Seismic analysis of a base-isolated reinforced concrete frame using high damping rubber bearings considering hardening characteristics and bidirectional coupling effect

High damping rubber bearings (HDRBs) are commonly used in base isolation systems, which can protect the structures from heavy earthquake damage by elongating natural vibration periods of the structures and improving the energy dissipation capacity of the systems. For the HDRBs, the stiffness hardening effect is exhibited at large shear strain, which is favorable to control the displacement of the bearings. Meanwhile, the horizontal hysteretic behavior in the two orthogonal directions is coupled and related to the path history. Here, the influence of the stiffness hardening and bidirectional coupling effects of the bearings on the seismic responses of the isolation system is studied. The full-scale quasistatic cyclic tests of two HDRB specimens with different dimensions were carried out under unidirectional and bidirectional loads. In the numerical analysis, the DHI and the classical Bouc-Wen model are used to simulate the hysteretic behavior of the HDRBs, in which the DHI model can capture the increased damping and stiffness, and bidirectional coupling properties. The parameters of both bearing models are determined according to the test results. Time-history analyses of a six-story reinforced concrete (RC) frame building with an HDRB isolation system are carried out for a suite of earthquake ground motions. The influence on seismic response of superstructure and isolation layer is analyzed with or without considering the hardening effect and bidirectional coupling effect of HDRBs. The results demonstrate that with consideration of the stiffness hardening and coupling effects, the peak responses of the superstructure significantly increase under high-intensity earthquakes. The analytical results suggest that the stiffness hardening and the effect of bidirectional coupling of the HDRBs should be considered in the design for HDRB isolation systems.