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Nonlinear static and dynamic analyses are increasingly being used to evaluate the seismic performance of structures. The level of detail that mathematical models should include depends on the required analysis. In a collapse performance evaluation of special steel moment frames, models must include material and element deterioration due to local geometrical instabilities that will allow triggering structural instability. However, capturing these effects in distributed plasticity models is challenging for two reasons: (1) the selection of a constitutive law that incorporates the geometrical phenomenon, and (2) the localization issues related to the material softening. This paper proposes a simple approach to implement phenomenological calibrated constitutive law that includes compression softening applied on a distributed plasticity model. Additionally, a regularization method is proposed to reduce mesh-sensitivity, ensuring an objective response. The constitutive law and the regularization method are tested under different modeling levels: material, element, and structure. Furthermore, the model is used to perform collapse analyses of 2-, 4-, 8-, 12-, and 20-story special steel moment frame buildings. The proposed model can incorporate critical features, such as flexure-axial force interaction, postpeak deterioration, and an objective global response regardless of the element discretization. [ABSTRACT FROM AUTHOR]

The objective of this paper is to provide details of the design and response of a full-scale cold-formed steel (CFS)-framed building that was tested under a series of dynamic excitations during different phases of construction. The seismic response of complete buildings framed from CFS is essentially unexplored, although significant work on the behavior of CFS members and subsystems, particularly shear walls has been conducted. The experiments described herein are the first tests of a CFS-framed building designed to North American seismic standards. Seismic testing was conducted using the twin shake tables at the University at Buffalo through the U.S. National Science Foundation Network for Earthquake Engineering Simulation program. The project was conducted in two phases. In the first phase, the building specimen was constructed and tested with only the structural components in place: CFS shear walls sheathed with oriented strand board (OSB), CFS gravity walls (unsheathed), and CFS-framed floor and roof diaphragms sheathed with OSB. Nondestructive testing and design basis earthquake-level testing were performed in this phase. In the second phase, a second building specimen was constructed, to identical specifications as in the first phase. However, the second building specimen was finished with nonstructural components including exterior sheathing of the perimeter gravity walls, gypsum-board sheathing of the interior of the perimeter gravity and shear walls, interior partition walls, ceilings, staircases, and exterior weatherproofing. Nondestructive testing was conducted at construction milestones, and the finished second-phase specimen was destructively tested at the maximum considered earthquake level. This paper presents the fundamental system-level response of these building specimens: period, percent damping, drift, and accelerations. The companion paper details component-level response. Overall, the work demonstrates the excellent performance of these structures under seismic excitation, while highlighting that this performance is related to the full system-level response and not just the designated elements in the lateral-force-resisting system. [ABSTRACT FROM AUTHOR]

The strength of thin-walled stainless steel columns has been investigated extensively over the last few years. In European standards, the concept of section classification for determining the cross section capacity is used. In this system, for Class 4 cross sections, the effective width method (EWM) must be used to account for the effect of local buckling. Because of the complexity and limitations of this method, other methods have been developed, such as the direct strength method (DSM) for cold-formed thin-walled profiles and the continuous strength method (CSM), initially established for members made of nonlinear metallic materials. In the CSM, to take advantage of strain hardening, a deformation-based design approach using a continuous relationship between the cross-sectional slenderness and the cross-sectional deformation capacity is used. To a large extent, the CSM yields accurate predictions, especially in the low slenderness range where the current DSM design procedures for members submitted to pure compression tend to produce conservative predictions for materials with pronounced strain hardening such as stainless steel alloys. The present paper presents an extension of the traditional DSM, which provides accurate design strength predictions in the low slenderness range for stainless steel thin-walled section columns failing by distortional, local, and overall buckling. It contains practical information concerning the reference experimental data and draws conclusions about the justification of the proposed analytical formula. The paper is divided into three main parts: the description of the database, the establishment of the design model, and a reliability analysis of the method. [ABSTRACT FROM AUTHOR]

This paper proposes an interval analysis scheme to map out the complete bound spectrum of the most maximum and most minimum responses of geometrically nonlinear elastoplastic structures subjected to both interval applied loads and interval inelastic material properties. The proposed heuristic method uses a finite-step holonomic formulation under pseudodisplacement control. Geometric nonlinearity is modeled using a conventional second-order approximation. The analysis thus determines directly the most maximum and most minimum bound solutions by processing a pair of optimization problems, known as interval mathematical programs with equilibrium constraints or interval MPECs. The simultaneous presence of complementarity constraints and interval data is the main cause of difficulties (associated with nonconvex and/or nonsmooth optimization programs) underpinning the interval MPECs considered. The simple solution approach proposed reformulates the interval MPECs into their noninterval nonlinear programming counterparts that can be processed by a smoothing regularization technique. The efficiency and robustness of the proposed interval analysis scheme are illustrated through a number of numerical examples motivated by various engineering applications, such as the safety assessment of multistory frames that are prone to geometric nonlinearity and interval uncertainties. [ABSTRACT FROM AUTHOR]

This paper investigates the performance of a novel semiactive damping device at mitigating nonsimultaneous multihazard loads. The device, termed modified friction device (MFD), has been previously proposed by the authors. It consists of a variable friction system based on automotive drum brake technology. The device has been demonstrated in a laboratory environment, and its dynamic behavior has been modeled. This model is used to conduct numerical simulations on two representative structures, one short building located in Japan and the other tall building located in Boston, MA. Simulated hazards include wind, blast, and seismic loads. Various control cases are considered, including semiactive control under five different sets of control weights, and passive viscous and passive friction to benchmark performance. Results show that the semiactive control cases outperforms all of the other cases for the vast majority of hazards and performance indices, provided that the right control weights are utilized. [ABSTRACT FROM AUTHOR]

