Study On Nonlinear Spectra And Input Energy Of Base-isolated Structure

To reduce or control the seismic response of structures, and make the buildings under earthquake generates small displacement and deformation, the current measures are mainly through shock isolation, this method does not require an external power supply, but through isolation layer to separate and consume a lot of energy, so that the energy input to the upper structure is reduced, thus ensuring the safety of the structure. This structure has simple structure、low cost, easy to maintain, satisfactory damping effect, so the structure has been widely used in various engineering fields.Since1950Housner proposed energy method, energy method is expanding rapidly in half a century, because the transfer, conversion and absorption (dissipation) of the energy is the basic characteristic about structural seismic response. In addition, compared with the traditional design method based on capacity and displacement, the velocity response of the structure is the connecting link between acceleration and displacement response, namely combining the velocity and frequency, we can express the acceleration and displacement response of the structure. Energy analysis method can study the base-isolated structures’nonlinear characteristics and properties under strong earthquake better. However, the structure’s energy calculation is relatively complex. So this paper is designed to find the relationship between velocity and energy, the purpose is to use structure’s velocity instead of energy to find a more simple index which can be used to describe the performance of the structure in the future. The specific work in this paper is as follows:(1) In this paper, we selected30seismic records from the United States Pacific Earthquake Engineering Research Center (PEER) strong earthquake database, the principle of choice is:the selected earthquake records’moment magnitude is greater than5.5, the focal distance less than20km, the focal mechanism strike-slip faults, soil shear wave velocity180-360m/s.(2) Because of the isolation layer of base-isolated structure, from the view of energy,firstly in this paper we discuss the simplified model of base-isolated structure, discuss the simplified model is SDOF or a two degree of freedom, and analysis the total input energy when the parameter is different. Through the study of structure’s total input energy to obtain the conclusion that the base-isolated structure can be simplified to SDOF. (3) Using MATLAB programming and state-space approach, we can get structure’s elastic velocity spectra, and then we study the characteristics of velocity spectra, give the fitting formula of designed velocity spectra. Then analyzes structure’s nonlinear velocity spectra when isolation layer’s hysteretic model is Bouc-Wen model, obtains the relationship between linear and nonlinear velocity spectra.(4) Studying the relationship between velocity spectra and equivalent velocity spectra. Supposing the ratio of energy and velocity is a, called structure’s a spectra. And then analyzing three element of earthquake---amplitude、spectral characteristic、holding time and structure’s dynamic characteristics---stiffness reduction coefficient and damping ratio to make a difference to a spectra.