive material response and then to develop analytical material models. These experimentally calibrated material models are used in structural level models to assess the performance of RC structures under seismic loading. Three storey, one bay prototype RC moment resisting frames (MRFs), reinforced with SMA composite and steel reinforcements, are subjected to sequential ground motions to study their nonlinear behavior and impact of aftershocks. These RC-MRF frames are subjected to incremental dynamic analysis using suite of ground motion records from main shock-aftershock earthquake sequences and a comparison is drawn based on accumulation of damage and residual drifts. It is found that the sequential incremental dynamic analysis technique is a good means to perform post main-shock damage assessment and to evaluate the response of reinforcement structures. Numerical results also show superior performance of SMA composite reinforced MRF in terms of dissipation of energy and accumulation of lower residual drifts.

Fiber Reinforced Polymer (FRP) has been widely used in strengthening civil engineering structures for decades. Well-established analytical procedures can be applied to design-strengthening systems for beams and columns in flexure, provided that special attention is paid to the bond between the concrete and FRP (Triantafillou, 1998). However, the study on shear behaviour is still immature. This paper presents the results of an ongoing project aiming at developing a rational model to evaluate the behavior of FRP-strengthened RC elements subjected to pure shear load. To achieve this goal, a series of large-scale tests were conducted using the Universal Panel Tester housed at the University of Houston (UH). In the past three decades, the UH research group proposed a series of analytical models to predict the shear behavior of RC members, of which the most recent is the Softening Membrane Model (SMM). The SMM has been proven to be able to predict the whole stress-strain curve of the RC member under pure shear (Zhu and Hsu, 2002). By utilizing the new constitutive law proposed by Moslehy (2010), a new SMM-FRP was developed. The current test results show a good agreement with the results calculated by SMM-FRP.

A challenge to the current progress towards sustainability of built environment is to quantitatively predict the long-term behavior of the critical concrete structures, e.g., large-span prestressed box girders, super-tall buildings, nuclear reactor containments, and radioactive waste repositories. An accurate prediction of multi-decade performance requires realistic approximation of the unique porous microstructure of concrete, which transforms with its hygrothermal evolution. To minimize the phenomenological formulation in constitutive modeling and ameliorate the computational burden in numerical simulation, a multi-scale model of concrete, with a focus pinned on mesoscale, is proposed in this study to break down the long-term behavior of concrete to the time-dependent mechanics of its different phases. In this model, concrete microstructure is represented by three material phases: coarse aggregate, mortar matrix, and the interfacial transition zone (ITZ) between the first two. In the framework of the classic continuum micromechanics, a Mori-Tanaka type homogenization scheme, enriched by continuous retardation spectrum method based on Laplace transform and Widder's approximation, is first formulated to capture the aging viscoelasticity of the representative unit cell of concrete which includes one aggregate only. The stress concentration in the ITZ, where micro-cracking will initiate and accumulate, is calculated by exterior point Eshelby solution, and then the corresponding damage strain in ITZ is taken into account as a smeared effective strain on the aggregate. The proposed multi-scale material model is incorporated into ABAQUS, and good results are obtained in simulations of some benchmark matrix-inclusion problems. Its effectiveness to realistically approximate the concrete long-term performance is also illustrated in a structural analysis of a prestressed concrete member.

Pipes carrying pressurized fluids are an important part of the civil infrastructure, and structural health monitoring (SHM) could ensure structural integrity by predicting and preventing structural failures. Guided wave ultrasonics is a good candidate for use in pipe SHM because guided waves can propagate long distances and are sensitive to structural damage such as cracks and corrosion losses. However, the multi-modal and dispersive characteristics of guided waves make it difficult to interpret their arrival records. Moreover, guided waves are also sensitive to environmental and operational variations, limiting the effectiveness of ultrasonic methods to detect pipe damage in a real environment. We introduce a damage detector based on singular value decomposition (SVD) that can identify a change of interest, caused by a mass scatterer that simulates subtle damage, under realistic environmental variations. We show the effectiveness and robustness of this method on experimental data collected on a pipe segment under realistic environmental and operational variations over a time period of several months.

The state-of-the-art in computational inverse mechanics approaches that address concepts in smart structure technologies, including nondestructive evaluation and shape changing technology, is presented. The primary focus is on techniques that utilize computational mechanics foundations (e.g., the finite element method) to simulate a structure's behavior, such as predicting sensor responses or predicting behavior during active shape change, and then utilize the simulation to solve the corresponding inverse problem. Through examples of computational methods for nondestructive evaluation and optimal design of morphing shape memory polymer structures, the benefits and challenges of implementing these computational inverse mechanics approaches is discussed, along with potential research needs to move into the future.

Load testing and displacement monitoring was performed on marble panels installed on a mock-up display at an active construction site located at a national tourist attraction. Applied loads simulated gravity, in-plane seismic, and out-of-plane wind forces. Displacements were measured and plotted in real-time under measured loads. A steel test frame with built-in boundary elements was designed for ease of installation and transition between load test setups. A description of the frame design, load application methods - including the application of low-magnitude, in-plane forces - instrumentation, and data reduction for the on-site tests will be discussed. Also discussed are challenges involved with performing this type of testing (typically performed in a laboratory setting) on a construction site at an operational national tourist attraction.

In PSD testing, measured signals from the experiment are used in command generation on the fly, thus making this method prone to error propagation. One way to assess the accuracy of the real-time (or fast) PSD test results is to assess the accuracy of the tracking of command displacements imposed by the hydraulic actuators. This paper introduces indicators that can be used to uncouple and quantify actuator lag/lead (i.e., phase errors) and undershoot/overshoot (i.e., amplitude errors) errors that occur while imposing the command displacements dynamically in real-time. These closed-form indicators provide monitors that are not experiment-specific and are simple to use. They use uncomplicated mathematical functions, which makes them suitable candidates for online implementation with the potential for incorporation into the control law to improve the actuator control. The procedure and the associated theoretical background are explained for each step and then the properties of the indicators are examined through several predefined command and measured signals with known characteristics. In addition, the performance of the indicators is evaluated by comparing the results with those revealed by the previous indicators.

This paper discusses the design and experimental evaluation of a novel seismic-resistant reinforced concrete (RC) coupled shear wall system. In this system, the widely-used unbonded post-tensioned floor slab construction method is adapted to couple (i.e., link) two RC wall piers, providing significant performance and construction benefits over conventional RC coupling beams in high seismic regions. Previous experiments of post-tensioned coupled wall structures are limited to floor-level coupling beam subassemblies. The current paper extends the available research to multi-story structures by presenting the design of an 8-story prototype test specimen consisting of two C-shaped shear walls. The design is validated through the testing of a simplified 15% scale specimen in the laboratory. The experimental specimen includes the foundation, the first three floors of the shear walls, and the associated coupling beams. The upper stories of the building are simulated with hydraulic jacks that supply the appropriate bending moment, shear, and axial forces at the top of the laboratory structure. This paper compares the measured load displacement response of the laboratory structure with predictions from design models. Experimental and design predictions of several key behavior parameters are shown to match well. Future work involves the construction and testing of large scale (40%) specimens to validate the approach. Ultimately, the measured information from the test specimens will be used in the development of validated design procedures and modeling/prediction tools for multi-story post-tensioned coupled wall structures.

This presentation introduces a new acceleration control method for shake tables that provides high-performance and robustness. Unlike the conventional displacement feedback control that is used in most servo hydraulic shake tables, the proposed control method adopts direct acceleration feedback control. While acceleration feedback is not practically employed due to stability issues associated with table drift, the proposed method allows the use of acceleration feedback by adding force feedback loop for stabilization. In this study, controllers for the acceleration and force feedback loops are designed using loop shaping techniques incorporating control-structure interaction. The proposed control method was experimentally investigated at the Johns Hopkins University under two loading conditions: without payload and with payload close to the capacity of the shake table. Experimental results showed that the proposed acceleration control method combining acceleration and force feedback provides better acceleration tracking and wider controllable frequency range than the conventional displacement feedback control in both loading conditions. This presentation will cover basic theory of the proposed method, experimental results, and requirements to upgrade existing shake tables with the proposed control method.

