Evaluation of the Seismic Behavior of Asymmetric Base Isolated Braced Steel Structures

Base isolation can improve the seismic performance of structures by reducing the seismic demands instead of increasing the capacity. Many of existing and new structures are asymmetric, thus the effect of asymmetry on the behavior of isolated structures should be studied. In asymmetric structures the horizontal distance between center of mass and center of rigidity, induce inherent torsional moment which can generate extra forces and deformations in the members. In the present research work the seismic behavior of asymmetric isolated concentric braced structures is investigated by nonlinear dynamic analyses using SAP2000 software. The behavior of fixed base and isolated asymmetric structures for different values of mass and stiffness eccentricities are studied and compared to each other. Based on the analysis results, base isolation has reduced story rotations and torsional amplifications in asymmetric structures in mass eccentric and stiffness eccentric structures. Also smaller torsional amplifications can be expected in stiffness eccentric structures than mass eccentric structures. Furthermore a new method for designing isolators in asymmetric structures is proposed which can reduce story rotations and torsional moments in such structures.