Multitiered steel concentrically braced frames (CBFs) are commonly used to provide lateral resistance for tall single-story commercial, performing arts, sports, and industrial buildings. The seismic response of these frames is studied in this paper. A set of seven special concentrically braced frames (SCBFs), ranging from 9 to 30 m tall with two to six tiers, located in a high seismic area was designed according to the 2010 AISC Seismic Provisions. Fundamental behavior of the two- and four-tiered frames was investigated using three-dimensional (3D) finite element models with shell elements, with particular focus on the buckling response of the columns. The seismic frame response and column stability were then studied more broadly for all frames using more computationally efficient 3D finite element models with fiber-based beam-column elements, which were validated against the shell element models. Multitiered CBFs designed according to current multistory CBF procedures are shown to develop drift concentration in a single tier and high in-plane column bending demand, which in some cases leads to flexural yielding and column instability. As potential solutions to this problem, alternate design strategies were studied and their seismic performance is also presented. Designing for higher seismic forces did not appreciably improve column stability, but use of fixed column bases or buckling-restrained braces provided improved distribution of drift over multiple tiers and reduced the occurrence of column instability. Unlike multistory braced frame seismic design, column flexural demands are more important in multitiered braced frames and must be considered in seismic design. [ABSTRACT FROM AUTHOR]

Current seismic design of bridges is based on 'one-time' performance of the bridges during the design seismic event. However, there might be a considerable probability of observing more than one damaging earthquakes in a bridge's service life. Bridge components are known to accumulate seismic damage and deterioration in their structural properties. In such a scenario, design criteria that account for the cumulative seismic damage of bridges over time and performance objectives that span more than one seismic event are needed. This paper computes the probability of occurrence of more than one damaging earthquake during a bridge's service life. Furthermore, it investigates the importance of accounting for cumulative seismic damage in seismic design. This paper develops a probabilistic model to compute the degraded deformation capacity of flexural reinforced concrete (RC) bridge columns as a function of cumulative low-cycle fatigue damage incurred in the past earthquakes. The model is developed for flexural RC columns because low-cycle fatigue is most significant for such columns. The proposed model accounts for the degradation in the ultimate curvature capacity and deformation capacity of RC columns associated to low-cycle fatigue. The proposed model is calibrated by using data from cyclic-load analyses of RC columns performed by using the finite element method (FEM). The FEM model accounts for the cracking and pinching of RC sections to properly simulate the degradation process. Finally, the proposed model is used to assess the fragilities of RC bridge columns conditioning on the values of low-cycle fatigue damage and deformation demand. It is found that in seismically active regions there is a significant probability of observing more than one damaging earthquake in a bridge's service life. Therefore, accounting for the cumulative seismic damage in the seismic design is important. It is also found that the cumulative seismic damage can significantly affect the reliability of bridge columns. The developed models are useful for a seismic design that properly accounts for long-term safety and reliability of bridges. [ABSTRACT FROM AUTHOR]

A model using lifetime functions is used to evaluate the probability of survival of bridge components. The possible outcomes associated with nondestructive inspections (NDIs) are incorporated in an event-tree model. Each time a bridge component is inspected, different decisions can be made. The use of a lifetime function for each component of the structural system enables one to express the probability that the component survives. In theory (i.e., perfect inspection), each NDI should be associated with two possible outcomes: survival or failure. In the first case, no damage is detected and the probability density function of time to failure is updated knowing that the component has survived until the inspection. In the second case, damage is detected and maintenance action is planned. In practice, NDIs are subjected to uncertainties (i.e., imperfect inspections) and detecting or not detecting damage depends on the inspection quality (i.e., probability of detection). For poor-quality inspections, there is a significant risk to overestimate the probability of safe performance. The aim of this paper is to provide a practical methodology for determining optimal NDI strategies for different components of steel bridges. The different types of inspections considered in this paper are visual, magnetic particle, and ultrasonic. An economic analysis is performed and NDI strategies are optimized by simultaneously minimizing both the expected inspection/maintenance cost (i.e., the sum of inspection and maintenance costs) and the expected failure cost. The proposed approach is applied to an existing steel bridge. [ABSTRACT FROM AUTHOR]

There are two goals in this paper: (1) the presentation of a comprehensive database of over 4,000 fundamental period and damping measurements from nearly 1,000 buildings; and (2) the derivation of predictive models for period and damping. The modeling approach presented in the companion paper, which accounts for the statistically nonconforming properties that characterize building data, is used herein to derive models which include two intrinsic sources of variability: that associated with multiple measurements from different excitations on the same building and variability between buildings that are nominally identical. The period models are in general agreement with those currently used, with model parameters that are in terms of height, material, and lateral force resisting system. The damping models show a decrease in damping as building height increases and, for steel and reinforced structures, a twofold increase in damping for earthquake excitations as opposed to other, lower amplitude excitations. [ABSTRACT FROM AUTHOR]

The 2005 AISC specification provides a new method for stability design of steel frame structures termed the direct analysis method (DM). This paper addresses the consideration of partial base fixity using the DM. A refined model of the column base moment-rotation response is developed using an adaptation of the component method (CM) of Eurocode 3 plus a representation of the foundation stiffness. The paper also summarizes and applies an extension of the DM for frames containing web-tapered members. An example clear-span gable frame is checked using several models of its nominally simple four-bolt base detail: ideally pinned, elastic based on G=10 from the AISC sidesway-uninhibited alignment chart, and elastic-perfectly plastic using the CM. It is demonstrated that the member strength unity checks and the frame deflections obtained by modeling of the column bases as elastic based on G=10 are accurate to slightly optimistic compared to the refined base model. It is found that the contribution of the significant initial stiffness of typical bases to the overall frame response is limited by the connection strength. [ABSTRACT FROM AUTHOR]

