Experimental Study On Axially Compressive Behavior Of CFRP Confined Concrete-filled Thin-walled Steel Tube Short Column

Concrete filled steel tube（CFST）composite structure is widely used in various projects because of its excellent mechanical properties,such as long-span buildings,high-rise buildings,municipal engineering and bridge engineering.With the research and application of high-strength concrete and high-strength steel,the research and application of thin-walled steel tube composite structure has attracted extensive attention.However,because the thin-walled steel tube is easy to buckling under the axial force,the stress state of the thin-walled steel tube in the thin-walled CFST composite column is different,which has a great influence on the performance of the composite column.In this paper,the mechanical properties,mechanical mechanisms and influencing factors of two types of thin-walled CFST composite columns are studied by means of experiments and numerical simulations.One is the thin-walled concrete-filled steel tubular composite column in which the thin-walled steel tube and the concrete bear vertical force at the same time,and the other is the confined concrete with thin-walled steel tube（CCST）composite column in which the thin-walled steel tube does not directly bear the vertical force.On the basis of the analysis of mechanism and influencing factors,in order to improve the mechanical properties of different forms of thin-walled CFST composite columns,the thin-walled CFST composite columns were reinforced by CFRP outsourcing,and their properties,failure forms and mechanisms were studied,the influencing factors were analyzed,and the calculation and analysis method of bearing capacity were proposed.（1）Through the axial compression test of thin-walled concrete-filled steel tubular composite columns with two types of composite columns,the effects of section form and stress state of steel tube on the mechanical properties of composite columns are explored.The experimental results show that the steel tube in the circular section column has better restraint effect on the core concrete and excellent mechanical properties,and the bearing capacity and deformation performance of the CCST column are higher than that of the CFST column;The restraint effect of square section composite column is poor,the bearing capacity and ductility are poor,and the mechanical performance of rectangular section composite column is the worst.Among them,the bearing capacity of CCST column is lower than that of CFST column,but the deformation performance is better than that of CFST column.（2）Through the axial compression experiment of CFRP confined thin-walled CFST composite columns and thin-walled CCST composite columns,the effects of the number of CFRP confined layers and section forms on the mechanical properties of composite columns are explored.The experimental analysis shows that with the increase of the number of CFRP confined layers,the bearing capacity and deformation capacity of composite columns increase,and the improvement of the bearing capacity of CFST columns is greater than that of CCST composite columns;The effect of CFRP restraint on the mechanical properties of composite columns with different sections is different,that is,the bearing capacity of composite columns with circular section is improved the most,and the bearing capacity of composite columns with rectangular section is improved the least.The improvement effect of CFRP on the mechanical properties of composite columns with different sections is different.The improvement effect of CFRP on circular section specimens is the best,while the improvement effect of rectangular section specimens is the worst.（3）The finite element numerical simulation analysis method is used to numerically simulate and analyze the axial compression performance of the thin-walled concrete-filled steel tubular composite columns with and without CFRP constraints.The simulation results show that the bearing capacity and ductility of circular section columns are the best,and the mechanical properties of CFST columns are worse than that of CCST columns;The mechanical properties of square and rectangular section specimens are poor,among which the rectangular section specimens are the worst,and the bearing capacity of CCST composite columns is lower than that of CFST composite columns,but the ductility is better.After CFRP restraint,the bearing capacity and ductility of composite columns are significantly improved.With the increase of the number of CFRP confined layers,the bearing capacity of composite columns is improved,and the improvement of the bearing capacity of CFST composite columns is greater than that of CCST composite columns.（4）Based on the experimental analysis and numerical analysis,the bearing capacity calculation formulas of two types of composite columns,thin-walled concrete-filled steel tubular composite columns and CFRP confined thin-walled concrete-filled steel tubular composite columns,are derived.The comparative analysis shows that the test results are in good agreement with the calculation results,which can be used to calculate the bearing capacity of members.