Mitigation of torsional responses in asymmetric base-isolated structures using an optimal distribution of isolators in base story

Abstract

The present research proposed an optimal design procedure of isolators in the base story of the asymmetric base-isolated structures to mitigate torsional responses. Instead of the distribution of isolators in the base story proportional to the load-bearing surface (case A), two optimal distributions of isolators in the base story, cases B and C, are proposed for this purpose. Case B gives an optimal distribution of isolators in the base story considering the arithmetic mean of the optimal horizontal effective stiffness of isolators for considered earthquakes and case C represents a weighted average of them according to the peak ground acceleration (PGA) of the considered earthquakes. A three-story reinforced concrete structure is adopted for numerical studies. Different eccentricities, e = 10, 20, 30%, in three directions are considered in the structural models. Nonlinear time-history analyses of the structure are carried out using OpenSees software. A particle swarm optimization (PSO) algorithm is utilized to find the optimal distribution of the isolators at the base story in such a way that the maximum torsion of floors is minimized. The simulation results show that case B, in comparison with those given for case A, results in a significant reduction in the maximum torsion of floors, especially in small eccentricities, at the cost of an increase in the maximum floor displacement responses. For most eccentricities, case C significantly reduces the maximum torsion of floors, while the increment of maximum floor displacement in the case has not exceeded 10%. Hence, the proposed approach in case C performs better than case B in the mitigation of torsional responses of asymmetric base-isolated structures. It is also found that by increasing the eccentricities of floors, the reduction average of the maximum torsion of floors is reduced.