This paper presents a feasibility study of multidegrees-of-freedom effective force testing (MDOF-EFT). The study is intended to facilitate the development of a force feedback controller and investigation of performance as well as robustness of MDOF-EFT. First, the dynamics of MDOF-EFT systems are analytically investigated. Analytical transfer functions of the control plant, the valve-to-force relations, showed that the plant is dynamically coupled and the natural frequencies of test structures are the transmission zeros of the plant. Using a set of model parameters from a previous study, a case study that includes controller design, numerical simulations and robust stability assessment is performed. A decoupling loop shaping (DLS) controller consisting of a pseudo inverse of the plant and second-order loop shaping controllers is adopted as the force feedback controller. It is shown that the DLS controller provides a stable control system while successfully decoupling the control loops and compensating the control-structure interaction. Numerical simulations demonstrate that the DLS controller enables tracking of static and dynamic forces for multiple actuators. Robust stability of MDOF-EFT with the DLS controller is assessed using Monte Carlo simulation. The stochastic simulation results show that the DLS controller is stable and robust, providing sufficient stability margins for uncertain models with maximum 50% errors in the estimated system parameters. This paper demonstrates that MDOF-EFT is feasible with the DLS controller and can be implemented in experimental laboratories. Copyright © 2013 John Wiley & Sons, Ltd.

Substructure hybrid simulation has been actively investigated and applied to evaluate the seismic performance of structural systems in recent years. The method allows simulation of structures by representing critical components with physically tested specimens and the rest of the structure with numerical models. However, the number of physical specimens is limited by available experimental equipment. Hence, the benefit of the hybrid simulation diminishes when only a few components in a large system can be realistically represented. The objective of the paper is to overcome the limitation through a novel model updating method. The model updating is carried out by applying calibrated weighting factors at each time step to the alternative numerical models, which encompasses the possible variation in the experimental specimen properties. The concept is proposed and implemented in the hybrid simulation framework, UI-SimCor. Numerical verification is carried out using two-DOF systems. The method is also applied to an experimental testing, which proves the concept of the proposed model updating method. Copyright © 2013 John Wiley & Sons, Ltd.

Real-time hybrid simulation represents a powerful technique capable of evaluating the structural dynamic performance by combining the physical simulation of a complex and rate-dependent portion of a structure with the numerical simulation of the remaining portion of the same structure. Initially, this paper shows how the stability of real-time hybrid simulation with time delay depends both on compensation techniques and on time integration methods. In particular, even when time delay is exactly known, some combinations of numerical integration and displacement prediction schemes may reduce the response stability with conventional compensation methods and lead to unconditional instability in the worst cases. Therefore, to deal with the inaccuracy of prediction and the uncertainty of delay estimation, a nearly exact compensation scheme is proposed, in which the displacement is compensated by means of an upper bound delay and the desired displacement is picked out by an optimal process. Finally, the advantages of the proposed scheme over conventional delay compensation techniques are shown through numerical simulation and actual tests. Copyright © 2013 John Wiley & Sons, Ltd.

Two new closed-form expressions representing the mean rate of exceedance of a given limit state are presented herein. These proposals overcome limitations that were identified with the original formulation of the well-known SAC/FEMA approach. The new expressions involve new parametric functions for the modeling of the seismic hazard data and for the demand evolution for increasing values of the earthquake intensity measure. Given the carefully selected parametric form of these functions, mathematical tractability is able to be maintained to establish two new closed-form solutions representing the mean rate of exceedance of a given limit state. The function proposed for the hazard exhibits nonlinear behavior in log-log space and is able to represent the actual hazard data over a wider range of earthquake intensity levels. The function proposed for the demand evolution addresses issues related to the inadequate performance of the SAC/FEMA approach when force-based demand parameters such as the shear force are considered. To illustrate the applicability of the new closed-form solutions, the probability of occurrence of several limit states is determined for a reinforced concrete structure. Copyright © 2013 John Wiley & Sons, Ltd.

Hydraulic actuators are typically used in a real-time hybrid simulation to impose displacements to a test structure (also known as the experimental substructure). It is imperative that good actuator control is achieved in the real-time hybrid simulation to minimize actuator delay that leads to incorrect simulation results. The inherent nonlinearity of an actuator as well as any nonlinear response of the experimental substructure can result in an amplitude-dependent behavior of the servo-hydraulic system, making it challenging to accurately control the actuator. To achieve improved control of a servo-hydraulic system with nonlinearities, an adaptive actuator compensation scheme called the adaptive time series (ATS) compensator is developed. The ATS compensator continuously updates the coefficients of the system transfer function during a real-time hybrid simulation using online 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 online updating of the coefficients of the system transfer function, the ATS compensator can effectively account for the nonlinearity of the combined system, resulting in improved accuracy in actuator control. A comparison of the performance of the ATS compensator with existing linearized compensation methods shows superior results for the ATS compensator for cases involving actuator motions with predefined actuator displacement histories as well as real-time hybrid simulations. Copyright © 2013 John Wiley & Sons, Ltd.