An experimental research project was conducted to evaluate the compression stability of high strength steel sections. Two buckling modes “local and overall buckling” and the interaction between them were investigated. To further study the structural behavior of the columns thoroughly, finite element analyses were performed using ABAQUS. This paper presents a finite element analysis on the postbuckling behavior of thin-walled compression members of high strength steel with box-shaped columns fabricated from cold-reduced high strength steel plates with nominal yield stress of 550 MPa in 0.42 and 0.60 mm thicknesses. The effect of the input parameters such as the degree of prescribed initial imperfection and the size of the element mesh on the convergence of the solution has been investigated and is discussed. The paper also shows the comparison of loads obtained numerically and experimentally with those predicted on the basis of AS/NZS4600 and the North American Specification. The accurate results of the numerical simulations show that the finite element analysis can be used to predict the ultimate loads of thin-walled members including the postbuckling behavior of thin-walled sections. It is demonstrated that the finite element analysis can therefore be used to design and optimize thin-walled sections of high strength steel. [ABSTRACT FROM AUTHOR]

This paper presents a material model for the analysis of concrete under multiaxial, cyclic loading conditions. An elastoplastic formulation, having a nonassociative flow rule to capture compression-dominated behavior, is combined with a rotating smeared-crack model to capture the tension-dominated behavior. The proposed formulation resolves the issues that exist in many available concrete material models related to properly capturing the crack opening and closing behavior and accounting for the effect of confinement on the strength and ductility under compression-dominated stress states. The accuracy of the model is validated through analyses for reinforced concrete components. A parametric study demonstrates the importance of accounting for the increase in ductility due to the confinement effect. Additional analyses elucidate the impact of using different yield surfaces for the elastoplastic model on the simulation results. [ABSTRACT FROM AUTHOR]

Thin steel plate shear walls (SPSWs) are an effective lateral force resisting system and are increasingly being used in structures. Engineers require the ability to predict both elastic and inelastic structural responses of SPSWs accurately by a simplified model that is convenient to use. Based on the strip model, this paper proposes a new simplified model for SPSWs, called the three-strip model, in which the infill plates of SPSWs are replaced by three tension-only strips. Comparisons of experimentally obtained pushover responses of SPSWs and that predicted by the proposed model are given and reasonable agreement is observed. A parametric study examines the effect of aspect ratio and column flexibility on the accuracy of the predicted inelastic behavior from the three-strip model. In addition, the capacity of the proposed model in predicting frame forces is verified based on test and strip model results. It was found that the three-strip model can provide generally accurate results for SPSWs and meanwhile reduce modeling workload and computational expense. [ABSTRACT FROM AUTHOR]

This paper deals with the development of shear capacity equations for composite truss beams (CTBs). These equations are obtained from a purposely developed mechanics-based shear model. As opposed to commonly used approaches for the design of these structural elements, where the shear capacity is attained when the first pair of web steel bars yields, in this case the shear capacity is expressed as that causing n pairs of web steel bars to yield before concrete crushes. In this way, the performance of CTBs is maximized. Two different capacity equations are proposed: one, derived from an analytical method, explicitly takes into account the contribution of concrete to shear capacity, and the other, following most current codes, does not consider such contribution. The accuracy of the proposed capacity equations has been verified against the results of nonlinear analyses and experimental tests conceived and conducted to this aim. The comparison confirmed both the consistency of the proposed mechanical model with the experimental observations, and the capability of the capacity equations to predict the shear capacity of CTBs. [ABSTRACT FROM AUTHOR]

This paper proposes a new kind of composite waffle slab that consists of orthogonal steel girders and flat RC slab. The steel beams help decrease the cracking that occurs with concrete, and using prefabricated RC slabs reduces the framework and speeds up the construction. An experimental investigation of steel-concrete composite waffle slabs is conducted, and the specimens are proved to have excellent ductility and load-bearing capacity. Then a mixed finite-element (FE) model combining the beam elements and shell elements is proposed to predict the behavior of composite waffle slab. Based on the numerical model, a series of parameter analyses are conducted, and the influences of different parameters on the mechanical behavior of the composite waffle slab are intensively investigated. Simplified formulas for the ultimate capacity of the composite waffle slab are derived based on the test observations and numerical analysis. Reasonable recommendations are proposed to help engineers design the composite waffle slab effectively. [ABSTRACT FROM AUTHOR]

This paper describes a test program on a wide range of aluminum alloy square hollow section stub columns containing central circular openings. A total of 28 compression tests was conducted by applying uniform axial load to the specimens, which were fabricated by extrusion of square hollow sections using 6061-T6 heat-treated aluminum alloys. The column strengths; failure modes (including local buckling failure and material yielding failure); strain distribution along the quarter of square hollow sections at midheight of the columns; and load versus axial shortening curves of test specimens were obtained from the experimental investigation. The test results were compared with the design strengths calculated by using the current design rules given in the design specifications and design rules proposed by other researchers for perforated carbon steel structural members. It is shown that most of the current design rules for perforated carbon steel structural members are generally inappropriate, with comparatively high scatter of predictions for the design of perforated aluminum alloy stub columns. [ABSTRACT FROM AUTHOR]

An improved formula to determine the equivalent viscous damping ratio of a single-degree-of-freedom (SDOF) bilinear oscillator is proposed in this paper, which simultaneously considers the influence of the initial period, ductility ratio, and strain hardening ratio. A large number of parameters are considered in order to present comprehensive results. Based on the assumption of secant stiffness, several equivalent linearization approaches are evaluated through parametric analyses. Then, optimization analyses are performed using a genetic algorithm to identify the variation trend of optimal damping ratios. A new formula of an equivalent viscous damping ratio is proposed by nonlinear regression analyses of the optimal damping ratios. Finally, the proposed approach is validated and the results show that the proposed approach leads to better estimates of maximum inelastic displacement response in the entire parameter space considered in this study when compared with other equivalent linearization approaches. [ABSTRACT FROM AUTHOR]

