Estimating Seismic Demands for Isolation Bearings with Building Overturning Effects

An earlier procedure that estimates the peak deformation in base-isolated buildings is extended to include overturning and thereby estimate the peak axial forces in individual isolators. Such tools can be used as part of a design procedure to predict and subsequently eliminate bearing tension by modifying the design. The procedure is based on nonlinear response history analysis of an isolated block using an advanced bearing model that incorporates the relation between axial load and bearing response, known as axial-load effects. Rocking of the structure and bearing axial-load effects are found to have little influence on the peak lateral bearing deformation; median response spectra are within 10% of those when rocking is neglected entirely. Furthermore, bearing axial-load effects can usually be neglected in determining the maximum and minimum bearing axial forces; cases are identified where the error in neglecting such effects exceed 10%. Because the structure has been modeled as rigid, the limitations of the procedure should be assessed for superstructure designs that allow significant structural deformation.