Seismic performance of exterior beam–column subassemblages of reinforced concrete structure designed and detailed on the basis of the provisions of Eurocode and Indian Standards at different stages of their evolution is evaluated. Performance of the subassemblages designed and detailed according to the three different stages of codal evolution (gravity load design, ‘Nonductile’, and ‘Ductile’) is evaluated through analytical formulations and experimental investigations. In the ‘NonDuctile’ specimens, it has been observed that the shear distortion and degradation in stiffness and strength are significantly high. Performance of the ‘Ductile’ specimens based on Eurocode and Indian Standards is almost similar in terms of strength and stiffness degradation. Nevertheless, the specimen designed on the basis of Indian Standard shows higher energy dissipation at a given drift ratio. In the analytical study, shear and flexural failure of members of subassemblage and shear failure of the joint are considered as possible modes of failure of the beam–column subassemblage. For evaluating the shear strength of the joint region, a soften strut-and-tie model is used. Analytically obtained strengths based on the failure criteria of different components of the specimens have been first validated with experimental results and then used to determine the strength of the specimens. The investigation could indicate even the mode of failure at local level. It is utmost important to mention here that even the ductile specimens dissipate most of the energy through the development of damage in the joint region, which is neither desirable nor safe for the stability of whole structure. Copyright © 2013 John Wiley & Sons, Ltd.

In this report, the capabilities of the adaptively shifted integration (ASI)-Gauss code in the analysis of the seismic responses of framed structures are verified and validated by comparing the results with detailed numerical simulations performed by the parallel finite element analysis code, E-Simulator, and with experimental results obtained by E-Defense. The numerical results obtained by both codes showed good agreement with the experimental results obtained by E-Defense. Furthermore, seismic waves with unnaturally large magnitudes are applied to a high-rise building model to demonstrate the ability of the ASI-Gauss code to analyze the collapse behaviors of building frames. Copyright © 2013 John Wiley & Sons, Ltd.

Although the ability to simulate accurately the detailed behavior of nonlinear isolation bearings and the effects of this nonlinearity on dynamic response of the isolated building is desirable, such detailed analyses are not feasible during initial design stages when bearing properties are being selected. However, it would be very beneficial to be able to estimate accurately key engineering demand parameters at the early stages of design to understand the dynamic response characteristics of the isolated structure and to balance and optimize the bearing and structural characteristics to achieve the performance goals set for the building. Unfortunately, classical modal response spectrum analysis methods do not provide accurate results for problems with large, nonclassical damping, as is characteristic of isolated buildings. To find a method capable of predicting peak building responses even with large nonclassical damping, generalized modal response spectrum analysis is implemented. The responses of several buildings having different heights and isolated by linear viscous as well as triple friction pendulum and single friction pendulum isolation systems are investigated. Generalized modal response spectrum analysis methods were found to give significantly better predictions for all systems compared with classical methods. The behavior of buildings isolated with single friction pendulum systems exhibiting sudden changes in stiffness could not be well predicted by either general or classical modal response spectrum analysis when effective damping was increased. Copyright © 2013 John Wiley & Sons, Ltd.

The self-centering energy dissipative (SCED) brace is a new steel bracing member that provides both damping to the structure and a re-centering capability. The goal of this study was to confirm the behavior of SCED braces within complete structural systems and to confirm the ability to model these systems with both a state-of-the-art computer model as well as a simplified model that would be useful to practicing engineers. To these ends, a three-story SCED-braced frame was designed and constructed for testing on a shake table. Two concurrent computer models of the entire frame were constructed: one using the opensees nonlinear dynamic modeling software, and a simplified model using the commercial structural analysis software sap2000. The frame specimen was subjected to 12 significant earthquakes without any adjustment or modification between the tests. The SCED braces prevented residual drifts in the frame, as designed, and did not show any significant degradation due to wear. Both numerical models were able to predict the drifts, story shears, and column forces well. Peak story accelerations were overestimated in the models; this effect was found to be caused by the absence of transitions at stiffness changes in the hysteretic model of the braces. Copyright © 2013 John Wiley & Sons, Ltd.

This paper presents numerical modeling techniques to capture the behavior of the triple friction pendulum (TFP) isolation bearing when rotation is permitted about the horizontal axes of the top and bottom components. This paper builds on a previous model for the TFP bearing presented by the authors that is based on the kinematic and constitutive relationships of the individual components of the TFP bearing. The effect of rotation on cyclic bearing behavior and seismic system behavior are investigated numerically for two cases: constant support rotation and variable support rotation. It is found that constant support rotations should be limited in amplitude to ensure standard TFP bearing behavior. Results suggest that flexible supports may not have a large effect on global structure performance as long as typical deformation limits for the supporting members are met. In cases of both constant support rotations and flexible supports, the hardening stages of TFP bearing behavior are diminished. Copyright © 2013 John Wiley & Sons, Ltd.

Supplemental damping is known as an efficient and practical means to improve seismic response of building structures. Presented in this paper is a mixed-integer programming approach to find the optimal placement of supplemental dampers in a given shear building model. The damping coefficients of dampers are treated as discrete design variables. It is shown that a minimization problem of the sum of the transfer function amplitudes of the interstory drifts can be formulated as a mixed-integer second-order cone programming problem. The global optimal solution of the optimization problem is then found by using a solver based on a branch-and-cut algorithm. Two numerical examples in literature are solved with discrete design variables. In one of these examples, the proposed method finds a better solution than an existing method in literature developed for the continuous optimal damper placement problem. Copyright © 2013 John Wiley & Sons, Ltd.