This paper presents a formal optimization methodology for the design of seismic retrofitting of three-dimensional irregular buildings. The damping coefficients of viscous dampers potentially allocated in given feasible locations as well as the stiffness of their supporting braces are adopted as design variables. The objective function minimizes a cost function of the dampers while constraints are added to limit various responses of interest to allowable values under a filtered white noise excitation (e.g., interstory drift at each location separately, total acceleration at each location separately, force of each damper, stress in each supporting brace, force/forces of each structural member, base shear, overturning moment at the base, side constraints on dampers' forces and braces' cross sections, etc.). A first-order optimization method is adopted for that purpose. The constraints on various normalized responses are condensed to a single constraint on their maximum value and the gradient required is efficiently derived analytically using the adjoint analytical method. Thus, a computational effort at the order of a single additional analysis is required for the evaluation of the gradient of the constraint regardless of the number of design variables considered or responses to be constrained. This efficient scheme enables a study on the effect of limiting the brace size with and without a limit on its stresses. [ABSTRACT FROM AUTHOR]

An experimental program was performed at IVALSATrees and Timber Institute on single and coupled cross-laminated (CLT) wall panels with different anchoring systems and different types of joints between adjacent panels. The mechanical properties of CLT walls were assessed and are critically discussed in the paper. The connector layout and the design of the screwed vertical joints were found to markedly affect the overall behavior of the structural system. The in-plane deformations of CLT panels were almost negligible, whereas concentration of forces and deformations mainly occurred in the connections. Advanced analytical models for nonlinear pushover analysis of CLT wall systems were developed and verified against test results. The models take into account all stiffness and strength components of connectors, as well as the bending and shear deformation of the panels. A parametric study of CLT wall systems with different aspect ratios and wall segmentation was performed, showing that segmentation of CLT walls decreases their stiffness and strength but significantly improves their deformation capacity. [ABSTRACT FROM AUTHOR]

Due to their typical high slenderness ratios, glass structural elements can be often subjected to buckling phenomena. Major difficulties in a correct estimation of their effective buckling strength and load-carrying behavior are generally given by a combination of multiple mechanical and geometrical aspects, especially in presence of laminated cross sections or interacting applied loads. In this paper, buckling experiments are performed on laminated glass beam-columns eccentrically compressed. Extended numerical and analytical comparisons are performed with test results in terms of Euler's critical loads or load-displacement paths. As shown, appropriate calibration of numerical and analytical models generally can provide good agreement between buckling predictions and experimental results. Viscoelastic numerical models, in particular, if well-calibrated in terms of mechanical [e.g., creep effects in polyvinyl butyral (PVB)-foils] and geometrical properties (e.g., initial imperfections, load eccentricities) can provide interesting correlation with experiments, both in the form of global loadcarrying behavior and ultimate loads. At the same time, simplified analytical methods based on the equivalent thickness concept can be used for rational analytical predictions--although in well-defined load-time and temperature conditions--and simplified buckling verification procedures. [ABSTRACT FROM AUTHOR]

An experimental program was performed at IVALSA Trees and Timber Institute on single and coupled cross-laminated (CLT) wall panels with different anchoring systems and different types of joints between adjacent panels. The mechanical properties of CLT walls were assessed and are critically discussed in the paper. The connector layout and the design of the screwed vertical joints were found to markedly affect the overall behavior of the structural system. The in-plane deformations of CLT panels were almost negligible, whereas concentration of forces and deformations mainly occurred in the connections. Advanced analytical models for nonlinear pushover analysis of CLT wall systems were developed and verified against test results. The models take into account all stiffness and strength components of connectors, as well as the bending and shear deformation of the panels. A parametric study of CLT wall systems with different aspect ratios and wall segmentation was performed, showing that segmentation of CLT walls decreases their stiffness and strength but significantly improves their deformation capacity. [ABSTRACT FROM AUTHOR]

This paper presents an improvement to the eigensystem realization algorithm (ERA) with natural excitation technique (NExT), which is called the ERA-NExT-AVG method. The method uses a coded average of row vectors in each Markov parameter for evaluating modal properties of a structure. The modification is important because, for the existing stochastic system identification methods, the state-space model, obtained from output sensor data, is usually overparameterized resulting in large systems. Solving such a problem can be computationally very intensive especially in the applications when using the computational capabilities of embedded sensor networks. As a way to improve the efficiency of the ERA-NExT method, the proposed method focuses on the number of components in a single Markov parameter, which can theoretically be minimized down to the number of structural modes. Applying the coded average column vectors as Markov parameters to the ERA, the computational cost of the algorithm is significantly reduced, whereas the accuracy of the estimates is maintained or improved. Numerical simulations are performed for a shear frame model subjected to Gaussian white noise ground excitation. The efficiency of the proposed method is evaluated by comparing the accuracy and computational cost of the estimated modal parameters using the proposed method, with several other stochastic modal identification methods including the ERA-observer Kalman filter identification, ERA-NExT, and autoregressive models. The performance of the method is then evaluated by applying it to ambient vibration data from the Golden Gate bridge, collected using a dense wireless sensor network, and its vertical and torsional modes are successfully and accurately identified. [ABSTRACT FROM AUTHOR]

The prediction of creep deformations of concrete and reinforced concrete structures has been a field of extensive research. Several different creep models have been developed. Most of the models were developed for constant concrete stresses; thus, in the case of varying stresses, a specific time integration is necessary. Currently, when modeling concrete creep, the engineering focus is on the application of sophisticated time-integration methods rather than choosing more appropriate creep models or considering the scatter of input parameters. For this reason, this paper presents a method to quantify the uncertainties of creep prediction originating from the selection of creep models, from the time-integration methods, and from input parameter uncertainty. By adapting variance-based global sensitivity analysis, a methodology is developed to quantify the influence of different sources of uncertainty. By application of the developed method, it is determined that selecting more appropriate creep models is of higher importance than choosing sophisticated time-integration methods. [ABSTRACT FROM AUTHOR]