Composite construction is commonly adopted in major projects now. Large size columns, beams and slabs are made compositely and connected together. Such connections in the past did not take advantage of composite behavior and designed as steel connections. Modern codes make provisions for design and construction of connections compositely. It is, therefore, essential to understand the behavior of such composite connections. This paper is concerned with beam to column composite connection subjected to cyclic loading. Results obtained from the finite element analysis of composite joints tested by other researchers are presented. The beam-to-column connections comprise of end plate welded to the beam and bolted to column flange. Analysis are carried out by finite element analysis through a computer code LUSAS. The connections are modeled with a mesh established as efficient and effective in terms of analysis time and accuracy of the results by a convergence study. The results are presented in the form of moment-rotation curves and the numerical results compared with the corresponding experimental values.

The critical load of a column, compression member or structure calculated from a linear elastic analysis of an idealized perfect structure, does not necessarily correspond with the load at which global instability of a real structure occurs. This calculated critical load of a member does not provide sufficient information to determine when failure, due to instability of the structure as a whole, will occur. To obtain this information it is necessary to consider the initial geometrical imperfections, eccentricities of loading, and the entire nonlinear load deflection behavior of the structure. However, except for rather straightforward structural framing systems, this process for determining the critical load is too cumbersome and time consuming to be used in practical engineering applications. Today's computer programs allow for the analysis of complex structures in which advanced analytical techniques are incorporated such as step-by-step large deformation analysis, local member buckling analysis, progressive collapse analysis, etc. It is just a natural progression that global stability analysis for structures be analyzed with these tools. The goal of this paper is to propose a practical method for stability evaluation and assessment of structures that are difficult to analyze with conventional methods, (e.g. the compression chord of a truss pedestrian bridge or a wind girt). This is done with the use of computer programs in which a second order analysis can be readily accomplished by taking into consideration end-restraints, reduced flexural stiffness and initial geometrical imperfections or out-of-plumbness.

Modeling uncertainty can have a significant impact on the assessed risk of earthquake-induced damage and collapse in a building obtained through the performance-based earthquake engineering framework. This paper quantifies the effect of modeling uncertainty on performance-based risk assessments, accounting for differences in software platforms, solution algorithm, element-types, and model parameter calculation or selection, using results of the 7-story reinforced concrete (RC) building blind prediction contest (2006) at University of California at San Diego (UCSD). The blind prediction test data provide a unique opportunity to quantify the influence of modeling uncertainty on structural response predictions. In this study, the bias and variability of predicted drift in the contest submissions are taken to represent modeling uncertainty. The modeling uncertainty quantified from the UCSD test submissions is combined with uncertainty due to record-to-record variability and a set of fragility curves are computed for different drift levels. The final fragility curves account for modeling and ground motion uncertainty. Results are compared with the results with no modeling uncertainty. The key contribution of this study is to investigate the uncertainty embedded in the response due to combination of ground motion and modeling uncertainty.

To evaluate the behavior of pre-tensioned concrete girders, a model that accurately simulates the different stages of the pre-stressing operation through a complete nonlinear analysis is needed. The proposed model is based on the computationally efficient two-field mixed formulation, in which deformations and forces are approximated within the finite element independently. The mixed formulation is usually characterized by its robustness under severe loading conditions and fast convergence with very few finite elements. The model for the pre-stressed concrete element is made up of three components: a beam-column element that describes the behavior of concrete and bonded mild reinforcements, a tendon element that describes the behavior of the pre-stressing tendons, and a bond element that describes the transfer of forces between the pre-stressing tendon and the beam-column element with special bond stress-slip relations. The inelastic response of the beam-column and tendon elements is based on section discretization into fibers with uniaxial hysteretic material models. Correlation studies of the proposed model with experimental results of high strength pre-tensioned specimens that were recently tested at the University of Houston Structural Research Laboratory are performed. The studies confirmed the accuracy and efficiency of the proposed model in describing the local and global behavior as well as the proper failure mode.

Residential buildings in coastal areas are often at risk to hurricanes which can result in both wind and surge damage. In the United States, economic losses average about $5.4 billion annually from hurricanes and greater than 75% of declared federal disasters are a result of flooding. Although current design codes do consider load combinations, these are generally for non-environmental (natural hazard) loading and focus on dead and live loads, and then their combination with each environmental load being considered individually. In this paper correlations between hurricane winds and wind-induced surge from a flood standpoint are considered in order to estimate the loss to residential construction due to combined wind and surge using assembly-based vulnerability. The method presented herein for estimating loss due to combined wind and surge in hurricanes may be used for design code assessment and calibration, retrofit planning for buildings in coastal areas, disaster planning purposes, and potentially for insurance underwriting.

This paper presents a standardized methodology to analyze the impact of disasters on the functionality of healthcare systems post-disaster. The methodology to collect loss-of-function data was developed in 2010 and tested after the Bío-Bío, Baja California, and Christchurch earthquakes of 2010 and 2011. The survey tool collects field data on the performance of critical building systems and infrastructure, assesses the impact of system and infrastructure failure on the ability of hospitals to keep functioning, and provides data that can enhance existing and future tools to assess the performance of healthcare facilities. These survey tools can be used by other reconnaissance teams to collect data from both the healthcare and engineering perspectives, as well as an event tree which links the two perspectives together. This multi-disciplinary approach, involving engineers, epidemiologists, and health and emergency managers, is necessary to properly analyze how structural and non-structural damage disrupts essential services provided by hospitals.

The study proposes a method utilizing 3D nonlinear dynamic analysis of buildings with the objective of achieving predictable seismic performance and building strengthening that translates to the owner's requirement. The frame model assumes that the nonlinear behaviour of the structure is concentrated in plastic hinges, modelled as spring system located at element ends. The nonlinear parameters of the spring systems are derived from the actual member section. The model is verified via agreement of elastic and nonlinear behavior with that obtained using conventional software. The dynamic simulation allows the analysis of progressive collapse sequence providing ease in the assessment of performance and identification of critical members. The ground acceleration data used is selected to be compatible with the project site's seismic design spectrum. Strengthening critical members is accomplished by recalculation of spring parameters based on the capacity increase and the material used. This paper analyzes a newly designed and an existing residential building using the proposed method and shows that it is capable of simulating the seismic performance and the potential collapse mechanism. The results show that increasing member dimension and applying CFRP are applicable to prevent collapse.

The 2010 Maule, Chile earthquake resulted in significant damage to many mid- and high-rise shear wall buildings. Damage data on many of these buildings was collected by post-disaster reconnaissance teams and construction drawings were obtained from local design firms. This data collection enabled the evaluation of damaged buildings to determine the lessons learned for the advancement of U.S. codes and standards. A study of five mid-rise buildings was carried out as part of this evaluation program; three structures were severely damaged and two of them ultimately demolished, one building sustained minor damage and was repaired, and one building was reported to have no structural damage. The objectives of the study included evaluation of current assessment tools (ASCE/SEI 31-03 and 41-06) and evaluation of current analysis methods to predict observed damage. Assessment of ASCE/SEI 31 consisted of performing Tier 1 checklist evaluation to identify potential deficiencies. Deficiencies were then evaluated using the Tier 2 linear dynamic procedures of ASCE/SEI 31 and 41. The results of these evaluations are presented, along with recommendations for additional evaluation criteria for deficiencies observed in the buildings studied. The ability of current analysis methods to predict observed damage was evaluated by performing nonlinear dynamic analysis of one building using OpenSees and Perform3D structural analysis softwares. The results of these different analysis procedures are presented in this paper.

This paper presents preliminary results of a broader ongoing experimental study on the seismic bond strengthening of the critical reinforcement splice region in wide reinforced concrete (RC) columns or bridge piers confined with carbon fiber reinforced polymer (FRP) laminates and having bending about their minor axis. Large lateral drifts induced by strong earthquakes may result in splitting bond failure of the spliced bars, or rapid bond degradation at the splice location, leading to possible structural failure of the columns or piers. Previous research has verified the effectiveness of external confinement using FRP composites for improving bond performance of the spliced reinforcement of columns or piers having circular cross section, or rectangular cross section subjected to cyclic loading about their major axis. The preliminary experimental program reported in this paper consists of testing two full-scale wide column specimens under extreme substandard splice detailing associated with relatively short splice length, small concrete covers, small transverse spacing between the spliced bars, and complete absence of internal steel ties within the splice region. The test results showed that, despite extreme substandard splice detailing, the use of external FRP confinement improved the bond strength of the spliced bars, delayed the bond stiffness degradation under cyclic loading, and increased the energy dissipation capacity of the wide columns. However, the corresponding improvements were not as effective as anticipated. Consequently, before a final conclusion is drawn, more tests will be carried out on similar specimens but using instead more realistic and practical splice details as well as internal transverse steel ties for satisfying the minimum detailing requirements of the ACI 318-11 for gravity load designed axial members.

