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Comparison of meta-heuristic approaches for the optimization of non-linear base-isolation systems considering the influence of superstructure flexibility
Abstract
The main aim of this study is to investigate the meta-heuristic approaches called Harmony Search (HS), Grey Wolf Optimizer (GWO), Teaching-Learning Based Optimization (TLBO), and Jaya Algorithm (JA) in the optimization process of non-linear base isolation systems under near-fault earthquakes considering the effect of the superstructure flexibility. The optimization processes were performed by achieving the objective function set as minimizing the peak top floor acceleration to peak ground acceleration ratio with and without base displacement limits. The context of the optimization process, the analytical model of the superstructure, and the number of isolators are considered as the design constants. The mechanical parameters of the non-linear isolation systems, such as the isolation system period, the total characteristic strength ratio, and the yield displacement, are determined as the independent design variables. According to the results obtained in this study, GWO, JA, and TLBO algorithms can be more suitable for solving such design problems among the considered algorithms. Although the objective function values are amplified with the increase in the superstructure flexibility, these values remain around 1.0, even for the most stringent base displacement limit. It can be expressed as a successful seismic performance, especially considering such strong near-fault ground motions, which are the most challenging types for seismically isolated buildings.
... For example, if the IIS is installed on a story that first reaches plastic deformations (e.g., Faiella et al. 2022), it would be too rigid after the yielding of this story, causing a non-optimal or even worse seismic response than in the case without IIS. In these situations, it would be necessary to carry out more detailed analyzes, adopting optimization methods in the time domain (e.g., Liu et al. 2018; Öncü-Davas et al. 2022;Donà et al. 2022) that can allow taking into account both the specific nonlinear behavior of the structure and the specific seismic scenario. ...
Inter-story isolation system (IIS) is increasingly being sought to add floors on top of existing buildings while controlling their base shear forces. One of the main issues with this application is the need to control the drift between the two structural parts and the acceleration of the superstructure, while optimizing the substructure performance. Therefore, this study investigates the use of the IIS as a structural Tuned Mass Damper (TMD), proposing a specific multi-objective optimization approach and providing new design equations for the IIS parameters. Unlike conventional TMD approaches, which focus on the performance of the primary structure (i.e., substructure), the proposed approach aims to consider the overall structural response. The optimal solutions obtained are then compared with the consistent ones currently available in the literature. Finally, time-history analyses of three simple case study structures are performed to validate these optimized solutions. The results demonstrate the potential of the IIS in improving the seismic performance of the substructure, as well as the possibility of limiting the isolation drifts and superstructure accelerations according to specific needs.
... Various isolated bearings have various damping effects on the structure, which can effectively reduce the earthquake response (Behzad Talaeitaba et al., 2021;Van-Thuyet, 2021;Ali et al., 2022;Wu et al., 2022). Recently, Öncü- Davas (2022) has investigated the optimization of a nonlinear foundation isolated system under near-fault earthquakes. Rostami (2021) investigated the lateral load distribution of static analysis of the base isolated building frames subject to far-fault and near-fault ground motions through three scaling methods. ...
The finite element model of a new staggered story isolated structure is established. Using the time-history analysis method, the dynamic response state of the structure at each time step is calculated by integrating the acceleration time-history data step-by-step. Three different types of seismic waves (ordinary seismic wave, near-fault impulse seismic wave, far-field quasi harmonic and long-period seismic wave) are input respectively for dynamic time history analysis. The result indicates that the new staggered story isolated structure has a good shock absorption effect under the action of three different types of seismic waves. There are certain differences in the shock absorption effect under the three kinds of ground motions. The seismic response under ordinary ground motions is minimal, but the seismic response of the structure increases in response to far-field quasi harmonic and long-period ground motions and the near-field fault pulse ground motions. Meanwhile, the inter-story shear force, inter story acceleration, inter-story displacement, damage, and the energy input are all increasing, However, compared with the aseismic structure, the inter-story shear force is reduced by 48%, the inter-story acceleration is reduced by 23%, the inter-story displacement is reduced by 48%, and the energy dissipation rate of the isolated layer is 65%. In addition, the isolated bearing is in good condition during occasional earthquakes under normal ground motion. However, the bearing exceeds the permissible range during near-fault impulse ground motion and far-field harmonic and long-period earthquakes. Therefore, special consideration should be given to the area where the far-field harmonic and long-period ground motion are involved.
The current study deals with the optimization of seismic isolation system parameters applied in base-isolated buildings. In this regard, to consider the more realistic condition, both near-fault and far-fault earthquake ground motions are taken into account by simultaneously imposing six different acceleration records during the direct time integration analysis of the structure. The minimization of the system's peak acceleration ratio is defined as the objective function of the optimization model. To get feasible results, the determining parameters of the isolator are restricted. Such that, while the isolation system period and damping ratio are kept within a predefined range, the maximum displacement of the system is monitored and limited during the optimization process. To solve the proposed optimization model, an enhanced version of the Deferential Evolution method so-called Fuzzy Differential Evolution incorporated Virtual Mutant (FDEVM) is employed. The FDEVM method applies a fuzzy decision-making mechanism to adopt its search behavior based on the governing conditions of the current problem. It also uses a new mutant vector called virtual mutant to contribute information of all population based on each agent's quality. So, the FDEVM works as a self-adaptive and parameter-free search algorithm. The acquired results show that the FDEVM works properly to find the optimal values for the dynamic parameters of the seismic base isolation system.
In this research, the seismic performance of a steel column damper is evaluated using cyclic loading tests of two one-story one-bay reinforced concrete (RC) frames before and after retrofit. The theoretical formulation and design procedure of the damper are explained first and then the details of the tests are described. The seismic performances of the test frames are evaluated in terms of hysteretic behavior, energy dissipation, crack pattern, failure mechanism, and damper behavior. The analytical model of the damper is established and verified using the experimental data. In order to further investigate the applicability of the developed damper for seismic retrofit, a case-study structure is chosen and retrofitted using the proposed damper. The seismic performance of the structure is evaluated and compared before and after retrofit in detail using pushover, nonlinear time-history, and fragility analyses. The results show that the presented damper can efficiently reduce inter-story drifts and damage of the structure. The details of modeling techniques and simulations given in this study can provide guidelines and insight into nonlinear analysis and retrofit of RC structures.
