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The pushover method is often used instead of the nonlinear time history (NTH) method for analyzing base-isolated structures due to its simplicity. However, pushover analysis cannot account for reversible earthquake effects on structural damage. An alternative fast method is the endurance time (ET) method, which has fewer limitations in considering the dynamic effects of earthquakes on structures. However, the accuracy of this method is questionable due to the inability to distinguish non-converged results of individual records. To clarify this question, this study investigates the application of the modified endurance time (MET) method, which is an enhanced version of the ET method under pounding. For this study, three base isolated special moment frames (SMFs) designed based on the design earthquake hazard level were analyzed under the earthquake hazard level with 2475-year return periods (EHL2475). The results of the MET and pushover methods were compared to each other with respect to the NTH analysis, which was considered the reference method. The results have indicated that the MET had less error than the pushover in predicting structural damage based on the maximum interstory drift ratio (IDR). In contrast to the pushover, the MET can predict nonstructural acceleration-sensitive element damage with errors of 12.3–20.8%.

A comparative study of performance of various isolation systems for liquid storage tanks is investigated under real earthquake ground motions. The various base isolation systems considered are the laminated rubber bearings (with and without lead core) and sliding isolation systems (with and without restoring force). The isolated liquid storage tank is idealized with three-degrees-of-freedom associated with convective, impulsive and rigid mass under uni-directional earthquake excitation. Since the force-deformation behaviour of the isolation systems is non-linear, as a result, the equations of motion are solved numerically by step-by-step method. In order to measure the effectiveness of the isolation systems, the seismic response of the isolated liquid storage tanks is compared with the corresponding response of non-isolated tanks. Further, the effectiveness of the isolation is also explored for wide range of practical liquid storage tanks considering the influence of tank aspect ratio. It is observed that the isolation systems are quite effective in attenuating the earthquake acceleration transmitted to the tank, which reduces the design seismic forces significantly. Further, it is also found that the sliding type isolation systems are more effective in controlling the response of liquid storage tanks in comparison to the elastomeric bearings. Among the various sliding systems, the resilient-friction base isolator is found to be most effective for seismic isolation of the tanks.

In contrast with the odd number based multiple tuned mass dampers (ON-MTMD) used conventionally, which is targeted at the central natural frequency, the arbitrary integer based multiple tuned mass dampers (AI-MTMD) is proposed for the convenient applications of MTMD by giving up the central natural frequency hypothesis. The total number of the TMD units constituting the AI-MTMD may be selected as an arbitrary integer according to the practical requirements. In terms of the dynamic magnification factors (DMF) of the AI-MTMD structure system, the criterion for evaluating the optimum parameters and effectiveness of the AI-MTMD is selected as the minimization of the minimum values of the maximum DMF of the structure with the AI-MTMD. Employing the maximum DMF of every mass block in the AI-MTMD, the stroke of the AI-MTMD is simultaneously evaluated. The results indicate that both the AI-MTMD and the ON-MTMD can practically render the same performance, thus demonstrating that the former can be more convenient in mitigating structural oscillations with respect to the ON-MTMD stuck to the central natural frequency hypothesis.

This paper describes the application of semiactive devices for controlling the earthquake response of two highway bridges of different cross sections and pier heights. Each of the bridges consists of a three-span continuous deck supported on the piers and abutments. Semiactive devices such as the magnetorheological damper, the variable friction damper and the variable stiffness device are considered as the control devices. These devices are inserted between the deck and piers or abutments of the isolated bridge. The semiactive device changes its properties according to the structural response and adds control forces to the system. Each pier supporting the bridge is modeled as a linear lumped mass system. The optimum parametric values of the semiactive dampers are evaluated and considered in analysis of the bridge. A comparative study is performed for different semi-active devices installed on the bridges under different seismic loadings in the longitudinal direction. The behaviors of the bridges with different semiactive isolation devices are compared with the corresponding nonisolated ones. The semiactive dampers are observed as an effective protective device in reducing the displacements of the isolation bridges as well as the base shear of the piers.