Post-earthquake reconnaissance and experimental research indicate that the beam-column joint is a crucial zone in reinforced concrete (RC) moment resisting frames. Due to their exceptional mechanical properties and high strength to weight ratio, fiber reinforced polymers (FRP) are considered to be one of the most promising materials for beam-column joint retrofitting in RC structures. In this study, the efficacy of joint retrofitting in upgrading the seismic performance of RC structures is numerically investigated through displacement-based adaptive pushover and inelastic dynamic time history analyses. Four typical FRP strengthening schemes between the beam and column are investigated and the corresponding structural global ductility and seismic performance for FRP joint-retrofitted RC frames are evaluated through both the seismic behavior factor and the inter-story drift ratio. The proposed method is demonstrated by seismic behavior of an intermediate-rise RC building under earthquake forces. The numerical results indicate that the proposed retrofitting scheme can greatly improve the seismic performance and global ductility of RC buildings.

There are many finite element modeling (FEM) techniques used in practice and research to interpret behavior and gather responses of structures subjected to earthquake excitations. The application of particular FEM techniques in a structure typically depends on the engineering demand parameters (EDPs) or structural responses of interest, along with other considerations regarding computational effort and level of fidelity required. A non-ductile reinforced concrete (RC) building may suffer from many design deficiencies when located in a seismically active region. In this paper, a localized shear and joint model are chosen to incorporate into the FEM in order to gather global, intermediate, and local EDPs that correlate with observed areas of concern in non-ductile RC buildings. Four numerical model combinations are developed to perform a probabilistic seismic demand analysis on the gathered EDPs. Based upon the results of these simulations, a new uncertainty value (σ_FEM) is established to reflect the uncertainty introduced by adopting different types of numerical models. The σ_FEM addresses an uncertainty that exists from assuming more simplified models of structural behavior, and allows researchers and industry to idealize numerical models for analysis while still reflecting this uncertainty.

Nonlinear seismic analysis techniques have increasingly been used in performance-based seismic engineering to analyze and design new and existing structures, especially in the case of analyzing structural collapse. It is evident that any engineered structure will experience significant nonlinear response prior to collapse, and this nonlinear response requires inclusion of both geometric nonlinearity and material nonlinearity in the analysis. All analysis software packages today, both commercial and non-commercial, claim to possess both geometric and material nonlinearity capabilities, but so far no study has been done to demonstrate that these software packages give reasonably consistent matching results. In this research, nonlinear response history analyses of two simplified structural models subjected to seismic excitation are performed to evaluate the output consistency of different computational software tools that are commonly used in research and practice today, in addition to the nonlinear modal analysis algorithm. To do so, simulated results are compared with one another to demonstrate the relative precision and consistency of these computing tools for analyzing structural collapse, and the pros and cons of the assumptions made among these tools are also identified.

Fluid viscous dampers are added to building to protect its structural, nonstructural components, and contents. Dampers are designed to resist forces produced by the maximum considered earthquake. The efficacy of dampers in providing seismic protection and the code-prescribed methodology for design of these units have been well established and have been validated by the excellent performance of buildings with dampers in the past earthquakes. However, little data is available for buildings with viscous dampers subjected to large earthquakes. A multi-year research project is currently investigating such cases. Initial analyses revealed that the limit states of viscous dampers have a significant effect on the response of the building to which they are installed. Analytical model of steel buildings with viscous dampers, incorporating damper limit states, have been prepared and subjected to incremental dynamic analysis to determine the collapse performance. To date, analysis has shown that performance of the buildings with viscous dampers was acceptable when subjected to large earthquake. Furthermore, the use of factors of safety to delay the onset of reaching limit states was seen to be both beneficial and cost effective.

Nonlinear response history analysis has generally been utilized in seismic design practice to analyze structures with unusual geometry that possess significant post-elastic ductility. Nonlinear analysis has also been used within a performance-based design framework to demonstrate adequate seismic performance and address non-conformance with prescriptive building code provisions. This paper presents a case study on the inelastic seismic response of a tall, round, concrete tower that exceeds code-prescribed height limitations for a concrete shear wall system. Features of the tower include an offset observation cab, with a unique cantilevered system and vertical post tensioning to provide self-centering during major earthquake events. Material nonlinearity, post tensioning effects and foundation elements were modeled explicitly. A suite of hazard-specific ground motions were selected and scaled to account for numerous factors, including soil nonlinearity. The seismic response of the tower was evaluated, with special attention devoted to the selection of appropriate performance goals and acceptance criteria.

Elastomeric and lead-rubber seismic isolation bearings have been widely used in the United States and around the world for the past thirty years. During earthquake ground shaking, these bearings will be subjected to vertical compressive loads due to gravity plus seismic forces, accompanied by simultaneous large lateral displacements. The design of isolation systems composed of these bearings, therefore, requires stability of the isolation system and individual bearings at the maximum displacement be demonstrated. The current codified procedure for assessing the stability of individual bearings uses a ratio of areas, referred to as the reduced area method, to determine the critical load capacity of the bearings at a given lateral displacement. This critical load capacity must be greater than a combination of vertical forces imposed on the bearing for stability to be demonstrated. While the reduced area method provides a simple means for estimating the critical load of a bearing at a given lateral displacement, it lacks a rigorous theoretical basis and has been shown to provide inconsistent predictions by comparison to data obtained from quasi-static testing of individual bearings. Dynamic stability testing of two isolation systems composed of four elastomeric bearings was performed at the University at Buffalo using the UB-NEES earthquake simulators to gain an improved understanding of the stability behavior of the individual bearings and global system under representative earthquake ground shaking. This paper presents: (1) a summary of the dynamic stability tests; (2) sample experimental results; and (3) a discussion of the implications for the assessment of stability in design. The new experimental results presented in this paper demonstrate: (1) instability in individual bearings does not necessarily lead to global instability; (2) the state of the global isolation system should be used to assess stability to economize the design; and (3) the reduced area method is not able to accurately predict the global stability of the isolation system.

Seismic assessment of steel frame structures is typically concerned with analytical modeling of the lateral load-resisting system only, ignoring the effect of composite action on its lateral stiffness and strength. Based on a recently developed database for deterioration modeling of steel beams with reduced beam sections (RBS), the effect of composite action on their bending strength and deterioration parameters (e.g. plastic rotation capacity, post capping rotation capacity, rate of cyclic deterioration) under cyclic loading can be quantified. These connections are widely used in United States in design of special steel moment resisting frames. A phenomenological model, which is able to simulate important deterioration modes when a steel component is subjected to cyclic loading, has been modified to incorporate slab effects on moment-rotation characteristics of composite steel beams. The effect of composite action on the collapse capacity of SMFs is demonstrated through a case study of an 8-story steel building.

This paper investigates collapse probability of steel buildings from mainshock-aftershock sequences, as part of the ongoing research to develop a framework to integrate aftershock hazard into Performance-Based Engineering (PBE). During earthquake events, aftershocks can be observed following the mainshock. Aftershocks have the potential to cause severe damage to buildings and threaten life safety, even only minor damage is present from a mainshock. To date, the description of seismic hazard in PBE has not included the probability of aftershocks. Three approaches to generate collapse fragility for steel buildings that sustain certain level of damage from a mainshock are used to investigate the effects of mainshock-aftershock sequences. It is found that structural collapse capacity may reduce significantly when the building is subjected to a high intensity mainshock. As a result, the structure is likely to collapse even if only a small aftershock follows the mainshock.

This paper presents the extension of two new displacement-based seismic design methods referred to collectively as Modal Displacement-Based Design (MDBD) to multistory one way asymmetric plan wall structures. The proposed design procedures are verified for a 10 story RC wall structure. The first method, called ‘Modal Pushover Design' (MPD), is iterative at the single degree of freedom (SDF) system response spectra (RS) level and the second, called ‘Direct Modal Pushover Design' (DMPD), is non-iterative. Advantages of MDBD methods include direct consideration of torsional and higher mode effects in the design procedure and lighter reliance on empirical capacity design equations for establishing design shear and elastic moment capacities.

