Observed Behavior of Seismically Isolated Buildings

Strong motion recordings from four seismically isolated buildings are used in time-invariant and time-variant modal identification analyses. The buildings are identified as equivalent, time-varying, linear dynamic systems characterized by their time-varying modal parameters (including fundamental-mode natural frequencies, damping ratios, and mode shapes). The identification analyses for each site are repeated using multiple earthquake recordings. In the case of one building, the inferred isolator stiffness and damping during several earthquakes is compared with experimental results. Spectral displacements of the ground motion at the effective fundamental period of the seismically isolated building are found to correlate well with two measures of isolator behavior—the maximum reduction in fundamental mode frequency during strong shaking, and the fraction of the peak total building deformation occurring within the isolators. While the recorded ground motions have much smaller amplitudes than the design basis motions, extrapolation of the observed trends in frequency reduction and relative isolator deflection to larger levels of shaking indicates performance estimates consistent with those assumed in design (which were based on laboratory testing of the isolators).