Axial behavior of steel tubes strengthened with outer steel tube–concrete jackets

Hollow steel tube (HST) columns are widely used in engineering structures due to their high load-carrying efficiency and excellent ductility. However, strengthening is often required when functional upgrading is needed or when load demands increase. This study investigates the axial compressive behavior and load–displacement response of HST columns strengthened using a composite strengthening technique consisting of an outer steel tube and a sandwiched concrete jacket (STSJC). Based on 19 axial compression tests, the effects of interlayer concrete strength, outer steel tube thickness, and steel strength on failure modes, load–displacement responses, strength indices, and ductility indices were systematically analyzed. The results indicate that the thickness of the outer steel tube has the most significant influence on performance enhancement, whereas increasing the concrete strength reduces ductility. A finite element model was established and validated against the experimental results, followed by a parametric analysis. The results reveal that the outer steel tube provides dominant confinement, inducing a triaxial stress state in the interlayer concrete and modifying the load-sharing mechanism, thereby significantly enhancing the post-peak load-carrying capacity. Finally, the load-carrying performance was evaluated based on both experimental and numerical results, confirming the applicability and significant engineering potential of the STSJC strengthening method.