Seismic response of 20‐story base‐isolated and fixed‐base reinforced concrete structural wall buildings at a near‐fault site

This paper investigates numerically the seismic response of six seismically base‐isolated (BI) 20‐story reinforced concrete buildings and compares their response to that of a fixed‐base (FB) building with a similar structural system above ground. Located in Berkeley, California, 2 km from the Hayward fault, the buildings are designed with a core wall that provides most of the lateral force resistance above ground. For the BI buildings, the following are investigated: two isolation systems (both implemented below a three‐story basement), isolation periods equal to 4, 5, and 6 s, and two levels of flexural strength of the wall. The first isolation system combines tension‐resistant friction pendulum bearings and nonlinear fluid viscous dampers (NFVDs); the second combines low‐friction tension‐resistant crosslinear bearings, lead‐rubber bearings, and NFVDs. The designs of all buildings satisfy ASCE 7‐10 requirements, except that one component of horizontal excitation, is used in the 2D nonlinear response history analysis. Analysis is performed for a set of ground motions scaled to the design earthquake and to the maximum considered earthquake (MCE). At both the design earthquake and the MCE, the FB building develops large inelastic deformations and shear forces in the wall and large floor accelerations. At the MCE, four of the BI buildings experience nominally elastic response of the wall, with floor accelerations and shear forces being 0.25 to 0.55 times those experienced by the FB building. The response of the FB and four of the BI buildings to four unscaled historical pulse‐like near‐fault ground motions is also studied. Copyright © 2013 John Wiley & Sons, Ltd.     

Seismic isolation of cable‐stayed bridges for near‐field ground motions

This study examines the efficacy of using seismic isolation to favorably influence the seismic response of cable‐stayed bridges subjected to near‐field earthquake ground motions. In near‐field earthquake ground motions, large amplitude spectral accelerations can occur at long periods where many cable‐stayed bridges have significant structural response modes. This combination of factors can result in large tower accelerations and base shears. In this study, lead–rubber bearing seismic isolators were modeled for three cable‐stayed bridges, and three cases of isolation were examined for each bridge. The nine isolated bridge configurations, plus three non‐isolated configurations as references, were subjected to near‐field earthquake ground motions using three‐dimensional time‐history analyses. Introduction of a small amount of isolation is shown to be very beneficial in reducing seismic accelerations and forces while at the same time producing only a modest increase in the structural displacements. There is a low marginal benefit to continue to increase the amount of isolation by further lengthening the period of the structure because structural forces and accelerations reduce at a diminishing rate whereas structural displacements increase substantially. In virtually all cases the base shears in the isolated bridges were reduced by at least 50several instances by up to 80individual near‐field records showed large variability from one record to the next, with coefficients of variation about the mean as large as 50assessing the characteristics of near‐field ground motion for use in isolation design of cable‐stayed bridges. Copyright © 2003 John Wiley & Sons, Ltd.     

Performance of base‐isolated reinforced concrete buildings under bidirectional seismic excitation considering pounding with retaining walls including friction effects

Seismic pounding of base‐isolated buildings has been mostly studied in the past assuming unidirectional excitation. Therefore, in this study, the effects of seismic pounding on the response of base‐isolated reinforced concrete buildings under bidirectional excitation are investigated. For this purpose, a three‐dimensional finite element model of a code‐compliant four‐story building is considered, where a newly developed contact element that accounts for friction and is capable of simulating pounding with retaining walls at the base, is used. Nonlinear behavior of the superstructure as well as the isolation system is considered. The performance of the building is evaluated separately for far‐fault non‐pulse‐like ground motions and near‐fault pulse‐like ground motions, which are weighted scaled to represent two levels of shaking viz. the design earthquake (DE) level and the risk‐targeted maximum considered earthquake (MCE_R) level. Nonlinear time‐history analyses are carried out considering lower bound as well as upper bound properties of isolators. The influence of separation distance between the building and the retaining walls at the base is also investigated. It is found that if pounding is avoided, the performance of the building is satisfactory in terms of limiting structural and nonstructural damage, under DE‐level motions and MCE_R‐level far‐fault motions, whereas unacceptably large demands are imposed by MCE_R‐level near‐fault motions. In the case of seismic pounding, MCE_R‐level near‐fault motions are found to be detrimental, where the effect of pounding is mostly concentrated at the first story. In addition, it is determined that considering unidirectional excitation instead of bidirectional excitation for MCE_R‐level near‐fault motions provides highly unconservative estimates of superstructure demands. Copyright © 2014 John Wiley & Sons, Ltd.     

