Response Of Base-isolated Structures Research Articles

Wavelet analysis of soil-structure interaction effects on seismic responses of base-isolated nuclear power plants

Seismic base isolation has been accepted as one of the most popular design procedures to protect important structures against earthquakes. However, due to lack of information and experimental data the application of base isolation is quite limited to nuclear power plant (NPP) industry. Moreover, the effects of inelastic behavior of soil beneath base-isolated NPP have raised questions to the effectiveness of isolation device. This study applies the wavelet analysis to investigate the effects of soil-structure interaction (SSI) on the seismic response of a base-isolated NPP structure. To evaluate the SSI effects, the NPP structure is modelled as a lumped mass stick model and combined with a soil model using the concept of cone models. The lead rubber bearing (LRB) base isolator is used to adopt the base isolation system. The shear wave velocity of soil is varied to reflect the real rock site conditions of structure. The comparison between seismic performance of isolated structure and non-isolated structure has drawn. The results show that the wavelet analysis proves to be an efficient tool to evaluate the SSI effects on the seismic response of base-isolated structure and the seismic performance of base-isolated NPP is not sensitive to the effects in this case.

Torsionally Seismic Behavior of Triple Concave Friction Pendulum Bearing

In this paper, attempts are made to investigate the responses of torsionally coupling base-isolated structure supported on Triple Concave Friction pendulum bearing (TFP) subjected to near field ground motions. Nonlinear dynamic analyses are conducted to study the effects of essential parameters such as superstructure and isolation system eccentricity, and the uncoupled torsional to lateral frequency ratio of superstructure on seismic responses of TFP isolated structures. Further, the responses of base-isolated structure using TFP are compared with the same base-isolated structure using friction pendulum system (FPS) as well as double concave friction pendulum (DCFP). Results indicate that TFP isolated structure significantly outperforms other types of friction bearings. Moreover, increasing effective period causes TFP isolated structures to behave better than the others in reducing torsional responses of structure. It is concluded that effectiveness of TFP isolators in respect to others bearings is negligible on seismic responses of structures through increasing effective damping.

The effects of peak ground velocity of near-field ground motions on the seismic responses of base-isolated structures mounted on friction bearings

This research has been conducted in order to investigate the effects of peak ground velocity (PGV) of near-field earthquakes on base-isolated structures mounted on Single Friction Pendulum (SFP), Double Concave Friction Pendulum (DCFP) and Triple Concave Friction Pendulum (TCFP) bearings. Seismic responses of base-isolated structures subjected to simplified near field pulses including the forward directivity and the fling step pulses are considered in this study. Behaviour of a two dimensional single story structure mounting on SFP, DCFP and TCFP isolators investigated employing a variety range of isolators and the velocity (PGV) of the forward directivity and the fling step pulses as the main variables of the near field earthquakes. The maximum isolator displacement and base shear are selected as main seismic responses. Peak seismic responses of different isolator types are compared to emphasize the efficiency of each one under near field earthquakes. It is demonstrated that rising the PGVs increases the isolator displacement and base shear of structure. The effects of the forward directivity are greater than the fling step pulses. Furthermore, TCFP isolator is more effective to control the near field effects than the other friction pendulum isolators are. This efficiency is more significant in pulses with longer period and greater PGVs.

Shaking Table Test Study on Seismic Responses of Base-Isolated Buildings under Near-Fault Ground Motions

The shaking table tests of a 1:7 scale model of three-floor steel frame base-isolated building was completed to study the seismic responses of base-isolated buildings under near-fault ground motions. Under the action of the typical near-fault seismic wave, the seismic responses of base-isolated structures increase with the increase of PGA. The maximum story displacements of super-structure decrease with increase of story. The velocity pulse has an adverse effect to acceleration responses of base-isolated structures. The isolation effect of base-isolated super-structures is still favorable under near-fault ground motions, but it will be necessary to add damping in isolation system or limit the displacement of bearings to prevent the excessive deformation of isolation layer.