The influence of high initial isolator stiffness on the response of a base-isolated benchmark building is investigated. The base-isolated building is modeled as a three-dimensional linear-elastic structure having three degrees-of-freedom at each floor level. The time-history analysis of this building is carried out by solving the governing equations of motion using Newmark-beta method along with an iterative predictor–corrector approach. The force–deformation behavior of the isolation system is modeled by a bilinear law, which can be effectively used to model all isolation systems in practice. Three near-field earthquakes with bidirectional ground motions are considered. Structural response parameters such as absolute top floor acceleration, base shear, and base displacement are chosen for investigating the effects of high initial isolator stiffness. It was observed that the high initial isolator stiffness of the isolation system excites the higher modes in the base-isolated building and increases the top floor acceleration. Such a phenomenon can be detrimental to the sensitive instruments placed in the isolated structure. On the other hand, both the base displacement and base shear reduce marginally due to increase in the initial isolator stiffness. Further, the influences of high initial isolator stiffness are found to dependent on the period and characteristic strengths of the base isolation system.

Structures designed against earthquake loads based on using control systems may experience explosions during their lifetime. In this paper, the performance of a hybrid control system composed of a low-damping base isolation and a supplemental magneto-rheological (MR) damper under external explosion has been studied. Base isolation system has the ability of decreasing the maximum structural response under blast loadings by shifting the period of the structure. In addition, MR damper improves the base isolation system performance by controlling the base drift of the structure. Hence, in this paper, the capability of a hybrid base isolation system equipped with an MR damper at the base has been evaluated in reducing the maximum structural response and base drift under external blast loadings. To determine the voltage of the semi-active MR damper, the H₂/Linear Quadratic Gaussian (LQG) and clipped-optimal control algorithms have been applied. For numerical simulations, a 10-storey shear frame subjected to blast loadings applied on different floors has been considered and the performance of the hybrid isolation system and MR damper has been studied. The results have proven the effectiveness of the hybrid control system in controlling the maximum response and base drift of the isolated structure against spherical external explosion. Furthermore, comparing the performance of the hybrid passive and semi-active base isolation systems indicates that the semi-active hybrid base isolation system is more effective in reducing the root-mean-square (RMS) value of the base drift. Similarly, it has been found that the semi-active hybrid base isolation system also performs better than the high-damping base isolation system.

For seismic isolated buildings, the isolators should be able to control peak deformation and have good restoring ability to return the building to its original position. Shape memory alloys (SMAs) are known for their properties of energy dissipation and deformation recovery, which make them appealing for use in isolation systems. To this end, the feasibility of SMA-based damping devices for use in an isolation system of low-rise frame buildings is studied in this paper. Numerical analyses were conducted and compared for buildings installed with SMA-based bearings (SMABs) and lead-rubber bearings (LRBs). A two-mass isolated frame building installed with these bearings was subjected to earthquakes corresponding to multilevel seismic hazards. The results show that the SMABs offer comparable seismic hazard mitigation efficacy to that of the LRBs while successfully restoring the building to the rest position after the earthquake. Furthermore, adding the energy dissipation capacity and properly using the strain hardening behavior of the SMAs are favorable for controlling isolation deformation while protecting the superstructure. The strain hardening behavior of the SMABs is beneficial for controlling deformation at the isolator level, which can still maintain isolation efficacy.

Shape Memory Alloys (SMAs) are now widely used as a damping element into the isolation systems. The pre-stressed SMAs exhibit hysteretic damping through a nonlinear flag-shaped hysteresis loop. Many nonlinear models of the SMA are available to depict such behavior. The nonlinear models require a lot of effort and computational time for the analysis of base-isolated structures. Therefore, the codes recommend that a nonlinear model can be replaced by an equivalent linear model in the analysis. Linearization is a method to convert the nonlinearity of a system into a system with analogues linear parameters. This paper proposes an empirical equation for a damping ratio to get a linear damping coefficient of the SMAs which can be used in the seismic analysis of base-isolated structures.

The evaluation of any damping ratio using the traditional system identification method does not give precise solutions due to variation in hysteretic parameters and the unpredictable nature of an earthquake. The empirical equation is proposed using a set of optimal statistical data obtained from the seismic analysis of a base isolated structure. Moreover, analysis of the base isolated structure using the newly modified equivalent elastic-viscous SMA model gives comparable and conservative results with a nonlinear SMA model as compared to the existing elastic-viscous SMA model. Since the hysteresis parameters are used to derive the empirical equation for the damping ratio, this equation is also applicable for any type of structure.

