Axial Compression Behavior of Concrete Columns Strengthened with UHPC-Filled Steel Tubes

To enhance the load-bearing capacity of conventional reinforced concrete (RC) columns and address the issue of longitudinal reinforcement buckling, this study proposes a novel composite column strengthened with small-diameter ultra-high-performance concrete-filled steel tubes (UHPCFST), in which the UHPCFST members replace the traditional longitudinal reinforcement. First, the mechanical behavior of UHPCFST was experimentally investigated. Results show that its stress–strain curve exhibits steel-like elastoplastic or strain-hardening characteristics after yielding. Subsequently, the axial compressive performance of the proposed column was studied through numerical simulation, with emphasis on the failure process, load–displacement response, and contribution of each constituent material at different loading stages. By comparing the longitudinal stress in concrete and the strain development in longitudinal reinforcement, steel tubes, and stirrups between conventional RC columns and the composite column, and by systematically varying parameters such as the steel ratio and the number of steel tubes, the influence of these parameters on the axial performance of the composite column was revealed. The results indicate that replacing longitudinal reinforcement with UHPCFST significantly improves the column performance. Compared to a conventional RC column with an equivalent reinforcement ratio, the proposed composite column exhibits an approximately 10% higher peak load capacity, a 182% increase in peak displacement, and a distinct biphasic response characterized by a double-peak pattern in its load–displacement curve. The first peak is contributed jointly by the surrounding concrete and the UHPCFST, while the second peak is mainly provided by the UHPCFST skeleton. This study offers a new perspective for improving the seismic resistance and load-carrying capacity of RC columns.