Capacity of semi-rigid composite joints in accommodating column loss

In the scenario of column loss, joints would be subjected to bending moment combined with a tensile force due to large vertical displacement, where tensile force plays a critical role in resisting progressive collapse of structure by providing the catenary force. In order to study the effect of tensile force on the behavior of semi-rigid composite joints in structures in the case of column loss, six semi-rigid flush endplate connections tests were conducted, which include pure flexural tests, pure tensile tests and combined flexural and tensile tests. The experimental results indicate that under pure bending moment condition, the semi-rigid composite joint displays sufficient rotation capacity for forming “catenary action”. It is characteristic of the semi-rigid composite joint that its moment capacity decreases in a linear manner together with the increase of tensile load. Also, the capacity of semi-rigid composite joints is compared with that of the full-welded rigid composite joints. The tensile strength of high-strength bolts would not exert any effect on the initial stiffness of semi-rigid joint, but bring about decline in the moment resistance and tensile resistance of semi-rigid joint. The joints tend to fail at “catenary phase” under tensile force. A simplified M-N correlation formula for composite joint is proposed to describe the behavior of joint in structures under column loss. The finite element model with material failure criterion can predict the fracture of bolt in semi-rigid composite joint.