The Effect of Isolation Layer Yield Stress on the Seismic Response of Base Isolated Frame Structure

At present the research on the effect of the isolation layer parameters on earthquake response of base-isolated structure is rarely. To solve this problem, this paper taking a reinforced concrete frame structure for example, used ANSYS software to analyze the response of base-isolated structure under earthquake. It focused on the relationship between the isolation layer yield stress and the isolation effects, and provided reference for reasonable selection of seismic isolation layer parameters.

Experimental Investigation on Mid-Story Isolated Structures

Isolation technique has been acceded as a part of the China Seismic Code for Design of Buildings. In this code, the limitations for using isolation design are very strict, superstructure must be regular and the isolation layer must be located on the top of base (base isolated structure). Because of the needs of architecture and function or the feasibility of technique, some limitations have been broken in recent projects. Sometimes isolated layer can be set on the intermediate story, so-called the mid-story isolated structure. According to the characteristic of structure, isolation layer of mid-story isolated structure is set on a place where the structure’s vertical stiffness is suddenly changed, as like the top of the first story, middle story, conversion story of the structure. Laminated rubber bearings (LRB) are adopted as an isolation layer. Because the isolation layer is set on intermediate story, the whole structure is divided into superstructure and substructure; the structure’s dynamic characteristics are changed. The mechanism of mid-story isolated structure appears different characteristic compared with base isolation. The aim of mid-story isolation is not only to reduce seismic responses of superstructure, but also to reduce seismic responses of the substructure. Theoretical analysis and the shaking table test of the mid-story isolated structure were carried. And the response of mid-story isolated structure is discussed by comparing with the response of base-isolated structure and base fixed structure. The key problems of mid-story isolated structure are the force condition and the interaction of the structure up and below the isolation layer. Many factors, such as the number of story, mass, stiffness of superstructure and substructure, parameter of the isolation layer, have influence on the seismic behavior of the mid-story isolated structure. The optimum combination relationship of these factors is presented and dynamic characteristics and dynamic responses are investigated.

The Application of Increment-Dimensional Blocks PIM on the Base-Isolated Structure

In this paper, seismic elastic and plastic responses of base-isolated structure are investigated, in which the bilinear restore-force model is employed for the isolation equipment. The mutual influences of both damping matrices and stiffness matrices are decoupled by using transition matrix. Based on the increment-dimensional precise integration method (PIM), the increment -dimensional blocks PIM is developed to separate structure characteristics and external forces, which can avoid the calculation of exponential matrices by PIM in each integration step. The results of numerical examples show that the efficiency of the new algorithm is very high in comparing with old method. The advantages of the new algorithm appear more obvious when the characteristics of large-scale structure have slightly changed, and only the external forces vary.

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 Double Concave Friction Pendulum Base-Isolated Structures considering Vertical Component of Earthquake

The current investigation has been conducted to examine the effect of vertical component of earthquake on the response of base-isolated structures using double concave friction pendulum (DCFP) system. The structure is idealized as a three-dimensional single-story building resting on a DCFP system. The coupled differential equations of motion are derived and solved in the incremental form using Newmark's step-by-step method of integration. Based on these equations, a computer program was developed to study the influence of vertical component of earthquake on the seismic response of isolated structure using DCFP system. To facilitate the investigation, an error function was defined and effect of various structural parameters on this function was studied under Tabas 1978, Northridge 1994 and Kobe 1995 earthquakes motions. The effect of vertical component of earthquake on the peak values of bearing displacement and base shear of the isolated structure is studied using the variation of essential parameters such as isolation period, superstructure period and friction coefficient of sliding surfaces. It is demonstrated that the maximum error caused by neglecting the vertical component of earthquake in determining the peak bearing displacement and base shear of the structure is 5 and 22 percent, respectively.

