Improvement of near-fault seismic isolation using a resettable variable stiffness damper

A seismic structure isolated by a conventional passive isolation system is usually a long-period structural system; therefore, although its dynamic response may be effectively mitigated in a regular earthquake, the responses may be considerably amplified in a near-fault earthquake with long-period characteristics, due to the low-frequency resonant effect. In order to overcome this problem, a sliding isolation system equipped with a new type of the semi-active damper called a resettable variable stiffness damper (RVSD) is proposed in this study. An RVSD damper is similar to a conventional resettable stiffness damper, except that it has a variable stiffness part. By controlling the variable stiffness, the damper force provided by the RVSD will follow a target force that is determined on-line by a general, active control law. As a result, the RVSD damper is able to prevent the abrupt changes of the damper force that inevitably exists in a conventional resettable damper. The harmonic and seismic responses of an isolation system with the RVSD are studied numerically and compared with the other types of isolation systems. The simulated results demonstrate that the RVSD is able to attenuate the low-frequency resonance behavior of the seismic isolation system induced by long-period ground motions. As compared with an isolation system with a conventional resettable damper, the study shows that isolation with the RVSD is superior in reducing the acceleration response due to a near-fault earthquake, while maintaining its effectiveness in the suppression of the isolator displacement.