Assessment of a technique for the torsional response reduction of seismic isolated asymmetric structures

This research studies the effectiveness of a bilinear hysteric elastomeric base-isolation system in decreasing the torsional moments generated in a superstructure due to the bidirectional actions of selected ground motions. A five-storey three-dimensional steel structure with asymmetry in both horizontal directions is assumed as the basic model. A parametric study shows that assuming an L-shaped plan for the buildings leads to more critical results. The eccentricity of the L-plan building is changed to a wide range of plan dimensions. Seven ground motions are selected based on the site specifications and the distance of the site to the fault rupture. The mean peak responses are compared to the corresponding responses of the fixed base system to investigate the effectiveness of base isolation. They are also compared with those of the corresponding symmetric system to highlight the effect of torsional coupling. The effects of the center of stiffness of the isolation system being coincident with the superstructure's center of mass are investigated as a technique for reducing the response. The torsional moments are shown to be reduced even in cases with large structural eccentricities and the response reduction has a greater impact in the lower storeys. When the superstructure's center of mass coincides with the base-isolation system's center of stiffness, this causes a reduction in the relative displacement of the exterior edges, the storey rotation and the amount of damage. Therefore, eliminating asymmetry at the base level is proposed as a practical tool to enhance the performance of asymmetric base-isolated buildings. © 2009 Asian Network for Scientific Information.