This paper describes an experimental investigation on the scaling of ground motion records for use in the nonlinear response history analysis of building structures. The study is based on the measured response from small-scale shake-table experiments of nonlinear multi-story building frame structures under ground motion suites that have been scaled using different methods. Four structural configurations with different fundamental periods and lateral strengths are investigated to assess the effectiveness of the scaling methods in reducing the dispersion in the seismic lateral displacement demands of the structures from a total of 720 shake-table tests. Four scaling methods are examined, including three methods that depend on a priori knowledge of the properties of the structure being analyzed, as well as a method based solely on the properties of the ground motions. The experimental results show that the method based solely on the properties of the ground motions generally produced demand results with less dispersion than the other evaluated scaling methods. However, the methods that rely on prior knowledge of the structure performed better at preserving the benchmark median demands for several of the cases considered.

Two-stage analysis has been performed by structural engineers since its introduction to the code in the 1988 Uniform Building Code. This analysis procedure simplifies the analysis and design of a flexible structure supported by a rigid structure while allowing proper base shear scaling and use of R-factors of the two structures. However, despite its original intent, two-stage analysis can prove to be difficult to implement in complicated structures. Through our experience with the design of two new hospitals and evaluation of one existing hospital, we highlight to practicing seismic engineers and code developers some challenges and recommendations regarding (a) code interpretation and intent of the two-stage analysis provisions, (b) application, idealization and modeling of boundary conditions and soil springs, and (c) use and limitations of analysis models and software.

Calibrating modeling software hysteretic behavior against experimental data can improve the overall accuracy of a nonlinear response history analysis. Commonly used design standards provide no direct guidance regarding the hysteretic behavior of inelastic hinges under cyclic loading, which leaves the task of defining the hysteretic curve and its defining characteristics to the analyst. Since the amount of energy dissipated by inelastic components during a nonlinear response history analysis provides a significant source of the total damping of the system, the proper modeling of the shape of the hysteretic curve is critical to the reliability of the analysis results. By comparing the hysteresis loops from test data to those generated by a widely-used analysis program, the authors developed methodologies to perform this calibration in an accurate manner. This rational, formal approach represents a significant improvement over the current treatment of this issue, where little or no effort is spent matching the shape of the hysteretic curve.

This paper summarizes studies of the ATC-84 Project related to the potential use of risk-based concepts and criteria in future editions of seismic codes. Risk-based concepts provide a quantitative basis to incorporate specific functional, economic and life-safety performance objectives in seismic codes. Risk-based concepts are part of the recommendations of the ATC-84 Project for re-formulation of seismic code design requirements that also include future development of improved values of seismic performance factors (SPFs) of current code-approved seismic force-resisting systems using the methodology set forth in FEMA P-695 Quantification of Building Seismic Performance Factors (FEMA 2009a). The findings of the ATC-84 Project are published in the National Institute of Standards and Technology report, NIST GCR 12-917-20 Tentative Framework for Development of Advanced Seismic Design Criteria for New Buildings (NIST 2012).

The use of viscous fluid dampers (VFDs) and buckling restrained braces (BRBs) is common in seismic regions. These devices are very successful in robust energy dissipation and they use different mechanisms in preventing damage to structural systems. These devices are not used together very often, but a hybrid system may act more efficiently for a wider range of earthquake magnitudes. The building benefits more from VFDs at low magnitude earthquake. For a moderate event, both VFDs and BRBs come into action; and, for extreme events, VFDs may help when BRBs tend to exhibit large maximum and residual drift. This paper presents a study using varying configuration of VFD and BRB in a reference test model. These devices are employed in alternate-diagonal configuration at each story and the number of these devices is varied. A series of nonlinear time history analysis is performed and various analysis responses are captured. The results from the study are used to understand dynamic characteristics and response analysis of such a hybrid system and how it may be employed to get the optimum response.

Building structures can be critically affected by impulsive loads such as blasts, collisions, gusts, and pulse-dominated earthquakes. The addition of nonlinear energy sinks (NESs) in buildings has been proposed as a means to rapidly and passively dissipate the energy in a system exposed to this type of loading. This rapid dissipation occurs because the essential nonlinearity of the NES allows it to resonate with any mode of the structure and engage in targeted energy transfer, the nearly one-way transfer of energy to the NES where it is locally dissipated. Additionally, the NES couples the modes of the structure and facilitates the transfer of energy from the lower modes of the structure to the higher modes, where it can be dissipated at a reduced time scale. In this study the experimental performance of a system of multiple NESs in a large 9-story test structure is discussed. Two different types of NESs are used, each of which employ a different type of restoring force; one type of NES utilizes a smooth restoring force that is roughly cubic, while the other utilizes a linear restoring force coupled with one-sided vibro-impacts. To load this system, an impulse-like ground motion is applied via a large shake table. The results of this study show that the system of NESs greatly improves the performance of the structure across a wide range of impulse amplitudes by reducing and very rapidly attenuating its response.

The aim of this paper is twofold. First, a practical methodology previously proposed for the optimal design of linear viscous dampers is modified here for the direct design of nonlinear viscous dampers. Then, a rigorous comparison is conducted of the seismic behavior of structures optimally retrofitted with linear viscous dampers versus that of structures optimally retrofitted with nonlinear viscous dampers.

Floor diaphragm in-plane flexibility in concrete buildings was ignored for simplicity by structural engineers in practical design until ASCE7 Building Standard (ASCE 2005) acknowledged that this assumption can result in considerable errors when predicting the seismic response of RC buildings with diaphragm plan aspect ratio greater than 3:1. Such provisions are supported by a comprehensive experimental and analytical research conducted at SUNY Buffalo and Lehigh University in late 1980's for solid reinforced concrete slab panels (i.e., without openings) where their hysteretic characteristics were experimentally evaluated using inelastic cyclic testing of slab panel subassemblies and were included in a developed computational tool (IDARC2) for inelastic dynamic analysis by using a three-parameter hysteretic model to account for the stiffness degradation, α, strength deterioration, β, and bond-slip pinching, γ. Subsequently, IDARC2 predictions of the inelastic performance of concrete floor diaphragms were verified by two shake-table tests of one-story concrete buildings (Reinhorn, 1988; Panahshahi, 1991 and 1994; and Kunnath, 1991). Due to lack of similar research on seismic response of concrete floor diaphragms with openings, in the current study the effects of varying theses three hysteretic parameters (± 25%) on seismic response of a three story reinforced concrete buildings with end walls with 4:1 plan aspect ratio with floor diaphragms openings located in St. Louis, Missouri, are investigated. It is concluded that although the floor diaphragms element experienced considerable amount of inelastic deformations, the maximum top story frame displacement, building base shear force and its distribution to interior frames did not change significantly by varying the three hysteretic parameters.

The seismic behavior of large low-rise buildings with rigid walls and flexible diaphragms will be dominated more by the diaphragm's seismic response than by the very stiff vertical walls. For practitioners, estimating the stiffness of large flexible diaphragms is important for computing building setbacks from property lines and adjacent structures as well as evaluating structural integrity under seismic loads. In addition, researchers attempting to accurately model a building's dynamic behavior need to assemble an accurate diaphragm stiffness prediction. The traditional diaphragm chord model consists of a single continuous line of axial resistance at the diaphragm boundaries; however, as this paper will demonstrate a collective series of structural members distributed across the diaphragm will function intentionally or unintentionally as a collective chord, adding significant flexural stiffness and reducing chord forces. In seismically active areas, masonry and concrete wall anchorage forces utilize code-mandated continuous cross-ties within the diaphragm, and often these cross-ties are sufficiently strong and stiff to unintentionally develop collective chord behavior whether in steel or in wood diaphragm systems. While neglecting this embedded collective chord behavior results in conservative chord and diaphragm drift designs, researchers or practitioners trying to predict seismic response of these buildings will potentially underestimate the true seismic response.

The floor plates of buildings play an important role in redistributing horizontal forces among vertical lateral-load carrying substructures. It is a customary in engineering practice to assume a rigid diaphragm constraint to model floors for lateral force analyses of building structures. The concept that building structures with flexible diaphragms behave differently from systems with rigid diaphragms is fundamental. The practice of assuming rigid diaphragm is not necessarily conservative. In the past, studies have been carried out, predominatly for low-rise building, to find out differences in response if floors were modelled as flexible diaphragm instead of rigid. The objective of this paper is to evaluate the impact of diaphragm flexibility on the structural response of a tall building. A detailed model of the floor that includes all primary structural members is constructed. The in-plane diaphragm stiffness of the detailed floor model is computed and used to arrive at an equivalent shell element floor model. This equivalent shell element model is then inserted in the 64-story diagrid tall building global model. The study shows that, structures modelled with flexible diaphragms can experience higher accelerations and displacements than structures modelled with rigid diaphragms, and their fundamental periods of vibration can be noticeably longer.