The accuracy of a series spring model to predict the peak displacement and displacement history of Triple Pendulum™ (TP) bearings in a strongly shaken, full-scale building is evaluated in this paper. The series spring model was implemented as a self-contained three-dimensional TP bearing element in OpenSees and is now available for general use. The TP bearing element contains the option for constant friction or a generalized friction model that accounts for the effect of instantaneous velocity and compression load on the friction coefficient. Comparison between numerical simulation and experimental data of a five-story steel moment frame building shows that the peak displacement of isolation system can generally be predicted with confidence using a constant friction coefficient model. The friction coefficient model accounting for the effect of axial load and velocity leads to minor improvement over the constant friction coefficient models in some cases. Copyright © 2013 John Wiley & Sons, Ltd.

In this paper, an analytical method is proposed to determine the dynamic response of 3-D rectangular liquid storage tanks with four flexible walls, subjected to horizontal seismic ground motion. Fluid–structure interaction effects on the dynamic responses of partially filled fluid containers, incorporating wall flexibility, are accounted for in evaluating impulsive pressure. The velocity potential in which boundary conditions are satisfied is solved by the method of separation of variables using the principle of superposition. The impulsive pressure distribution is then computed. Solutions based on 3-D modeling of the rectangular containers are obtained by applying the Rayleigh–Ritz method using the vibration modes of flexible plates with suitable boundary conditions. Trigonometrical functions that satisfy boundary conditions of the storage tank such that the flexibility of the wall is thoroughly considered are used to define the admissible vibration modes. The analysis is then performed in the time domain. Moreover, an analytical procedure is developed for deriving a simple formula that evaluates convective pressure and surface displacements in a similar rigid tank. The variation of dynamic response characteristics with respect to different tank parameters is investigated. A mechanical model, which takes into account the deformability of the tank wall, is developed. The parameters of such a model can be obtained from developed charts, and the maximum seismic loading can be predicted by means of a response spectrum characterizing the design earthquake. Accordingly, a simplified but sufficiently accurate design procedure is developed to improve code formulas for the seismic design of liquid storage tanks. Copyright © 2013 John Wiley & Sons, Ltd.

Fundamental period of vibration appears to be one of the most critical parameter for the seismic design of buildings because this period strongly affects the magnitude of seismic forces. In this paper, an empirical formula for estimating the fundamental period of reinforced concrete structures is recommended, on the basis of the vibration analysis of 20 different real building configurations. These structures have already been constructed in Greece, and they are analyzed by using in detail 3-D finite element models and modal eigenvalue analysis. These models take into account the presence of external and internal infill walls, which are usually ignored as nonstructural elements. This neglect leads to unreliable evaluation of period because the infill walls' contribution to the lateral stiffness and therefore to the fundamental period of vibration is also ignored. Furthermore, taking into account that the flexibility of soil elongates the fundamental period, the soil–structure interaction effect is also considered. To achieve a unique, simple, and effective empirical expression for the fundamental period of vibration, a comprehensive nonlinear regression analysis is applied for the datasets of buildings under consideration. This empirical expression is also compared with the similar expressions from the pertinent literature. Copyright © 2013 John Wiley & Sons, Ltd.

This paper presents the main results of the evaluation of residual inter-story drift demands in typical moment-resisting steel buildings designed accordingly to the Mexican design practice when subjected to narrow-band earthquake ground motions. Analytical 2D-framed models representative of the study-case buildings were subjected to a set of 30 narrow-band earthquake ground motions recorded on stations placed in soft-soil sites of Mexico City, where most significant structural damage was found in buildings as a consequence of the 1985 Michoacan earthquake, and scaled to reach several levels of intensity to perform incremental dynamic analyses. Thus, results were statistically processed to obtain hazard curves of peak (maximum) and residual drift demands for each frame model. It is shown that the study-case frames might exhibit maximum residual inter-story drift demands in excess of 0.5%, which is perceptible for building's occupants and could cause human discomfort, for a mean annual rate of exceedance associated to peak inter-story drift demands of about 3%, which is the limiting drift to avoid collapse prescribed in the 2004 Mexico City Seismic Design Provisions. The influence of a member's post-yield stiffness ratio and material overstrength in the evaluation of maximum residual inter-story drift demands is also discussed. Finally, this study introduces response transformation factors, T_p, that allow establishing residual drift limits compatible with the same mean annual rate of exceedance of peak inter-story drift limits for future seismic design/evaluation criteria that take into account both drift demands for assessing a building's seismic performance. Copyright © 2013 John Wiley & Sons, Ltd.

In this paper, the effects of pulse period associated with near-field ground motions on the seismic demands of soil–MDOF structure systems are investigated by using mathematical pulse models. Three non-dimensional parameters are employed as the crucial parameters, which govern the responses of soil–structure systems: (1) non-dimensional frequency as the structure-to-soil stiffness ratio; (2) aspect ratio of the superstructure; and (3) structural target ductility ratio. The soil beneath the superstructure is simulated on the basis of the Cone model concept. The superstructure is modeled as a nonlinear shear building. Interstory drift ratio is selected as the main engineering demand parameter for soil–structure systems. It is demonstrated that the contribution of higher modes to the response of soil–structure system depends on the pulse-to-interacting system period ratio instead of pulse-to-fixed-base structure period ratio. Furthermore, results of the MDOF superstructures demonstrate that increasing structural target ductility ratio results in the first-mode domination for both fixed-base structure and soil–structure system. Additionally, increasing non-dimensional frequency and aspect ratio of the superstructure respectively decrease and increase the structural responses. Moreover, comparison of the equivalent soil–SDOF structure system and the soil–MDOF structure system elucidates that higher-mode effects are more significant, when soil–structure interaction is taken into account. In general, the effects of fling step and forward directivity pulses on activating higher modes of the superstructure are more sever in soil–structure systems, and in addition, the influences of forward directivity pulses are more considerable than fling step ones. Copyright © 2013 John Wiley & Sons, Ltd.