Inter-storey seismic isolation is increasingly gaining attention. One of the main related issues is the need to limit the relative displacement between substructure and superstructure, while maintaining a good seismic performance of the superstructure. As shown in some studies, fluid viscous dampers (FVDs) mounted in isolation systems are effective in reducing isolator deflection but can be harmful by amplifying inter-storey drifts and floor accelerations. Additionally, the effectiveness of FVDs for inter-storey applications was investigated only recently, and specific approaches for their optimisation and performance evaluation are missing. Therefore, this paper proposes a method for the optimal multi-objective design of FVDs, based on the definition of appropriate surrogate response models, which allows for rationally comparing the FVD effects for a wide range of dampers and structures. In particular, the optimal FVD parameters are provided in a dimensionless form, so that they can be predicted by design equations of general validity within the range of the structures analysed. This method is applied to a stock of regular structures with various vibration periods of superstructure, isolation and substructure, examining a linear and a non-linear isolation system and a set of natural records, in order to comprehensively assess the effects of FVDs and their non-linearity on the seismic performance of these structures. Finally, prediction models of optimal FVD parameters are provided based on the results obtained and are applied to three case studies as an example.
In this study a new hysteretic damper for seismic retrofit of soft-first story structures is proposed and its seismic retrofit effect is evaluated. The damper consists of one steel column member and two flexural fuses at both ends made of steel plates with reduced section, which can be placed right beside existing columns in order to minimize interference with passengers and automobiles in the installed bays. The relative displacement between the stories forms flexural plastic hinges at the fuses and dissipate seismic energy. The theoretical formulation and the design procedure based on plastic analysis is provided for the proposed damper, and the results are compared with a detailed finite-element (FE) model. In order to apply the damper in structural analysis, a macromodel of the damper is also developed and calibrated by the derived theoretical formulas. The results are compared with the detailed FE analysis, and the efficiency of the damper is further validated by the seismic retrofit of a case study structure and assessing its seismic performance before and after the retrofit. The results show that the proposed hysteretic damper can be used effectively in reducing damage to soft-first story structures.
Seismic isolation systems exposed to far-fault earthquakes can reduce floor accelerations and story drift ratios to acceptable levels. However, they exhibit different structural performances in each earthquake due to different excitation frequency contents. By optimizing the isolation system parameters, their performance may be maintained at the best level under different far-fault earthquakes. In this study, the optimization of the parameters of the nonlinear isolation system of a 5-story benchmark building is performed by Teaching-Learning Based Optimization (TLBO) algorithm to minimize peak floor accelerations under historical far-fault earthquakes with and without exceeding a specified base displacement limit. According to the results of the analyses, it can be said that TLBO algorithm is a robust algorithm with low standard deviations for determining optimum nonlinear isolation system parameters.
In many Finite Element platforms, a classical global damping matrix based on the elastic stiffness of the system (including isolators), is usually developed as part of the solution to the equations of motion of base-isolated buildings. Analytical and numerical investigations illustrate that this approach can lead to the introduction of unintended damping to the isolated mode and spurious suppression of the first-mode responses. A nonclassical damping model (i.e., an assembly of the superstructure and isolation system damping sub-matrices) might also fail to accurately specify the intended damping values to the first and/or higher vibration modes. For example, the use of Rayleigh damping approach to develop the superstructure damping sub-matrix can lead to the undesired addition of damping to the isolated mode arising from the mass-proportional component of the superstructure damping. On the other hand, the improper use of nonclassical stiffness-proportional damping (e.g., determining the proportional damping coefficient, β_k, based on the first mode) can result in specifying significant damping to the higher-modes and the unintended mitigation of the higher-mode responses. Results show that a nonclassical stiffness-proportional model in which β_k is determined based on the second modal period of the base-isolated building can reasonably specify the intended damping to the higher modes without imparting undesirable damping to the first mode. The nonclassical stiffness-proportional damping can be introduced to the numerical model through explicit viscous damper elements attached between adjacent floors. In structural analysis software such as SAP2000®, the desired nonclassical damping can be also modeled through specifying damping solely to the superstructure material.
Seismic response of five-story frame structure supported by lead-rubber bearings isolation system is investigated subjected to near-fault ground motions. The main structure is modeled as a simple linear multi-degrees-of-freedom vibration system with lumped masses, excited by near-fault ground motions in the horizontal direction. The variation curves of peak top floor acceleration and peak bearing displacement of isolated building are plotted under different yield shear coefficient. The objective function selected for optimality is to maximize the seismic energy dissipated by the lead-rubber bearings. The main constraint conditions selected for optimality are the minimization of both peak bearing displacement and peak top floor acceleration. Optimum parameters of lead-rubber bearing isolation system are investigated and found that optimum yield shear coefficient of lead-rubber bearings is found to be in the range of 0.10–0.14 under near-fault ground motions. Optimum yield shear coefficient decreases with the increase of second isolation period. Optimum yield shear coefficient of lead-rubber bearings with higher yield displacement is larger than that of lead-rubber bearings with low yield displacement. Optimum ratio of pre-yield stiffness to post-yield stiffness of lead-rubber bearings is found to be in the range of 16–35. Optimum stiffness ratio increases proportionally with the decrease of yield displacement. Optimum stiffness ratio increases slightly with the increase of yield shear coefficient. Excluding the effect of pre-yield stiffness, the optimum second isolation period is recommended to be in the range of 4–6 s.
This paper investigates the possibility of using active control devices for the seismic protection of museum artifacts subjected to sliding and rocking. An actively base isolated system is compared to a passively base isolated one constituted by a spring-dashpot system, that can represent a simplified modeling of both a base isolator or a flexible case-supporting system.
Results show that active base isolation has superior performance than the passive one for the seismic protection of museum's artifacts. Moreover, due to their adaptability and robustness, active control devices can be adopted for different works of art in different site exposure conditions.
Structures can be designed to withstand severe earthquake forces
by providing ductility and energy dissipation capacity to the
structural elements, thus allowing damage in the structural elements
and invariably in the nonstructural elements. Another approach which
is being rapidly adopted all-around the world is the concept of
base isolation, wherein the flexibility and the energy dissipation
capacity are provided by a specially designed isolation system that
is placed between the superstructure and the foundation. These
isolation systems can be designed to essentially limit the nonlinear
behavior to the isolation level, imposing little or no ductility
demand on the superstructure.
The study of three dimensional behavior of base isolated
structures requires a comprehensive analytical model. The analytical
model should be capable of addressing highly nonlinear behavior of
isolation systems such as sliding systems and elastomeric bearing
systems (with biaxial effects). The existing analytical models and
solution algorithms cannot accurately analyze sliding systems or
combined elastomeric-sliding systems.
