Estimation of base isolated structure responses

D M Lee,I C Medland

doi:10.5459/bnzsee.11.4.234-244

Abstract

The dynamic responses of a set of elastic six storey shear structures mounted on a bilinear hysteretic Base Isolation (B.I.) System and forced by a wide range of different 'types’ and 'strengths’ of real and simulated earthquakes, are studied. The responses of the same set of structures, without the Base Isolation System, to the same range of earthquakes are also computed for comparative purposes. It is found that for the B.I. system investigated, not only are the maximum shear forces in an isolated structure reduced to, on average, 0.2 of their values without B.I., but also that the maximum shear responses of isolated structures can be closely estimated from a wide variety of different parameters, selected from ground motion and elastic response spectrum data of the forcing earthquakes. Ninety-five percent confidence intervals are determined for the linear least squares regression relations between structure responses and earthquake parameters. For the isolation system considered the maximum displacement across the isolation system during an earthquake is shown to be very closely correlated with the maximum structure shear force.

Highlights

The Base Isolation (B.I.) System ^ is a recently developed, practical and convenient form of 'soft first storey' for shielding a structure from earthquake attack
The maximum shear force, S, in an isolated structure forced by a given earthquake can be accurately predicted from data such as the Housner Spectral Intensity, which gives only a general indication of the energy of the earthquake
S for an isolated structure is closely correlated with the value at

Summary

INTRODUCTION

The Base Isolation (B.I.) System ^ is a recently developed, practical and convenient form of 'soft first storey' for shielding a structure from earthquake attack. Investigation of a 1 Soft Storey Isolation System' Kelly et al (i2) suggested a design procedure utilising effective periods and damping factors of the isolated structure and the 30, 35, 40 and 45% damped single d.o.f. elastic response spectra of the forcing earthquake in an iterative, mode superposition approach. While this procedure gave reasonable estimates of the expected maximum structure shears it did not give good estimates of the maximum base displacement as it could not take nonlinear base shifting into account. The responses of unisolated structures are considered for comparative purposes

STRUCTURE MODELS AND PARAMETERS USED

EARTHQUAKE RECORDS U S E D

Selected Parameters

CONCLUSIONS