Experimental and computational verification of reasonable design formulae for base‐isolated structures

It is clear that base isolation is a sensible strategic design in attenuating the responses of a structural system induced by ground motions. The design of seismically isolated structures is mainly governed by the Uniform Building Code (UBC) published by the International Conference of Building Officials. The UBC code emphasizes a simple, statically equivalent design method that displacements of an isolated structure are concentrated at the isolation level. Therefore, the superstructure nearly moves as a rigid body and the design forces of elements above isolators are based on the behaviour of isolators at the design displacement. However, in the UBC code, the distribution of inertial (or lateral) forces over the height of the superstructure above isolation has been found to be too conservative for most isolated structures. In view of this, two simple and reasonable design formulae for the lateral force distribution on isolated structures have been proposed in this paper. Results obtained from a full‐scale isolated structure tested on the shaking table and numerical analyses of two additional examples verify the suitability of design formulae. It is illustrated that the proposed formulae can predict well the lateral force distribution on isolated structures during earthquakes. Copyright © 2003 John Wiley & Sons, Ltd.     

Testing of dynamically substructured,base‐isolated systems using adaptive control techniques

Experimental techniques for testing dynamically substructured systems are currently receiving attention in a wide range of structural, aerospace and automotive engineering environments. Dynamic substructuring enables full‐size, critical components to be physically tested within a laboratory (as physical substructures), while the remaining parts are simulated in real‐time (as numerical substructures). High quality control is required to achieve synchronization of variables at the substructuring interfaces and to compensate for additional actuator system(s) dynamics, nonlinearities, uncertainties and time‐varying parameters within the physical substructures. This paper presents the substructuring approach and associated controller designs for performance testing of an aseismic, base‐isolation system, which is comprised of roller‐pendulum isolators and controllable, nonlinear magnetorheological dampers. Roller‐pendulum isolators are typically mounted between the protected structure and its foundation and have a fundamental period of oscillation far‐removed from the predominant periods of any earthquake. Such semi‐active damper systems can ensure safety and performance requirements, whereas the implementation of purely active systems can be problematic in this respect. A linear inverse dynamics compensation and an adaptive controller are tailored for the resulting nonlinear synchronization problem. Implementation results favourably compare the effectiveness of the adaptive substructuring method against a conventional shaking‐table technique. A 1.32% error resulted compared with the shaking‐table response. Ultimately, the accuracy of the substructuring method compared with the response of the shaking‐table is dependent upon the fidelity of the numerical substructure. Copyright © 2009 John Wiley & Sons, Ltd.     

Full‐scale shaking table test of a base‐isolated medical facility subjected to vertical motions

Base isolation is a well known technology that has been proven to reduce structural response to horizontal ground accelerations. However, vertical response still remains a topic of concern for base‐isolated buildings, perhaps more so than in fixed‐base buildings as isolation is often used when high performance is required. To investigate the effects of vertical response on building contents and nonstructural components, a series of full‐scale shaking table tests were conducted at the E‐Defense facility in Japan. A four‐story base‐isolated reinforced concrete building was outfitted as a medical facility with a wide variety of contents, and the behavior of the contents was observed. The rubber base isolation system was found to significantly amplify vertical accelerations in some cases. However, the damage caused by the vertical ground motions was not detrimental when peak vertical floor accelerations remained below 2 g with three exceptions: (1) small items placed on shelves slid or toppled; (2) objects jumped when placed on nonrigid furniture, which tended to increase the response; and (3) equipment with vertical eccentricities rocked and jumped. In these tests, all equipment and nonstructural components remained functional after shaking. Copyright © 2013 John Wiley & Sons, Ltd.     

