Passive supplemental damping in a base-isolated structure provides the necessary energy dissipation to limit isolation system displacement. However, damper forces can become quite large as the passive damping level is increased, resulting in the requirement to transfer large forces at the damper connections to the structure which may be particularly difficult to accommodate for retrofitted structures. One method to limit the level of damping force, while simultaneously controlling the isolation system displacement, is to utilize a hybrid isolation system containing semi-active dampers in which the damping coefficient can be modulated. The effectiveness of such a hybrid seismic isolation system for earthquake hazard mitigation is investigated in this paper. The system is examined through an analytical and computational study of the seismic response of a bridge structure containing a hybrid isolation system consisting of elastomeric bearings and semi-active dampers. The general results of the study show that hybrid seismic isolation systems may prevent or significantly reduce structural damage during a seismic event. Furthermore, it is shown that such systems are capable of controlling the peak deck displacement of bridges, and thus reducing the required length of expansion joints.
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