Study On Ground Motion Input For Seismic Design Of Tall Building Structures With Deep Pile Foundation

For a long time, the rigid ground assumption has been used for earthquake response analysis in the seismic design in which only superstructure is considered in the model and the free field surface input of ground motion is adopted. However, as for the case of tall building with deep pile foundation, the structural input would be highly different from free field ground motion, so the conventional method of surface ground motion input based on rigid ground assumption needs further and detailed examination.Centered on the problem above, this paper first contains a review about the research of pile-soil-superstructure interaction, then a verification of parameters and model is made by using ANSYS LS-DYNA, we made a free field earthquake response analysis and a comparison between conventional input and integral finite element method results of structural earthquake response, based on response spectrum analysis we also made a estimation about the error of conventional input.Below are the conclusions:The natural vibration period of the pile-soil-superstructure integral model is longer than that of the superstructure model based on rigid ground assumption, the gap enlarges with the softness of the field soil and diminishes with the height of the building. The natural vibration period increment in the integral model on single layer soil with similar thickness and equivalent shear wave velocity is smaller than that in the model on multi-layer soil whose rigidity increases with depth.Earthquake response, inner force and deformation from conventional input method are higher than those from integral bedrock input method; the gap enlarges with the softness of the field soil.The distribution shape of the error with storey becomes more complex with the increasing of the storey number.There are mainly 4 influencing factors for the error of the conventional input method: the spectrum characteristic of the bedrock wave input, the spectrum characteristic of the free field ground earthquake response, the natural frequency of first a few order especially primary natural frequency of the conventional and the integral finite element model.The conventional input method is reliable and feasible in most cases, but at the lower part of the building, the story drift, the moment at the upper end of the columns, the moment and shear force at the end of the beams located at middle and side spans lack enough reliability, the same case happen the axial force in all middle columns and side columns, which demands special attention and requires adjustment on the results from conventional input method.At the end of the paper we made a summary about the deficiency of present research and perspectives of subsequent study.