Seismic base isolation of asymmetric shear buildings

Abstract

Effects of alternative designs of bilinear base isolation systems on the seismic response of ten, five and two-storey shear buildings, with one axis of symmetry consisting of three parallel frames, are investigated. Analyses with the El Centro and Taft records predict appreciably lower shear forces and torques compared with the unisolated case, whereas lateral roof displacements are likely to increase for the lower structures. Optimum levels of yield force, and secondary to primary slope ratio of the isolation system hysteretic force displacement relationship, are discussed. Locating the centres of rigidity and the yield forces of the bearings at the mass centre of the superstructure appears to be the best strategy for reducing twist. Other alternatives, however, may reduce base motion and inter-storey shear even more. It is shown that increased shear forces and very large displacements may result when the 1977 Rumanian earthquake record is applied, suggesting that a decision regarding the use of base isolation depends on the earthquake characteristics at the site.