This report deals with the development of a comprehensive
analytical model and solution algorithm for nonlinear dynamic
analysis of three dimensional base isolated structures and the
development of computer program 3P-BASIS.
A new analytical model and solution algorithm involving the
pseudo-force method is developed. New biaxial and uniaxial models
of isolation elements are developed. The novelty of the analytical
v
model and solution algorithm is its capability to capture the highly
nonlinear frictional behavior of sliding isolation systems in plane
motion.
Nonlinear behavior is restricted to the base and the superstructure
is considered to be elastic at all times. The nonlinear
isolation system may consist of elastomeric andjor sliding bearings,
linear springs and viscous elements. The solution algorithm consists
of the pseudo-force method with iteration. Comparison of the computed
results with experimental results is presented for verification.
A six story reinforced concrete base isolated structure is analyzed
to demonstrate the efficiency of the algorithm.
A simple yet powerful optimization algorithm is proposed in this paper for solving the constrained and unconstrained optimization problems. This algorithm is based on the concept that the solution obtained for a given problem should move towards the best solution and should avoid the worst solution. This algorithm requires only the common control parameters and does not require any algorithm-specific control parameters. The performance of the proposed algorithm is investigated by implementing it on 24 constrained benchmark functions having different characteristics given in Congress on Evolutionary Computation (CEC 2006) and the performance is compared with that of other well-known optimization algorithms. The results have proved the better effectiveness of the proposed algorithm. Furthermore, the statistical analysis of the experimental work has been carried out by conducting the Friedman’s rank test and Holm-Sidak test. The proposed algorithm is found to secure first rank for the ‘best’ and ‘mean’ solutions in the Friedman’s rank test for all the 24 constrained benchmark problems. In addition, for solving the constrained benchmark problems, the algorithm is also investigated on 30 unconstrained benchmark problems taken from the literature and the performance of the algorithm is found better.
This work proposes a new meta-heuristic called Grey Wolf Optimizer (GWO) inspired by grey wolves (Canis lupus). The GWO algorithm mimics the leadership hierarchy and hunting mechanism of grey wolves in nature. Four types of grey wolves such as alpha, beta, delta, and omega are employed for simulating the leadership hierarchy. In addition, the three main steps of hunting, searching for prey, encircling prey, and attacking prey, are implemented. The algorithm is then benchmarked on 29 well-known test functions, and the results are verified by a comparative study with Particle Swarm Optimization (PSO), Gravitational Search Algorithm (GSA), Differential Evolution (DE), Evolutionary Programming (EP), and Evolution Strategy (ES). The results show that the GWO algorithm is able to provide very competitive results compared to these well-known meta-heuristics. The paper also considers solving three classical engineering design problems (tension/compression spring, welded beam, and pressure vessel designs) and presents a real application of the proposed method in the field of optical engineering. The results of the classical engineering design problems and real application prove that the proposed algorithm is applicable to challenging problems with unknown search spaces.
In this paper the efficiency of various dissipative mechanisms to protect structures from pulse-type and near-source ground motions is examined. Physically realizable cycloidal pulses are introduced, and their resemblance to recorded near-source ground motions is illustrated. The study uncovers the coherent component of some near-source acceleration records, and the shaking potential of these records is examined. It is found that the response of structures with relatively low isolation periods is substantially affected by the high-frequency fluctuations that override the long duration pulse. Therefore, the concept of seismic isolation is beneficial even for motions that contain a long duration pulse which generates most of the unusually large recorded displacements and velocities. Dissipation forces of the plastic (friction) type are very efficient in reducing displacement demands although occasionally they are responsible for substantial permanent displacements. It is found that the benefits by hysteretic dissipation are nearly indifferent to the level of the yield displacement of the hysteretic mechanism and that they depend primarily on the level of the plastic (friction) force. The study concludes that a combination of relatively low friction and viscous forces is attractive since base displacements are substantially reduced without appreciably increasing base shears and superstructure accelerations. Copyright © 2000 John Wiley & Sons, Ltd.
Seismic isolation, with its capability of reducing floor accelerations and interstory drifts simultaneously, is recognized
as an earthquake resistant design method that protects contents of a building along with the building itself. In research
studies, superstructures of seismically isolated buildings are commonly modeled as idealized shear buildings. Shear building
representation corresponds to an idealized structure where the beams are infinitely stiff in flexure and axially inextensible;
columns are axially inextensible; and rigid floors are supported on these columns. Although it is more convenient to model
and analyze a shear building, such an idealization may influence the seismic responses of seismically isolated buildings.
This study presents a comparison of the seismic performances of seismically isolated buildings with superstructures modeled
as shear buildings to those with full three dimensional superstructures. Both linear and nonlinear base isolation systems
with different isolation periods and superstructures with different number of stories are considered.
KeywordsShear building–Seismic isolation–Base isolation–Earthquake engineering
Structural systems often show nonlinear behavior under severe excitations generated by natural hazards. In that condition,
the restoring force becomes highly nonlinear showing significant hysteresis. The hereditary nature of this nonlinear restoring
force indicates that the force cannot be described as a function of the instantaneous displacement and velocity. Accordingly,
many hysteretic restoring force models were developed to include the time dependent nature using a set of differential equations.
This survey contains a review of the past, recent developments and implementations of the Bouc-Wen model which is used extensively
in modeling the hysteresis phenomenon in the dynamically excited nonlinear structures.
The inter-storey seismic isolation technique is becoming increasingly attractive for the seismic risk mitigation of buildings, also as alternative strategy to base isolation. The reasons for applying this technique can be various and of different nature, such as: architectural concerns, feasibility of construction, and performance benefits. An interesting application of this technique concerns its use for adding upper storeys to existing buildings, avoiding the increase of seismic forces on the substructure, or even reducing them. Therefore, this technique could also be effective for seismic retrofit applications, using the mass of the superstructure as a nonconventional TMD for controlling the dynamic response of the substructure.
Among the main issues concerning this application, there is the need to control the relative displacement between the two structural parts and the acceleration of the superstructure, while improving the seismic performance of the substructure.
In this paper, a multi-objective optimization method for the dynamic characteristics of the additional superstructure is presented, which uses a TMD approach and considers the performances of the substructure, isolation system and superstructure. A 3-storey (3 DOF) case study structure was taken as a reference, analysing a wide range of isolated masses, isolation periods and damping ratios. Time-history analyses are finally performed, based on the optimization results, in order to assess the effectiveness of both the optimization method and the isolation technique, also considering the structural non-linearity.
