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 build ings are identified as equivalent, time-varying, linear dynamic systems cha racterized by their time varying modal parameters (including fundamental-mo de natural frequencies, damping ratios, and mode shapes). The identificatio n analyses for each site are repeated using multiple earthquake recordings. In the case of one building, the inferred isolator stiffness and damping d uring several earthquakes is compared with experimental results. Spectral d isplacements of the ground motion at the effective fundamental period of th e seismically isolated building are found to correlate well with two measur es of isolator behavior-the maximum reduction in fundamental mode frequency during strong shaking, and the fraction of the peak total building deforma tion occurring within the isolators. While the recorded ground motions have much smaller amplitudes than the design basis motions, extrapolation of th e observed trends in frequency reduction and relative isolator deflection t o larger levels of shaking indicates performance estimates consistent with those assumed in design (which were based on laboratory testing of the isol ators).