Concentrically braced frames (CBF) are among the most popular structural systems used in Eastern Canada for low- and middle-rise buildings. Across southern Quebec, in the area of St. Lawrence and Ottawa River valleys, existing vulnerable buildings, designed and built prior to the development of modern seismic design codes are at risk due to an increase in earthquake activity. This study is focused on the seismic assessment of 3- and 6-storey CBF buildings located in Montreal and Quebec that were designed based on the 1980 Canadian code provisions. Using incremental dynamic analysis and the fitted fragility curves, the probability of failure of selected structures are estimated. It is concluded that both low- and middle-rise CBF buildings located in Montreal and Quebec are exposed to earthquake damage risk.

The structural response of steel moment resisting frames (MRFs) is greatly dependent on the behavior of beam-to-column joints, according to a properly detailed beam-bolts-plates-column structural chain, in light of capacity design principles. A modeling procedure for bolted top-and-seat angle components and connections for potential use in seismic MRFs is presented herein. Although these partially-restrained connection systems have been demonstrated to provide economic savings, they are not currently certified to be used for moment resistance in any major building specification jurisdiction. Examples of full-scale moment resisting connection systems, experimentally tested in past programs, have been numerically analysed, focusing on top-and-seat angle components, which were observed to control the global response of the joint in terms of failure mechanisms, limiting the displacement ductility capacity and dissipation energy capabilities of the whole resisting system. Refined non-linear solid FEM models, accounting for the influence of friction, pretension of bolts, prying and relative slippage of components through highly non-linear contact elements, have been developed to reproduce the cyclic-reversal test protocol.

The ends of the beams in a special moment resisting frame (SMRF) undergo significant inelasticity during large earthquakes. Due primarily to a lack of data and a concern for premature fracture, fasteners are prohibited in this region of the beam which is referred to as "the protected zone". However, the restriction on fasteners in the protected zone creates issues in construction and, as a result, unauthorized attachments are not uncommon. Full-scale beam-to-column tests have been conducted to investigate the cyclic behavior of SMRF connections with powder actuated fasteners (PAF) applied in the protected zone. Both bolted extended end plate connections and reduced beam section connection configurations were tested. Several specimens included PAF in typical fastening patterns that might represent deck attachment to the top flange of the beam or connection of cold-formed steel partition walls to the underside of the beam. One specimen included a grid of PAF over both flanges to investigate the effect of PAF that are applied in the local buckle locations. The behavior of specimens with PAF is compared to that of control specimens. For the configurations examined in this study, PAF were not found to have a significant effect on hysteretic behavior.

Hollow structural sections (HSS) are currently underutilized in seismic applications in structures. To address this, a connection design methodology is derived for both unreinforced and reinforced, fully welded, HSS-to-HSS moment connections. Experimental testing of two unreinforced HSS-to-HSS connections with unmatched and matched beam and column widths is conducted under cyclic loads to failure. The hysteretic behavior shows that these connections are limited in their ability to isolate inelasticity to the beam member and panel zone region. A finite element parametric study also is used to better understand the effect of different parameters on internal and external reinforced HSS-to-HSS connection performance under cyclic loads. Based on the finite element models, reinforced connections show the ability to develop plastic hinging in the beam members with a normalized moment capacity greater than unity.

Real-time hybrid simulation replicates structural responses under earthquakes through integrating physical testing of experimental substructures and numerical modeling of analytical substructures. Experimental studies, however, indicated that actuator delay-induced tracking errors are often inevitable even when a sophisticated actuator delay compensation technique is applied. Reliability assessment of real-time hybrid simulation results is, therefore, necessary to correctly interpret for structure performance evaluation. This, however, could be difficult since exact structural response is often not available for an immediate comparison. This paper proposes a reliability assessment approach for real-time hybrid simulation results. Statistical distribution of actuator delay values corresponding to certain accuracy level established for linear elastic single-degree-of-freedom structures is modified to account for structural nonlinearity. An existing tracking indicator is then utilized to incorporate the probabilistic distribution of this actuator delay to assess the reliability of real-time hybrid simulation results. Real-time hybrid simulations of a steel moment resisting frame with an elastomeric damper are used to demonstrate the application of proposed approach.

Effective force testing (EFT) is one of the force-based experimental methods for performance evaluation of structures that incorporate dynamic force control using hydraulic actuators. While previous studies have shown successful implementations of force control, controllable frequency ranges are limited to low frequencies (10 Hz). This study presents the EFT method using a robust loop shaping force feedback controller that can extend the frequency range up to 25 Hz. This study investigates the dynamic properties of hydraulic actuators and the design of a loop shaping controller that compensates for control-structure interaction as well as suppresses the effect of oil-column resonance. The designed loop shaping controller was successfully implemented into an EFT setup at the Johns Hopkins University. An experimental investigation was performed with linear and nonlinear test structures. Results showed that the loop shaping controller provided excellent force tracking performance as well as robustness for dynamic force loadings.

The hybrid simulation method for earthquake engineering, which combines numerical simulation and physical experimentation, was developed to evaluate the seismic performance of civil structural systems. While significant improvement has been made in hybrid simulation techniques, there remain great challenges in computational algorithms and control compensations for real-time operation. Recently a versatile hybrid testing system was built at Western Michigan University consisting of a shake table, an actuator/reaction system and an advanced hybrid testing controller. Such testing system is capable of conducting various real-time hybrid simulation experiments such as displacement-based pseudodynamic substructure testing as well as force-based real-time dynamic hybrid testing. Due to its versatility and easy operation at the benchmark scale, the testing system is particularly suitable for the development of advanced hybrid simulation techniques and earthquake engineering education/outreach activities. The development of this testing system including both hardware and software integration is described. A three story shake table substructure test and several slow and real-time pseudodynamic tests are presented as examples to demonstrate the system's capability and feasibility.

Structural testing often involves the use of servo-hydraulic actuators. The inherent nonlinearity of a servo-hydraulic actuator as well as the nonlinear response of experimental specimens results in an amplitude-dependent behavior of the entire servo-hydraulic system, making it difficult to accurately control the actuator. The existence of the nonlinear response in the servo-hydraulic system can be a critical issue that must be addressed to achieve a successful real-time hybrid simulation involving a large-scale experimental substructure controlled by multiple actuators. In order to achieve improved control of servo-hydraulic systems with nonlinearities, an adaptive time series (ATS) compensator is introduced in this paper. The ATS compensator continuously updates the coefficients of the system transfer function during a real-time hybrid simulation using on-line real-time linear regression analysis. Unlike most existing adaptive methods, the system identification procedure of the ATS compensator does not involve user-defined adaptive gains. Through the on-line updating of the coefficients of the system transfer function, the ATS compensator can effectively account for nonlinearities in the servo-hydraulic system and experimental specimen, resulting in improved accuracy in actuator control. In this study, the exceptional performance achieved in actuator control using the ATS compensator is demonstrated through a real-time hybrid simulation of a large-scale 3-story steel frame structure with large-scale magneto-rheological (MR) dampers.

Flood and flood-induced scour have been long recognized as a leading cause of bridge failure to the river-crossing bridge inventory in the United States. Two other types of combined hydraulic impact and scour hazard that have drawn much attention are due to surging of ocean flow triggered by either seismic or hurricane energy. Regardless of the causes, the resulting scour greatly modifies the boundaries of the near-field soil for foundations. However, the combined effects of hydraulic impact and scouring on modification on structural characteristic have not been comprehensively studied. In this paper, we conduct a preliminary 1-g small scale experiment, wherein a generic foundation-structure system is placed in a laboratory flume channel and is subjected to forced vibration. Test results successfully reveal the formation of foundation scour, the failure of the structure, and the progressively modified dynamic characteristics of the soil-structure system.

