Effectiveness of isolation system on reduction of seismic responses of primary and secondary structures

Abstract

The effectiveness of the isolation system installed at the support of the secondary structure and at the base of the primary structure is evaluated for reducing seismic responses of the primary and the secondary structures under various earthquakes in this study. A constant average acceleration method which is one of the direct integration methods is used for numerical integration of the equation of motion. Matrix condensation technique reducing other degrees of freedom than the horizontal displacement components is utilized to compose the stiffness matrix for computational efficiency. Also the interaction effect between the primary and the secondary structures is considered in the analysis. Three analytical models used in this study to evaluate the effectiveness of the isolation system for the structures are as follows : fixed-base primary structure with fixed-support secondary structure, base-isolated primary structure with fixed-support secondary structure, and fixed-base primary structure with support-isolated secondary structure. The seismic responses of the structures are analyzed to identify their behavior due to the characteristics of input ground motion with various peak accelerations and frequency contents such as El Centro 1940, Taft 1952, Mexico 1985, Pacoima Dam 1971, and artificially generated earthquakes. The effective horizontal stiffness of the isolation system on reducing the seismic responses is so evaluated under several earthquakes that the structural acceleration and displacement become small simultaneously. The analysis results show that the structural responses of the baseisolated structure are less than those of the fixed base structure with from 53.3\% to 83.7\% and still much less than the ground motion. The responses of the fixed support secondary structure on the base isolated primary structure and the support-isolated secondary one on the fixed-base primary one are much less than those of the fixed support secondary one on the fixe...