The effect of collision between adjacent reinforced concrete building frames under multiple earthquakes is investigated in this paper. The four planar frames and the nine different pairs of adjacent reinforced concrete structures of the first companion paper are also examined here, under five real seismic sequences. Such a sequence of earthquakes results in a significant damage accumulation in a structure because any rehabilitation action between any two successive seismic motions cannot be practically materialised because of lack of time. Various parameters are investigated, such as the maximum horizontal displacement of top floor, ductility of columns, permanent displacements and so on. Furthermore, four different separation gaps between the building frames are considered to determine their influence on the behaviour of these frames. It is concluded that in most of the cases, the seismic sequences appear to be detrimental in comparison with the single seismic events. Copyright © 2013 John Wiley & Sons, Ltd.

The effect of different structures configurations on the collision between adjacent planar RC building frames subjected to strong earthquakes is examined in this paper. Two 5-storey and two 8-storey frames, regular or with setbacks, are combined together to produce nine different pairs of adjacent RC structures. These pairs of buildings are subjected to six strong ground motions that are absolutely compatible with the design process. Various parameters are investigated such as maximum displacements, permanent displacements, members' ductility and internal forces and interstorey drift ratios. It is concluded that the effect of collision of adjacent frames seems to be unfavourable for most of the cases and, therefore, the structural pounding phenomenon is rather detrimental than beneficial. Copyright © 2013 John Wiley & Sons, Ltd.

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 three-story, nine-story 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 of buildings studied, the largest vulnerability was identified for the three-story building. To achieve a consistent margin of safety against collapse, a significantly lower R factor is required for the low-rise SCBFs (three-story), whereas the mid-rise and high-rise SCBFs (nine-story and 20-story) may continue to use the current value of 6, as provided in ASCE-07. Copyright © 2013 John Wiley & Sons, Ltd.

In order to investigate the seismic failure characteristics of a structure on the liquefiable ground, a series of shaking table tests were conducted based on a plaster model of a three-story and three-span subway station. The dynamic responses of the structure and ground soil under main shock and aftershock ground motions were studied. The sand boils and waterspouts phenomena, ground surface cracks, and earthquake-induced ground surface settlements were observed in the testing. For the structure, the upward movement, local damage and member cracking were obtained. Under the main shock, there appeared longer liquefaction duration for the ground soil while the pore pressure dissipated slowly. The acceleration amplification effect of the liquefied soil was weakened, and the soil showed a remarkable shock absorption and concentration effect with low frequency component of ground motion. However, under the aftershock, the dissipation of pore pressure in the ground soil became obvious. The peak acceleration of the structure reduced with the buried depth. Dynamic soil pressure on the side wall was smaller in the middle and larger at both ends. The interior column of the model structure was the weakest member. The peak strain and damage degree for both sides of the interior column exhibited an ‘S’ type distribution along the height. Moreover, the seismic response of both ground soil and subway station structure exhibited a remarkable spatial effect. The seismic damage development process and failure mechanism of the structure illustrated in this study can provide references for the engineers and researcher. Copyright © 2013 John Wiley & Sons, Ltd.

Modern highway bridges in Illinois are often installed with economical elastomeric bearings that allow for thermal movement of the superstructure, and steel fixed bearings and transverse retainers that prevent excessive movement from service-level loadings. In the event of an earthquake, the bearing system has the potential to provide a quasi-isolated response where failure of sacrificial elements and sliding of the bearings can cause a period elongation and reduce or cap the force demands on the substructure. A computational model that has been calibrated for the expected nonlinear behaviors is used to carry out a parametric study to evaluate quasi-isolated bridge behavior. The study investigates different superstructure types, substructure types, substructure heights, foundation types, and elastomeric bearing types. Overall, only a few bridge variants were noted to unseat for design-level seismic input in the New Madrid Seismic Zone, indicating that most structures in Illinois would not experience severe damage during their typical design life. However, Type II bearing systems, which consist of an elastomeric bearing and a flat PTFE slider, would in some cases result in critical damage from unseating at moderate and high seismic input. The sequence of damage for many bridge cases indicates yielding of piers at low-level seismic input. This is caused by the high strength of the fixed bearing element, which justifies further calibration of the quasi-isolation design approach. Finally, the type of ground motion, pier height, and bearing type were noted to have significant influence on the global bridge response. Copyright © 2013 John Wiley & Sons, Ltd.

