Effect of superstructure modeling assumptions on the seismic performance of seismically isolated buildings

Recent studies on the seismic performance of seismically isolated buildings with braced frames designed using the minimum criteria of the ASCE 7-16 standard resulted in observations that seismically isolated buildings have unacceptable collapse performance due to either insufficient isolator displacement capacity or excessive inelastic structural deformations. The collapse performance and the overall performance could be improved by increasing the displacement capacity of the isolators and by limiting inelastic action in the superstructure while still providing ductile detailing. These observations may have been dependent on the modeling assumptions for the superstructure. This article investigates the effect of various modeling methods that are commonly used in research and design practice on the seismic performance of a six-story braced seismically isolated building with two different isolation systems, and a comparable nonisolated six-story building, all designed to meet the minimum criteria of ASCE 7-16. Performance measures considered in this study include story drift ratios, residual story drift ratios, floor accelerations, isolator horizontal displacement, and collapse. The study found that for seismically isolated buildings and for the modeling assumptions considered, only the inclusion of the effect of gravity frames has a beneficial effect of reducing the occurrence of small to moderate peak floor accelerations while there is insignificant effect on the other performance measures and particularly the computed probabilities of collapse in the MCE_R. Also, the different modeling assumptions were found to have a more important effect on the seismic performance of nonisolated buildings than that of comparable seismically isolated buildings.