This study addresses the optimum design of seismic isolated structures via metaheuristic search methods. Three recently developed bio-inspired search methods, namely crow search (CSA), whale optimization (WOA) and grey wolf optimizer (GWO), were employed to develop efficient design optimization algorithms for parameter optimization of seismic isolated structures. The developed design optimization algorithms have been applied to optimize a shear frame model with a base isolation system, where the main objective was to obtain isolation system parameters resulting in minimum roof acceleration without exceeding the isolation system displacement limits. The linear isolator system parameters, namely the damping ratio and isolator period, were optimized based on the time history analyses. The optimization procedure was performed for various isolation displacement limits and damping ratio ranges. The results obtained for this isolated structure by the three design optimization algorithms were compared with previously published results in the literature in order to achieve a verification of the developed algorithms. One of the main outcomes of this study is that the high damping ratio does not always guarantee optimum isolation parameters, while an optimum damping ratio is crucial for achieving the minimum roof acceleration.
In this paper, the genetic algorithm (GA) optimization method is proposed for optimizing the parameters of base isolation system. For genetic algorithm optimization, program in C++ has been constructed to find the optimized parameters of the base isolation system. The aim of this study is to find the parameters of base isolation building within a defined range to simultaneously minimize the acceleration at roof and that of the base isolation model, which subjected to historic ground motion and resting on various soil conditions namely; hard, medium, and soft soils. According to IS code, nine cases have been investigated for various isolation system damping ratios and different soil conditions.
Dynamic analysis of time history has been performed for examining the performance of base isolation model using ETABS software. The performance analysis of base isolation model, after using the optimum Parameters proved that the parameters of base isolation system
determined by genetic algorithm are true optima and effective not only in the objective
function of optimization, but also on others dynamic response such as displacement, drift,
and base shear.
Code provisions are intended to guide engineers to design new buildings adequately protected against structural failures; however, in critical facilities (such as power plants, military headquarters, hospitals, etc.) the post-emergency operability is maintained not only preserving structural integrity but also safeguarding the content functionality. As a result, the present work aims to investigate the use/effectiveness of a method to retrofit existing strategical buildings: in detail, the suggested procedure consists in installing cutting-edge dampers to isolate selected floor systems on a set of multi-storey frames, decoupling fragile nonstructural contents from the seismic source.
Initially, taking advantage of peculiar properties of innovative shape-memory alloy materials, we design dampers capable to remain elastic for low-intensity dynamic events, combining 3D finite element numerical simulations to experimental tests performed at the Italian National Research Council (CNR).
Subsequently, visco-elastomeric dampers are proposed in order to achieve comparable dissipative performances.
Finally, we calibrate equivalent monodimensional FE link-like elements to study building global response and to reproduce the dynamics of floor isolation systems (FISs). Results provide evidence on the local and global seismic response attenuation due to FISs, in terms of stress and deformation reduction. Floor isolation effectiveness is highlighted, comparing visco-elastomeric with SMA dampers, both in a deterministic and in a statistic perspective.
The use of fluid viscous dampers (FVDs) together with isolators, frequent in near-fault buildings, is effective in reducing displacements of the isolation layer. Such a hybrid system is also beneficial in the case of inter-storey isolation with the aim of limiting P-Δ effects. However, previous research on base isolation shows that this additional damping may also be detrimental, as inter-storey drifts and floor accelerations may increase. This paper analyses the effectiveness of FVDs for enhanced seismic performance of systems with inter-storey isolation. A seven-floor building, with natural and lead rubber bearings between the second and third levels, was used as a case study, and a multi-objective optimal design was performed to identify the best damper parameters. In particular, time-history analyses with various natural records were carried out and two competing objectives were examined: minimisation of the deflection of the isolation layer and minimisation of the total drift of the superstructure. The results show not only the effectiveness of optimal FVDs but also the fact that their optimal linearity degree depends to a great extent on the non-linear seismic response of the structure, i.e., on the type of earthquake. The simplest design approach, consisting of applying an optimization algorithm for each design accelerogram, did not seem, in this case, to be sufficient to identify the best overall design solution. The design consequences of these findings are discussed.
Structural control can be used for protecting buildings and its vibration-sensitive contents from earthquakes. Seismic isolation is a passive control system that lowers effective earthquake forces by utilizing flexible bearings. However, supplemental damping in the isolation system may become necessary to reduce large isolator displacements under near-fault earthquakes. Semi-active dampers are preferred over passive dampers because of their capacity in minimizing possible amplifications in floor accelerations due to increased damping. Semi-active dampers are also preferred over the active ones because of their higher stability and lower power consumption. On the other hand, seismic performance of semi-active isolation may vary due to variations in the mechanical properties of semi-active devices and/or seismic isolators. Such uncertainties alongside the uncertainties associated with ground motion parameters should be taken into consideration to develop a realistic picture of the behavior of seismically isolated buildings equipped with semi-active control devices. The objective of this study is to examine the effectiveness of semi-active isolation in protecting vibration-sensitive equipment and integrity of a structure by considering the aforementioned uncertainties and present the reliability of semi-active seismic isolation under near-fault earthquakes. For this purpose, this paper introduces a method that uses synthetically generated near-fault earthquakes and Monte-Carlo Simulations. This method is used to determine the reliability of a 3-story and a 9-story benchmark buildings with semi-active isolation systems under near-fault earthquakes of various magnitudes and varying fault distances. The results are presented in the forms of comparative plots of probability of failure and reliability.
The Rolling-Ball Rubber-Layer (RBRL) system was developed to enable seismic isolation of lightweight
structures, such as special equipment or works of art, and is very versatile, a great range of equivalent natural
frequencies and coefficients of damping being achievable through choice of the system parameters.
The necessity to have a simple and effective design procedure has led to a new parametric experimentation
at Tun Abdul Razak Research Centre (TARRC) on the rolling behaviour of the RBRL system and
load–deflection behaviour of the recentering springs. The experimental results, together with theories for
the rolling resistance of a loaded steel ball on a thin rubber layer and the lateral load–deflection behaviour
of cylindrical rubber springs, are used to develop a general design method for the RBRL system, which
allows the system to be tailored to the specific application.
Sinusoidal test results are presented for the small-deflection behaviour of the system, influenced by the
presence of a viscoelastic depression on the rubber tracks beneath each ball, and an amplitude-dependent
time-domain model is proposed, based on these results and on the steady-state behaviour of the system.
The model is validated through comparison with previously performed shaking-table tests. Attention is here
restricted to uniaxial behaviour.