This paper proposes a probabilistic approach for the pre-event assessment of seismic resilience of bridges, including uncertainties associated with expected damage, restoration process, and rebuilding/rehabilitation costs. A fragility analysis performs the probabilistic evaluation of the level of damage (none, slight, moderate, extensive, and complete) induced on bridges by a seismic event. Then, a probabilistic six-parameter sinusoidal-based function describes the bridge functionality over time. Depending on the level of regional seismic hazard, the level of performance that decision makers plan to achieve, the allowable economic impact, and the available budget for post-event rehabilitation activities, a wide spectrum of scenarios are provided. Possible restoration strategies accounting for the desired level of resilience and direct and indirect costs are investigated by performing a Monte Carlo simulation based on Latin hypercube sampling. Sensitivity analyses show how the recovery parameters affect the resilience assessment and seismic impact. Finally, the proposed approach is applied to an existing highway bridge located along a segment of I-15, between the cities of Corona and Murrieta, in California. Copyright © 2013 John Wiley & Sons, Ltd.

In the design and assessment of structures, the aspects regarding the future performance are gaining increased attention. A wide range of performance measures is covered by ‘sustainability’ to reflect these aspects. There is the need for well established methods for quantifying the metrics of sustainability. In this paper, a framework for assessing the time-variant sustainability of bridges associated with multiple hazards considering the effects of structural deterioration is presented. The approach accounts for the effects of flood-induced scour on seismic fragility. Sustainability is quantified in terms of its social, environmental, and economic metrics. These include the expected downtime and number of fatalities, expected energy waste and carbon dioxide emissions, and the expected loss. The proposed approach is illustrated on a reinforced concrete bridge. The effects of corrosion on reinforcement bars and concrete cover spalling are accounted. The seismic fragility curves at different points in time are obtained through nonlinear finite element analyses. The variation of the metrics of sustainability in time is presented. The effects of flood-induced scour on both seismic fragility and metrics are also investigated. Copyright © 2013 John Wiley & Sons, Ltd.

Marginal wharves are key components in providing functionality of port facilities. Ports are central components of the US economy. Earthquake damage to a port can disrupt the economic stability. Therefore, port facilities must be able to quickly return to full operation shortly after a seismic event. Prior studies have shown that integrity of marginal wharves may be compromised by excessive soil movement and structural damage. The latter is often localized at pile-to-wharf connections and in the pile body buried within the soil. Recent research has resulted in an improved connection design that mitigates damage. This study was undertaken to evaluate the full seismic performance of marginal wharves including both conventional and damage-resisting connections. A series of finite element models of a representative pile-supported wharf facility were created. The models varied in their moment-resisting pile-to-wharf connections. A total-stress analysis approach was used to capture the soil response along with p–y, t–z, and Q–z soil–structure interaction springs. Validated connection interface elements were integrated with non-linear frame elements to simulate the marginal wharf structure and substructure. Non-linear static pushover and dynamic time history analyses, for three different hazard levels, were performed. The results of the numerical simulations were used to assess the performance of the marginal wharf including estimates of crane damage and port downtime. Copyright © 2013 John Wiley & Sons, Ltd.

The study presented in this paper addresses the issue of engineering validation of Graves and Pitarka's (2010) hybrid broadband ground motion simulation methodology with respect to some well-recorded historical events and considering the response of multiple degrees of freedom (MDoF) systems. Herein, validation encompasses detailed assessment of how similar is, for a given event, the seismic response due to comparable hybrid broadband simulated records and real records. In the first part of this study, in order to investigate the dynamic response of a wide range of buildings, MDoF structures are modeled as elastic continuum systems consisting of a combination of a flexural cantilever beam coupled with a shear cantilever beam. A number of such continuum systems are selected including the following: (1) 16 oscillation periods between 0.1 and 6 s; (2) three shear to flexural deformation ratios to represent respectively shear-wall structures, dual systems, and moment-resisting frames; and (3) two stiffness distributions along the height of the systems, that is, uniform and linear. Demand spectra in terms of generalized maximum interstory drift ratio (IDR) and peak floor acceleration (PFA) are derived using simulations and actual recordings for four historical earthquakes, namely, the 1979 M_w 6.5 Imperial Valley earthquake, 1989 M_w 6.8 Loma Prieta earthquake, 1992 M_w 7.2 Landers earthquake, and 1994 M_w 6.7 Northridge earthquake. In the second part, for two nonlinear case study structures, the IDR and PFA distributions over the height and their statistics, are obtained and compared for both recorded and simulated time histories. These structures are steel moment frames designed for high seismic hazard, 20-story high-rise and 6-story low-rise buildings. The results from this study highlight the similarities and differences between simulated and real records in terms of median and intra-event standard deviation of logs of seismic demands for MDoF building systems. This general agreement, in a broad range of moderate and long periods, may provide confidence in the use of the simulation methodology for engineering applications, whereas the discrepancies, statistically significant only at short periods, may help in addressing improvements in generation of synthetic records. Copyright © 2013 John Wiley & Sons, Ltd.