Previous analytical, numerical, and experimental studies have been conducted to assess the behavior of semi-rigid partial-strength beam-column subassemblies and the results showed desirable behavior. However, realistic evaluation of semi-rigid steel frames requires a system-level examination to allow for investigating the effect of local behavior of the connection on the global response of the structural system. Most of previously conducted studies on system-level assessment utilized finite element analysis with line elements representing the beams and columns and rotational springs representing the connections using an idealized moment-rotation relationships. One of the shortcomings of this approach, in addition to idealizing the momentrotation relationships, is that the interaction between the beam and column flanges and the angles comprising the connection is not accounted for. This paper discusses an advanced hybrid simulation application for the seismic assessment of steel frames with top and seat-angle with double web-angle (TSADWA) connections using advanced experimental setup and a new detailed multi-resolution finite element model. The findings of the simulations with focus on connection behavior and frame response is highlighted. The results provide valuable insight to engineers into the expected behavior of steel frames with TSADWA connections under moderate-to-high seismic demands.

Hollow structural sections (HSS) are currently underutilized in seismic applications in structures. To address this, a connection design methodology is derived for both unreinforced and reinforced, fully welded, HSS-to-HSS moment connections. Experimental

This is a case study where the goal is to predict the vibration environment due to a footfall excitation in the vicinity of a replacement Magnetic Resonance Imaging (MRI) unit on a hospital floor. A new MRI unit with a weight of 19,000 lbs replaces an existing, much heavier unit with a weight of 82,000 lbs. Dynamic characteristics of the floor will change when the new unit is installed, affecting the vibration environment of the unit that is generated by nearby footfalls. In this study we use physical heel drop measurements of the existing conditions to calibrate a computer model that we then use to predict the vibrations and compare the calculated vibrations with the criteria for the new unit. The results show that the vibration levels predicted for the new MRI unit do not exceed its vibration criterion. The paper will present detailed analytical model and analysis procedure, together with conclusions.

Stadium structures may suffer from vibration serviceability problems due to light weight and rapid constructions as well as considerations such as improved line of sight and increased capacity. In this context, Structural Health Monitoring (SHM) data can be implemented to track and evaluate performance of such structures during different events. This paper presents findings from a Structural Identification (St-Id) implementation to a football stadium to evaluate the structural performance by means of a detailed Finite Element (FE) model validated using experimental data. The stadium was monitored for three years to determine the vibration levels during different games and different events, e.g. goals, interceptions and playing a particular song. It is observed that certain events and long periods of playing particular songs generate vibration levels that create uncomfortable situations for the spectators based on the design codes. Laboratory studies were conducted to determine the forcing functions experimentally due to jumping with the rhythm of a song that was often played in the stadium. The FE model of the stadium was developed and validated using the modal analysis results from the ambient vibration data. The experimentally obtained loading functions were used with the FE model to simulate the behavior under spectators' loading.

Surface train and subway-induced vibrations have been a concern in buildings located in the vicinity of railway lines. Operation of sensitive equipment in laboratories and precision manufacturing facilities can be affected by undesired levels of building vibration. New residential building developments in proximity to railway lines have been in question for human comfort. Cost effective mitigation techniques require a good understanding of the nature of these railway-induced base excitations of buildings. In order to understand the characteristics of surface train and subway-induced building floor vibrations, exploratory site vibration measurements were performed at seven full-scale buildings in the Boston area. Tri-axial vibration measurements were made on grade slab of buildings and in adjacent open fields at the same distance from the tracks. Vibration measurements were also made at an internal column location at the foundation level of one building. These vibration measurements were quantified and compared. It is found that in case of train-induced building vibrations, open field data can be used in design of buildings. However due to inconsistency in observations, similar conclusion is not drawn in case of subway-induced building vibration. Findings from these observations in the open field and constructed facilities responses should accordingly be considered in design phase of buildings.

Vibration serviceability due to rhythmic loading generated by a crowd is a widely acknowledged design consideration for assembly-type structures such as stadium grandstands. However, guidance for serviceability design is rarely incorporated into a governing code. To aid designers in the process of designing for serviceability due to rhythmic crowd loading, the current state of knowledge is summarized. Topics to be presented are organized based on the various components contributing to vibration serviceability assessment: 1) excitation, 2) structural response, and 3) assessment/design. The excitation component will speak to a crowd loading to be used for design and the many factors that influence the crowd loading including the synchronization effects of a crowd, the number of harmonic components to be included, and the effects that the motion of the structure has on the crowd motion. The structural response component will focus on the estimation of the dynamic properties of a structure and how these properties are affected by human-structure interaction. The final component, assessment and design, will explain the current assessment measures for vibration serviceability including vibration dose values and RMS acceleration. The acceptable limits for these measures will be discussed as well as the various design approaches to prevent serviceability issues.

Floor vibration serviceability is a primary design consideration for steel framed floors and numerous evaluation methods have been proposed over the last 35 years or so. However, there is relatively little information available on the accuracy of the methods. Therefore, the objective of this study is to investigate the accuracy of the following five well-known methods: Modified Reiher-Meister, Murray Criterion, Hugo Bachmann's Method, AISC Design Guide 11 Chapter 4, and the SCI P354 Simplified Method. The methods listed above were used to evaluate the bays in an extensive 122 bay database to determine the accuracy of prediction of natural frequencies and overall acceptability. The SCI P354 most accurately predicted the natural frequency, with an average measured-to-predicted ratio of 1.01 (24% COV). The AISC Design Guide 11 most accurately predicted floor bay acceptability, with a 91% agreement with observations.

Optimization results are presented for improving energy dissipation capacity of passive seismic control devices under cyclic deformation. The locations and thicknesses of the stiffeners, as well as the aspect ratio, of a shear-type hysteretic steel damper are optimized to improve energy dissipation capacity under static cyclic deformation. A general-purpose finite element software package is used for elastoplastic analysis, and a heuristic optimization algorithm called tabu search is utilized for optimization after discretization of variables into integer values. The objective function is the dissipated energy before the maximum value of equivalent plastic strains among all elements reaches the upper bound under the prescribed loading condition, and constraints on reaction force and moment are incorporated by penalty function approach. The material parameters of the low-yield-point steel are identified through optimization also using tabu search.

Several practical tools for the characterization of the optimal layout of material in a structure have been developed in recent years based on both numerical and analytical approaches. Within the numerical methods, discrete and continuum methodologies have been successfully applied in a variety of applications. This paper revisits such methods and demonstrates their relevance in the conceptual design of high-rise buildings, long span structures and smaller specialty structures. In particular, an example of application to the design of a canopy structure for a currently under-construction high-rise is described in details.

The optimization of the design of steel frame structures has received significant attention in recent research. With increased computing capability and the development of innovative stochastic optimization methods such as genetic algorithms, ant colony optimization, simulated annealing, and harmony search among others, reasonable solutions for the highly nonlinear, multivariable steel frame weight minimization problem can be more readily obtained. Based on the harmony search method developed by Geem et al (2002), this paper presents the design-driven harmony search (DDHS) algorithm and demonstrates its improved performance, relative to previous studies, in consistently obtaining accurate solutions to steel frame optimization problems with minimal computational effort. The performance of DDHS is compared against that of other stochastic algorithms, as reported in prior studies, in two benchmark problems - the optimization of a ten-story 568-member space frame and the optimization of an eight-story 744-member steel space frame.

The optimal design of complex engineering structures is often challenging because of large problem size and the need to simplify objective and constraint functions in order to find a solution using available optimization algorithms. In this study, the design of built-up wide flange sections for plate girders is explored to expand the scope of the optimization formulation by including more practical design aspects. In addition to choosing optimal plate sizes to satisfy strength and serviceability requirements, alternative configurations such as stiffened vs. unstiffened webs, hybrid girders, and fully braced vs. unbraced vs. intermediately braced spans can be treated through the use of categorical and discrete variables. These variables augment the continuous variables and give the designer more control over the outcome of the design. The addition of these elements makes the posed optimization problem highly nonlinear and nonsmooth. The solution of such a problem can be problematic using gradient-based methods. In order to include these types of variables in the optimization formulation, a mixed variable programming approach is used. The algorithm utilized is the Mesh Adaptive Direct Search (MADS). This is a derivative-free method with robust performance on nonsmooth problems. MADS is a globally convergent nonlinear programming method that is guaranteed to find a local minimizer that satisfies limited first- and second-order optimality conditions. The method easily makes the extension to mixed variable programming through the use of extended polling and neighborhood functions that give an explicit ordering to different values in a category. Finally, an objective function is utilized that incorporates the relative cost of different materials and captures labor cost by including the cost ratio of fabrication and material. In addition to the raw tonnage cost, activities associated with fabrication such as welding, cutting and grinding, and surface preparation, are included in the objective, making the design "least cost" rather than just least weight.

