Character Analysis Of Varying Rigidity Of Rigid Pile Composite Foundation Under Seismic Load

The design of tall building foundation is a complex problem, Especially inthe areas of poor soil, it will cause uneven settlement of foundation, coupledwith the increasing of the height of the structure and the upper load, which canmake the design of foundation more difficult, foundation deformation occurs theinternal large and external small dish distribution, it makes the foundation andsuperstructure have a greater internal forces. To solve the problems, designerspropose the concept of balance and imbalance of varying rigidity of rigid pilecomposite foundation, considering the superposition stiffness of piles and soil,variable stiffness composite foundation can take full advantage of the nature ofsoil and piles, it reachs the bearing capacity and seismic requirements offoundation and reduces structure uneven settlement,which has been graduallyextended in the actual project.Based on ABAQUS finite element analysis software, the paper probes theoverall finite element dynamic analysis of rigid pile composite foundation-raftfoundation–structure, by using soil-structure dynamic interaction method,and analyzes comparatively of the seismic performance of variable stiffness ofrigid pile composite foundation and the seismic response of upper structure,including the following aspects: (1) The paper achieves the soil-upper structure interaction analysis by usingthe finite element method, the whole finite element model of rigid pilecomposite foundation-raft foundation-superstructure is established by usingABAQUS finite element software. and discusses the key technologies ofpile-soil-structure system dynamic interaction.(2)The paper calculates seismic modal of the integrated model by using theblock Lanczos method, and the results shows that the natural frequency willdecrease and cycle will increase with the decreasing of vertical supportingstiffness of raft on both sides.(3)The paper analyzes preliminary the principle of variable stiffness rigidpile composite foundation. Raft settlement presented dishing distributioncharacteristics under the weight load, it is expressed as a larger settlement in thecentre and a smaller settlement at the edge. It can effectively leveling raftfoundation settlement, reduce differential settlement and internal forces aftervariable rigidity design.(4)The paper analyzes the seismic dynamic response of variable rigidityrigid pile composite foundation by using time history analysis method. Themaximum bending moment and relative horizontal displacement of side piles aregreater than the middle of the pile before variable rigidity design. Moment andhorizontal displacement of long piles is much larger than short piles, there aremultiple peaks of moment in long pile, the maximum bending moment occurs atvariable stiffness after variable rigidity design. It is designed to enhance thestrength of the side piles and the piles at variable stiffness.(5)The paper analyzes the seismic dynamic response of foundation andsuperstructure by using time history analysis method before and after the designof variable rigidity of rigid pile composite foundation. Raft of differentialsettlement and internal forces will increase under earthquake, additionaldifferential settlement and internal forces of raft will increase with thedecreasing of vertical support stiffness on both sides. The upper structure of seismic response will increase with the decreasing of vertical support stiffnesson both sides. The paper summarizes the pros and cons of seismic performanceon variable stiffness rigid pile composite foundation, offfers some valuablespecific recommendations for actual project.