The base-isolation technology has been increasingly applied in high-rise buildings in recent years. Previous studies have shown that the unfavorable wind-induced response can be effectively reduced by a certain degree of hysteretic energy dissipation after the yielding of the isolation system under strong wind load. However, a large relative inelastic displacement at the isolation level will occur under this condition, which is likely to exceed the width of seismic gap, and cause a structural impact with the adjacent structures (i.e. retaining wall or other barriers). Such wind-induced impacts may have detrimental consequences on the structural performance and have not been investigated much before. This study prospectively investigated the wind-induced impact behavior of base-isolated high-rise buildings with adjacent structures. The multi-story superstructure is modeled as a linear elastic shear building, while the isolation system is represented by bilinear hysteresis restoring force model. An impact element combined with linear spring and damper is used. The story wind load is determined by its cross power spectral density (PSD). Responses for two kinds of typical impact including crosswind and along-wind impact are examined by time history analysis. The results demonstrate that the impact will increase the fluctuation components of superstructure displacement, shear force and bending moment to some extent compared with that of the base-isolated building without impact. The floor accelerations, especially floors close to the base, are significantly increased during impact. The impact has no effect on the mean component of each response. At last, a parametric analysis is carried out to explore the influences of various impact parameters on the inelastic impact response. The results of this study can help to better understand the impact behavior of base-isolated high-rise buildings under strong wind excitation and be useful for performance-based wind resistance design.

This study investigates the feasibility of utilizing the friction pendulum system based inter-storey isolation (FPS-I) strategy to replace the friction pendulum system base isolation (FPS-B) for high-rise structures’ vibration control against earthquakes. Both experimental verifications and computational analysis are carried out. A scaled nine-storey experimental model structure is constructed in accordance with the third generation Benchmark problem, and three aspects variant FPS with different slideway radius configurations are designed and manufactured based on the geometric similarity criterion. To assess the dynamic characteristics of FPS-B structure and FPS-I structure, four typical ground motions and four different intensities of peak ground acceleration (PGA) are considered. The findings show that FPS-I can effectively suppress the superstructure’s acceleration as well as affecting the lower substructure’s response. When the same earthquakes occur, the vibration reduction effect of FPS-I strategy is achievable between 50 and 60%, which is obviously superior to FPS-B scheme. The FPS-I technology is observed to have an even greater effectiveness on the entire structure’s vibration reduction during strong earthquakes than the traditional FPS-B technology. The basic mode as well as the higher-order mode responses of the high-rise structure can be controlled, resulting in the seismic response of the entire FPS-I structure at lower levels. The first-order mode contributes the most to the superstructure’s floor acceleration response. The location of the isolation layer changes the dynamic characteristics of the structure substantially. Finally, the finite element models for FPS-B structure and FPS-I structure are developed. It is demonstrated through the mutual comparison of experimental and numerical results that the finite element model is sufficient accurate for parametric studies. The numerical model can reproduce the dynamic characteristics of both isolation strategies with high fidelity. This research emerges the benefits of FPS with inter-storey isolation to address the issue of high-rise structures being prone to be over turned in the case of base isolation.

Base isolation proves to be an effective vibration control strategy for buildings. However, the base-isolation floor (BIF) may undergo substantial displacements. To improve the seismic performance of base-isolation system, this paper proposes a multi-performance economical optimization procedure (MPEOP) for exploring the optimal parameters of base isolation system with tuned inerter negative stiffness damper (BIS-TINSD) subjected to non-stationary seismic excitation. The simplified analysis model for BIS-TINSD is developed, followed by the formulation of the state-space representation. The non-stationary seismic excitation is represented as a stationary Gaussian process with a time-modulating function based on the Clough–Penzien spectrum, and combining the equations of motion with seismic excitation results in the augmented state-space representation of structure-damper-excitation, followed by the formulation of differential Lyapunov equation. The multi-objective performance economical index is proposed, in which the performance optimization objective is defined as a weighted combination of base isolation floor displacement and superstructure acceleration, and the economic optimization objective is set as the support stiffness. The Pareto optimal fronts (POF) are adopted to deal with optimal objectives. The examination of optimal design parameters is extended under real earthquake records. The results demonstrate the effectiveness of the MPEOP, and indicate the advantages of the BIS-TINSD compared with the inerter amplify damper (IAD) and negative stiffness inerter damper (NSID).

