Research On Axial Compression Performance Of X-joints With Square Chord And Circular Braces

The joints with circular hollow section(CHS)braces and square hollow section(SHS)chord(CHS-SHS)is composed of square steel tube and circular steel tube by welding.Because of its convenient connection,beautiful appearance and good mechanical performance,it is widely used in practical engineering.But at present,the research on it is less,so the research on the joints with circular hollow section(CHS)braces and square hollow section(SHS)chord(CHS-SHS)is very urgent.This paper mainly studies the X-joints with circular hollow section(CHS)braces and square hollow section(SHS)chord(CHS-SHS),including empty CHS-SHS X-joints,the X-joints with concrete-filled CHS braces and SHS chord(CFCHS-SHS),as well as the X-joints with CHS braces and concrete-filled SHS chord(CHS-CFSHS)The axial compression performance of three types of CHS-SHS X-joints was studied by applying axial compression to the brace direction.A total of 60 specimens were designed,including 20 empty CHS-SHS X-joints,20 CFCHS-SHS X-joints and 20CHS-CFSHS X-joints.The effect of two influential factors was studied,including the brace diameter to chord width ratio(β=d/b),chord width to double wall thickness ratio(γ=b/2T)on the axial compression behavior of three types of CHS-SHS X-joints.In addition,the failure modes,load-displacement curves and stress distribution curves of three types of joints are analyzed.Finally,the ultimate bearing capacity,initial stiffness and ductility coefficient of three types of CHS-SHS X-joints were compared.The results show that the initial stiffness and ultimate bearing capacity of empty CHS-SHS X-joints increase with the increase of β value and the decrease of γ value,and the ductility coefficient increases slightly with the increase of β value,while γ value has little effect on the ductility coefficient.The ultimate strength and initial stiffness of CFCHS-SHS X-joints increase with the increase of β value and the decrease of γ value,and the ductility coefficient increases slightly with the increase of β value,while the influence of γ value is relatively small.Also,the ultimate bearing capacity and initial stiffness of CFCHS-SHS X-joints increase with the increase of β value,the influence ofγ value on the ultimate bearing capacity and initial stiffness is very small,and the influence of β and γ value on the ductility coefficient is also very small.By comparing with the experimental results,the ultimate bearing capacity,initial stiffness and ductility coefficient of empty CHS-SHS X-joints and CFCHS-SHS X-joints are similar,while the ultimate strength and initial stiffness of CHS-CFSHS X-joints are greater than those of empty CHS-SHS X-joints and CFCHS-SHS X-joints,and the ductility coefficient is slightly than that of empty CHS-SHS X-joints and CFCHS-SHS X-joints.Finally,based on the analysis of the test results,the design formulas of empty CHS-SHS X-joints and CFCHS-SHS X-joints are proposed by using the yield line model,and the rationality and accuracy of the formulas are verified.In addition,it is found that the total cross-section yield strength of brace is very accurate for predicting the ultimate strength of CHS-CFSHS X-joints under brace axial compression.