This paper presents an optimization approach to the design of simply supported, post-tensioned, prestressed concrete I-girder bridges. The objective is to minimize the total cost of the structure, considering cost of materials, fabrication, and installation. For a particular girder span and bridge width, the design variables considered for cost minimization of the bridge system are girder spacing, various cross-sectional dimensions of the girder, number of strands per tendon, number of tendons, tendon layout and configuration, slab thickness, slab rebar, and shear rebar for the girder. Explicit constraints on the design variables are developed on the basis of geometric requirements, practical conditions for construction, and code restrictions. Implicit constraints for design are formulated as per the American Association of State Highway and Transportation Officials (AASHTO) Standard Specifications. The optimization problem is characterized by having a combination of continuous, discrete, and integer sets of design variables and multiple local minima. An optimization algorithm, evolutionary operation (EVOP), is used that is capable of locating directly with high probability the global minimum without requiring information on gradient or subgradient of the objective function. The present optimization approach is used for a real-life bridge project, leading to a feasible and acceptable design resulting in around 35% savings in cost per square meter of the deck area. Computational time required for optimization of the present problem is only a few seconds. Because constant design parameters have influence on the optimum design, this cost minimization procedure is performed for a range of such parameters. [ABSTRACT FROM AUTHOR]

Nondestructive dynamic methods are a useful tool in structural engineering. Their purpose is twofold. First, they provide guidance on the validation of a mathematical model of the structure under investigation. Second, repeated tests over time can indicate the emergence of possible damage occurring during the structure's lifetime. Recent technological progress has generated extremely accurate and reliable experimental methods, enabling a good estimate of dynamic behavior of a structural system. Although experimental techniques are now well-established, the interpretation of measurements still lags somewhat behind. This particularly concerns structural identification due to its nature of inverse problem. In addition, when identification techniques are applied to the study of real-world structures, additional obstacles arise given the complexity of the structural behavior, the inaccuracy of the analytical models used to interpret experiments, measurement errors, and incomplete field data. This study discusses some of these aspects with reference to a series of harmonic forced-vibration tests carried out on the new headquarters of the Hypo Bank (Tavagnacco, northeast Italy). The special structural typology of the tilted building and the complex distribution of the resisting structural members have made interesting, and at the same time quite complicated, the interpretation of the real dynamic behavior of the construction. The present paper (Part I) is devoted to the experimental programme. The use of the measured dynamic data in the calibration of numerical models of the Hypo Bank complex is presented and discussed in Part II. [ABSTRACT FROM AUTHOR]

This paper presents a computational procedure to evaluate the shear strength contribution provided to a reinforced concrete (RC) beam by a system of near-surface mounted (NSM) fiber reinforced polymer (FRP) strips. This procedure is based on the evaluation of (1) the constitutive law of the average-available bond-length NSM FRP strip effectively crossing the shear crack, and (2) the maximum effective capacity it can attain during the loading process of the strengthened beam. Because of complex phenomena such as (1) interaction between forces transferred through bond to the surrounding concrete and the concrete fracture, and (2) interaction among adjacent strips, the NSM FRP strip constitutive law is largely different than the linear elastic one characterizing the FRP behavior in tension. Once the constitutive law of the average-available bond-length NSM strip is reliably known, its maximum effective capacity can be determined by imposing a coherent kinematic mechanism. The self-contained and ready-to-implement set of analytical equations and logical operations is presented along with the main underlying physical-mechanical principles and assumptions. The formulation proposed is appraised against some of the most recent experimental results, and its predictions are also compared with those obtained by a recently developed more sophisticated model. [ABSTRACT FROM AUTHOR]

This paper discusses the progressive collapse resistance of steel-concrete composite floors in which steel beams are attached to columns through shear tabs. This is a common type of system used for the gravity bay portions of steel buildings. The study is conducted using computational simulation models validated through extensive comparisons to disparate test data. The models are used to investigate key parameters influencing the robustness of generic composite floors subjected to the removal of a center column. In particular, the effects of deck thickness, steel reinforcement, and the numbers of bolts in the shear tab connection on the behavior of the system are studied as a function of the loading scheme. The simulation results show that the majority of collapse resistance comes from the steel deck and that, for the system considered, increasing connection strength by adding more bolts might not be that beneficial in increasing overall collapse strength. The dynamic impact factor, which is widely used to account for dynamic effects within a static design framework, is also computed and the DIF value recommended in existing design guidelines is evaluated. [ABSTRACT FROM AUTHOR]

This paper presents an effective numerical reliability-based optimization technique for the design of base-isolated concrete building structures under spectrum loading. Attempts have been made to automate the integrated spectrum analysis, reliability analysis, and design optimization procedure and to minimize the total cost of the base-isolated building subjected to multiple design performance criteria in terms of the story drift of the superstructure and lateral displacement of the isolation system or corresponding reliability constraints. In the optimal design formulation, the cost of the superstructure can be expressed in terms of concrete member sizes while assuming all these members to be linear elastic under a specified design earthquake. However, the base isolation is assumed to behave nonlinearly and its cost can be related to the effective horizontal stiffness of each isolator. Based on the principle of virtual work, the drift responses and corresponding reliability indexes can be explicitly formulated and the integrated optimization problem can be solved by an optimality criteria method. The technique is capable of achieving the optimal balance between the costs of the superstructure and isolation systems while the seismic drift performance or corresponding reliability of a building can be simultaneously considered. An illustrative example shows that conventional deterministic design optimization cannot ensure designs with satisfactory reliability levels, whereas the reliability-based design optimization can achieve the objective when uncertainties are considered. It is believed that such an optimization technique provides an effective tool for seismic design of building structures. [ABSTRACT FROM AUTHOR]