In this paper, the responses of multidegree of freedom (MDOF) structures on sliding foundations, subjected to harmonic or random base motions, are investigated taking into consideration the variations of friction forces. The variation of friction force is a consequence of variation of friction coefficient, which depends on such parameters as relative velocity and the existing pressure. Modelling of the friction force under the foundation raft is accomplished by using a fictitious rigid link with a rigid-perfectly plastic material. This results in identical equations of motion for the sliding structure, both in the sliding and nonsliding (stick) phases and considerably decreases the required number of time steps for the nonlinear analysis. Since the force in the link is of constant value, to consider the varying friction force, a compensatory force, which is the difference between the exact friction force and the constant force in the rigid link, is applied to the foundation raft. A model of variable friction coefficient for Teflon–steel interfaces is used for the assessment of the method and the results are compared with existing literature, through which, the capability of the method is illustrated. It is shown that by using exact model of friction lower values for the superstructure responses are predicted compared with those obtained by using Coulomb friction model. Furthermore the effect of the stiffness of the structure on the differences between the results of the two models is also studied.

The main activities and the relevant results of an experimental program aimed at evaluating the dynamic characteristics of a large seismically isolated building are described. The building is situated in Potenza (Italy) and is the largest of five seismically isolated blocks of the University of Basilicata. Cyclic shear tests on scaled isolators and some small amplitude free vibration tests were preliminarily carried out. The latter ones were aimed at getting a preliminary linear characterisation of the structure. Finally, many in situ release tests on the isolated structure were carried out, using a mechanical device purposely designed to statically move the building and then suddenly release it, thus making it vibrate freely. The variations of the dynamic characteristics of the system while the oscillation amplitude decreases have been evaluated by using the "Short Time Fourier Transform (STFT)". The results were elaborated in terms of stiffness and equivalent damping and compared to the results of the laboratory tests on scaled isolators, finding an excellent agreement.

This paper presents the concept of constant strength design spectra for the design of base-isolated structures; particularly those structures using isolators with a bilinear hysteretic behaviour when subjected to dynamic loading. The constant strength design spectra relate peak accelerations, velocities, displacements and effective isolated natural periods for bilinear systems with a given yield strength and post yield stiffness. Constant strength design spectra could be useful for the design of base isolators with bilinear hysteretic behaviour, as these devices can be designed for fixed yield strength and post yield stiffness. The concept of constant strength design spectra and its application for the design of base isolated structures is illustrated with case studies of specific structures.

The objective of the present work is to examine advantages and drawbacks of different types of isolation systems, when seismic isolation is used as a protection strategy against damage to internal equipment and contents. The starting point of the study is the big experimental program of table tests on reduced-scale R/C structural models, carried out within the MANSIDE (Memory Alloys for New Seismic Isolation DEvices) project. Seven identical 1:3.3-scaled, 3-storey frames were tested, including two fixed-base models and four base-isolated models with different isolation systems, namely: (1) rubber isolators, (2) steel-hysteretic system and (3), re-centring SMA (Shape Memory Alloy) system. In this study the internal equipment is regarded as an elastic single degree of freedom, with 2% equivalent viscous damping. Therefore, the capability of fixed-base and base-isolated models with different isolation systems to protect light secondary systems is evaluated by comparing the floor response spectra obtained from the storey accelerations recorded during shaking table tests. Three different PGA's are considered, about 0.15g, 0.3g and 0.5g, respectively. All the shaking table tests are also simulated with an accurate numerical model, to validate and better understand the experimental results. It is found that each type of isolation system reduces considerably the seismic effects on internal equipments in wide frequency regions. However, tuning effects may arise in specific frequency ranges, corresponding to the first mode in structures equipped with quasi-elastic (rubber) isolation systems, and to higher modes in structures equipped with elasto-plastic (steel) and nonlinear re-centering (SMA) isolation systems.

