Study On The Residual Resistance Of Concrete-filled Steel Tube After Impact

Concrete-filled steel tube columns are widely used in various engineering constructions because of their high bearing capacity and good seismic performance.In practical engineering,the concrete filled steel tube column will be damaged after the impact load,resulting in the bearing capacity of the concrete filled steel tube column decline.Therefore,it is of great significance to study the residual resistance of concrete-filled steel tube column after impact.In this paper,the axial compression test is carried out on the concrete filled steel tube column after impact,and the residual resistance of the concrete filled steel tube column after impact is studied by using ABAQUS software as follows:(1)The axial compression tests and mechanical properties of concrete-filled steel tube columns under impact were carried out.The effects of axial compression ratio,impact energy and steel tube wall thickness on the bearing capacity,initial stiffness coefficient and displacement ductility coefficient of concrete-filled steel tube under axial compression load are considered.The results show that when the axial compression ratio increases from 0 to 0.2,the bearing capacity and displacement ductility coefficient of concrete-filled steel tube increase with the increase of axial compression ratio by 20.05%and 15.66%,respectively.When the axial compression ratio increases from 0.2 to 0.4,the bearing capacity and displacement ductility coefficients of concrete-filled steel tubular columns begin to decrease by 3.56%and 5.21%,respectively,and the initial stiffness coefficients increase with the increase of the axial compression ratio.Steel tube wall thickness can improve the mechanical properties of concrete-filled steel tube column.The bearing capacity,displacement ductility coefficient and initial stiffness coefficient of concrete-filled steel tube column with steel tube wall thickness of7mm are increased by 35.46%,22.89%and 7.84%,respectively,compared with those of concrete-filled steel tube column with steel tube wall thickness of 5mm.With the increase of impact energy,the axial compressive properties of concrete-filled steel tube columns decrease.(2)The finite element model of concrete-filled steel tubular column after impact is established and verified.The finite element model of concrete-filled steel tube column under axial compression load is established and compared with the test results obtained from axial compression test to ensure the accuracy and reliability of the finite element model.By adjusting the grid size of concrete-filled steel tube column,the influence of different element sizes on the calculation accuracy of concrete-filled steel tube column was studied,and the final mesh size was divided into 15mm.The results show that:The average and standard values of Pu/Pus are1.12 and 0.02,respectively.These results indicate that the finite element numerical simulation model established in this paper is reasonable and accurate,and can be used to further analyze the bearing capacity and deformation of concrete-filled steel tubular columns under different impact parameters.(3)The influence parameters of the residual resistance of concrete-filled steel tube column after impact were analyzed.Based on the reasonable finite element model,the mechanical properties of concrete-filled steel tube column under axial load were analyzed,and the effects of concrete strength grade,impact mass,impact height,steel tube strength grade,impact position,boundary conditions and length diameter ratio on the residual resistance of concrete-filled steel tube column after impact were studied.The results show that the strength grade of concrete has little effect on the residual resistance.When the strength grade of concrete increases from C50 to C65,the residual resistance only increases by 1.44%.When the impact mass increases from330kg to 630kg,the residual resistance decreases by 10.64%.When the impact height increases from 4m to 7m,the residual resistance decreases by 202.56k N.The residual resistance of concrete-filled steel tube column using Q420 is 39.54%higher than that of Q235.The closer the impact position is to the mid-span,the smaller the residual resistance is.As the degree of freedom of boundary conditions increases,the residual resistance decreases.(4)Study the residual resistance of concrete-filled steel tube column after impact.Combined with the calculation formulas of the bearing capacity of concrete-filled steel tube members in various countries,the bearing capacity values of the unimpacted concrete-filled steel tube columns calculated by national codes are compared with the bearing capacity values of the unimpacted concrete-filled steel tube columns calculated by finite element method.Finally,the suggested formula of the bearing capacity of concrete-filled steel tube members of British Standards Institute BS5400 is selected.Based on this formula,a calculation formula for the residual resistance of concrete-filled steel tubular column after impact is proposed.According to this formula,the residual resistance N_d of the studied concrete-filled steel tubular column after impact is in good agreement with the calculated value N,the error is within 15%,and the mean value of N/N_d is 0.001,and the variance of N/N_d is 0.001.The coefficient of variation of N/Nd is0.031,indicating that the formula of residual resistance of concrete-filled steel tube column after impact is reasonable.