The mission of seismic isolation is two folds: (i) protecting the integrity and (ii) protecting the contents of a structure by reducing floor accelerations to target limits and concurrently keeping base displacements below practical and economical limits. To this end, while seismic isolation has proven to be successful under far-field earthquakes, its success in case of near-field earthquakes is being questioned for over a decade now; the main reason being the threat of excessive base displacements due to the presence of long period large velocity pulses. Lowering isolation period and increasing isolation damping aiming to reduce base displacements unfortunately may result in a reduced seismic performance in terms of floor accelerations particularly under far-field earthquakes. And the success level of such a precaution under near-field earthquakes depends both on the fault-distance and the pulse period to the isolation period ratio. Thus, the aim of this study is to evaluate the performance limits of buildings equipped with seismic isolation systems of different characteristics when subjected to near-field ground motions at different fault distances with different velocity pulse periods. Accordingly, a methodology for assessing the performance limit of a seismic isolation system design that explicitly considers the seismic actions on the contents of the superstructure in the near-field region is introduced. Benchmark buildings with base isolation systems of different isolation periods and characteristic force ratios are subjected to synthetically developed near-field earthquake records at different fault-distances with different velocity pulse periods and their seismic performances are reported.
The seismic isolation, as a passive control method of structures and an innovative approach in resistant seismic design, significantly reduces the response of structural systems induced by strong ground motions. The performance of these systems in Near Fault (NF) ground motions with large pulses is different from that of Far Fault (FF) ground motions. It is indicated that the effects of near field earthquake with large velocity pulses can bring the seismic isolation devices to critical working conditions. To overcome this problem, the application of viscous damper is represented as an approach to improve the performance of these systems in near field earthquakes. In the present paper, through modeling six base isolated structures, the seismic responses were studied under near field and far field earthquakes. Nonlinear time history analyses were performed, applying the finite element software of ABAQUS to study the influence of various values of supplemental damping (5% ∼ 40%) on the responses of base isolated structures. The results demonstrated that, generally, by increasing supplemental damping ratio under near field earthquakes, the base displacement decreases; however, the stories relative displacements and accelerations increase. It is observed that by increasing supplemental damping ratio under far field earthquakes, the base displacement and the stories relative displacements decreases; however, the stories accelerations increase. Based on the analyses, these variations proved to be more prevalent under near field earthquakes and taller structures.
Recent studies show that slender structures with shallow foundations located on soil medium can benefit from rocking isolation effects during strong earthquakes. In such condition, foundation uplifting and soil yielding provide supplemental energy dissipation potential at substructure level. As a result, the structural demands would be significantly reduced. In this study, building structures with various geometrical properties mounted on surface raft foundations are examined. A set of 91 component pairs of near-fault forward-directivity ground motions recorded at soft as well as dense sites are selected. Three dimensional nonlinear soil–structure interaction (SSI) including foundation uplifting and soil yielding is considered. The results show that the protective effects of rocking isolation can play vital role in survival of medium-to-high-rise building structures subject to catastrophic earthquakes which are excessively greater than design limits. Evidently, rocking isolation has enhanced the elastic structural demands up to 50% for low-aspect-ratio as well as 75% for high-aspect-ratio structures. Such beneficial effects keep the superstructure in significantly larger safety margins. In addition, site effects on seismic demands of rocking structures, as well as liquefaction potential in case of buildings located on soft site are investigated.
In this article, the optimization of isolation system parameters via the harmony search (HS) optimization method is proposed for seismically isolated buildings subjected to both near-fault and far-fault earthquakes. To obtain optimum values of isolation system parameters, an optimization program was developed in Matlab/Simulink employing the HS algorithm. The objective was to obtain a set of isolation system parameters within a defined range that minimizes the acceleration response of a seismically isolated structure subjected to various earthquakes without exceeding a peak isolation system displacement limit. Several cases were investigated for different isolation system damping ratios and peak displacement limitations of seismic isolation devices. Time history analyses were repeated for the neighbouring parameters of optimum values and the results proved that the parameters determined via HS were true optima. The performance of the optimum isolation system was tested under a second set of earthquakes that was different from the first set used in the optimization process. The proposed optimization approach is applicable to linear isolation systems. Isolation systems composed of isolation elements that are inherently nonlinear are the subject of a future study. Investigation of the optimum isolation system parameters has been considered in parametric studies. However, obtaining the best performance of a seismic isolation system requires a true optimization by taking the possibility of both near-fault and far-fault earthquakes into account. HS optimization is proposed here as a viable solution to this problem.
The response of multi‐storey structures can be controlled under earthquake actions by installing seismic isolators at various storey levels. By vertically distributing isolation devices at various elevations, the designer is provided with numerous options to appropriately adjust the seismic performance of a building. However, introducing seismic isolators at various storey levels is not a straightforward task, as it may lead to favourable or unfavourable structural behaviour depending on a large number of factors. As a consequence, a rather chaotic decision space of seismic isolation configurations arises, within which a favourable solution needs to be located. The search for favourable isolators' configurations is formulated in this work as a single‐objective optimization task. The aim of the optimization process is to minimize the maximum floor acceleration of the building under consideration, while constraints are specified to control the maximum interstorey drift, the maximum base displacement and the total seismic isolation cost. A genetic algorithm is implemented to perform this optimization task, which selectively introduces seismic isolators at various elevations, in order to identify the optimal configuration for the isolators satisfying the pre‐specified constraints. This way, optimized earthquake response of multi‐storey buildings can be obtained. The effectiveness of the proposed optimization procedure in the design of a seismically isolated structure is demonstrated in a numerical study using time‐history analyses of a typical six‐storey building. Copyright © 2012 John Wiley & Sons, Ltd.
This paper presents the benchmark problem definition for seismically excited base-isolated build- ings. The objective of this benchmark study is to provide a well defined base isolated building with a broad set of carefully chosen parameter sets, performance measures and guidelines to the participants, so that they can evaluate their control algorithms. The control algorithms may be passive, active or semi-active. The benchmark structure considered is an eight story base isolated building similar to ex- isting buildings in Los Angeles, California. The base isolation system includes both linear and nonlinear bearings and control devices. The superstructure is considered to be a linear elastic system with lateral- torsional behavior. A new nonlinear dynamic analysis program has been developed and made available to facilitate direct comparison of results of different control algorithms.
