Seismic Response Of Base-isolated Buildings Research Articles

Seismic response of base-isolated buildings: exploring isolator properties

Seismic response of base-isolated buildings: exploring isolator properties

The effect of an improved passive bumper on the seismic response of the base-isolated structure: Numerical simulations and optimization design

In this paper, a passive bumper, called Flexible Limit Protective Device (FLPD) which was proposed by the author before, was improved to better control the seismic response of the base-isolated structure when the isolation layer exceeds its limit deformation. To distinguish from the FLPD, the improved FLPD is called a Flexible Energy Dissipating Device (FEDD), and the FEDD has stronger dissipation capacity than the FLPD. Above all, the compression behavior of FEDDs is experimentally investigated and the experiment data of FEDDs are compared to that of FLPDs. Subsequently, the nonlinear model of FEDDs is obtained and verified using the experimental data. This obtained model was used to study the effect on the seismic response of the base-isolated structure (multi-degree-of-freedom system), namely isolator displacement and inter-story drift angle. An elite non-dominated sorting algorithm method (NSGA-II) is applied to optimize the design of FEDDs, the gap between isolators and FEDDs and stiffness of FEDDs. Finally, the seismic response of base-isolated buildings equipped with FEDD with optimal parameters is numerically simulated. The results show that FEDDs with optimal parameters can effectively reduce the isolator displacements, and keep the inter-story drift angle within the safe range when the isolation layer exceeds its limit deformation during earthquakes.

Investigations of the effects of a passive bumper on the seismic response of base-isolated buildings: Experimental study and parameter optimization

The horizontal displacement values experienced by the isolation layer of base-isolated buildings can exceed the allowable range and cause failures during the rare or very-rare earthquakes. Excessive horizontal displacements of the isolation layer may cause collisions between the building and retaining walls of the isolation ditch and even cause the collapse of the isolated building. This paper proposes a cost-effective, easy-to-build, passive bumper device, called Flexible Limit Protective Device (FLPD) in order to act as shock-absorbers. Through numerical simulations and experiments, the nonlinear behavior of the FLPD is investigated. Subsequently, through structural simulations, the effectiveness of using FLPDs is studied. The elitist non-dominated sorting genetic algorithm (NSGA-II) is used to optimize the design of FLPDs, and the response of structures equipped with optimized FLPDs are simulated numerically. The results indicate that proposed optimized FLPDs can effectively work as shock-absorbers for base-isolated structures. This paper can provide a guideline for the design of shock-absorbers.

Seismic response control of multi-story base-isolated buildings using a smart electromagnetic friction damper with smooth hysteretic behavior

Abstract This paper studies the use of a smart/semi-active electromagnetic friction damper (SEMFD) for the control of seismic response of multi-story base-isolated buildings. The SEMFD consists of a ferromagnetic plate and two similar arrays of thick rectangular ferromagnetic-core coils (FCs) connected in series. The FCs are attached to the two sides of the ferromagnetic plate through two non-magnetic friction pads. The force in the damper is developed because of the friction between the friction pads and the ferromagnetic plate when the FCs moves relative to ferromagnetic plate. The normal force between the friction pad and the ferromagnetic plate is caused by the attractive magnetic interactions between the FCs arrays and the ferromagnetic plate. The magnitude of this force is controlled by a proposed semi-active controller that is capable of varying the current flowing through the FCs in such a way that it is able to avoid stick-slip motion by smoothing the nonlinear hysteretic behavior of the SEMFD. The capability of the SEMFD and the proposed semi-active controller to control the seismic response of base-isolated buildings is demonstrated by implementing them into the dynamic model of a six-story base-isolated building supported on lead-rubber bearings (LRBs). The numerical results show that the SEMFD and its semi-active controller are capable of limiting the displacement of the base floor without noticeably increasing the inter-story drifts and absolute accelerations of the floors. It is demonstrated that the parallel actions of the LRBs and the SEMFD in the base floor create a smart base isolation system that is capable of fully protecting a multi-story building from ground motions with intensities at a level as that of the ASCE 7–10 Maximum Considered Earthquake (MCE).

