Seismic behavior of torsionally coupled base-isolated structures

Abstract

The torsion in the seismic behaviour of base-isolated structures due to two horizontal components of ground motion was studied, simultaneously. To this end, the variation in essential parameters, such as the number of storeys, the ratio of the uncoupled torsional frequency to the lateral frequency in the superstructure and isolation system, the ratio of the eccentricity to the radius of gyration, the height-wise discrepancy in the eccentricities and the non-monotonous height distribution of the mass and stiffness were investigated. The structure was considered to be a shear building structure idealised using a spring and a concentrated mass. The behaviour of the seismic isolation system was simulated using a bilinear inelastic model, which is an appropriate model with which to consider interactions in two directions. Non-linear dynamic analysis was performed using seven ground motions. The results indicated that the eccentricity to the gyration radius ratio is an important parameter influencing the performance of torsional structures. It was demonstrated that a structure having bidirectional eccentricity has greater dynamic torsion in comparison with a structure having unidirectional eccentricity. Also, even a small eccentricity may cause significant dynamic torsion related to static torsion, especially in torsionally stiff base-isolated structures.