This paper evaluates the seismic structural and non-structural performance of self-centering and conventional structural systems combined with supplemental viscous dampers. For this purpose, a parametric study on the seismic response of highly damped single-degree-of-freedom systems with self-centering flag-shaped or bilinear elastoplastic hysteresis is conducted. Statistical response results are used to evaluate and quantify the effects of supplemental viscous damping, strength ratio and period of vibration on seismic peak displacements, residual displacements and peak total accelerations. Among other findings, it is shown that decreasing the strength of nonlinear systems effectively decreases total accelerations, while added damping increases total accelerations and generally decreases residual displacements. Interestingly, this work shows that in some instances added damping may result in increased residual displacements of bilinear elastoplastic systems. Simple design cases demonstrate how these findings can be considered when designing highly damped structures to reduce structural and non-structural damage.
Based on a Markov-vector formulation and a Galerkin solution procedure, a new method of modeling and solution of a large class of hysteretic systems (softening or hardening, narrow or wide-band) under random excitation is proposed. The excitation is modeled as a filtered Gaussian shot noise allowing one to take the nonstationarity and spectral content of the excitation into consideration. The solutions include time histories of joint density, moments of all order, and threshold crossing rate; for the stationary case, autocorrelation, spectral density, and first passage time probability are also obtained. Comparison of results of numerical example with Monte-Carlo solutions indicates that the proposed method is a powerful and efficient tool.
Design Example for a High-Damping Rubber Bearing Design Example for a Lead-Plug Bearing
This study encompasses the assessment of the equivalent linear (EL) analysis procedure for the design of seismic isolated bridges (SIB) subjected to near-fault ground motions with forward rupture directivity effect. The assessment procedure involves the comparison of the seismic response quantities obtained from EL analyses with those obtained from nonlinear time history (NLTH) analyses. The effect of several isolator and near-fault ground motion properties are considered in the assessment of the EL analysis procedure both individually and in the form of three dimensionless parameters. Regression analyses of the acquired data are then performed to appraise the effect of these parameters on the accuracy of the EL analysis results. It is found that the accuracy of the EL analysis results is affected by the magnitude of the near-fault ground motion, distance of the structure from the fault, post-elastic stiffness and yield strength of the isolator. Next it is demonstrated that the effective damping (ED) equation currently used in the design of SIBs must incorporate these parameters for a more accurate estimation of seismic response quantities. Finally, a new ED equation that includes such parameters is formulated and found to improve the accuracy of the EL analyses.
The analytical seismic response of multi-story buildings isolated by the friction pendulum system (FPS) is investigated under near-fault motions. The superstructure is idealized as a linear shear type flexible building. The governing equations of motion of the isolated structural system are derived and the response of the system to the normal component of six recorded near-fault motions is evaluated by the step-by-step method. The variation of top floor absolute acceleration and sliding displacement of the isolated building is plotted under different system parameters such as superstructure flexibility, isolation period and friction coefficient of the FPS. The comparison of results indicated that for low values of friction coefficient there is significant sliding displacement in the FPS under near-fault motions. In addition, there also exists a particular value of the friction coefficient of FPS for which the top floor absolute acceleration of the building attains the minimum value. Further, the optimum friction coefficient of the FPS is derived for different system parameters under near-fault motions. The criterion selected for optimality is the minimization of both the top floor acceleration and the sliding displacement. The optimum friction coefficient of the FPS is found to be in the range of 0.05 to 0.15 under near-fault motions. In addition, the response of a bridge seismically isolated by the FPS is also investigated and it is found that there exists a particular value of the friction coefficient for which the pier base shear and deck acceleration attain the minimum value under near-fault motions.
In this paper, suppression of the dynamic response of tall buildings, supported on elastomeric bearings with both linear and nonlinear behaviors, is studied. The isolated building is modeled as a shear-type structure having one lateral degree of freedom at each story level. The elastic supports are modeled as an additional degree of freedom having three unknown parameters: mass, stiffness, and damping ratio. The main objective of the paper is to find the optimal values of the parameters of the base isolation system, using genetic algorithms (GAs), to simultaneously minimize the displacement of the building's top story and that of the base isolation system. In order to simultaneously minimize the objective functions, a fast and elitist non-dominated sorting genetic algorithm (NSGA-II) approach is used to find a set of Pareto-optimal solutions. The optimal values of the parameters of the base isolators, namely: their mass, stiffness, and damping ratio are evaluated using the GAs by taking into account the nonlinearity of the isolator bearings to minimize the objective functions. Moreover, in order to solve the undesirable horizontal displacement of the lead-rubber bearings, a new method called the “independent story” (IS) system is proposed in this investigation. This system works as a big tuned mass damper (TMD) system, without using any additional damping or stiffness devices except those of the structure itself. Either one full story of the building or even a part of one story can be considered the IS system. For a numerical example, a ten-story building located in Mashhad, Iran is chosen. From the numerical study, the NSGA-II approach was found to be strongly effective in evaluating the optimal values of the parameters of the isolator bearings and minimizing the structural responses.
There is no consensus at the present time regarding an appropriate approach to model viscous damping in nonlinear time-history analysis of base-isolated buildings because of uncertainties associated with quantification of energy dissipation. Therefore, in this study, the effects of modeling viscous damping on the response of base-isolated reinforced concrete buildings subjected to earthquake ground motions are investigated. The test results of a reduced-scale three-story building previously tested on a shaking table
are compared with three-dimensional finite element simulation results. The study is primarily focused on nonlinear direct-integration time-history analysis, where many different approaches of modeling viscous damping, developed within the framework of Rayleigh damping are considered. Nonlinear direct-integration time-history analysis results reveal that the damping ratio as well as the approach used to model damping has significant effects on the response, and quite importantly, a damping ratio of 1% is more appropriate in simulating the response than a damping ratio of 5%. It is shown that stiffness-proportional damping, where the coefficient multiplying the stiffness matrix is calculated from the frequency of the base-isolated building with the post-elastic stiffness of the isolation system, provides reasonable estimates of the peak response indicators, in addition to being able to capture the frequency content of the response very well. Furthermore, nonlinear modal time-history analyses using constant as well as frequency-dependent modal damping are also performed for comparison purposes. It was found that for nonlinear modal time-history analysis, frequency-dependent damping, where zero damping is assigned to the frequencies below the fundamental frequency of the superstructure for a fixed-base condition and 5% damping is assigned to all other frequencies, is more appropriate, than 5% constant damping.
