Study On Seismic Behavior Of RC Frame Substructure With Cast-in-place Floor On Independent Foundation

In the structural design of multi-storey and high-rise concrete frame structures in China,it is usually assumed that the bottom column is fixed,and then the reaction force of the superstructure is applied to the underground foundation for foundation design.This design concept does not consider the interaction between the foundation and the superstructure.It is found that the failure of the frame column at the bottom of the structure is more serious than that of the frame beam,and a large number of plastic hinges appear at the end of the frame column,and the failure of the top of the bottom column is significantly more serious than that of the bottom column.Meanwhile,the actual yield mechanism of the structure is not consistent with the yield mechanism of "strong column and weak beam" required in the seismic design.There are many factors affecting the yield mechanism of frame structure,among which the important factor is the constraint degree of foundation to the bottom column.Winkler foundation model is an ideal elastic foundation model.Based on Winkler foundation,the rubber-wood combination simulation is designed.On this basis,the reinforced concrete frame substructure test with cast-in-place floor slab is carried out to evaluate the seismic performance and yield mechanism of the structure,and the finite element simulation is used to verify and supplement the test results.In this paper,an integral cast-in-place frame substructure with slab is fabricated.The rubber-wood composite is used to simulate the elastic foundation.Low cyclic cyclic load tests are carried out to study the seismic performance of the structure considering the soil-structure interaction.The experimental results show that the displacement Angle between layers of the structure ranges from 1/21 to 1/18 when the specimen is damaged.The ductility coefficient of the whole structure and the interlayer displacement is between 4.05 and 4.56.The stiffness corresponding to the overall yield point is about 35% of the cracking stiffness,and the stiffness corresponding to the peak point is about 16% of the cracking stiffness.Under the peak load,the positive and negative rotation angles of the middle column foundation are1/96 and 1/104,respectively,and the positive and negative rotation angles of the side column foundation are 1/100 and 1/118,respectively.Compared with the test results of the plane frame,the yield point and peak point of the frame structure with plate increase by 30.5% and 16.2%,respectively.The yield mechanism of the structure has changed from "beam hinge mechanism" to "partial beam hinge mechanism",and the difference between the vertical loading displacement of the side column and middle column foundation and the base reaction force value is reduced.The finite element model was established by ABAQUS software,comparing the results of the finite element analysis with the experiment,the numerical simulation results are in good agreement with the hysteretic curve and skeleton curve in the actual test,the initial stiffness of specimens is similar,and the simulated yield load and ultimate load have little difference from the test value,the relative error is less than 10%,and there is a certain error between the simulated and experimental values of yield displacements and ultimate displacements,the maximum error is 25.5 percent.Under the condition of two kinds of support,Soil-structure interaction model（SSI model）and rigid foundation assumption model（RFA model）differences began to appear in the initial loading stage,initial stiffness model of RFA is 3.1 times of SSI model,yield load and peak load were similar,the relative error of 12.8% and 0.8%respectively,but the feature point displacement value deviation is large,the yield displacement and peak displacement relative error of 166.0% and 50% respectively.The ABAQUS software was used to analyze the parameters of the model,and the main parameters included: foundation bed coefficient,reinforcement ratio of column longitudinal reinforcement,column axial compression ratio and cast-in-place floor width,so as to study the differences of hysteretic curve,skeleton curve,yield mechanism and failure mode under different parameters.As the coefficient of soil foundation decreases,the initial stiffness of the structure decreases,the yield load increases,and the yield displacement increases at the initial loading stage.At the later loading stage,the yield displacement increases,and the yield load is basically unchanged.The change of reinforcement ratio of longitudinal bars has a great effect on the hysteretic curve and skeleton curve of the structure.With the increase of reinforcement ratio of longitudinal bars,the yield load,yield displacement,peak load and peak displacement of the frame increase in different degrees.With the increase of the axial compression ratio,the initial stiffness decreases slightly,and the yield displacement increases within 5%.At the middle stage of loading,the hysteretic curve and the skeleton curve gradually coincide.At the late stage of loading,with the increase of the axial compression ratio,the structural load increases slightly,and the increase range is within 5%.As the width of the floor increases,the initial stiffness increases slightly at the initial stage of loading,and the yield displacement and yield load both increase within 10%.At the later stage of loading,the structure load increases with the width of the floor,and the increase range is 33% ～ 50%.