Seismic isolation is one of effective and widespread technology that can reduce response acceleration of superstructure. The most popular isolation device is laminated rubber bearing. In general it has linear stiffness against seismic load in the design deformation range. However it has nonlinear stiffness in case of large deformation. That is a hardening property in horizontal direction and a softening property in vertical direction. In the meantime, application of probabilistic safety assessment to seismic evaluation is attracting attention in order to improve seismic resistant reliability. In seismic PSA, accurate numerical simulation is required for not only design basis earthquake but also huge earthquakes that exceed design level. Therefore to express above-mentioned nonlinearity of rubber bearings is required. Thus this paper describes nonlinear simulation of a seismic isolated structure. Both the hardening characteristics of horizontal direction and the softening characteristics of vertical direction are expressed by multilinear models. Additionally the analysis model of horizontal direction considers a habituation effect of rubber. Response of nonlinear simulation is compared with linear simulation. As a result, it was confirmed that nonlinearity of rubber bearings debases the isolation performance.