Effects of the long-term behaviour of isolation devices on the seismic response of base-isolated buildings

Seismic isolation is widely used around the world for the protection of buildings, but its long-term behaviour was not considered in detail in past seismic design of isolated structures. Nevertheless, the American seismic code has recently introduced an explicit procedure aimed at evaluating upper-bound and lower-bound values of isolation system properties. Yet there are few studies on the evolution and extent of the deterioration of elastomeric (e.g., high-damping rubber bearings) and sliding (e.g., flat sliding bearings) isolators during their lifetime and on their impact on the seismic behaviour of the superstructure. To investigate this problem, six-storey–reinforced concrete buildings, base isolated with high-damping rubber bearings acting alone or in combination with flat sliding bearings, are designed without considering the variability of mechanical properties of the isolation system due to ageing and air temperature. Then, based on experimental results from accelerated ageing tests at high temperature, mathematical models are implemented to account for oxidation of elastomers and friction changes. The variability of mechanical properties at different mean temperatures, due to seasonal thermal variations, is also considered. Finally, fragility curves are developed for the base-isolation system and superstructure on the basis of nonlinear dynamic analysis of the degraded test structures.

Influence of Near-Fault Ground Motions with Fling-Step and Forward-Directivity Characteristics on Seismic Response of Base-Isolated Buildings

ABSTRACT The behavior of base-isolated buildings under near-fault (NF) ground motions with fling-step and forward-directivity characteristics is investigated using three-dimensional nonlinear finite element models. Nonlinear response history analysis (NLRHA) is also carried out after removing the dominant pulse from the NF ground motion records. The results obtained for 10-story base-isolated buildings with and without shear walls, indicate that NF ground motions with fling-step and forward-directivity characteristics produce large inter-story drift ratios and floor accelerations, compared to the case when the dominant pulse is removed. Additional NLRHA carried out using artificial pulses shows that the isolator and superstructure responses are larger for pulses with fling-step characteristics, compared to pulses with forward-directivity characteristics.

Supplemental dampers in base-isolated buildings to mitigate large isolator displacement under earthquake excitations

The effect of viscous, viscoelastic, and friction supplemental dampers on the seismic response of base-isolated building supported by various isolation systems is investigated. Although base-isolated buildings have an advantage in reducing damage to the superstructure, the displacement at the isolation level is large, especially under near-fault ground motions. The influence of supplemental dampers in controlling the isolator displacement and other responses of base-isolated building is investigated using a multi-storey building frame. The coupled equations of motion are derived, solved and time history analysis is carried out on a building modeled with fifteen combinations of five isolation systems and three passive dampers. The seismic responses are compared with that of the fixed-base and base-isolated buildings. Based on the results, it is concluded that supplemental dampers are beneficial to control the large deformation at the isolator level. Parametric study is conducted and optimum ranges of damper parameters to achieve reduced isolator displacement without adverse effect on the other responses are determined. Further, it is concluded that the combination of the resilient-friction base isolator (R-FBI) and viscous damper is the most effective in reducing the bearing displacement without significant increase in superstructure forces.

Modelling curved surface sliding bearings with bilinear constitutive law: effects on the response of seismically isolated buildings

This research work is related to the evaluations of some modelling assumptions of the stick–slip phase of the Friction Pendulum™ System (FPS) on the seismic response of base-isolated buildings. Although several theoretical and experimental research findings show the complexity of the FPS nonlinear behaviour, the current international seismic design codes allow the designer to model the FPS nonlinear response using simple bilinear constitutive relationship in order to reduce the computational effort for modelling and performing the analyses. Therefore, the problem that has to be faced by practitioners and researchers is related to the way of modelling the stick–slip phase and the corresponding friction phenomena with a simple bilinear constitutive model. This research work has been encouraged by some experimental evidence and performed for investigating, through a large parametric study, the influence that many modelling assumptions may have on the global and local responses of buildings. The study has been carried out without claiming to be innovative but for being a valuable tool in the design practice and, mainly, in modelling the nonlinear response of the FPS with the bilinear simplification allowed by design codes.

Computer-aided investigation of special issues of the response of seismically isolated buildings

This paper presents indicative results from the numerical investigation of two special issues of the seismic behaviour of base-isolated buildings, using custom-made software that utilizes modern objectoriented design approaches. The first issue concerns the modelling of the nonlinear behaviour of seismic isolation systems, focusing on the lead rubber bearings (LRBs), which are among the most commonly used seismic isolation systems. In particular, the inaccuracies between the actual behaviour of the LRBs, which can be more precisely represented by the Bouc–Wen model, and the usage of a bilinear inelastic model, which is often used in practice, are assessed through numerical simulations and parametric analyses. The second issue concerns potential pounding of base-isolated buildings with adjacent structures, when the available clearance around a seismically isolated building is limited, during very strong earthquakes. The consequences of potential pounding and the influence of certain parameters on the overall seismic response of base-isolated buildings are also assessed through numerical simulations and parametric analyses using custom-made software.

