Comparative study on axially-loaded round-cornered square high-strength steel tube confined UHPC columns

This study investigates the axial capacity of round-cornered square high-strength steel tube confined UHPC (HSSTCU) columns with the load primarily borne by the inner concrete. 9 round-corned square HSSTCU specimens with different steel strength grades and thicknesses were tested under axial compression, and compared with the same cross-sectional concrete-filled steel tube (CFST). Additionally, a finite element model (FEM) and parametric analysis were employed to examine the axial compression behavior, load bearing analysis, material utilization, and stress distribution. The results show that the round-cornered HSSTCU columns usually exhibit buckling damage failure at both ends, and the three typical load-displacement curve types are closely linked to the confinement index. The HSSTCU columns generally show smaller ultimate bearing load compared to the CFST columns with same cross-sectional parameters, but more superior ductility with higher residual bearing capacity after peak-load and better deformation capacity. The confinement index shows almost linear correlations on axial bearing capacity and material efficiency, up to 84.7% and 141% improvements with increasing confinement index from 0.717 to 2.386, respectively. The confined stress on the core concrete diffuses from the centre of the section towards the corners, intensifying near the corners. To accurately predict the bearing capacity of HSSTCU columns, a simplified formula was derived and validated using experimental results, literature data, and FEM simulations.