Seismic behaviour of asymmetric base isolated structures with various distributions of isolators

The paper tries to contribute to a better understanding of the behaviour of base isolated asymmetric structures. Numerous variants of originally symmetric four storey RC frame building isolated by a simple lead rubber bearing base isolation system with various distributions of isolators were considered as test examples. The symmetrical structural variant and appropriate LRB bearing properties were designed according to Eurocode 2 and 8. The asymmetric variants were produced by shifting the centre of mass CM toward one side of the building. Additional “torsionally unrestrained” and “torsionally restrained” sub-variants of each building variant were obtained by changing the mass distribution, while total mass sum remained unchanged. For the base isolation system we have considered six different distributions of bearings characterized by the position of the centre of isolators CI in respect to the centre of mass CM of the superstructure. Two symmetric (Uniform and Peripheral distribution) and four asymmetric distributions of isolators (called CI = CM, CI = CM/2, CI = −CM/2 and CI = −CM) were included in the analyses. The paper analyses the positive and negative effects of different bearing distributions to the displacements and rotations of the superstructure as well as to the base isolation system and tries to determine the most favourable distribution of isolators that is able to balance the effects of introduced eccentricities. The results obtained by 3D nonlinear dynamic analyses are presented as an average of maximums for ten selected ground motions and three different scalings. They indicate that all six considered distributions of bearings, however differently, substantially reduce the unfavourable torsional effects, which are with different extent transferred from the superstructure to the base isolation system. It was further observed that CI = CM distribution, favoured by common building codes, is best only for accommodating the torsional effects in the base isolation system. A significantly different conclusion was found observing the nonlinear behaviour of the superstructure, where CI = CM distribution might cause more damage in the flexible side frames.