Stability of elastomeric seismic isolation bearings

Elastomeric seismic isolation bearings are subjected to large axial loads a nd lateral displacements during strong earthquakes. The existing Koh-Kelly model for elastomeric bearings accounts for axial load effects on horizonta l stiffness. This linear model is based on small displacements and rotation s and predicts stable postcritical behavior or increasing critical load wit h increasing horizontal displacement; however, unstable postcritical behavi or is observed in the bearing test results presented in this study. The ana lytical model developed in this study, based on the Koh-Kelly model, includ es large displacements, large rotations, and nonlinearity of rubber, and it predicts unstable postcritical behavior. The formulation of the analytical model, calibration, and verification using experimental results are presen ted. It is shown that: (1) the critical load reduces with increasing horizo ntal displacement; and (2) the horizontal stiffness reduces with increasing horizontal displacement and axial load. It is also shown that the critical load capacity at a horizontal displacement equal to the width of the beari ng is not equal to zero, as predicted by the approximate procedure used in design, but higher.