A two-phase research program has been undertaken to investigate fundamental natural periods of concentrically braced frames (CBFs) designed according to Eurocode 8 (EC8). In the first phase of the program, over 83,700 buildings were designed, and the accuracy of the lower bound expressions given in well-known design specifications was evaluated. The results indicated that the lower bound expressions given in EC8 and National Building Code of Canada (NBCC) are acceptable. Although all structures had periods longer than the ones estimated by the EC8 expression, a few structures had shorter periods than the ones estimated by the NBCC expression. In general, the lower bound expressions given in EC8 and NBCC were found to provide over conservative estimates for most cases. In the second phase of the program, a simple hand method was developed to estimate the fundamental natural periods of CBFs designed to EC8. This method requires the use of inelastic top story drift ratio as a parameter to quantify stiffness characteristics. The drift ratios were extracted from the design pool developed as a part of the first phase and represented by simple mathematical relationships. Evaluation of the proposed method indicated that the method is accurate in providing estimates of the fundamental period. To safeguard against providing unconservative estimates, the method was modified to arrive at a new lower bound expression, which significantly improves the estimates compared with the ones provided by the existing expressions. Copyright © 2013 John Wiley & Sons, Ltd.

This paper develops a novel ground motion selection procedure for nonlinear time history analysis of critical structures. The skyline query originated from computer science is first introduced, including its concept and related algorithms. Then, the ground motion selection procedure based on skyline query is developed. Meanwhile, a new five-dimensional vector-valued intensity measure is defined as a critical ingredient of the selection procedure to measure the damage potential of ground motions. Finally, the process of the selection procedure is illustrated through examples of three shear models, and its efficiency is also validated. Through the examples of three shear models, the ground motion selection procedure based on skyline query proposed in this paper is proven to be capable of selecting a limited set of ground motions with high damage potentials for the nonlinear time history analysis purpose. Copyright © 2012 John Wiley & Sons, Ltd.

The objective of the study presented in this paper is to investigate the effects of masonry infills on the shear demand and failure of columns for the case when reinforced concrete frames with such infills are modeled by means of simplified nonlinear models that are not capable of the direct simulation of these effects. It is shown that an approximate simulation of the shear failure of columns can be achieved through an iterative procedure that involves pushover analysis, post-processing of the analysis results using limit-state checks of the components, and model adaptation if shear failure of columns is detected. The fragility parameters and the mean annual frequency of limit-state exceedance are computed on the basis of nonlinear dynamic analysis by using an equivalent SDOF model. The proposed methodology is demonstrated by means of two examples. It was shown that the strength of the four-story and seven-story buildings and their deformation capacity are significantly overestimated if column shear failure due to the effects of masonry infills is neglected, whereas the mean annual frequency of limit-state exceedance for the analyzed limit states is significantly larger than that estimated for the case if the shear failure of columns is neglected. Copyright © 2012 John Wiley & Sons, Ltd.

A procedure for estimating the peak response of SDOF systems equipped with passive supplemental dampers using code-defined uniform hazard spectra is presented. The proposed procedure makes use of an improved equivalent linearization method that unifies the treatment of supplemental hysteretic and viscous-viscoelastic damping and can be readily implemented in a computer script. Nonlinear time-history analyses demonstrated that the proposed method makes significantly more reliable predictions than other common equivalent linearization methods for systems with supplemental dampers. Using the proposed procedure, performance spectra, which are plots of normalized response of SDOF systems with dampers, can be generated for practical performance-based design. To obtain a more complete description of the system performance, a simplified method for estimating the residual drift is also verified using results of extensive nonlinear time-history analyses. Copyright © 2012 John Wiley & Sons, Ltd.

The seismic performance tests of a full-scale five-story passively controlled steel building were conducted on the E-Defense shaking table in Japan in March 2009. Before the tests, a blind prediction contest was held to allow researchers and practitioners from all over the world to construct analytical models and predict the dynamic responses of the steel frame specimen equipped with buckling-restrained braces (BRBs) or viscous dampers (VDs). This paper presents the details of two refined prediction models made and results obtained before the tests. When the proposed analytical modeling techniques are adopted as in the two refined prediction models, the overall prediction accuracy is about 90%. Sensitivity studies conducted after the tests are also presented in this paper. The effects of varying each modeling feature on the response simulation accuracy have been investigated. The analytical results suggest that considering concrete full-composite actions for beam members could improve prediction accuracy by about 20% against using the simplified bare steel beam model. Adopting refined BRB stiffness computed from incorporating finite-element gusset stiffness only improves the overall prediction accuracy by 0.9%. Considering the BRB dynamic loading test results for analytical BRB strength reduces the error by 1.9%. For the VD frame, incorporating the brace and VD stiffness could improve the overall prediction accuracy by about 15%. Copyright © 2012 John Wiley & Sons, Ltd.

The task of selecting and scaling an appropriate set of ground motion records is one of the most important challenges facing practitioners in conducting dynamic response history analyses for seismic design and risk assessment. This paper describes an integrated experimental and analytical evaluation of selected ground motion scaling methods for linear-elastic building frame structures. The experimental study is based on the shake table testing of small-scale frame models with four different fundamental periods under ground motion sets that have been scaled using different methods. The test results are then analytically extended to a wider range of structural properties to assess the effectiveness of the scaling methods in reducing the dispersion and increasing the accuracy in the seismic displacement demands of linear-elastic structures, also considering biased selection of ground motion subsets. For scaling methods that are based on a design estimate of the fundamental period of the structure, effects of possible errors in the estimated period are investigated. The results show that a significant reduction in the effectiveness of these scaling methods can occur if the fundamental period is not estimated with reasonable certainty. Copyright © 2012 John Wiley & Sons, Ltd.

