Effect of the change of foundation stiffness on the dynamic characteristics of base-isolated structure

Abstract

The safety of an isolated structure built on the soft soil ground under the action of carthquakes is of major concern because the current seismic design of isolated structures has not considered the motions of the foundations caused by the effects of the soil-isolated structure dynamic interaction (SISI). On this basis, a shaking-table test method for base-isolated structures on change of soil foundation stiffness was proposed and implemented. The foundation stiffness was controlled by the duration compression ratio and intensity of the input ground motion based on the influence of the increase in the excess pore water pressure ratio on the stifiness of the saturated sandy foundation. Meanwhile, the influence laws of foundation stiffness on the dynamic characteristics of base-isolated structures were summarized and analyzed. The results showed that the first-order natural frequency of base-isolated structures on change of foundation stiffness decreased with an increase in the relative stiffness ratio of the structure-soil foundation (R_S), while its damping ratio increased significantly. The seismic isolation efficiency of the seismic isolation layer and the amplification effect on the rotational angular acceleration of the pile cap were significantly weakened. Meanwhile, under the same conditions, for the soil foundation with relatively small stiffness, the amplitudes of the bending moment and horizontal lateral displacement in the middle and upper parts of the pile remarkably increase because of the efects of the ISI. The research results of this test provide a certain scientific basis and reference for the seismic design of base-isolated structures considering the SISI effects.

Keywords

base-isolated structure / change of foundation stiffness / dynamic characteristics of structure shaking-table test / soil-structure interaction

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