Nonlinear response of a torsionally coupled base-isolated structure

Torsion in base-isolated structures with inelastic elastomeric isolation systems owing to earthquakes is studied under various parametric variations using the finite element method. Parameters include: those that characterise the number of storeys; the ratio of uncoupled torsional fundamental frequency to lateral frequency in a superstructure; the ratio of uncoupled torsional frequency to lateral frequency in an isolation system; and the stiffness eccentricity in the superstructure and in the isolation system. The effect of these parameters on the torsional behaviour of a superstructure is investigated under different ground motions. It is shown that the total superstructure response is reduced significantly owing to the existence of a base isolation system. Torsional amplification significantly depends on: the isolation system and superstructure eccentricities; the lateral and torsional flexibilities of the superstructure; the ratio of uncoupled torsional frequency to lateral frequency of a superstructure; and the isolation system. The results of the parametric study help in the understanding of the behaviour of torsionally coupled base-isolated structures. In addition the study demonstrates that the asymmetric of superstructure cannot be ignored, even if the isolation system is symmetric.