Wavelet‐based characterization of design ground motions

With the recent emergence of wavelet‐based procedures for stochastic analyses of linear and non‐linear structural systems subjected to earthquake ground motions, it has become necessary that seismic ground motion processes are characterized through statistical functionals of wavelet coefficients. While direct characterization in terms of earthquake and site parameters may have to wait for a few more years due to the complexity of the problem, this study attempts such characterization through commonly available Fourier and response spectra for design earthquake motions. Two approaches have been proposed for obtaining the spectrum‐compatible wavelet functionals, one for input Fourier spectrum and another for input response spectrum, such that the total number of input data points are 30–35% of those required for a time‐history analysis. The proposed methods provide for simulating ‘desired non‐stationary characteristics’ consistent with those in a recorded accelerogram. Numerical studies have been performed to illustrate the proposed approaches. Further, the wavelet functionals compatible with a USNRC spectrum in the case of 35 recorded motions of similar strong motion durations have been used to obtain the strength reduction factor spectra for elasto‐plastic oscillators and to show that about ±20% variation may be assumed from mean to 5 and 95% confidence levels due to uncertainty in the non‐stationary characteristics of the ground motion process. Copyright © 2002 John Wiley & Sons, Ltd.     

Effects of seismic isolation on the seismic response of a California high‐speed rail prototype bridge with soil‐structure and track‐structure interactions

With the launch of the high‐speed train project in California, the seismic risk is a crucial concern to the stakeholders. To investigate the seismic behavior of future California High‐Speed Rail (CHSR) bridge structures, a 3D nonlinear finite‐element model of a CHSR prototype bridge is developed. Soil‐structure and track‐structure interactions are accounted for in this comprehensive numerical model used to simulate the seismic response of the bridge and track system. This paper focuses on examining potential benefits and possible drawbacks of the a priori promising application of seismic isolation in CHSR bridges. Nonlinear time history analyses are performed for this prototype bridge subjected to two bidirectional horizontal historical earthquake ground motions each scaled to two different seismic hazard levels. The effect of seismic isolation on the seismic performance of the bridge is investigated through a detailed comparison of the seismic response of the bridge with and without seismic isolation. It is found that seismic isolation significantly reduces the deck acceleration and the force demand in the bridge substructure (i.e., piers and foundations), especially for high‐intensity earthquakes. However, seismic isolation increases the deck displacement (relative to the pile cap) and the stresses in the rails. These findings imply that seismic isolation can be promisingly applied to CHSR bridges with due consideration of balancing its beneficial and detrimental effects through using appropriate isolators design. The optimum seismic isolator properties can be sought by solving a performance‐based optimum seismic design problem using the nonlinear finite‐element model presented herein. Copyright © 2016 John Wiley & Sons, Ltd.     

Vertical vibration isolator having piecewise‐constant restoring force

We present a vertical vibration isolator having a piecewise‐constant restoring force, which belongs to a class of passive and nonlinear vibration isolators. In vertical vibration isolation, direct use of low‐stiffness elements leads to unacceptably large deformations due to self‐weight. To overcome the difficulty, we apply a combination of constant‐force springs, each of which sustains a constant load regardless of its stretch. By arranging the constant‐force springs, so that the isolator has a piecewise‐constant restoring force, we alleviate the problem of the excessive deformation caused by self‐weight, provide stability at the static equilibrium state along with the self‐centering capability, and realize a large stroke while keeping the mechanism simple and compact. Further, we attempt to limit the response acceleration within a tolerance regardless of the frequency spectrum and the magnitude of earthquake ground motions. We demonstrate the effectiveness of the present isolator through shaking table tests and numerical simulations. Copyright © 2009 John Wiley & Sons, Ltd.     

近断层地震动作用下隔震结构的优化设计

首先讨论了近断层脉冲型地震动的特点,并以台湾集集地震实际脉冲型近震记录为地震动输入,应用含潜在约束策略的序列二次规划算法,对安装铅芯橡胶隔震支座的钢筋混凝土框架隔震结构的隔震器参数和上部结构构件截面几何尺寸进行一体化优化设计,然后输入E l Centro(1940)、Taft(1952)地震波对优化后的隔震结构进行地震反应分析。计算结果表明,对考虑脉冲型近断层地震动作用的隔震结构进行参数优化设计后,该隔震结构能同时满足脉冲型和普通非脉冲型近震作用的结构设计需求。     

