Base-isolated building with asymmetries due to the isolator parameters

The effects of torsional coupling, due to isolator parameters, on the seismic response of base-isolated buildings are presented. The isolated building is modelled as a single-storey structure mounted on different isolation devices such as elastomeric and sliding systems involving non-linear restoring forces. The governing equations of motion for the uncoupled and torsionally coupled system are derived and solved in time domain by Newmark's method of integration to obtain lateral-torsional displacement response. The displacement response of the isolated system with different combinations of structural configurations, isolation systems and the ratio of uncoupled torsional to lateral frequency of the system is investigated. A comparison of the response of the torsionally coupled base-isolated building is made with the corresponding response obtained from torsionally uncoupled base-isolated building. In addition, a parametric study is conducted to observe the effect of superstructure flexibility on the displacements in torsionally coupled base-isolated building. The eccentricities arising due to the asymmetries in the isolation stiffness and/ or yield strength of the isolators are compared with the eccentricity in the system as specified by the Uniform Building Code (UBC 1997). It is observed that the torsional coupling arising due to the dissimilarity in the isolator properties considerably influences the seismic response of the base-isolated building. Effects of superstructure eccentricity are found diminishing when the isolation eccentricities exist. The design bearing displacement suggested by the UBC static formula incorporating accidental torsion is found conservative for the isolation eccentricities arising due to the dissimilarity amongst the isolators