Experimental-numerical study of FRP-concrete-steel double-skin tubular columns under combined compression-torsion loads

Fiber-reinforced polymer (FRP)-concrete-steel double-skin tubular columns (DSTCs) offer an outstanding structural solution, particularly in applications demanding robust corrosion resistance, exceptional strength-to-weight performance under complex stress states. Although extensive research has been dedicated to elucidating the mechanical (compressive, flexural, seismic, and impact) behavior, studies addressing their response under compression-torsion remain scarce. This study evaluated the compression-torsion performance of circular DSTCs through experimental and numerical investigations. DSTCs were tested with varying axial load ratios and FRP confinement levels. Detailed analyses were conducted on failure modes, torque versus torsional angle relations, strain versus twist response, and ultimate strength. The results demonstrated that the torsional behavior of DSTCs under compression-torsion was significantly improved compared with that of pure torsion members due to the further activation of FRP tube confinement. Additionally, the torsional resistance of DSTCs initially exhibited enhancement with the axial load ratio but then decreased after reaching a peak magnitude. Subsequently, a finite element model was developed to further elucidate the effect of critical parameters and load distribution on normalized axial force-torque ( N / N u - T / T u ) interaction curves. The full-range torsional mechanism of DSTCs under compression-torsion can be categorized into three stages: elastic stage, plastic stage, and plastic hardening stage. Additionally, the N / N u - T / T u curves expanded outward as the torque contribution of sandwiched concrete increased. Finally, a prediction method for the N / N u - T / T u curves was formulated, offering valuable references for engineering applications.