Torsional balance of seismically isolated asymmetric structures

Seismic isolation systems reduce the destructive effects of earthquakes through introducing a flexible interface between the superstructure and the foundation. Numerous studies, both analytical and experimental, have shown that such devices can control the translational response of the superstructure. However, no generally consistent design recommendations to control the torsional response are available. The objective of this paper is to present a methodology that lead to achieve optimal torsional control. The assumption behind is that the isolation–superstructure system can be split into an isolated base and superstructure subjected to the filtered ground motion excitation produced by the base response. The superstructure acts as a rigid body subjected this filtered acceleration responses which are applied as quasi-static inputs into the superstructure. Under rather weak assumptions, this simplification provides excellent results in estimating the response of the system. Such response has been used in this study to choose the optimal eccentricity and torsional stiffness parameters of the isolation system that minimize the lateral-torsional response of the superstructure. Results are obtained using probabilistic techniques and show that the response of the superstructure may be substantially improved if the isolation system is torsionally flexible, and if the center of stiffness of the isolated base should lie in the vicinity of the (average) center of stiffness of the superstructure.