Empirical study on reduction in input motions to improve the seismic performance based design of buildings

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Nonlinear soil-structure interaction effects under strong ground motions are studied to consider in the seismic design.First, effectiveness of soil-structure interaction in earthquake response reduction of buildings is studied by earthquake response analyses using representative ground motion record of recent domestic or foreign big earthquakes. In the analyses, simple sway-rocking models are used and non-linearity of buildings is considered. It is assumed that buildings are supported by direct foundation on the elastic uniform soft soil. From the analyses, it is pointed out that the damage reduction effects by soil-structure interaction greatly depend on the ground motion characteristics, number of stories and seismic performance of buildings.Next, cyclic loading tests for pile foundation in dry sand are conducted to investigate the nonlinearity in the pile group effects in large pile-head-displacement region. The experimental parameters are the number of piles, loading rate, relative density of soil and alignment of pile group. The major findings obtained from tests are summarized as follows: 1) the soil around piles deformed significantly even at small amplitude, 2) shear force-displacement relationship, resultantly secant stiffness and equivalent damping factor, at pile head shows significant nonlinearity even in very small displacement range, 3) shear force at pile head hold almost constant value after the soil get failed, 4) variation in pile group efficiency with increase in pile-head displacement is not so large, 5) the ratio of the shear force at pile head to the total horizontal load is affected by the yield of piles and the failure of the soil as well as the number of piles, pile location, soil density, and loading direction.