Axial compression performance and optimum design of round-cornered square CFST with high-strength materials

Abstract

This study investigates the axial compressive behaviour and optimum design of round-cornered square concrete-filled steel tubes (CFST) with ultra-high performance concrete (UHPC) and high-strength steel (abbreviated as RSUCFST). 11 RSUCFST specimens are designed with various steel strength grades, steel thicknesses and cross-sectional sizes for axial compressive experiments. The results show that the RSUCFST stub columns typically experience shear, mid-height buckling, multiple local buckling and weld failures. The load-displacement curves are quantitated into 5 types based on different confinement indices. Utilizing appropriately thicker and higher-strength steel tubes could likely better match the inner UHPC in designing RSUCFST stub columns, while simply enlarging the cross-sectional size considerably improves the ultimate bearing capacity but weakens the ductility of RSUCFST. The optimum steel ratio (αsc) of 0.09–0.4, width-to-thickness ratio (B/t) less than 45(235/fy)0.5 and confinement index (ξ) of 1.0–2.5 are recommended for RSUCFST stub columns by considering axial compressive behaviour, material efficiency and current code suggestions. Lastly, an analytical predicting model of axial bearing capacity of RSUCFST stub column is proposed according to the confinement in round-cornered square cross-section and well validated by experimental results.