Response spectrum matching is commonly used to generate ground motions with response spectra matching a scenario target spectrum. There is some debate in the literature about whether spectrum-matched motions lead to biased structural analysis results. Furthermore, there are no objective, quantitative criteria available for deciding whether a ground motion has been manipulated excessively by spectrum matching, and whether large modification may also lead to bias. This study investigates both of these issues by presenting the results of structural analysis using two reinforced concrete moment frame models and two earthquake scenarios, with suites of unmatched and matched ground motions. Through comparison with a robust benchmark, it is shown that no significant bias is introduced by spectrum matching. The period range and target damping values for matching are also investigated, and matching up to three times the fundamental period is shown to be beneficial in reducing dispersion in the results. Finally, these analyses were also used to investigate whether large changes in the ground motion lead to biased analysis results. Several potential measures of change are investigated, including those based on peak absolute ground motion, cumulative squared ground motion (absolute or normalized), and input energy into single-degree-of-freedom systems. Although no systematic, statistically significant correlation is found for the analysis results in terms of any of these measures of change, tentative criteria are proposed, which may be used by analysts to aid in the decision of whether to accept or reject a spectrum-matched motion. Copyright © 2012 John Wiley & Sons, Ltd.

Steel rectangular section columns with stiffened plates are commonly used for elevated highway bridges in the urban areas of Japan. The seismic design of bridge piers is usually performed by dynamic analysis in the horizontal direction using various independent directional seismic acceleration data. However, this simple treatment does not reflect the effect of bilateral loading as a structural response to inelastic interaction. In this study, unidirectional and bidirectional loading hybrid tests were conducted to examine the seismic response and performance of square cross-sections of steel bridge piers subjected to bidirectional seismic accelerations. Comparison of the results of unidirectional and bidirectional loading tests revealed that the maximum load is the same as the average of unidirectional loading in the NS and EW directions; however, the maximum response displacement and residual displacement increase in proportion with hard to soft ground types. Moreover, a modified seismic design is proposed considering these bidirectional loading effects. Copyright © 2012 John Wiley & Sons, Ltd.

In this paper, a contribution of various types of masonry infill to the behaviour of reinforced concrete frames under lateral loads is presented. As a part of the bigger project, ten one-bay, one-storey reinforced concrete frames were designed according to the EC8, built in a scale 1:2.5, infilled with masonry and tested under constant vertical and cyclic lateral load. The masonry wall had various strength properties, namely, high strength hollow clay brick blocks, medium strength hollow clay brick blocks and low strength lightweight autoclaved aerated concrete blocks. There were no additional shear connectors between the masonry and frame. The results showed that the composite ‘framed wall’ structure had much higher stiffness, damping and initial strength than the bare frame structure. Masonry infill filled in the load capacity gap from very low (0.05%) to drifts when the frame took over (0.75%). The structures behaved as linear monolithic elements to drifts of 0.1%, reached the maximum lateral capacities at drift of 0.3%, maintained it to drifts of 0.75% and after that their behaviour depended on the frame. Masonry infill had severe damage at drift levels of about 0.75% but contributed to the overall system resistance to drifts of about 1%. At that drift level, the frame had only minor damage and was tested to drifts of about 2% without any loss of capacity. Improvement of the ‘infill provisions’ in the codes could be sought by taking into account the contribution of a common masonry that reduces expected damages by lowering the drift levels. Copyright © 2012 John Wiley & Sons, Ltd.

The concentrically braced frame (CBF) structure is one of the most efficient steel structural systems to resist earthquakes. This system can dissipate energy during earthquakes through braces, which are expected to yield in tension and buckle in compression, while all other elements such as columns, beams and connections are expected to behave elastically. In this paper, the performance of single-storey CBFs is assessed with nonlinear time-history analysis, where a robust numerical model that simulates the behaviour of shake table tests is developed. The numerical model of the brace element used in the analysis was calibrated using data measured in physical tests on brace members subjected to cyclic loading. The model is then validated by comparing predictions from nonlinear time-history analysis to measured performance of brace members in full scale shake table tests. Furthermore, the sensitivity of the performance of the CBF to different earthquake ground motions is investigated by subjecting the CBF to eight ground motions that have been scaled to have similar displacement response spectra. The comparative assessments presented in this work indicate that these developed numerical models can accurately capture the salient features related to the seismic behaviour of CBFs. A good agreement is found between the performance of the numerical and physical models in terms of maximum displacement, base shear force, energy dissipated and the equivalent viscous damping. The energy dissipated and, more particular, the equivalent viscous damping, are important parameters required when developing an accurate displacement-based design methodology for CBFs subjected to earthquake loading. In this study, a relatively good prediction of the equivalent viscous damping is obtained from the numerical model when compared with data measured during the shake table tests. However, it was found that already established equations to determine the equivalent viscous damping of CBFs may give closer values to those obtained from the physical tests. Copyright © 2012 John Wiley & Sons, Ltd.

A novel set of SAC/FEMA-style closed-form expressions is presented to accurately assess structural safety under seismic action. Such solutions allow the practical evaluation of the risk integral convolving seismic hazard and structural response by using a number of idealizations to achieve a simple analytical form. The most heavily criticized approximation of the SAC/FEMA formats is the first-order power-law fit of the hazard curve. It results to unacceptable errors whenever the curvature of the hazard function becomes significant. Adopting a second-order fit, instead, allows capturing the hazard curvature at the cost of necessitating new analytic forms. The new set of equations is a complete replacement of the original, enabling (a) accurate estimation of the mean annual frequency of limit-state exceedance and (b) safety checking for specified performance objectives in a code-compatible format. More importantly, the flexibility of higher-order fitting guarantees a wider-range validity of the local hazard approximation. Thus, it enables the inversion of the formulas for practically estimating the allowable demand or the required capacity to fulfill any design objective. Copyright © 2012 John Wiley & Sons, Ltd.