The stochastic response of base-isolated building considering the uncertainty in the characteristics of the earthquakes is investigated. For this purpose, a probabilistic ground motion model, for generating artificial earthquakes is developed. The model is based upon a stochastic ground motion model which has separable amplitude and spectral non-stationarities. An extensive database of recorded earthquake ground motions is created. The set of parameters required by the stochastic ground motion model to depict a particular ground motion is evaluated for all the ground motions in the database. Probability distributions are created for all the parameters. Using Monte Carlo (MC) simulations, the set of parameters required by the stochastic ground motion model to simulate ground motions is obtained from the distributions and ground motions. Further, the bilinear model of the isolator described by its characteristic strength, post-yield stiffness and yield displacement is used, and the stochastic response is determined by using an ensemble of generated earthquakes. A parametric study is conducted for the various characteristics of the isolator. This study presents an approach for stochastic seismic response analysis of base-isolated building considering the uncertainty involved in the earthquake ground motion.

The objective of this study is to establish a system for selecting the optimum friction material to meet the seismic performance requirements of a liquefied natural gas tank with a friction pendulum system (FPS). A methodology for determining the optimum frictional material using seismic fragility analysis is suggested, and it is applied to materials with various frictional coefficients for FPS. Seismic fragility curves with two different limit states are developed to determine the optimum friction material, and a methodology for combining fragility curves is proposed. The analysis shows that a lower friction coefficient for FPSs is more appropriate for preventing failure in FPSs and the superstructure investigated in this study.

In this paper, the effectiveness of seismic isolation using lead rubber bearings (LRBs) and friction pendulum systems (FPSs) for slender and broad, grounded and elevated tanks (two seat locations of isolation systems in elevated tanks) is investigated under bi-directional excitation of up to 10 records of earthquakes. An analytical mechanical model for a flexible, concrete cylindrical tank, taking into consideration the effect of wall mass and sloshing, is used. The assumptions underlying the mechanical model are basic equations of the motion according to the theory of fluid dynamics. So, the assumptions are more realistic and the results therefore are more accurate than the previous models. The results show that by selecting the best mechanical properties of base isolation systems, reductions of seismic base shear in the grounded broad tanks are around 35% and 30% and for the grounded slender tanks are around 55% and 58% in the x- and y-directions, respectively. Maximum total hydrodynamic pressure is reduced considerably by about 40% on average in all grounded and elevated models. As a result of isolation, elevated tanks are concluded to be better candidates in terms of more effective application of seismic isolation.

The base isolated system takes advantage of enhancing the earthquake-proof performance of the structure by means of reductions of input seismic motions. In order to perform reliable evaluation of the response of structures under dynamic loads such as earthquakes, it is necessary to examine their nonlinear response characteristics. In this study, the uncertain parameter effects on the base isolated system related to the nonlinear dynamic response are examined with Monte Carlo Simulation. It is suggested that the uncertain parameters provide significant roles on the maximum responses evaluations of the base-isolated system.

Impact response of structures supported on the sliding systems to real earthquake ground motion is investigated. The base-isolated structure is modeled as shear building and adjacent structure as a spring-dashpot (impact) element. The seismic response is obtained by deriving differential equations of motion of the structure with sliding system and solved using Newmark's step-by-step integration technique in iterations. It is found that performance of sliding systems is hampered with impact. Investigations are carried out to find out the effect of parameters of adjacent structure such as stiffness and gap distance on response of base-isolated structure under consideration. Adversity of impact increases with rising stiffness of adjacent structure and is found quite predominant parameter. Damping in adjacent structure mainly shows influence on bearing displacements which reduce marginally with increased damping. Impact response is obtained under varied flexibility of building for different sliding systems to study the performance during impact. It is observed that with increased flexibility, the superstructure acceleration increases. Also, the superstructure accelerations increase with increments in gap distance up to a certain value and for further widening in gap distance, declining trend is observed. For the changes in base mass to floor mass ratio the responses shows meager deviation. It is concluded that superstructure accelerations increases due to the occurrence of impact in base-isolated structures, ultimately resulting into reduction in efficiency of sliding isolation systems.