Research On Mechanical Behavior Of Concrete-filled Square Steel Tubular Middle-long Columns With Inner I-shaped CFRP Subjected To Bi-axial Eccentric Loading

The rapid development of modern construction industry has promoted the emergence of composite structure with high performance materials.It is a new structural form to place Ishaped CFRP profile in the core area of concrete-filled steel tube(CFST).At present,there is relatively little research on the mechanical properties of this new composite structure,and the related research bi-axial eccentric loading state is still in its infancy.Therefore,it has become a critical issue to fully understand the working mechanism and optimize the design of this new composite structure.In this paper,the working mechanism of concrete-filled square steel tubular(CFST)columns bi-axial eccentric loading with I-shaped CFRP profile is studied deeply by means of finite element simulation and experimental analysis,and the calculation formula of bearing capacity of the new composite structure bi-axial eccentric loading is deduced on the basis of parameter analysis.In this paper,the following research work has been carried out.: The experimental study of concrete-filled square steel tubular(CFST)columns with I-shaped CFRP profile bi-axial eccentric loading was carried out,the phenomena of the test were analyzed,the relationship curves of load deformation,load strain,load stress and load Poisson's ratio were obtained.the failure modes of the members were compared,and the effects of slenderness ratio and eccentricity on the ultimate bearing capacity and mechanical properties of the members were investigated.The bearing capacity and deformation condition obtained by finite element analysis were compared with those obtained by test,and the correctness of the finite element model was verified.On this basis,the force analysis and parameter analysis of the composite column in the whole process were carried out.,and the stress change state of the material,the failure form,the load of the material and the interaction between the materials were studied.The influence of parameter change on the bearing capacity and stiffness of composite columns was studied,and the variation law of mechanical properties of composite columns under different parameters was summarized.The finite element calculation data were sorted out,and the expression of bearing capacity of concrete-filled square steel tubular columns with built-in I-shaped CFRP was fitted,which was compared with the test results,and the applicability of the formula was discussed.The results show that the specimens of concrete-filled square steel tubular(CFST)columns with inner I-shaped CFRP profiles bi-axial eccentric loading are unstable at the middle section and brittle failure of CFRP profile occurs at the middle section,and the load lateral deflection curves of typical members can be divided into four stages according to the stress state of the members at different times: elastic-plastic section,elastic-plastic section,plastic strengthening section and falling section.The existence of CFRP profile further strengthens the restraint effect on concrete and improves the compressive capacity of concrete.The restraint effect of steel tube on concrete is mainly concentrated in the bending angle area of steel tube compression side,followed by the bending angle area of steel tube tension side,the ultimate bearing capacity and initial stiffness of composite columns increase with the increase of steel ratio,concrete strength and steel yield strength,and decrease with the increase of slenderness ratio and eccentricity.The ultimate bearing capacity and initial stiffness of composite columns increase with the increase of steel content,concrete strength and steel yield strength,and decrease with the increase of slenderness ratio and eccentricity.Based on the unified theory of CFST,the formula for calculating the bearing capacity of composite columns under biaxial bias is derived,and the applicability and accuracy of the formulas are verified.