Effect of Rocking Foundation Input Motion on the Nonlinear Response Characteristics of Superstructure

Abstract eng:
In this study, it is tried to determine the response characteristics of the superstructures according to embedment depth and mechanical properties of the superstructure. A new lumped parameter model (LPM) is constructed depend on the impedances of embedded foundations having different embedment depth placed on the elastic half-space modeled by thin layers for the Poisson Ratio value equals to 0.42 and shear wave velocity value equals to 100 and 200 m/s to represent the soft soil conditions. After that non-linear earthquake response analyses by using the proposed LPM model are carried out considering with and without rocking foundation input motion (RFIM) to get effects of RFIM on the ductility demands of structures. Analyses are done for different embedment ratios (e/r) which equal to 0, 0.5, 1.0, and 2.0. It is concluded about the embedment depth that when ductility capacities under the fixed based condition (μfix) is small, the responses of buildings with natural periods more than 2 seconds except for the e/r = 2 under considering the rocking foundation input motion are smaller than those of the fixed-base model. It is estimated that the input ground motion is reduced because the slender building with spread foundation on soft ground is assumed and the rocking spring of the soil is relatively small and the natural period of the coupled system becomes long. However, if the embedment is deep (e/r = 2.0) considering the rocking input motion, it is seen that the response is increasing because of the small rocking stiffness. By increasing ductility factor values, the effect of RFIM becomes more important especially for high-rise buildings having deep embedment ratios. The reason of this phenomenon is considered that equivalent elastic stiffness of superstructure becomes softer for increasing values of ductility capacity, therefore inertial interaction becomes less important and the additional force coming from the rocking motion becomes more important on the response of superstructure. Therefore, RFIM should be considered for the collapse limiting design especially for the high rise building having deep embedded foundations for this case.

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