Axial compressive behavior of circular UHPC filled high-strength steel tube (UFHST) short columns

: There is an increasing interest in using ultra-high performance concrete (UHPC) and high-strength steel in steel-concrete composite structures. This paper makes a contribution towards this by experimentally investigating the axial compressive behavior of circular UHPC filled high-strength steel tube (UFHST) short columns. A total of nine UFHST columns were tested under axial compression. The test parameter was the steel tube diameter-to-thickness ratio (13 to 32). The steel yield stress was 961 MPa (139.35 ksi), and the compressive strength of UHPC (without steel fibers) was 142 MPa (20.59 ksi). Results from the test indicated that the strength of the UFHST columns increased with decreasing steel tube diameter-to-thickness ratios. Two limit states were observed from the tests, i.e., shear failure of the concrete infill and local buckling of the steel tube. The governing limit states depend on the steel tube diameter-to-thickness ratio. Moreover, the strength enhancement and ductility of UFHST columns were discussed by using performance indices such as concrete contribution ratio (CCR), strength index (SI) and ductility index (DI). It was demonstrated that UFHST columns with thicker tube wall had better ductility, while leading to less strength enhancement or composite action.  (4) Here, The confinement factor ξ is the nominal strength ratio of steel tube to concrete infill; A s and A c are the cross-sectional areas of steel tube and concrete infill, respectively; f y and f c are the steel yielding stress and the cylinder concrete compressive strength, respectively; N u,filled represents the axial bearing capacity of UFHST columns; N u,hollow denotes the axial bearing capacity of hollow steel tubes measured in the test; 𝛿 u is the axial shortening at N u,filled ; 𝛿 95% is the axial shortening when the load falls to 95% N u,filled (95% N u,filled is used instead of 85% N u,filled , because the load drop after the peak load is less than 15% for some specimens in this paper, such