This paper describes an analytical study that was conducted to establish an accurate, yet efficient, algorithm for transporting fire data across disparate time scales between the fire and structural domains. In particular, a high-resolution computational fluid dynamics simulation was conducted using the computational fluid dynamics software Fire Dynamics Simulator (FDS) to measure the surface flux acting on an unprotected steel beam exposed to a heptane pool fire. Data from FDS was coupled to a heat transfer analysis in ABAQUS to determine the temperatures within the beam. Two subcycling algorithms were studied based on the conventional assumption of weak coupling (i.e., that the fire affected the structural response but the structural response did not affect the fire response). The first algorithm used a traditional subcycling technique in which surface fluxes from FDS were sampled at the requisite time steps in the thermal analysis. The second algorithm was formulated to produce a time-averaged flux at each time step in the thermal analysis. Results demonstrate that the time-averaged flux yields much greater accuracy than the time-sampled flux. Furthermore, the study shows that the time scale in the thermal analysis can be significantly relaxed if the energy equivalence of the surface flux is maintained.

Fire risks pose significant threats to the integrity and stability of the multi-story steel structures. The robustness of these structures against a fire hazard requires further attention. The progressive collapse of a high-rise structure is detrimental not only to the inhabitants in the building but also to the surrounding infrastructure. The collapse of World Trade Center Twin Towers showed that the impact location and the fire distribution could cause symmetrical as well as asymmetrical types of total collapse. This study investigates the uncoupled structural-thermal response of a 49-story, steel, high-rise structure. The structural load carrying system of the high-rise structure is assumed to be a moment resisting frame. The results show that the structural response and the progressive collapse differ depending on the fire spread and it does not significantly change due to the fire location as long as the fire is contained on a single floor. This study intends to provide a better understanding of the effect of fire loading leading to the collapse mechanism of a multi-story steel structure.

The occurrence of fires following an earthquake in steel structures is a significant issue in areas of moderate-to-high seismicity. The current seismic design philosophy permits a certain degree of damage during earthquakes, making the structures more vulnerable when exposed to the additional demand of fire loading. Evidence from previous earthquakes has shown high variability in the damage sustained by active and passive fire resistance systems and structural loss due to fires following earthquakes has been observed. This paper provides insight into the effects of the local behavior of reduced beam section (RBS) connections during fire following earthquakes on the global response of steel moment resisting frames. This is achieved through investigating the response of a 9-story steel frame designed for the Los Angeles region using nonlinear finite element analyses. The models are developed and analyzed under representative loading scenarios and the results are presented and their implications discussed.

Probability-based structural fire load and its application to building design is intended to be a practical and useful tool to structural and fire prevention engineers, architects and educators. The objective of the article is to bridge the gap between prescriptive and performance-based methods and to simplify complex computer analyses to the point that the probabilistic structural fire load will have a simple approximate analytical expression. The main audience is practicing structural and fire prevention engineers and researchers. Probabilistic structural analysis methods provide a means to quantify the inherent risk of a design and assess the sensitivities of dimensionless uncertain parameters, which now have discrete random values. This article provides systematic (step-by-step) applied, probabilistic analysis of structural fire load that is presented here as a dimensionless temperature-time random function. The main findings are as follows: mean values of maximum temperatures; variances; safety (risk) quantification; acceptable probability of failure (or minimum reliability); confidence probability; partial safety factor; correlation functions and the corresponding spectral functions; and the so-called "first-occurrence time problem". They are presented in this article in dimensionless forms. Therefore, they are intrinsic in each fire severity category. In probability-based structural fire load design, the structural reliability formulation is presented in such a way as to make it practical for design by engineers who may not be familiar with reliability concepts or have access to the necessary statistical data.

This paper focuses on the fire behavior and collapse resistance of mid-rise steel buildings with composite floor systems. Two 10-story steel buildings with different structural configurations: (i) lateral load resisting system on the perimeter, and (ii) lateral load resisting system on the interior. Theyare designed (for structural design and fire protection) according to American design practices. Finite Element (FE)-based numerical techniques were used to model and simulate the behavior of these buildings in fire conditions. The fire conditions are simulated by assigning structural components the temperature values corresponding to the design fire event. The numerical technique involved macro level spring models for the connections. These connections were capable of modeling temperature dependent coupled multi-axial force-displacement response along with a coupled failure criterion. Beam elements and a combination of beam and shell elements are used for modeling columns, and composite slab systems, respectively. Explicit dynamic analysis technique is employed to simulate the structural response. According to the simulation results, gravity columns are the most important components for overall structural stability in fire conditions. If all the structural components were protected for equal FRR value, gravity columns were found to be the first to fail. If the columns were sufficiently protected, further heating causes failure of the connections at the discontinuous end of gravity beams in flexure where the gravity beam had reached its combined (positive moment capacity at the mid span plus negative moment capacity of the shear connections at the ends) flexural capacity. It was also observed that by using stronger connections, this type of failure could be delayed or avoided. If the connections survive the heating mode, there is a risk of connection failure in the cooling phase too. This risk is higher for interior connections. In the cooling phase, as the beams start to shrink, there is a tension demand on connections. It was analytically tested that by increasing the ductility of the connection, this type of failure can be delayed or avoided.

This paper presents an analytical investigation of the behavior of concrete shear walls coupled together using unbonded post-tensioned precast concrete coupling beams under monotonic and reversed-cyclic lateral loading. At the beam-to-wall interface, steel top and seat angles are utilized to yield and dissipate energy during an earthquake. As the structure is laterally displaced, the non-linear deformations occur primarily as a result of the opening of gaps at the beam-to-wall interfaces. This system offers many advantages over monolithic cast-in-place beams such as reduced reinforcement detailing, the ability to undergo large nonlinear rotations without a significant amount of damage, and self-centering capabilities. A parametric investigation is presented to establish the effects that different structural design parameters have on the lateral strength and deformation capacities and demand. Additionally, the stiffness, strength, and ductility characteristics along with the energy dissipation and self-centering capabilities of the system under reversed cyclic loading are investigated.

A technique called "circularization" [where segmental, circular, concrete covers made of different concrete strengths (40 MPa, 80 MPa and 100 MPa)] was used to change a square column to a circular column. The behavior of the strengthened specimens under different loading conditions [including concentric loading, eccentric loading (25 mm and 50 mm) and flexural bending] was investigated. The experimental results demonstrate that using high-strength concrete (HSC) for the additional covers to strengthen existing square reinforced concrete (RC) columns provides higher load-carrying capacity than covers made of normal strength concrete. The highest capacity of the strengthened columns was four times higher than that of the corresponding reference column under concentric loading. Theoretical interaction diagrams were established and showed a good agreement with the experimental results.

This paper describes the development of the slotted beam detail. The design, construction, testing and results of two large experimental programs investigating the seismic performance of the slotted beam detail, described herein as Phase I and II, are presented. In Phase I, a two-storey, two-by-one bay, reinforced concrete slotted beam superassembly was tested under quasi-static cyclic loading. The specimen was designed, constructed and erected to closely replicate New Zealand construction techniques for a typical multi-storey building, which enabled the practicality of the detail to be assessed. A demanding biaxial loading protocol was applied to enable performance assessment of the slotted beam connections. In addition, the three-dimensional interactions between structural elements in the lateral load resisting system were examined. In Phase II, five 3-dimensional exterior beam-column subassemblies with floor slabs were tested under a similar regime to Phase I. Two subassemblies were directly extracted from the superassembly tested in Phase I and tested to determine the residual capacity of the subassemblies, following a large earthquake. The tests on the remaining three subassemblies investigated viable options for, and performance of, repair retrofit solutions for reinforced concrete slotted beam connections following large earthquakes. The retrofit solutions utilised tension-compression yielding, sliding friction and lead extrusion external energy dissipation devices. The tests also served to assess the performance of the replaceable dampers for new build applications.