Theory and experimental study for sliding isolators with variable curvature

Because a conventional isolation system with constant isolation frequency is usually a long‐period dynamic system, its seismic response is likely to be amplified in earthquakes with strong long‐period wave components, such as near‐fault ground motions. Seismic isolators with variable mechanical properties may provide a promising solution to alleviate this problem. To this end, in this work sliding isolators with variable curvature (SIVC) were studied experimentally. An SIVC isolator is similar to a friction pendulum system (FPS) isolator, except that its sliding surface has variable curvature rather being spherical. As a result, the SIVC's isolation stiffness that is proportional to the curvature becomes a function of the isolator displacement. By appropriately designing the geometry of the sliding surface, the SIVC is able to possess favorable hysteretic behavior. In order to prove the applicability of the SIVC concept, several prototype SIVC isolators, whose sliding surfaces are defined by a sixth‐order polynomial function, were fabricated and tested in this study. A cyclic element test on the prototype SIVC isolators and a shaking table test on an SIVC isolated steel frame were all conducted. The results of both tests have verified that the prototype SIVC isolators do indeed have the hysteretic property of variable stiffness as prescribed by the derived formulas in this study. Moreover, it is also demonstrated that the proposed SIVC is able to effectively reduce the isolator drift in a near‐fault earthquake with strong long‐period components, as compared with that of an FPS system with the same friction coefficient. Copyright © 2011 John Wiley & Sons, Ltd.     

Study on effects of damping in laminated rubber bearings on seismic responses for a ⅛ scale isolated test structure

The effects of damping in various laminated rubber bearings (LRB) on the seismic response of a ?‐scale isolated test structure are investigated by shaking table tests and seismic response analyses. A series of shaking table tests of the structure were performed for a fixed base design and for a base isolation design. Two different types of LRB were used: natural rubber bearings (NRB) and lead rubber bearings (LLRB). Three different designs for the LLRB were tested; each design had a different diameter of lead plug, and thus, different damping values. Artificial time histories of peak ground acceleration 0.4g were used in both the tests and the analyses. In both shaking table tests and analyses, as expected, the acceleration responses of the seismically isolated test structure were considerably reduced. However, the shear displacement at the isolators was increased. To reduce the shear displacement in the isolators, the diameter of the lead plug in the LLRB had to be enlarged to increase isolator damping by more than 24%. This caused the isolator stiffness to increase, and resulted in amplifying the floor acceleration response spectra of the isolated test structure in the higher frequency ranges with a monotonic reduction of isolator shear displacement. Copyright © 2002 John Wiley & Sons, Ltd.     

Optimized friction pendulum and precast-prestressed pile to base-isolate a Chilean masonry house

In this study friction pendulum system (FPS) bearings and precast-prestressed pile (PPP) isolators are considered as base isolation devices for a Chilean confined masonry house. The house is numerically modeled using a multiple degree-of-freedom approach that is calibrated with experimental data. Dynamic behavior of the FPS and PPP isolators is simulated using analytical formulations based on laboratory testing. Optimization of the isolators is performed using an earthquake that is generated to match the design spectrum for the house based on Chilean seismic code. A non-dominated sorting genetic algorithm (NSGA-II) is applied to carry out the optimization. Seismic response of the base-isolated structure subjected to a suite of ground motions is compared to the performance of the traditionally-constructed structure by means of several performance indices (PIs). Numerical simulations indicate that the PPP isolation system is more effective in reducing the base and structural shear, interstory drift, and floor acceleration of the structure than the FPS isolation system, although both systems result in substantial reductions of the response.     

Component and shaking table tests for full‐scale multiple friction pendulum system

The use of base isolation in developed countries including the U.S. and Japan has already been recognized as a very effective method for upgrading the seismic resistance of structures. In this study, an advanced base‐isolation system called the multiple friction pendulum system (MFPS) is investigated to understand its performance on seismic mitigation through full‐scale component and shaking table tests. The component tests of the advanced Teflon composite coated on the sliding surface show that the friction coefficient of the lubricant material is a function of the sliding velocity in the range of 0.03–0.12. The experimental results also indicate that there were no signs of degradation of the sliding interface observed after 2000 cycles of sliding displacements. A full‐scale MFPS isolator under a vertically compressive load of 8830 KN (900 tf) and horizontally cyclic displacements was tested in order to assess the feasibility of the MFPS isolator for its practical use. After 248 cycles of horizontal displacement reversals, the behaviour of the base isolator was almost identical to its behaviour during the first few cycles. The experimental results of the shaking table tests of a full‐scale steel structure isolated with MFPS isolators show that the MFPS device can isolate seismic transmitted energy effectively under soft‐soil‐deposit site earthquakes with long predominant periods as well as strong ground motions with short predominant periods. These test results demonstrate that the MFPS isolator possesses excellent durability and outstanding earthquake‐proof capability. Furthermore, the numerical results show that the mathematical model proposed in this study can well predict the seismic responses of a structure isolated with MFPS isolators. Copyright © 2006 John Wiley & Sons, Ltd.     