The nonlinear seismic response of base-isolated framed buildings subjected to near-fault earthquakes is studied to analyze the effects of supplemental damping at the level of the isolation system, commonly adopted to avoid overly large isolators. A numerical investigation is carried out with reference to two- and multi-degree-of-freedom systems, representing medium-rise base-isolated framed buildings. Typical five-story reinforced concrete (RC) plane frames with full isolation are designed according to Eurocode 8 assuming ground types A (i.e., rock) and D (i.e., moderately soft soil) in a high-risk seismic region. The overall isolation system, made of in-parallel high-damping-laminated-rubber bearings (HDLRBs) and supplemental viscous dampers, is modeled by an equivalent viscoelastic linear model. A bilinear model idealizes the behavior of the frame members. Pulse-type artificial motions, artificially generated accelerograms (matching EC8 response spectrum for subsoil classes A or D) and real accelerograms (recorded on rock- and soil-site at near-fault zones) are considered. A supplemental viscous damping at the base is appropriate for controlling the isolator displacement, so avoiding overly large isolators; but it does not guarantee a better performance of the superstructure in all cases, in terms of structural and non structural damage, depending on the frequency content of the seismic input. Precautions should be taken with regard to near-fault earthquakes, particularly for base-isolated structures located on soil-site.
A versatile, simulation-based framework for risk assessment and probabilistic sensitivity analysis of base-isolated structures is discussed in this work. A probabilistic foundation is used to address the various sources of uncertainties, either excitation or structural, and to characterize seismic risk. This risk is given, in this stochastic setting, by some statistics of the system response over the adopted probability models and stochastic simulation is implemented for its evaluation. An efficient, sampling-based approach is also introduced for establishing a probabilistic sensitivity analysis to identify the importance of each of the uncertain model parameters in affecting the overall risk. This framework facilitates use of complex models for the structural system and the excitation. The adopted structural model explicitly addresses nonlinear characteristics of the isolators and of any supplemental dampers, and the effect of seismic pounding of the base to the surrounding retaining walls. An efficient stochastic ground motion model is also discussed for characterizing future near-fault ground motions and relating them to the seismic hazard for the structural site. An illustrative example is presented that emphasizes the results from the novel probabilistic sensitivity analysis and their dependence on seismic pounding occurrences and on addition of supplemental dampers. Copyright © 2011 John Wiley & Sons, Ltd.
Stochastic response of buildings isolated by lead–rubber bearings (LRB) is investigated. The earthquake excitation is modelled by non-stationary random process (i.e. uniformly modulated broadband excitation). The stochastic response of isolated building frames is obtained using the time-dependent equivalent linearization technique as the force–deformation behaviour of the LRB is highly non-linear. The non-stationary response of isolated structure is compared with the corresponding stationary response in order to study the influence of non-stationary characteristics of earthquake motion. For a given isolated building system and excitation, it is observed that there exists an optimum value of the yield strength of LRB for which the root mean square absolute acceleration of superstructure attains the minimum value. The optimum yield strength of LRB is obtained under important parametric variations such as isolation period and damping ratio of the LRB and the frequency content and intensity of earthquake excitation. It is shown that the above parameters have significant effects on the optimum yield strength of LRB. Finally, closed-form expressions for the optimum yield strength of LRB and corresponding response of the isolated structure are proposed. These expressions were derived based on the model of isolated structure with rigid superstructure condition subjected to stationary white-noise excitation. It was observed that there is a good comparison between the proposed closed-form expressions and actual optimum parameters and response of the isolated building system. These expressions can be used for initial optimal design of LRB for building system. Copyright © 2008 John Wiley & Sons, Ltd.
In the current code requirements for the design of base isolation systems for buildings located at near-fault sites, the design engineer is faced with very large design displacements for the isolators. To reduce these displacements, supplementary dampers are often prescribed. These dampers reduce displacements, but at the expense of significant increases in interstorey drifts and floor accelerations in the superstructure. An elementary analysis based on a simple model of an isolated structure is used to demonstrate this dilemma. The model is linear and is based on modal analysis, but includes the modal coupling terms caused by high levels of damping in the isolation system. The equations are solved by a method that avoids complex modal analysis. Estimates of the important response quantities are obtained by the response spectrum method. It is shown that as the damping in the isolation system increases, the contribution of the modal coupling terms due to isolator damping in response to the superstructure becomes the dominant term. The isolator displacement and structural base shear may be reduced, but the floor accelerations and interstorey drift are increased. The results show that the use of supplemental dampers in seismic isolation is a misplaced effort and alternative strategies to solve the problem are suggested. Copyright © 1999 John Wiley & Sons, Ltd.
This paper presents the effect of isolator and substructure properties as well as the frequency characteristics and intensity of the ground motion on the performance of seismic-isolated bridges (SIBs) and examines some critical design clauses in the AASHTO Guide Specification for Seismic Isolation Design. For this purpose, a parametric study, involving more than 800 non-linear time history analyses of simplified structural models representative of typical SIBs, is conducted. The results from the parametric study are then used to derive important design recommendations and conclusions that may be used by bridge engineers to arrive to a more sound and economical design of SIBs. It is found that the SIB response is a function of the peak ground acceleration to peak ground velocity ratio of the ground motion. Thus, the choice of the seismic ground motion according to the characteristics of the bridge site is crucial for a correct design of the SIB. It is also found that the characteristic strength of the isolator may be chosen based on the intensity and frequency characteristics of the ground motion. Furthermore, the isolator post-elastic stiffness is found to have a notable effect on the response of SIBs. Copyright © 2005 John Wiley & Sons, Ltd.
A random vibration method is developed for the response analysis of hysteretic structural systems under stochastic two-dimensional earthquake excitations. The biaxial hysteretic restoring force is modelled by coupled non-linear differential equations; the response statistics are obtained using the equivalent linearization technique. The validity of the proposed model is appraised using available biaxial loading tests of reinforced concrete columns. A parametric study was performed to examine the significance of the effects of biaxial interaction under earthquake excitations. A practical method to evaluate the extreme response statistics is also presented.
This paper presents the benchmark problem definition for seismically excited base-isolated buildings. The objective of this benchmark study is to provide a well-defined base-isolated building with a broad set of carefully chosen parameter sets, performance measures and guidelines to the participants, so that they can evaluate their control algorithms. The control algorithms may be passive, active or semi-active. The benchmark structure considered is an eight-storey base-isolated building similar to existing buildings in Los Angeles, California. The base isolation system includes both linear and nonlinear bearings and control devices. The superstructure is considered to be a linear elastic system with lateral–torsional behavior. A new nonlinear dynamic analysis program has been developed and made available to facilitate direct comparison of results of different control algorithms. Copyright © 2005 John Wiley & Sons, Ltd.
For the public welfare and safety, buildings such as hospitals, industrial facilities, and technology centers need to remain
functional at all times; even during and after major earthquakes. The values of these buildings themselves may be insignificant
when compared to the cost of loss of operations and business continuity. Seismic isolation aims to protect both the integrity
and the contents of a structure. Since the tolerable acceleration levels are relatively low for continued services of vibration-sensitive
high-tech contents, a better understanding of acceleration response behaviors of seismically isolated buildings is necessary.