The current National Design Specification (NDS) for Wood Construction design methods for dowel-type connectors are based on four-yield modes for connections in double shear. These yield modes were formulated for use with steel dowels as fasteners. Hence design with nonferrous fasteners, such as wood pegs, common in timber frame joinery, is not addressed. Wood pegs, while large in diameter, are considerably more flexible than steel dowels of the same size. A fifth failure mode is proposed for use with wood pegs. This new failure mode, called Mode V yielding, which is an effective cross-grain dowel failure, has been observed in physical testing as well as numerical modeling. The objective of this paper is to establish a design procedure for the Mode V yielding of pegs. The procedure is calibrated to the level of performance (reliability) expected from the other yield modes. A regression equation to relate the effective cross-grain yield capacity of pegs to specific gravity is also developed for use with the Mode V design equation. A reliability analysis indicates that the proposed Mode V design equation can be used in conjunction with existing NDS yield equations for design of wood doweled connections. [ABSTRACT FROM AUTHOR]

Measurements from load tests may lead to numerical models that better reflect structural behavior. This kind of system identification is not straightforward due to important uncertainties in measurement and models. Moreover, since system identification is an inverse engineering task, many models may fit measured behavior. Traditional model updating methods may not provide the correct behavioral model due to uncertainty and parameter compensation. In this paper, a multimodel approach that explicitly incorporates uncertainties and modeling assumptions is described. The approach samples thousands of models starting from a general parametrized finite-element model. The population of selected candidate models may be used to understand and predict behavior, thereby improving structural management decision making. This approach is applied to measurements from structural performance monitoring of the Langensand Bridge in Lucerne, Switzerland. Predictions from the set of candidate models are homogenous and show an average discrepancy of 4-7% from the displacement measurements. The tests demonstrate the applicability of the multimodel approach for the structural identification and performance monitoring of real structures. The multimodel approach reveals that the Langensand Bridge has a reserve capacity of 30% with respect to serviceability requirements. [ABSTRACT FROM AUTHOR]

The research into modeling walking-induced dynamic loading and its effects on footbridge structures and people using them has been intensified in the past decade after some high profile vibration serviceability failures. In particular, the crowd induced loading, characterized by spatially restricted movement of pedestrians, has kept attracting attention of researchers. However, it is the normal spatially unrestricted pedestrian traffic, and its vertical dynamic loading component, that is the most relevant for vibration serviceability checks for most footbridges. Despite the existence of numerous design procedures concerned with this loading, the current confidence in its modeling is low due to lack of verification of the models on as-built structures. This is the motivation behind reviewing the existing design procedures for modeling normal pedestrian traffic in this paper and evaluating their performance against the experimental data acquired on two as-built footbridges. Additionally, the use of Monte Carlo simulations is also investigated. Possible factors that cause discrepancies between measured and calculated vibration responses, including possibility of existence of pedestrian-structure dynamic interaction, are discussed. [ABSTRACT FROM AUTHOR]

The need to identify the physical properties of a structure given its force-response (input-output) relationship is driven primarily by the need to validate the approximate solution models, such as finite-element models. This paper proposes a method, which combines the structural system identification and model updating techniques, for the damage detection of a steel frame structure and a RC frame. The damage detection procedure consists of two steps: (1) identifying the system dynamic characteristics using the subspace identification (SI) technique from input/output measurements and (2) developing a damage assessment method for structural members (including joints) based on a progressive finite-element model updating and a large-scale optimization using a nonlinear least-square technique. The proposed method was verified through a shaking table experimental study using: (1) a 1/4-scale six-story steel frame structure by loosening the connection bolts for damage simulations and (2) a two-story RC frame subject to different levels of ground excitations back to back. As demonstrated by experimental results, the proposed damage detection method, based on the combination of SI technique and the model updating approach, is very effective for the damage assessment of frame structures. The method not only can detect the damage locations but also can quantify the damage severities. [ABSTRACT FROM AUTHOR]

This paper proposes a new equation for the effective length factor (k-factor) for reinforced concrete columns in braced frames. The new formula is valid both for normal and high-strength concrete. The equation was obtained from a sensitivity analysis performed on a two-dimensional nonlinear finite-element numerical model that takes into account the inelastic behavior of the concrete columns (cracking, yielding, and second order effects). The numerical model was calibrated with 44 experimental tests performed by the writers’ research group. A comparative study was carried out between the numerical model and different national design codes, displaying important differences with respect to all of them: the ACI code (from 37 to -3%), the Spanish code EHE (from 26 to -9.26%), and the Eurocode 2 (from 14 to -14%). It was decided to propose two additional simplified equations: one for checking and the second for design. [ABSTRACT FROM AUTHOR]

Modeling the energy dissipation capacity of shear-yielding members is important in the evaluation of the seismic response of earthquake resistant structural systems. This paper presents the model of a frame element for the hysteretic behavior of these members. The model is based on a three-field variational formulation with independent displacement, stress, and strain fields. The displacement field is based on the Timoshenko beam theory. The nonlinear response of the element is derived from the section integration of the multiaxial material stress-strain relation at several control points along the element, thus accounting for the interaction between normal and shear stress and the spread of inelastic deformations in the member. With the derivation of the axial force-shear-flexure interaction of short members from the material response the proposed model is general, in contrast to existing concentrated plasticity models that require parameter calibration for different loading and support conditions. Furthermore, the model does not suffer from shear locking and does not require mesh refinement for the accurate representation of inelastic member deformations. Correlation studies of analytical results with available experimental data of the hysteretic behavior of shear-yielding members confirm the capabilities of the proposed model. Its computational efficiency makes it suitable for large scale simulations of the earthquake response of structures with shear-yielding members. [ABSTRACT FROM AUTHOR]

