Torsional Displacements in Base-Isolated Buildings

Abstract

An analytical study of the effects of torsional coupling on the seismic response of a base-isolated building is presented. The isolated structure is modeled as a rigid deck supported on axially inextensible bearings. The governing equations of motion for the coupled lateral-torsional response of the system are derived. The eccentricity in the system is that specified by the Uniform Building Code (UBC). The displacement response of the isolated system with different combinations of building configuration, isolation damping, and the ratio of uncoupled torsional to lateral frequency of the system is investigated. The response of the isolated structure under a variety of near-fault and other earthquake ground motions is compared to that obtained by use of response spectrum analysis. In the response spectrum analysis the accuracy of several modal combination rules is evaluated. It is shown that torsional coupling can influence the response of the isolated structure, but if the layout of the isolation bearings is such that the torsional frequency is larger than the lateral frequency, the effect is reduced and the usual modal combination rules work well. It is also shown that in this case, the UBC static formula for the additional isolator displacements due to torsion is conservative.