Earthquake Induced Pounding-Involved Response of Base-Isolated Buildings Incorporating Soil Flexibility

This research aims to get some insight into the influence of supporting soil on the seismic response of base-isolated buildings impacting with the neighbouring retaining wall. Two ground motion records from two different regions are used as excitations. A nonlinear impact model is incorporated to capture the pounding force. In addition, an efficient discrete-element model is applied to represent the rotational and horizontal movements of the soil. The dynamic response of the isolated building resting on different flexible base soils and hits the adjacent abutment and the corresponding response of the building resting on a rigid base soil are presented. Analysis of the results expressed in the form of time-history and peak response for various values of structural natural periods, gap sizes, and base-isolated periods is carried out. The results show a considerable influence of the simultaneous effect of base-isolated pounding and base soil flexibility on the dynamic response of isolated buildings during earthquakes.

Intensity measures for the seismic response prediction of base-isolated buildings

Base isolation has become a widely applied technique for protecting buildings located in highly seismic areas. Due to the strongly non-linear constitutive behaviour typical of many isolation devices, the seismic response of base-isolated buildings is usually evaluated through non-linear dynamic analysis. In this type of analysis a suitable set of ground motions is needed for representing the earthquake loads and for exciting the structural model. Many methods can be found in the literature for defining the ground motions. When natural accelerograms are used, the methods mainly differ from each other based on the intensity measures used for scaling the records to the defined earthquake intensity level. Investigations have been carried out for evaluating the predictive capability of the intensity measures used in these methods: while many studies focused on ordinary buildings, only a few focused on base-isolated ones. The objective of this paper is to evaluate the most commonly used intensity measures, which are currently available in the literature, with respect to their capability to predict the seismic response of base-isolated buildings. Selected for the investigation are two frame structures characterized by a different number of storeys and base-isolated with systems having different properties. Two sets of accelerograms, consisting of ordinary and pulse-like near-fault records, are used in the analyses and in the evaluation of the intensity measures. Modified versions of existing intensity measures are also proposed, with the intent of improving the correlations between the considered intensity measures and response quantities.

Utilization of object-oriented programming, design patterns and java for simulating earthquake-induced poundings of base-isolated buildings

Base-isolated buildings experience large horizontal relative displacements during strong earthquakes due to the excessive fl exibility that is purposely incorporated, through seismic bearings, at their bases. When the available clearance around a base-isolated building is limited, there is a possibility of the building pounding against the surrounding moat wall or adjacent structures. Considering the nonlinearities involved in this structural impact problem, it is evident that the effects of potential pounding on the overall seismic response of base-isolated buildings during earthquake excitations should be investigated numerically through appropriate simulations. Object-oriented programming (OOP), design patterns (DPs), and the Java programming language have been utilized in order to design and implement a fl exible and extendable software application that can be effectively used to perform the necessary numerical simulations and parametric studies of base-isolated buildings that may experience structural poundings during earthquake excitations. The aim of this paper is twofold: (i) to explain the signifi cant advantages of utilizing OOP, DPs, and Java in structural analysis software and (ii) to use the developed software to study earthquake-induced poundings of base-isolated buildings.

Adaptive control of base-isolated structures against near-field earthquakes using variable friction dampers

This paper investigates the effectiveness of two adaptive control strategies for modulating the control force of variable friction dampers (VFDs) that are employed as semi-active devices in combination with laminated rubber bearings for the seismic protection of buildings. The first controller developed in this study is an adaptive fuzzy neural controller (AFNC). It consists of a direct fuzzy controller with self-tuning scaling factors based on neural networks. A simple neural network is implemented to adjust the input and output scaling factors such that the fuzzy controller effectively determines the command voltage of the damper according to current level of ground motion. A multi-objective genetic algorithm is used to learn the shape of the activation functions of the network. The second controller is based on the simple adaptive control (SAC) method, which is a type of direct adaptive control approach. The objective of the SAC method is to make the plant, the controlled system, track the behavior of the structure with the optimum performance. Here, SAC methodology is employed to obtain the required control force which results in the optimum performance of the structure. For comparison purposes, an optimal linear quadratic Gaussian (LQG) controller is also developed and considered in the simulations together with maximum passive operation of the friction damper. The results reveal that the developed adaptive controllers can successfully improve the seismic response of base-isolated buildings against various types of earthquake.