Performance of seismic‐isolated bridges with and without elastic‐gap devices in near‐fault zones

In this paper, the efficiency of providing elastic‐gap devices (EGDs) to improve the performance of seismic‐isolated bridges (SIBs) in near‐fault (NF) zones is investigated. The device is primarily made of an assembly of circular rubber bearings and steel plates to provide additional elastic stiffness to the SIB upon closure of a gap. The EDG is intended to function at two performance levels under service and maximum considered design level (MCDL) NF earthquakes to reduce isolator displacements while keeping the substructure forces at reasonable levels. A parametric study, involving more than 500 nonlinear time history analyses of realistic and simplified structural models of typical SIBs, is conducted using simulated and actual NF ground motions to investigate the applicability of the proposed solution. It is found that providing EGD is beneficial for reducing the isolator displacements to manageable ranges for SIBs subjected to MCDL NF ground motions regardless of the distance from the fault and characteristics of the isolator. It is also found that providing EGD resulted in an improved performance of the isolators in terms of the reduction of heat generated by the isolators. Further analyses conducted using a realistic structural model of an existing bridge and five NF earthquakes confirmed that EGD may be used to reduce the displacement of the isolators while keeping the substructure base shear forces at reasonable ranges for SIBs located in NF zones. Copyright © 2008 John Wiley & Sons, Ltd.     

Optimal design and seismic performance of tuned mass damper inerter (TMDI) for structures with nonlinear base isolation systems

The tuned mass damper inerter (TMDI) couples the classical tuned mass damper (TMD) with an inerter, a mechanical device whose generated force is proportional to the relative acceleration between its terminals, thus providing beneficial mass‐amplification effects. This paper deals with a dynamic layout in which the TMDI is installed below the isolation floor of base‐isolated structures in order to enhance the earthquake resilience and reduce the displacement demand. Unlike most of the literature studies that assumed a linearized behavior of the isolators, the aim of this paper is to investigate the effectiveness of the TMDI while accounting for the nonlinearity of the isolators. Two nonlinear constitutive behaviors are considered, a Coulomb friction model and a Bouc‐Wen hysteretic model, representative of friction pendulum and of lead‐rubber‐bearing isolators, respectively. Optimal design is based on the stochastic dynamic analysis of the system, by modeling the base acceleration as a Kanai‐Tajimi filtered stationary random process and resorting to the stochastic linearization technique to handle the nonlinear terms. Different tuning criteria based on displacement, acceleration, and energy‐based performance indices are defined, and their implications in a design process are discussed. It is proven that the improved robustness of the TMDI reduces its performance sensitivity to the tuning frequency and to the earthquake frequency content, which are well‐known shortcomings of TMD‐like systems. This important feature makes the TMDI particularly suitable for nonlinear base‐isolated structures that are affected by unavoidable uncertainties in the isolators' properties and that may experience changes of isolators effective stiffness depending on the excitation level.     

Seismic response of sliding equipment and contents in base‐isolated buildings subjected to broadband ground motions

Base isolation has been established as the seismic design approach of choice when it comes to protecting nonstructural contents. However, while this protection technology has been widely shown to reduce seismic demands on attached oscillatory equipment and contents (EC), its effectiveness in controlling the response of freestanding EC that are prone to sliding has not been investigated. This study examines the seismic behavior of sliding EC inside base‐isolated buildings subjected to broadband ground motions. The effect of isolation system properties on the response of sliding EC with various friction coefficients is examined. Two widely used isolation models are considered: viscously damped linear elastic and bilinear. The study finds isolation to be generally effective in reducing seismic demands on sliding EC, but it also exposes certain situations where isolation in fact increases demands on EC, most notably for low friction coefficients and high earthquake intensities. Damping at the isolation level is effective in controlling the EC sliding displacements, although damping over about 20% is found to be superfluous. The study identifies a physically motivated dimensionless intensity measure and engineering demand parameter for sliding equipment in base‐isolated buildings subjected to broadband ground motions. Finally, the paper presents easy‐to‐use design fragility curves and an example that illustrates how to use them. Copyright © 2014 John Wiley & Sons, Ltd.     

