IS LINEARIZATION OF BILINEAR MODELS FOR BASE ISOLATION SYSTEMS MEANINGFUL ?

Abstract

Base isolation is a seismic design concept that affords a high level of protection to a structure from the damage caused by earthquakes. Base isolation with High Damping Rubber Bearings (HDRB) is today considered an important strategy for the protection of buildings of strategic or monumental interest subject to earthquakes. Well designed base isolation systems allow for damage reduction in structural elements for strong ground motions and for protection of sensitive equipment for moderate earthquakes. The seismic response of base isolated structures is usually analyzed by using non-linear analysis or by linear analysis using equivalent stiffness and equivalent hysteretic damping. The accurate prediction of the seismic behaviour of base isolated structure depends strongly on the accuracy of the mathematical model representing the nonlinear mechanical properties of the isolation bearings. In this study, the literature on the development and investigations of base isolation systems, nonlinear and linear models of isolation bearings are reviewed. There are some provisions for linear models of isolation bearings available in various codes. The aim of this study is to check the effectiveness of linear model provided in ATC-40 by comparison with results of nonlinear analyses. Time history analyses are carried out using bilinear model and also by plausible linear models for the isolator. The seismic gap observed using bilinear model is compared with that obtained using the plausible linear models in order to determine the rules for selection of linear model properties. DRAIN-2DX computer program has been used for the present study. The acceleration time history records of eight earthquake ground motions, synthetic time history compatible with IS: 1893-2002 response spectrum (Zone V, Hard Soil), and scaled records of Northridge earthquake ground motion to get displacement ductility ratio from 2 to 8 have been used for time history analyses. A comparison is presented between the results obtained using time history analyses and provisions made in ATC-40.