Static behavior of TOBs bolted endplate connection to strengthened HSST with fixed thread length

Five thread-anchored one-side bolts connected endplate connections to Hollow Square Steel Tube (TOBEC-HSSTs) with various strengthening measures were tested to investigate the structural performance under monotonic loads. The thread anchored length was 1.2 times the bolt diameter for all specimens. The connection behaviour were influenced by the endplate thicknesses and the strengthening measures (i.e. the internal polygonal tube, the internal polygonal tube with infilled concrete, and the cross-H section stiffener with infilled concrete). The failure patterns, moment-rotation curves, and strain response of joints were obtained through tests. It was found that (1) 1.2 times the bolt diameter was taken as the safe thread assembled length, which can avoid the premature failure of hole threads not before the fracture of other components under monotonic loads. (2) The presence of the infilled concrete can improve the rigidity and strength of the connection by 84% and 23%, respectively, compared to the reference joint without the infilled concrete, and the thickness of the column wall can be reduced when the concrete was infilled into hollow steel tube. (3) For the connections with thin endplates, the cross-H section stiffener exhibited 8.8% and 3.5% higher preliminary stiffness and yield bending moment than the inner polygonal tube, but the cross-H section stiffener had 14.3% lower rotation capacity. (4) Thickening endplates can change the failure modes of the connections to beam plastic hinge formation, enhance the preliminary stiffness and yield bending moment of joints by 2.4% similar to 11%, but decrease the rotation capacity of joints by 54% compared to thin endplates. Based on experimental results, the calculation methods of the bending moment bearing capacity for TOBEC-HSSTs considering the strengthening effect of internal stiffeners and infilled concrete under different failure patterns were proposed with errors no greater than 15%.