This paper presents an analytical investigation on the peak lateral displacement demands of "hybrid" precast concrete panelized shear wall structures under seismic loading. The hybrid wall structure investigated uses a combination of mild [i.e., Grade 400 (U.S. Grade 60)] steel bars and high-strength unbonded post-tensioning (PT) strands for lateral resistance across horizontal joints. The analytical work follows from a previous experimental validation of the hybrid wall system for ACI 318 (2011) adoption as "special" reinforced concrete shear walls based on the testing of six 40%-scale precast test specimens (four solid and two perforated walls) under reversed-cyclic lateral loading (Smith et al. 2012). The broad goal of the analytical program was to accurately reproduce and predict the hysteretic behavior of the hybrid system. Upon successful validation of the analytical models based on the measured behaviors of the test specimens, nonlinear dynamic time-history analyses of a full-scale prototype structure were conducted under an exhaustive ensemble of selected ground motion records. The dynamic analysis results provide a critical link between the prescribed lateral displacements that were used in the design and validation testing of the wall specimens per ACI ITG-5.1 (2007) and the expected seismic demands from earthquake loading. Ultimately, these results further support the successful code approval of hybrid precast concrete shear walls for moderate and high seismic regions of the United States.

Current seismic design practices and risk assessment methods have not included the effect of potential post-earthquakes on building structures. However, recent seismic events have demonstrated that even small magnitude aftershocks may cause severe damage or collapse on softened structures as a result of a mainshock. To account for the cumulative damage potential and increased vulnerability of reinforced concrete building frames under multiple earthquakes, this paper proposed an analytical tool, aftershock probabilistic seismic demand model required for computing aftershock fragility functions. The analytical models of non-ductile reinforced concrete frames are built in the nonlinear finite element platform of OpenSees. Incremental dynamic analysis approach is used to generate mainshock ground motions that can produce mainshock-damaged conditions in terms of maximum interstory drifts of interest. In the next step, back-to-back time history analyses for mainshock-aftershock sequences are performed in order to develop a probabilistic seismic demand model in an aftershock environment. The demand model can be characterized in terms of an aftershock intensity measure and an initial damage state associated with a mainshock response by using a suite of ground motions representative of the seismic hazard in the region. Furthermore, these demand models are compared with different levels of existing damage conditions (different mainshocks and same aftershocks).

This paper presents a study on the seismic response of multi-tiered, braced steel frames. The seismic demand and stability of the columns in a 4-tiered special X-braced frame structure located in a high seismic area and designed according to current AISC seismic provisions is evaluated. The drift demand in the individual bracing tiers and the combined column axial force and in-plane flexural demand are evaluated through nonlinear time history analysis. Detailed finite element analysis of one column under demands extracted from time history analysis is carried out to investigate the buckling response of the column. The results show that high in-plane bending demand is induced in the columns. Flexural yielding caused by non-uniform tier drift demand may detrimentally affect the torsional-flexural stability of the columns.

This paper deals with the design for stability of steel concentrically X-bracing frames built with HSS bracing members. Traditional slotted-tube connections with bolted shear splices are used for the bracing members. Both single- and double-shear splices are investigated. Full-scale quasi-static cyclic test program showed that instability of the discontinuous braces initiated through bending deformations of the connection plates, prior to overall buckling of the bracing members. Elastic stability analysis is performed to assess the effective buckling length and compressive resistances of the bracing members with double shear connections. Brace resistances are also obtained from an analytical model that accounts for inelasticity effects and geometrical imperfections of the brace connections. Numerical predictions are compared to test results and a numerical example is included to illustrate the method.

Special concentrically braced frames (SCBFs) are commonly used for seismic design of buildings. Their large elastic stiffness and strength efficiently sustains the seismic demands during smaller, more frequent earthquakes. During large, infrequent earthquakes, SCBFs exhibit highly nonlinear behavior due to brace buckling and yielding and the inelastic behavior induced by secondary deformation of the framing system. These response modes reduce the system demands relative to an elastic system without supplemental damping using a response modification coefficient, commonly termed the R factor. More recently, procedures put forth in FEMAP695 have been made to quantify the R factor through a formalized procedure that accounts for collapse potential. The primary objective of the research in this paper was to evaluate the approach for SCBFs. An improved model for SCBFs that permits simulation of brace fracture was used to conduct response history analyses. A series of 3-, 9- and 20-story SCBFs were designed and evaluated. Initially, the FEMAP695 method was conducted to estimate collapse and the corresponding R factor. An alternate procedure for scaling the multiple acceleration records to the seismic design hazard was also evaluated. The results show significant variation between the two methods. Of the three variations building studied, the large vulnerability was identified for the 3-story building. To achieve a consistent margin of safety against collapse, a significantly lower R factor is required for the low-rise SCBFs (3-story), while the mid-rise and high-rise SCBFs (9- and 20-story) may continue to use the current value of 6, as provided in ASCE-07.

Concrete is used in nearly every structure we build today, including buildings, bridges, homes and infrastructure. With greater emphasis placed on sustainability in recent years, structural engineers are faced with the challenge of meeting traditional design criteria in addition to evolving criteria that support sustainable construction. Performance-based specifications for concrete can substantially help meet this new challenge. Prescriptive requirements, such as minimum cement content or maximum water-to-cement ratio, are among many common specification requirements that can increase the environmental footprint of concrete. This paper outlines how concrete performance can be improved while lowering environmental footprint by implementing performance-based specifications.

This paper investigates the immediate and long-term service-load deflection behavior of reinforced concrete beams using recycled concrete aggregate (RCA) as replacement for natural coarse aggregate (e.g., crushed limestone, gravel). Test results from eighteen large-scale, simply-supported beams are discussed, incorporating the effects of reinforcement details and varying amounts of RCA up to the full replacement of the coarse aggregate by direct volume replacement. The beams were subjected to service-level loads both above and below the cracking limit while remaining within the linear elastic range of the steel in tension and concrete in compression. Data from companion creep and shrinkage cylinder tests are also presented. The results show significant increases in both the immediate and time-dependent deflections with increased amounts of RCA. The measured immediate beam deflections compared reasonably well with ACI-318 and Eurocode predictions; however, there were significant discrepancies in the time-dependent deflections.

Recent discussions within the Sustainable Structural Engineering community have focused on the theoretical benefits that may be realized by designing structures with an enhanced level of resilience against natural and man-made disasters worldwide. These benefits may be realized using metrics that include all three pillars of sustainability: social, economic, and environmental. The authors have developed an analysis methodology that allows quantification of the benefits of performance-based seismic design in terms of the latter two pillars. This methodology involves a marriage between the sciences of seismic loss estimation andnvironmental life cycle assessment (LCA). On the loss estimation side, a HazUS-based approach is used. On the LCA side, an economic input-output approach is used. The benefits of using these approaches to meet the application objectives of the authors are discussed. A case study is presented that offers valuable lessons to the SE community in working toward improved design strategies that effectively minimize the potential environmental impacts of our buildings. Quantification of the benefits of these improved strategies allows the opportunity for the SE to contribute further to a project's LEED certification by pursuing Innovation & Design credits, an effort currently being undertaken by the authors that is discussed.

The Judicial Council of California and the Administrative Office of the Courts (AOC) strives for enhanced seismic performance goals consistent with long-term sustainable design strategies in the design of new trial court facilities located in California regions of high seismicity. Enhanced seismic performance objectives have been achieved through the use of viscous damping device (VDD) technologies combined with steel moment frames in tall building structural systems. The nonlinear viscous damping devices are utilized to economically control damage in tall buildings when subjected to moderate to major earthquake ground shaking by significantly limiting building story drifts, floor accelerations and base shear demands. This paper, meant to appeal to practitioners seeking to meet long-term environmental and sustainable design goals, summarizes the use of VDD in the design of major court facilities including the 12-story San Bernardino Justice Center (SBJC) (under construction 2012) and the 24-story San Diego Central Courthouse (SDCC) (scheduled for construction in 2013). This paper emphasizes how the seismic performance objectives were achieved in the structural analysis and design, as well as, the methods utilized during the design phases to demonstrate long-term enhanced seismic performance through the use of seismic risk assessments, life-cycle cost analysis and carbon footprint environmental analysis tools.

The Sustainable Form Inclusion System (SFIS) is a sustainable innovation for void inclusion in reinforced and post-tensioned concrete systems. SFIS utilizes non-biodegradable, post-consumer polystyrene (e.g. Styrofoam) to produce light-weight void forms which can be cast into concrete flat slabs. SFIS reduces the amount of concrete and rebar required for the structural systems, reduces the gravity and lateral seismic demands, and gives new life to difficult waste products. Development and detailing of the SFIS system are discussed with consideration to constructability and sustainability. Then, two case studies which utilize SFIS as part of a post-tensioned concrete flat slab gravity framing system are presented. The first case study developed the SFIS system for use in a high-rise office building. The second example utilizes SFIS in a long-span green roof for large, column-free spaces.