In this paper, a simplified system identification procedure is developed to investigate the dynamic characteristics of base-isolated buildings equipped with lead-rubber bearings (LRBs). The multistory superstructure is assumed to be linear on the account of substantial reduction in seismic forces due to the installation of LRBs for which a bilinear hysteretic model is considered. The hysteretic model is in turn characterized by a backbone curve by which the multivalued restoring force is transformed into a single-valued function. With the introduction of backbone curves, the system identification analysis of inelastic structures is simplified to a large extent. The proposed algorithm extracts individually the physical parameters of each floor and the bearing system that are considered useful information in the structural health monitoring. A numerical example is conducted to demonstrate the feasibility of using the proposed technique for physical-parameter identification of partially inelastic based-isolated buildings. [ABSTRACT FROM AUTHOR]

This paper presents an analytical model for the failure of lightly reinforced concrete slabs under elevated temperature, considering simply supported boundary conditions with planar edge restraints. This model is typically applicable to the failure assessment of composite floor slabs under fire, where the steel deck is assumed to lose strength relatively quickly, leaving a lightly reinforced concrete slab, as supported by experimental evidence. The proposed model accounts for membrane action, which arises at large slab deformations, and important it presents a rational failure criterion based on the rupture of the steel reinforcement. In this respect, this is the first analytical slab model to consider the influence of bond between steel and concrete on reinforcement rupture, while also dealing with elevated temperatures, including the potentially negative effects of thermal curvature. Based on principles of mechanics, detailed analytical forms of the model are first presented and verified for each of the ambient and elevated temperatures cases. In each case, a simplified form of the analytical model, which is more suitable for practical design-oriented application, is also proposed and verified. The successful verification of the proposed analytical models, demonstrated against the results of appropriate nonlinear finite-element analysis, paves the way for their application in the design of composite floor slabs with planar edge restraints under fire. [ABSTRACT FROM AUTHOR]

The benefits associated with structural control include the mitigation of undesired structural responses and reduction in the probability of damage to structural components during seismic events. Structural control systems in current use depend on extensive wired communication systems to connect sensors and actuators with a centralized controller. While wired architectures are appropriate when control systems are small, the cost and installation complexity of tethered systems increases as the control system grows large (i.e., defined by hundreds of nodes). Alternatively, wireless sensors are proposed for use in large-scale structural control systems to keep costs low and to improve system scalability. Wireless sensors are capable of collecting state data from sensors, communicating data between themselves, calculating control actions, and commanding actuators in a control system. However, bandwidth and range limitations of the wireless communication channel render traditional centralized control solutions impractical for the wireless setting. While computational abilities embedded with each wireless sensor permit fully decentralized control architectures to be implemented, strategic utilization of the wireless channel can improve the performance of the wireless control system. Toward this end, this paper presents a partially decentralized linear quadratic regulation control scheme that employs redundant state estimation as a means of minimizing the need for the communication of state data between sensors. The method is validated using numerical simulations of a seismically excited six-story building model with ideal actuators. Additional experimental validation is conducted using a full-scale physical realization of the six-story building. A wireless sensor network commanding magnetorheological dampers is shown to be effective in controlling a multistory structure using the partially decentralized control architecture proposed. [ABSTRACT FROM AUTHOR]

This paper presents research on seismic design of steel special moment resisting frames (SMRFs) with supplemental elastomeric dampers. The SMRF properties and damper design criteria are varied to illustrate their effects on the supplemental damper design. The SMRFs are based on current seismic design practices, and the supplemental dampers are designed using a simplified design procedure (SDP). The SDP allows the structure with dampers to be designed for specified performance objectives, where the structure is intended to reach selected performance levels, defined in terms of story drift and member response, at specified seismic input levels. Nonlinear dynamic time history analyses show that SMRFs with dampers designed using the SDP achieve the specified seismic performance objectives, and that elastomeric dampers are more effective when used in a more flexible SMRF. The SDP permits various seismic performance objectives and corresponding design criteria to be specified. Criteria that allow some inelastic behavior (R=2), but limit the story drift to 1.5% under the design basis earthquake are shown to lead to the most effective damper design. [ABSTRACT FROM AUTHOR]

Complementary to recent studies on the cross-section behavior of elliptical hollow sections, investigations into the member response have also been performed and described in this paper. The first stage of these investigations involved a series of precise column buckling tests on hot-finished steel elliptical hollow sections. In total, 24 flexural buckling tests about the minor and major axes were carried out. Specimen sizes and lengths were carefully chosen in order to cover a wide range of both cross section and member slendernesses. The nondimensional column slenderness of the test specimens varied between 0.19 and 1.58. Measurements were taken of specimen geometry, global initial geometric imperfections, and tensile material properties. Key results from the tests including the full load-deformation histories have been presented and discussed. The test results have also been supplemented by 158 numerically generated results, allowing a wider range of geometries to be investigated. Design rules for the member buckling resistance of elliptical hollow section columns have been proposed and verified by means of reliability analysis. The results presented are the first member buckling tests on elliptical hollow sections, and represent part of the development of comprehensive structural design rules for these sections. [ABSTRACT FROM AUTHOR]

Reliability-based assessment of seismic performance of wood shear walls needs to consider many uncertainties arising from earthquake ground motions, material properties, carried mass, and analytical models. Using a response surface method with importance sampling, this paper presents a seismic reliability analysis on eight types of diagonal-braced and structural-panel-sheathed shear walls which are commonly used in modern post-and-beam wood buildings. A recently developed wood shear wall model was used to simulate the seismic response of the shear walls. The shear wall model parameters were calibrated and verified using a reversed cyclic test database. Peak wall drift was chosen as the performance criteria to estimate the failure probabilities of the walls with respect to two performance expectations. The results presented herein showed that the seismic reliability of the structural-panel-sheathed walls was higher than that of the diagonal-braced walls. [ABSTRACT FROM AUTHOR]

