Experimental Investigations On Concrete-filled Steel Tubular X-joints Under Axial Compression

Concrete-filled steel tubular joint(CFST) is formed by hollow section branch with concrete-filled main pipe by welding, the concrete in three-phase compression under the restriction of steel tube, giving full play to the compression performance, concrete can effectively prevent local buckling of main pipe, steel-concrete composite structure has become a development direction of current structure system.Based on the tests X-joints of grounting, nonlinear finite element analysis on the joints is made in order to study failure mode, ultimate capacities and influence of geometric parameters on the ultimate capacities of the X-joints.Firstly, experimental research on X-joints of grounting under axial force and is conducted. The static loading test of 22 square nodes and 22 circular nodes are studied. Results of these tests show that calculation ultimate capacity of joints adopting National Regulation can’t be well used to calculate the ultimate of branch pipes under axial compression; the joint strength and initial stiffness benefit from the increase of the β ratio and decrease of the χ ratio; when the rectangle joints with large hollow ratio(χ), ultimate capacity of SS type node is the highest value comparing with SC type node and SSR type node; when the circular joints with large hollow ratio(χ), ultimate capacity of CC type node is the highest value comparing with CS type node and CSR type node.Secondly, Finite Element(FE) validation researches on capacity of X-joints of grounting are performed. Failure modes and load-displacement curves are compared between test and FE, reasonable calculation model and accurate constitutive relationship of materials are selected in orde to precisely simulate experimental value. Comparison between Abaqus and experimental results proves validation of FE method available based on the FE models.Afterwards, FE parameter analysis on 124 rectangle joints of grounting and 124 circular joints of grounting is performed in order to study the effects of brace to chord width ratio(β), hollow ratio(χ), and internal member to chord thickness ratio, cross sections of chord and strength grade of concrete on the structural behaviour of grouting X-joints under axial compression.Based on the results of finite element analysis, each variable impact on ultimate capacitie of node rule is summarized. Design formulas are developed for the joints through one-variable nonlinear regression analyses. The equations are proved to be reliable by comparing with the results of parameter analysis.Finally, the defects of the article are summarized and the direction for future is pointed out.