Experimental Study On Static Behavior Of Concreted Filled Circular Section X-Joints

Concrete filled steel tube structure is characteristic of good bearing capacity,ductility, easy construction, anti-seismic capability and fire-resistance because ofteam play of concrete and steel. So it has been used extensively in bridges, high-risebuilding and all kinds of structures. At the same time, tube filling concrete is a kindof ideal strengthening scheme. The joint as an iMPaortant part of the structure, incoMPalex stress, has been the focus of research. However, at present the study ofconcreted filled circular hollow section (CHS) joints were focused on the T, N, Kjoints, rarely involving X joint. There is no reasonable, reliable calculation methodand basis for X joint ultimate strength design.On the basis of Concrete filled CHS X-joints mode tests, the mechanical behavior,joint failure modes, joint ultimate capacities are presented in the paper. The effect ofconcrete in chord and brace on the ultimate capacity and stiffness of joints isdiscussed. Parametric formulae for the axial coMParession ultimate capacity aregiven.Firstly, experimental research on static behavior of concreted filled CHS X-joints isconducted. The tests of16specimens of CHS X-joints with chord concreted filledand16joints with brace concreted filled were carried out under axial coMParession(AC), in-plane bending (IPB) and out-of-plane bending (OPB) separately. The testprocedure and joints’ failure mechanism are presented. The effect of concrete inchord and brace on joints’ ultimate capacity and stiffness is discussed. Test resultsshowed that the principal failure mode of joints with brace concrete-inside was plasticfailure of the chord member, in which the IPB joints also failed by punching shearfailure of chord wall and the OPB joints by brace failure. And joints with chordconcrete-inside primarily failed by local buckling of the brace member, in that theIPB and OPB joints had brace failure as well. The calculated results of ultimatecapacity of CHS X-joints according to the current codes can’t correctly reflect theultimate bearing capacity of concreted filled CHS X-joints. CoMPaared to CHSjoint’s theory ultimate capacity, concrete in chord can greatly enhance the joint’s AC and OPB ultimate capacity, and the maximum iMParoving of the joint’s AC and OPBultimate capacity is respectively more than500percent and130percent, the IPBultimate capacity is increased by10~50percent. Concrete in brace has little effect onjoint’s AC and IPB ultimate capacity, and maximize OPB ultimate capacity by60percent. And concrete filled in chord has an obvious constraint function in for localdeformation of chord wall, significantly iMParove the radial stiffness as well asjoint’s coMParessive, in-plane and out-plane bending stiffness by coMPaaring withconcrete filled in brace. The limit deformation criteria used in judging ultimatebearing capacity of CHS X-joints do not apply to CHS X-joints with chord concretedfilled, but can be considered reference to CHS X-joints with brace concreted filled.Following the experimental and theoretical analysis studies on concrete filled CHSX-joints under axial coMParession, new strength equations are developed forconcrete filled CHS X-joint’s AC ultimate capacity. By statistical analysis for thejoints in the paper, it is proved that the given formulae are accurate and reliable.At last, the problems those require further studies are discussed.