Performance of square UHPC filled high-strength steel tubular columns under axial compression: Experiment and theory analysis

In this paper, 12 ultra-high performance concrete (UHPC) filled high-strength steel tubular (UHPCFST) short columns with square-section were subjected to axial compression tests. The effects of confinement factor (ξ) on the axial compression performance of UHPCFST were discussed, and the failure modes, axial load–strain curves, and strain responses were discussed. The results showed that the specimens mainly had two failure modes: shear failure and waist drum failure, and all specimens showed good ductility and high post-peak bearing capacity, indicating that high-strength steel tubes could provide higher restraint for UHPC and form a high-strength and high-performance steel-UHPC system. When ξ ≥ 1.42, the strength index (SI) increased significantly with the increase of ξ. When ξ ≤ 1.06, the SI was close to 1. With the increase of ξ, the ductility and toughness of the specimens were improved. Finally, considering the non-uniformity of the confinement of the core UHPC by the square-section steel tube, a calculation model of the ultimate compressive capacity of UHPCFST specimens with square-section was proposed. Compared with the seven classical models, the calculation model proposed in this paper was more accurate.