Seismic behavior of TOBs bolted endplate connection to strengthened CFST column

When using the Thread-anchored One-side Bolts (TOBs) which are a type of one-side bolts anchored through the tube wall hole threads instead of the nuts, the conventional Concrete Filled square Steel Tube (CFST) with the limited wall thickness fails to provide the sufficient thread anchored length to resist the tensile force of TOBs. Meanwhile, the joint region of the relatively thin tube wall is prone to deform outward to a large degree, which is crucial to the safety of the whole structure. To solve the above issues, in this work an experimental investigation on TOBs bolted Endplate Connection to strengthened CFST (TOBEC-CFST) under cyclic loads is conducted by putting a stiffener inside the CFST. Six specimens were designed to study the effects of following parameters: (a) stiffener type (the inner polygonal tube stiffener or the cross-H section stiffener); (b) thread anchored length (18 mm and 24 mm); and (c) bolt diameter (M16, M20 and M22). The experimental results revealed that the hole thread fracture can be avoided by increasing the stiffener flange thickness to extend the hole thread length and the inner stiffener can improve the behavior of the panel zone of the connection. Compared to the inner polygonal tube stiffener, the cross-H section stiffener can effectively enhance the overall integrity of the connection because of the presence of the web transferring force directly to the opposite tube wall, contributing to a higher bearing capacity and energy dissipation capacity of the joints. Increasing the bolt diameter can improve the bearing capacity and energy dissipation capacity of TOBEC-CFSTs, but the degree of the improvement does not increase linearly with bolt diameter. Based on test results, to enhance the overall mechanical behavior of TOBEC-CFSTs in seismic zones, the cross-H section stiffener is recommended provided for sufficient thread length. •Thread-anchored One-side Bolts bolted Endplate Connections under cyclic loads were tested.•Inner polygonal tube and cross-H section stiffener were used to strengthen Concrete Filled square Steel Tube (CFST).•Internal strengthening component effectively prevents hole thread failure of connection.•Effects of strengthened component type, thread anchored length and bolt diameter were presented.