Rocking bearings with variable frequency for near-fault seismic isolation

A near-fault earthquake usually possesses a long-period pulse-like waveform that can result in an excessive isolator displacement in a conventional isolation system. To alleviate this problem, a rocking bearing with a variable frequency, called polynomial rocking bearing (PRB), is proposed in this study. Because the isolation stiffness of the PRB is a function of the isolator displacement, so the isolation frequency of a PRB isolator becomes variable. In addition, the isolation frequency is merely determined by the geometric parameters, and is independent of the isolated structural weight. When the rocking surface of the PRB is defined by a six-order polynomial function, the restoring force of a PRB bearing will have a softening behavior followed by a hardening behavior, which aims to suppress the maximum responses of the structural acceleration and isolator displacement. The result of numerical simulation has demonstrated that due to the property of variable frequency, the proposed bearing is able to effectively mitigate the resonance effect due to a low-frequency excitation. The result also shows that the PRB is particularly effective in suppressing the isolator displacement induced by a near-fault earthquake with a long-period pulse.