Estimating the seismic response of base-isolated buildings including torsion, rocking, and axial-load effects

A procedure based on rigorous nonlinear analysis is developed to estimate the peak response of an isolation system, offering an alternative to equivalent-linear methods in current U.S. building codes. The governing equation is reduced to a form such that the median normalized deformation of the system due to an ensemble of ground motions is found to depend on the isolation period, and the strength normalized by peak ground velocity. Because the dispersion of normalized deformation for an ensemble of ground motions is small, the median normalized deformation is a meaningful estimate of response. Equivalent-linear methods underestimate the peak deformation by up to 50%.; The basic procedure for lead-rubber bearings is extended to more complex models of the isolated building. Friction pendulum (FP) systems are treated by reducing the yield displacement in the bilinear force-deformation relation. A variety of asymmetric-plan buildings are represented by introducing eccentricity in an idealized, rectangular system. The resultant design equation estimates the peak deformation among all isolators within 10% of the median.; Existing models for isolation bearings neglect the following observed behaviors: (1) decreasing lateral stiffness with increasing axial load, (2) decreasing vertical stiffness with increasing lateral deformation, and (3) inadequacy of yield strength in lightly loaded lead-rubber bearings. A series of bearing models are thus developed to include these observed axial-load effects. The constant-strength model modifies an existing linear two-spring model to include bilinear force-deformation, and the variable-strength model incorporates an empirical equation for time-varying yield-strength. By comparing response history analyses using advanced and traditional models, bearing axial-load effects are found to have little influence on the peak bearing deformation or axial forces. Even if rocking is neglected entirely, median response spectra for lateral deformation are within 10% of those when axial-load effects are included.; This dissertation culminates in a final three-dimensional (3D) analysis of the response of the isolated block. Accidental torsion in the isolation system resulting from axial-load effects is found to be insignificant. Deformation design equations are simple and accurate, but those for axial forces are complicated and can err on the order of 25% for asymmetric-plan systems.