Torsional Strengthening of Base - Isolated Asymmetric Structures by Increasing the Flexible Edge Stiffness of Isolation System

In this paper, torsional behavior of asymmetric structures with fixed base and isolated base has been compared. Studied Structures are three-dimensional, three and eight story steel structures whose nonlinear time history analysis was conducted based on the records of El Centro earthquake (1940), Tabas earthquake (1978), and Bam earthquake (2003). Analysis results show the efficiency of seismic isolations to reduce the rotation of asymmetric structure stories. However, increasing the eccentricity reduces the effect of isolations on decreasing torsion. This effect is negligible for large eccentricities. By increasing the eccentricity and the period of the isolation system, displacements of isolations located on the flexible edge will also rise. Practical solutions to strengthen torsional of the base-isolated asymmetric structures have been proposed. To assess the effectiveness of proposed practical solutions on strengthening base-isolated asymmetric structures, four structural models (models A, B, C, D) are defined. For rehabilitation the new Low-rise asymmetric structures or retrofitting the existing Low-rise structures, it is necessary to reduce the torsion of isolation system by increasing the flexible edge stiffness (using model C). To strengthen the torsion of higher base-isolated asymmetric structures, model D (simultaneous increase in flexible edge stiffness of the superstructure system and isolation system) is suggested. In these structures, increasing the flexible edge stiffness of superstructure stories, conditional on approximate the center of stiffness of the base story (isolation system to the center of mass, leads to torsional strengthening and reduces the torsion of stories.