Axial Compression Behavior Of Round-ended Concrete Filled Steel Tubular Short Columns With Binding Bars

CFST composite members make full use of the combination advantages of steel tube and concrete materials.The steel tube can provide a good restraining effect on the core concrete.At the same time,the construction is convenient,economical and reasonable.The section form of CFST composite members is also evolving to the shape of circle,rectangle,multi cavity and special shape to meet the different use requirements.At present,the research on CFST at home and abroad is mainly focused on circular and rectangular sections,but less on round-ended sections.At present,there are few experimental and theoretical studies on the axial compression of Round-ended concrete-filled steel tubular short columns with binding bars(RCFST),and the existing studies show that the two ends of round-ended concrete-filled steel tubular are strongly restrained,and the middle straight section is weakly restrained,while in other cross-section forms of concrete-filled steel tubular,the binding bars can better improve and improve the mechanical properties of concrete-filled steel tubular In order to improve the interface bond performance of the component,the binding bars is added to the round-ended concrete-filled steel tube.This paper studies the working mechanism of RCFST with binding bars under axial compression.Finally,the recommended formula of bearing capacity under axial compression is given and its rationality is verified.The specific work is as follows:(1)The axial compression tests of 11 RCFST short columns with binding bars and 3 ordinary RCFST short columns were carried out to study the influence of axial spacing of binding bars,diameter of binding bars,number of binding bar rows and section steel ratio on the axial compression performance of RCFST short columns.It can be seen from the results that with the increase of section steel ratio,pull rod diameter and pull rod row number,the bearing capacity and ductility of the members are improved,and the ultimate strain of RCFST short column with binding bars decreases with the increase of height width ratio,which can be increased by 41.6%at most compared with the ordinary members.When the ordinary RCFST short column without binding bars is damaged,Near the upper and lower end of the formation of the drum deformation,resulting in shear failure.With the increase of the setting density of the binding bar,the buckling deformation of the member is more and more close to the midspan section,and the distribution of the buckling deformation is more uniform,and the internal concrete and the binding bar have shear failure.(2)Compared with other concrete filled steel tube sections,the concrete filled steel tube with round-ended with binding bars is more complicated in mechanical properties.The RCFST tube with binding bars is divided into two sections,i.e.rectangular section and semicircle section finally;the recommended formula of bearing capacity of RCFST with binding bars is put forward,and the rationality of the formula is verified.Using the finite element software,57 model components are analyzed.The results show that with the increase of section steel ratio,bar diameter and bar density,the bearing capacity and ductility of the component are improved,while the increase of concrete strength can improve the bearing capacity of the component but lead to the decrease of ductility.(3)Based on the experimental study of circular End-shaped concrete-filled steel tubular short columns with binding bars under axial compression,the round-ended section is divided into rectangular area in the middle and semicircle area at both ends by using the superposition principle.Through regression analysis,a calculation method suitable for the bearing capacity of RCFST with binding bars is proposed.Under the condition that the whole process of finite element refinement simulation analysis is in good agreement with the experimental results,on this basis,the parameter expansion analysis is carried out,and the main parameters that affect the bearing capacity of RCFST with binding bars are further studied,and the calculation formula obtained by regression test is verified.Based on the double shear unified strength theory and limit equilibrium theory,the bearing capacity calculation program based on the fiber model method is compiled in Python language.Finally,compared with American ACI(2005),European EC4(2004)and technical code for concrete filled steel tubular structures(gb50936-2014),the design suggestions of RCFST short columns with binding bars are given.