Behavior of eccentrically-compressed square reinforced UHPC-inner CFST composite short columns: Experimental investigation and theoretical analysis

Reinforced ultra-high performance concrete (UHPC) encased concrete-filled steel tubular (RU-CFST) columns, combining peripheral UHPC and an inner CFST, were previously proposed to enhance the ductility of UHPC columns under compression. This approach has been validated through various axial compression tests. This study aims to investigate the behaviors of eccentrically-compressed square RU-CFST short columns. An eccentric loading test program involving seven RU-CFST short columns was carried out. Test variables included the eccentricity ratio, steel tube diameter, longitudinal reinforcement ratio, and stirrup spacing. The mechanical failure, composite, and cracking behaviors of RU-CFST short columns, featuring both compression-controlled and tensile-controlled failure modes, were thoroughly analyzed. Test results showed that increasing the eccentricity ratio shifted the failure mode from compression-controlled to balanced and then to tensile-controlled. The RU-CFST column with a compression-controlled failure mode showed a sudden load drop at peak but retained significant residual bearing capacity. Increasing the longitudinal reinforcement ratio and steel tube diameter enhanced the ultimate bearing capacity of RU-CFST columns, while reducing stirrup spacing improved ductility. Based on the test and theoretical analysis results, a simplified calculation method that accounts for UHPC tensile strength was developed to predict the ultimate bearing capacity of square RU-CFST short columns under compression-controlled and tension-controlled failures. • The effects of parameters on the performances of RU-CFST columns were investigated. • The failure mechanisms of eccentrically-compressed RU-CFST column were analyzed. • Calculation method estimating RU-CFST column under eccentric loading was developed.