Advanced numerical modeling of cold-formed stainless steel members, from manufacturing to full-range response under applied loading, requires knowledge of the stress-strain relationship of the material over a wide range of tensile and compressive strains. Although a number of stress-strain models have been developed for stainless steels, they are only capable of accurate predictions either over a limited strain range or for the tensile stress-strain behavior only. This paper presents a three-stage stress-strain model for stainless steels, which is capable of accurate predictions over the full ranges of both tensile and compressive strains. The new stress-strain model is defined using the three basic Ramberg-Osgood parameters and is based on a careful interpretation of existing experimental data. The accuracy of the proposed model is demonstrated by comparing its predictions with experimental stress-strain curves. These comparisons also clearly demonstrate the advantage of the proposed model over the only existing full-range stress-strain model. [ABSTRACT FROM AUTHOR]

This paper presents a plain strain analytical model, based on the elasticity theory, to determine the confining pressures of transverse reinforcements on the concrete core of a reinforced concrete member. The analytical evaluation of the confining pressures was first carried out on reinforced sections with square and circular stirrups, and subsequently on reinforcement configurations of greater complexity with square and rectangular stirrups and supplementary cross ties. Finally, the model has been used to evaluate the confining pressures applied by external wrapping in any material [fiber-reinforced polymer (FRP), S-glass, steel, etc.] and to design better combinations of techniques and confinement materials. In order to obtain the stress–strain curves due to passive confinement, an analogy between square and circular sections has been introduced. In this way, any active confinement model derived by triaxial tests on cylindrical specimens can be used. The model has been validated by comparing its predictions with results from existing models and experimental tests. [ABSTRACT FROM AUTHOR]

This paper presents and discusses the results of a numerical study on seismic deformation demand to sheathed cold-formed steel structures. It is part of a wider research, involving also physical tests on oriented strand board-sheathed stud walls. Numerical results have been obtained using an ad hoc mathematical model of the hysteresis response of sheathed stud walls, also taking into account the strong nonlinearity of the response for small lateral displacements and the strong pinching of hysteresis loops. This model has been calibrated against data coming from physical tests, using both old available experimental results and newly obtained ones. Numerical results have been elaborated on in order to both judge the seismic performance of the structure under increasing seismic intensity levels and develop an ad hoc cyclic loading protocol for physical tests. [ABSTRACT FROM AUTHOR]

This paper proposes an analytical approach to calculating failure surfaces in rectangular reinforced concrete cross sections with symmetrical reinforcement when submitted to axial loads and biaxial bending. The analytical method is valid for concrete with a strength of between 25 and 80 MPa. The failure surface is obtained by means of reference generatrices that lie on two directrices corresponding to two known axial loads and on the maximum and minimum axial capacity of the section. The proposed expression was assessed with results obtained from experimental tests (from the literature) and from a numerical model. This approach enables the section capacity to be verified and the reinforcement designed to a sufficient degree of accuracy for everyday professional practice. The method is easy to use and may be extensively applied to reinforced concrete columns in buildings. [ABSTRACT FROM AUTHOR]

This paper presents a formal probabilistic framework for seismic design and assessment of structures and its application to steel moment-resisting frame buildings. This is the probabilistic basis for the 2000 SAC Federal Emergency Management Agency (FEMA) steel moment frame guidelines. The framework is based on realizing a performance objective expressed as the probability of exceeding a specified performance level. Performance levels are quantified as expressions relating generic structural variables “demand” and “capacity” that are described by nonlinear, dynamic displacements of the structure. Common probabilistic analysis tools are used to convolve both the randomness and uncertainty characteristics of ground motion intensity, structural “demand,” and structural system “capacity” in order to derive an expression for the probability of achieving the specified performance level. Stemming from this probabilistic framework, a safety-checking format of the conventional “load and resistance factor” kind is developed with load and resistance terms being replaced by the more generic terms “demand” and “capacity,” respectively. This framework also allows for a format based on quantitative confidence statements regarding the likelihood of the performance objective being met. This format has been adopted in the SAC/FEMA guidelines. [ABSTRACT FROM AUTHOR]

This paper proposes simple models that can simulate the hysteretic behavior of shear panels made of low-yield steel, which exhibits significant strain-hardening under load reversals. The models are based on either the bilinear or the Ramberg-Osgood model, and combine kinematic and isotropic hardening in a linear manner by the introduction of a weighting coefficient. The accuracy of these models is demonstrated by comparing hysteresis curves obtained from these models with experimental hysteresis curves, which have been supplied by a quasi-static loading test with a prescribed deformation history and by a pseudodynamic test. Programming of these models is found to be very easy, involving only slight modification of the programs developed for the bilinear and the Ramberg-Osgood models. Thus, these models can readily be incorporated into inelastic dynamic response analysis of structures having shear panels made of low-yield steel as hysteretic dampers. [ABSTRACT FROM AUTHOR]

Genetic-algorithm (GA)–based structural optimization can be parallelized to a high degree on new generation of scalable distributed-memory multiprocessors. In this paper, a mixed computational model is presented for GA-based structural optimization of large space structures on massively parallel supercomputers. Parallelism is exploited at both coarse-grained design optimization level in genetic search using the multiple-instruction–multiple-data model of computing and fine-grained fitness function evaluation level using the single-instruction–multiple-data model of computing. The latter model involves the development of a data-parallel iterative preconditioner-conjugate-gradient algorithm for the solution of the resulting system of linear equations. The model has been implemented on Connection Machine CM-5 and applied to optimization of large space steel structures subjected to the constraints of the American Institute of Steel Construction's allowable stress design specifications. The model and concurrent algorithm developed in this research is highly scalable. A peak performance of 2.4 giga–floating-point operations per second is achieved using 512 processors for a structure consisting of 4,016 elements. [ABSTRACT FROM AUTHOR]

The author comments on a paper about the analytical model of ground motion pulses developed for design and assessment of seismic protective systems. He notes the limitedness of the significance of the closed-form response expressions from the proposed model to elastic linear structures. He observes the good match of the model with the actual ground velocity and displacement and its mismatch in the actual acceleration.