Behavior Of Diaphragm-through Connection Between Concrete-Filled Square Tubular Column And Steel Beam

Diaphragm-through connection between concrete-filled square steel tubular column and steel beam has been recommended in technical specification for structures with concrete-filled rectangular steel tube members (CECS159:2004). However, because studies on diaphragm-through joints are very limited in our country, loading bearing capacity and seismic behavior are not well understood and calculating theory and design method are not well developed. Therefore, it is significant for theory and practice to study mechanical behavior of this type of joints.Joint types of concrete-filled square steel tubular column with steel beam are summarized and their characteristics are analyzed. On the basis of analysis of problems existing in details of diaphragm-through joint, two new types of joints—circular fillet type and fillet slope type are proposed to improve the connecting behavior of diaphragm with steel beam flange.In order to investigate the connecting behavior of concrete-filled square steel tubular column with steel beam tensile flange, sixteen cruciform specimens of diaphragm-through joint are studied by static tensile test. Stress transfer mechanism in the connecting zone is analyzed and comparing analysis is executed on the measured value and the calculated value of load bearing capacity. Research results indicate that joints of circular fillet type and fillet slope type have high load-bearing capacity and good ductility. Main factors that influence the load-bearing capability of joint are diameter of concrete cast hole, diaphragm thickness, and width-to-thickness ratio of the tube. Load-bearing capability calculated from the theoretical formula presented in specification of Japan is conservative for the specimens with the tube filled in concrete, but overly estimates the strength of the specimens without concrete.Finite element models involving geometric large deformation, contact problems between the steel and the concrete and materials nonlinear are presented and behavior of diaphragm-through joints under tensile loading is analyzed. The yield mechanism is studied, and on the basis of parameter analysis, formula for calculating tensile capacity is proposed, which take account of the force that transferred to the diaphragm plate and tube. Comparison analysis indicates that calculating results agree well with experimental results.Seven full-scale cruciform specimens are tested under low-reversed cyclic loading. The failure process and feature of each specimen are described and load-carrying capacity, ductility, energy dissipation, rigidity degeneration are analyzed. The results show that the specimens of diaphragm-through joint have stable hysteretic characteristic and enough energy dissipation capacity. Details of the steel beam flange with the diaphragm have obvious effect on the ductility, energy dissipation and rigidity degeneration, and the seismic performance of joint of circular fillet type and fillet slope type are better than those reinforced by taper plate welded to the beam flanges and diaphragm. Since all the specimens fail in the beam, diaphragm thickness, diameter of concrete cast hole and width-to-thickness ratio of the tube have less influence on joint seismic behavior, however, the concrete filled in the tube can greatly reduce shearing deformation of joint panel zone and improve the seismic performance of diaphragm-through joint.Nonlinear finite element analyses of diaphragm-through joint are conducted under low-reversed cyclic loading. Stress distribution in the joint panel zone and the damage mechanism is accurately simulated and results of finite element analyses agree well with results of experiment. Finally, some suggestions for improving joint design are presented on the basis of parameter analysis of the hysteretic behavior of diaphragm-through joint.