Stiffness controllable isolation system for near-fault seismic isolation

A seismic structure isolated by a conventional passive isolation system is usually a long-period structural system with a fixed fundamental vibration frequency. Even though conventional isolation systems may effectively mitigate the dynamic responses of structures in a regular earthquake, they may also encounter a low-frequency resonance problem when subjected to a near-fault earthquake that usually has a long-period pulse-like waveform. This long-period wave component may result in an enlargement of the base displacement as well as decrease isolation efficiency. To overcome this problem, a sliding base isolation system with controllable stiffness is proposed in this study. By varying the stiffness of the isolation system, the restoring force provided by the system can be controlled by a proposed semi-active control method that is developed based on active feedback control. The result of numerical simulation in this paper has shown that the proposed system is able to effectively mitigate the effect of low-frequency resonance induced by a near-fault earthquake. As a result, the base displacement and super-structure acceleration of the isolated structure can be reduced simultaneously, which is a major improvement over the conventional passive isolation system. (C) 2007 Elsevier Ltd. All rights reserved.