Study On Dynamic Characteristics And Performance Of Base Isolated Structure On Soft Ground

Under the strong earthquake,a large number of buildings were seriously damaged,resulting in huge economic losses and casualties.The research shows that the bad foundation soil will have a non negligible impact on the seismic response of the structure.Under the seismic excitation,the foundation soil will be in a viscoelastic,plastic and nonlinear working state.This phenomenon will also have an impact on the seismic response law of the base isolated structure,and the dynamic interaction between soil foundation isolated structure can not be ignored.In order to study the variation law of seismic response of foundation isolation structure on soft foundation,based on the basic principle of soil foundation isolation structure dynamic interaction,shaking table model tests of two groups of pile foundation isolation structure on soft foundation represented by liquefiable foundation and variable stiffness foundation are designed and carried out in this paper;At the same time,based on the shaking table model test parameters,the three-dimensional finite element model of dynamic response of isolation structure on liquefiable foundation is established by using ABAQUS software platform.The numerical simulation results are compared with the experimental results to verify the reliability of the numerical model and calculation method;Finally,taking the actual project as the background,the three-dimensional full-scale refined finite element model of base isolated frame structure with small aspect ratio is established by using ABAQUS.Five sites are designed and extended,and the seismic behavior level division under soft foundation is given for the structure.The main research contents and conclusions of this paper are as follows:(1)Through the shaking table model test of pile foundation isolation structure on liquefiable foundation,the seismic response of pile group foundation and soil interaction system,the vibration characteristics of foundation and isolation layer and the seismic response law of isolation structure are systematically analyzed.After the foundation liquefaction,the dynamic characteristic parameters of the isolated structure are basically close to those of the non isolated structure on the non liquefied foundation,and the dynamic behavior of the whole structure system changes from the isolated structure to the non isolated structure;(2)Through the shaking table model test of pile foundation isolation structure on variable stiffness foundation,the influence law of foundation stiffness change on the dynamic characteristics of foundation isolation structure is revealed,and the approximate fitting formula of structure soil relative stiffness ratio and the first-order natural vibration frequency and damping ratio of isolation structure is given;(3)The three-dimensional finite element model of base isolated structure on liquefiable foundation is established by ABAQUS.The calculation results of model test and numerical simulation of liquefiable foundation structure are compared and analyzed,and the influence of SSI effect on the seismic response of base isolated structure is analyzed.The conclusions of numerical simulation and model test show that the base isolated structure designed based on rigid foundation assumption does not meet the requirements of seismic design after foundation liquefaction.(4)Based on the actual project,five different site categories are designed for the base isolation frame structure with small aspect ratio on soft soil site,and the three-dimensional finite element full-scale model is established to evaluate the damage degree of the structure.The inter story displacement angle and isolation layer displacement are selected as the quantitative indexes of seismic behavior level,and the inter story displacement angle,isolation layer displacement The prediction formulas of three kinds of indexes of floor maximum acceleration and the relative stiffness ratio of structure foundation are proposed,and the limit values of story displacement and isolation layer displacement of base isolation structure with small height width ratio considering site category under horizontal earthquake are proposed.