Effect of tuned mass damper on displacement demand of base-isolated structures

The third author of this paper has previously proposed the installation of a tuned-mass damper (TMD) to reduce the displacement demand on a base isolated structure. The TMD consists of a mass-dashpot-spring subsystem that is attached to the isolated superstructure, analogous to a pendulum. The present paper examines the effectiveness of this scheme and determines optimal parameters for the design of the TMD. Both the base-isolated structure and the TMD are modeled as single-degree-of-freedom, linear oscillators. The optimal TMD parameters are determined by considering the response of the base-isolated structure, with and without the TMD, to a white-noise base acceleration. Such an excitation is representative of broadband ground motions having a nearly constant intensity over a duration several times longer than the period of the base-isolated structure. It is found that, under such an excitation, a reduction of the order of 15%–25% in the displacement demand of the base-isolated structure can be achieved by adding the TMD. Next, the responses of an example base-isolated structure with and without an optimally designed TMD to selected suites of far- and near-field recorded accelerograms are determined. The study shows that for far-field ground motions the effectiveness of the TMD is more or less similar to that predicted by the white noise model, whereas for near-field ground motions the effectiveness of the TMD is less, i.e. of the order of 10% or less. Reasons for this result are described.

Influence of isolator characteristics on the response of base-isolated structures

The influence of isolator characteristics on the seismic response of multi-story base-isolated structure is investigated. The isolated building is modeled as a shear type structure with lateral degree-of-freedom at each floor. The isolators are modeled by using two different mathematical models depicted by bi-linear hysteretic and equivalent linear elastic–viscous behaviors. The coupled differential equations of motion for the isolated system are derived and solved in the incremental form using Newmark’s step-by-step method of integration. The variation of top floor absolute acceleration and bearing displacement for various bi-linear systems under different earthquakes is computed to study the effects of the shape of the isolator hysteresis loop. The influence of the shape of isolator force-deformation loop on the response of isolated structure is studied under the variation of important system parameters such as isolator yield displacement, superstructure flexibility, isolation time period and number of story of the base-isolated structure. It is observed that the code specified equivalent linear elastic–viscous damping model of a bi-linear hysteretic system overestimates the design bearing displacement and underestimates the superstructure acceleration. The response of base-isolated structure is significantly influenced by the shape of hysteresis loop of isolator. The low value of yield displacement of isolator (i.e. sliding type isolation systems) tends to increase the superstructure accelerations associated with high frequencies. Further, the superstructure acceleration also increases with the increase of the superstructure flexibility.

Design Tool for Base-Isolated Structures Modeled as Single-Degree-of-Freedom Oscillators

A stochastic equivalent linearization technique combined with a ground response spectrum approach is proposed to approximate the inelastic response of base-isolated structures. These structures are modeled by nonlinear oscillators with a single degree of freedom. The main advantage of the proposed methodology is the fast calculation of approximate results in comparison with the slow, but more accurate time history analyses. The Bouc-Wen constitutive model is used to represent the inelastic behavior of the isolators. The equations of motion are linearized by an iterative stochastic technique involving the a-priori unknown response statistics. At each iteration step, the modal contributions from one real and one pair of complex conjugate modes are combined by a response spectrum approach to obtain the maximum responses of interest. The process requires the use of conventional spectra (pseudo-acceleration and relative velocity) as well as the relative displacement spectrum of a massless oscillator. Floor response spectrum results above the isolators are calculated by the proposed approach and are compared against the results obtained by a simulation involving time history analyses.

An Approximate Procedure for Solving Base-Isolated Structures

Dynamic analysis of several shear-type structures with base isolation indicates that the response of these structures follows their fundamental mode shape. Based on this observation, this paper uses an approximate procedure for computing the response of base-isolated structures. The procedure consists of modeling the structure and its base by a two-degree of freedom system, one representing the base and the other the structure. The response from the two-degree of freedom model and mode shapes of the structure are used to compute the response of the structure to earthquake excitation. The approximate procedure is simple, requires substantially less computational time than other methods, and gives results that are in excellent agreement with those from direct integration. Nonlinear properties and nonproportional damping are easily included in the model. Savings of approximately 54–77 percent in computational time result by using the approximate model.

Rigid-Body Response of Base-Isolated Structures

The problem of control of rigid body response of structural systems such as nuclear islands, which are mounted on a gird of bearings with three dimensional damping and stiffness properties, is analyzed. A parametric study is carried out to show the effects of the dimensionless parameters of the foundation on the transmissibilities of the structure due to harmonic ground motion. The characteristics of an optimal base isolation are discussed and results are presented in a form which may be of use in preliminary designs.