ASSESSMENT OF SEISMIC PERFORMANCE OF SEISMICALLY ISOLATED BUILDINGS

Seismically isolated buildings in the United States are currently designed and analyzed by the procedures of the ASCE/SEI 7-16 standard, Chapter 17. The ASCE standard requires that the isolation system be detailed to accommodate the displacement demand calculated in the Risk-Targeted Maximum Considered Earthquake (MCE_R), where this displacement is the average of peak values calculated in seven nonlinear response history analyses. The procedure permits the use of a response modification coefficient (R_I factor) between 1.0 and 2.0 depending on the seismic force-resisting system used. The ASCE/SEI 7-16 standard specifies that the minimum required strength for isolated superstructure and the allowable story drift for the design are determined based on calculations using the MCE_R spectrum. The authors have investigated the reliability of the ASCE/SEI 7-16 provision by studying an archetypical 6-story perimeter frame seismically isolated buildings designed with special concentrically braced frames (SCBF) and special moment resisting frames (SMF) for a location in California using the minimum criteria of ASCE/SEI 7-16 and also the other enhanced design criteria. Comparable non-isolated structures, also with braced and moment frames, were designed based on the minimum criteria of ASE/SEI 7-16 and studied. This paper summarizes the results of these studies that have been recent published. The seismic isolation systems considered in this study consist of triple or double friction pendulum isolation bearings. Moat walls for limiting the isolation system displacement and preventing isolator collapse were also considered in some studies. The seismic performance was assessed in terms of the probability of collapse in the MCE_R and in terms of annual exceedance of engineering demand parameters (peak story drift ratio, maximum residual story drift ratio, floor acceleration). The seismic demand parameters used relate to damage of structural components, non-structural components and contents. The spectral shape effects are explicitly considered by utilizing conditional spectra. Conclusions are presented on the required displacement capacity of seismic isolation systems and the strength of the superstructure to meet minimum criteria and to achieve improved performance.