Nonlinear modelling of HDRBs in the seismic analysis of retrofitted and new base-isolated r.c. buildings

The seismic protection of r.c. buildings by base-isolation systems is often achieved using elastomeric devices like the high-damping-rubber bearings (HDRBs). When these buildings are subjected to severe ground motions which can occur at sites located near active faults, the seismic demand of their superstructure can be amplified in a significant way and the interaction between shear and axial laws of the elastomeric bearings can become quite complex. Sophisticated models of the axial, shear and rotational behaviour of HDRBs, supported by experimental tests and numerical simulations, are proposed in the literature to describe their nonlinear response. However, few numerical studies are available to compare the influence of such models on the nonlinear response of base-isolated r.c. buildings under near-fault earthquakes. In this work, nonlinear incremental dynamic analyses of two typical r.c. framed structures, one retrofitted and the other one new, are carried out considering simplified and refined force–displacement laws of HDRBs. To this end, nine earthquakes, corresponding to three strong near-fault seismic events, extracted from the Pacific Earthquake Engineering Research center database, are selected to highlight effects of correlation between their horizontal and vertical components. Numerical results suggest that the nonlinear behaviour of HDRBs can be described in an equivalent way by using an upper and lower bound approach.