Seismic demands on secondary systems in base‐isolated nuclear power plants

Numerical models of a sample nuclear power plant (NPP) reactor building, both conventionally constructed and equipped with seismic protective systems, are analysed for both safe shutdown and beyond‐design‐basis earthquake shaking at two coastal sites in the United States. Seismic demands on secondary systems are established for the conventional and seismically isolated NPPs. The reductions in secondary‐system acceleration and deformation demands afforded by the isolation systems are identified. Performance spaces are introduced as an alternate method for evaluating demands on secondary systems. The results show that isolation systems greatly reduce both the median and dispersion of seismic demands on secondary systems in NPPs. Copyright © 2007 John Wiley & Sons, Ltd.     

Prediction of the maximum credible ground motion in Singapore due to a great Sumatran subduction earthquake: the worst‐case scenario

Although Singapore is located in a low‐seismicity region, huge but infrequent Sumatran subduction earthquakes might pose structural problems to medium‐ and high‐rise buildings in the city. Based on a series of ground motion simulations of potential earthquakes that may affect Singapore, the 1833 Sumatran subduction earthquake (M_w=9.0) has been identified to be the worst‐case scenario earthquake. Bedrock motions in Singapore due to the hypothesized earthquake are simulated using an extended reflectivity method, taking into account uncertainties in source rupture process. Random rupture models, considering the uncertainties in rupture directivity, slip distribution, presence of asperities, rupture velocity and dislocation rise time, are made based on a range of seismologically possible models. The simulated bedrock motions have a very long duration of about 250 s with a predominant period between 1.8 and 2.5 s, which coincides with the natural periods of medium‐ and high‐rise buildings widely found in Singapore. The 90‐percentile horizontal peak ground acceleration is estimated to be 33 gal and the 90‐percentile horizontal spectral acceleration with 5% damping ratio is 100 gal within the predominant period range. The 90‐percentile bedrock motion would generate base shear force higher than that required by the current design code, where seismic design has yet to be considered. This has not taken into account effects of local soil response that might further amplify the bedrock motion. Copyright © 2002 John Wiley & Sons, Ltd.     

Accidental torsion due to overturning in nominally symmetric structures isolated with the FPS

Overturning of a structure causes variations in the normal loads of the isolators supporting that structure. For frictional isolators, such variation leads to changes in the frictional forces developed and, hence, in the strength distribution in plan. For frictional pendulum system (FPS) isolators, it also causes changes in the pendular action, i.e. in the stiffness distribution of the isolation interface. Therefore, although the structure is nominally symmetric it develops lateral–torsional coupling when it is subjected to two horizontal components of ground motion. This coupling is denoted herein as accidental torsion due to overturning, and its effect in the earthquake response of nominally symmetric structures is evaluated. Several parameters are identified to control this coupling, but the most important are the slenderness of the structure and the aspect ratio of the building plan. Results are presented in terms of the torsional amplification of the deformations of the isolation base and the interstorey deformations of the superstructure. The FPS system is modelled accurately by including true large deformations and the potential uplift and impact of the isolators. Impulsive as well as subduction‐type ground motions are considered in the analysis, but results show small differences between them. An upper bound for the mean‐plus‐one standard deviation values of the torsional amplifications for the base due to this accidental torsion is 5%. This implies that for design purposes of the isolation system such increase in deformations could probably be neglected. However, the same amplification for the interstorey deformations may be as large as 50%, depending on the torsional stiffness and slenderness of the superstructure, and should be considered in design. In general, such amplification of deformations decreases for torsionally stiffer structures and smaller height‐to‐base aspect ratios. Copyright © 2003 John Wiley & Sons, Ltd.