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This study examines the roles of soil-structure interaction (SSI), higher modes, and damping in a base-isolated structure built on multiple layers of soil overlying a half space. Closed-form solutions for the entire system, including a superstructure, seismic isolator, and numerous soil layers overlying a half-space, were obtained. The formulations obtained in this study simply in terms of well-known frequencies and mechanical impedance ratios can explicitly interpret the dynamic behavior of a base-isolated structure interacting with multiple soil layers overlying a half-space. The key factors infl uencing the performance of the isolation system are the damping ratio of the isolator and the ratio of the natural frequency of the fi xed-base structure to that of the isolated structure by assuming that the superstructure moves as a rigid body. This study reveals that higher damping in the base isolator is unfavorable to higher mode responses that usually dominate the responses of the superstructure and that the damping mechanism plays an important role in transmitting energy in addition to absorbing energy. It is also concluded that it is possible to design a soft soil layer as an isolation system for isolating vibration energy.

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This paper is aimed at the performance evaluation of the multiple direction-optimized friction pendulum system (MDO-FPS) with multiple sliding interfaces on seismic mitigation through a series of shaking table tests of a full scale building isolated with MDO-FPS isolators. Experimental tests of a three-story steel building of 40 tons in total weight, 3m and 4.5m in length and width, respectively, in two horizontal directions and 9m in height, subjected to various types of earthquakes were carried out to investigate the efficiency of the isolators in reducing structural responses during earthquakes. Results obtained from the shaking table tests demonstrate that the roof accelerations, base shears, column shear forces have been remarkably reduced while compared to the responses of a traditionally designed structure.