Cumulative absolute velocity (CAV) is an important ground motion intensity measure used in seismic hazard analysis. Based on the Next Generation Attenuation strong motion database, a simple ground-motion prediction equation is proposed for the geometric mean of as-recorded horizontal components of CAVs using mixed regression analysis. The proposed model employs only four parameters and has a simple functional form. Validation tests are conducted to compare the proposed model with the recently developed Campbell–Bozorgnia (CB10) model using subsets of the strong motion database, as well as several recent earthquakes that are not used in developing the model. It is found that the predictive capability of the proposed model is comparable with the CB10 model, which employs a complex functional form and more parameters. The study also corroborates previous findings that CAV has higher predictability than other intensity measures such as the peak ground acceleration. The high predictability of CAV warrants the use of the proposed simple model as an alternative in seismic hazard analysis. Copyright © 2012 John Wiley & Sons, Ltd.

Existing design procedures for determining the separation distance between adjacent buildings subjected to seismic pounding risk are based on approximations of the buildings' peak relative displacement. These procedures are characterized by unknown safety levels and thus are not suitable for use within a performance-based earthquake engineering framework.

This paper introduces an innovative reliability-based methodology for the design of the separation distance between adjacent buildings. The proposed methodology, which is naturally integrated into modern performance-based design procedures, provides the value of the separation distance corresponding to a target probability of pounding during the design life of the buildings. It recasts the inverse reliability problem of the determination of the design separation distance as a zero-finding problem and involves the use of analytical techniques in order to evaluate the statistics of the dynamic response of the buildings. Both uncertainty in the seismic intensity and record-to-record variability are taken into account.

The proposed methodology is applied to several different buildings modeled as linear elastic single-degree-of-freedom (SDOF) and multi-degree-of-freedom (MDOF) systems, as well as SDOF nonlinear hysteretic systems. The design separation distances obtained are compared with the corresponding estimates that are based on several response combination rules suggested in the seismic design codes and in the literature. In contrast to current seismic code design procedures, the newly proposed methodology provides consistent safety levels for different building properties and different seismic hazard conditions. Copyright © 2012 John Wiley & Sons, Ltd.

Dynamic characteristics of structures — viz. natural frequencies, damping ratios, and mode shapes — are central to earthquake-resistant design. These values identified from field measurements are useful for model validation and health-monitoring. Most system identification methods require input excitations motions to be measured and the structural response; however, the true input motions are seldom recordable. For example, when soil–structure interaction effects are non-negligible, neither the free-field motions nor the recorded responses of the foundations may be assumed as ‘input’. Even in the absence of soil–structure interaction, in many instances, the foundation responses are not recorded (or are recorded with a low signal-to-noise ratio). Unfortunately, existing output-only methods are limited to free vibration data, or weak stationary ambient excitations. However, it is well-known that the dynamic characteristics of most civil structures are amplitude-dependent; thus, parameters identified from low-amplitude responses do not match well with those from strong excitations, which arguably are more pertinent to seismic design. In this study, we present a new identification method through which a structure's dynamic characteristics can be extracted using only seismic response (output) signals. In this method, first, the response signals’ spatial time-frequency distributions are used for blindly identifying the classical mode shapes and the modal coordinate signals. Second, cross-relations among the modal coordinates are employed to determine the system's natural frequencies and damping ratios on the premise of linear behavior for the system. We use simulated (but realistic) data to verify the method, and also apply it to a real-life data set to demonstrate its utility. Copyright © 2012 John Wiley & Sons, Ltd.

Buckling restrained braces (BRBs) are very effective in dissipating energy through stable tension–compression hysteretic cycles and have been successfully experimented in the seismic protection of buildings. Their behavior has been studied extensively in the last decades and today the level of performance guaranteed by these devices and the technological constrains that have to be fulfilled to optimize their behavior are well known. Furthermore, several companies in the world have developed their own BRBs and are now producing them. In spite of this, many seismic codes (for instance, the EuroCode 8) do not stipulate provisions for the design and construction of earthquake-resistant structures equipped with BRBs. This discourages the structural engineering community from using these devices and seriously limits their use in structural applications. In this paper a procedure for the seismic design of steel frames equipped with BRBs is proposed. Furthermore, the paper presents a numerical investigation aimed at validating this design procedure and proposing the value of the behavior factor q that should be used for this structural type. To this end, a set of frames with BRBs is first designed by means of several values of q. Then, the obtained frames are subjected to a set of accelerograms compatible with the elastic response spectrum considered in design. The seismic response of the frames is determined by nonlinear dynamic analysis and represented in terms of the ductility demand of BRBs and the internal force demand of nondissipative members (beams and columns). Finally, the largest value of q that leads to acceptable seismic performance of the analyzed frames is assumed as adequate. The value of q is given in the paper as a continuous function of the assumed ductility capacity of the BRBs. Copyright © 2012 John Wiley & Sons, Ltd.