Effects of Spatial Variable Ground Motions on the Seismic Response of Base-Isolated Building

Aimed to base-isolated building with large plane dimension, the change laws of seismic response for base-isolated building under spatial variable ground motions were researched. Firstly, the artificial spatial variable earthquake time histories were generated using spectral representation method based on code response spectrum. Then the 3-D FEM modal of one based-isolated building with large plane dimension was established and the seismic response of based-isolated building under spatially ground motion was studied by nonlinear time history analysis. The mitigation effects of based-isolated building with large plane dimension were compared each other at the cases of uniform excitation, non-uniform excitation considering only wave passage effect, non-uniform excitation considering both the wave passage effect and incoherence effect, multi-component uniform excitation, multi-component non-uniform excitation considering the wave passage effect and multi-component non-uniform excitation considering both the wave passage effect and incoherence effect. The results show that the seismic response of base-isolated structure with large plane dimension under the uniform excitation is relative safety. When the base-isolated building with large plane dimension is designed by time history analysis, the spatial variability of earthquake ground motion effects can be considered.

Seismic response of base-isolated buildings including soil–structure interaction

This study investigates the effect of soil–structure interaction (SSI) on the response of base-isolated buildings. The equations of motion are formulated in the frequency domain, assuming frequency-independent soil stiffness and damping constants. An equivalent fixed-base system is developed that accounts for soil compliance and damping characteristics of the base-isolated building. Closed-form expressions are derived, followed by a thorough parametric study involving the pertinent system parameters. For preliminary design, the methodology can serve as a means to assess effective use of base isolation on building structures accounting for SSI. This study concludes that the effects of SSI are more pronounced on the modal properties of the system, especially for the case of squat and stiff base-isolated structures.

Base-Isolated Building with Asymmetries Due to the Isolator Parameters

The effects of torsional coupling, due to isolator parameters, on the seismic response of base-isolated buildings are presented. The isolated building is modelled as a single-storey structure mounted on different isolation devices such as elastomeric and sliding systems involving non-linear restoring forces. The governing equations of motion for the uncoupled and torsionally coupled system are derived and solved in time domain by Newmark's method of integration to obtain lateral-torsional displacement response. The displacement response of the isolated system with different combinations of structural configurations, isolation systems and the ratio of uncoupled torsional to lateral frequency of the system is investigated. A comparison of the response of the torsionally coupled base-isolated building is made with the corresponding response obtained from torsionally uncoupled base-isolated building. In addition, a parametric study is conducted to observe the effect of superstructure flexibility on the displacements in torsionally coupled base-isolated building. The eccentricities arising due to the asymmetries in the isolation stiffness and/or yield strength of the isolators are compared with the eccentricity in the system as specified by the Uniform Building Code (UBC 1997). It is observed that the torsional coupling arising due to the dissimilarity in the isolator properties considerably influences the seismic response of the base-isolated building. Effects of superstructure eccentricity are found diminishing when the isolation eccentricities exist. The design bearing displacement suggested by the UBC static formula incorporating accidental torsion is found conservative for the isolation eccentricities arising due to the dissimilarity amongst the isolators.

Use of a Viscoelastic Model for the Seismic Response of Base-Isolated Buildings

Due to recent developments in elastomer technology, seismic isolation using elastomer bearings is rapidly becoming an acceptable design tool to enhance structural seismic margins and to protect people and equipment from earthquake damage. With proper design of isolators, high-energy seismic input motions can be transformed into low-frequency, low-energy harmonic motions, and the accelerations acting on the isolated building may be significantly reduced. Several alternatives exist for the modeling of the isolators. This study is concerned with the use of a viscoelastic model to predict the seismic response of base-isolated buildings. The in-house finite element computer code, SISEC has been modified to incorporate a viscoelastic spring element, and several simulations are performed. Then, the computed results have been compared with the corresponding observed data recorded at the test facility.

Seismic response of base-isolated buildings using high damping rubber bearings.

Seismic response of base-isolated buildings using high damping rubber bearings.