In an effort to shed light to this issue, following are investigated via bi-directional time history analyses of seismically
isolated benchmark buildings subject to historical earthquakes: (i) the distribution of peak floor accelerations of seismically
isolated buildings subject to seismic excitations in order to find out which floors are likely to sustain the largest accelerations;
(ii) the influence of equivalent linear modeling of isolation systems on the floor accelerations in order to find out the
range of possible errors introduced by this type of modeling; (iii) the role of superstructure damping in reducing floor accelerations
of seismically isolated buildings with flexible superstructures in order to find out whether increasing the superstructure
damping helps reducing floor accelerations notably. Influences of isolation system characteristics and superstructure flexibility
are both taken into account.
KeywordsSeismic base isolation–Linear and nonlinear isolation–Floor accelerations–Superstructure damping–Isolation system characteristics
Base isolation is a quite sensible structural control strategic design in reducing the response of a structural system induced by strong ground motions. It is clear that the effects of near-fault (NF) ground motions with large velocity pulses can bring the seismic isolation devices to critical working conditions. In the present paper, nonlinear time history analyses were performed using a commercial structural analysis software package to study the influence of isolation damping on base and superstructure drift. Various lead-rubber bearing (LRB) isolation systems are systematically compared and discussed for aseismic performances of two actual reinforced concrete (RC) buildings. Parametric analysis of the buildings fitted with isolation devices is carried out to choose the appropriate design parameters. The efficiency of providing supplemental viscous damping for reducing the isolator displacements while keeping the substructure forces in reasonable ranges is also investigated.
This paper presents the results of a series of numerical simulation studies on seismic responses of secondary systems in base-isolated structures including equipment-structure interactions. A three-storey building is used as the primary structure, while the equipment is modelled as a single-degree-of-freedom linear system. A number of base isolation systems such as the Laminated Rubber Bearing, the Pure Friction, the Resilient-Friction, and the Electricite de France system are considered. Several earthquake records including the N00W component of El Centro 1940, the S16E components of Pacoima Dam 1971, and the N90W component of Mexico City 1985 earthquakes are used as ground accelerations. Acceleration and deflection response spectra of the secondary system under different conditions are evaluated and the effects of equipment — structure interactions are studied. It is shown that the use of base isolation provides considerable protection for structural contents. However, peak responses of the secondary systems vary substantially depending on the base isolation system used. Among the base isolation systems considered, the Laminated Rubber Bearing system appears to be remarkably effective in reducing peak responses of secondary systems under a variety of conditions.
The paper tries to contribute to a better understanding of the behaviour of base isolated asymmetric structures. Numerous variants of originally symmetric four storey RC frame building isolated by a simple lead rubber bearing base isolation system with various distributions of isolators were considered as test examples. The symmetrical structural variant and appropriate LRB bearing properties were designed according to Eurocode 2 and 8. The asymmetric variants were produced by shifting the centre of mass CM toward one side of the building. Additional “torsionally unrestrained” and “torsionally restrained” sub-variants of each building variant were obtained by changing the mass distribution, while total mass sum remained unchanged. For the base isolation system we have considered six different distributions of bearings characterized by the position of the centre of isolators CI in respect to the centre of mass CM of the superstructure. Two symmetric (Uniform and Peripheral distribution) and four asymmetric distributions of isolators (called CI = CM, CI = CM/2, CI = −CM/2 and CI = −CM) were included in the analyses. The paper analyses the positive and negative effects of different bearing distributions to the displacements and rotations of the superstructure as well as to the base isolation system and tries to determine the most favourable distribution of isolators that is able to balance the effects of introduced eccentricities. The results obtained by 3D nonlinear dynamic analyses are presented as an average of maximums for ten selected ground motions and three different scalings. They indicate that all six considered distributions of bearings, however differently, substantially reduce the unfavourable torsional effects, which are with different extent transferred from the superstructure to the base isolation system. It was further observed that CI = CM distribution, favoured by common building codes, is best only for accommodating the torsional effects in the base isolation system. A significantly different conclusion was found observing the nonlinear behaviour of the superstructure, where CI = CM distribution might cause more damage in the flexible side frames.
Response of structures supported on the sliding systems under near-fault earthquake ground motion is investigated. The fault normal and parallel components are applied in two horizontal direction of the system. The superstructure is considered to be rigid and the frictional forces mobilized at the interface of the sliding system are assumed to be velocity dependent. The interaction between frictional forces of sliding system in two horizontal directions is duly considered and coupled differential equations of motion of the system are solved in the incremental form using step-by-step method with iterations. The iterations are required due to dependence of the frictional forces on the response of the system. The response of the system is analyzed to investigate the performance of sliding systems under near-fault motion. In addition, the effects of velocity dependence and bi-directional interaction of frictional forces on the response of isolated system are also investigated. The response of sliding system due to fault normal and parallel components is found to be more or less uncorrelated and the resultant sliding base displacement is mainly contributed by the fault normal component. It is also observed that the dependence of friction coefficient on relative velocity of system does not have noticeable effects on the peak response of the isolated system. However, if the effects of bi-directional interaction of frictional forces are neglected then the sliding base displacements under near-fault motion will be underestimated which can be crucial from design point of view.
The effects of near-field ground motions with large velocity pulses have motivated passive damping requirements for the protection of seismically isolated structures. Structures in which the first mode damping exceeds 20% or 30% typically do not exhibit classical modes, and simulation via a simple superposition of uncoupled second order equations is not possible. When the damping is produced by viscous or linear visco-elastic devices, we can, however, gain insight into the dynamic behavior of these structures using a convenient first-order formulation and frequency domain methods. When the damping effects are created by non-linear mechanisms such as yielding or friction, the behavior of the structure is amplitude dependent and analyses are commonly carried out in the time domain. In this paper, frequency domain analysis and earthquake time history analysis are applied to study the influence of isolation damping on higher-mode effects and inter-story drift ratios. Because higher mode effects, plan irregularities, and bi-directional ground motions are all important attributes of the dynamic behavior of these structural systems, a simple comparison of isolation damping mechanisms can not be carried out via simple single or two degree of freedom realizations. In order to incorporate these important details in the study of the dynamic behavior of these structures, a set of 8-story proto-type building models with L-shaped floor plans, different isolation periods, isolation damping characteristics, and levels of isolation stiffnesses are examined.