A novel FRP–steel–UHPC composite column system for marine structures: Axial compressive behavior and analytical model

To address the requirements for high‐performance and green marine engineering structures, this paper presents a novel FRP–steel–UHPC composite tube confined seawater sea‐sand concrete column (FSU–SSC). Axial compression tests were performed on 12 specimens to examine the effects of FRP layers, FRP type, steel tube thickness, and UHPC shell thickness on the mechanical behavior. The test results demonstrate that the FSU–SSC exhibits excellent synergistic interaction among components. The UHPC shell effectively restrains local buckling of the steel tube, optimizes interfacial stress transfer, and produces more uniform cracking, thereby significantly improving ductility and confinement efficiency. Under equivalent hoop stiffness, GFRP provides superior load capacity and deformability relative to CFRP. ANOVA results reveal that FRP layers dominate the ultimate bearing capacity with a contribution ratio of 81.29%, followed by steel tube thickness (9.36%) and UHPC thickness (7.26%). Finally, an ANOVA regression model and a mechanics‐based analytical model are proposed for the performance evaluation and preliminary design of FSU–SSC composite columns. The predictions from the two models are compared against experimental results, and satisfactory accuracy is achieved. The findings provide design references for the engineering application of such composite columns in marine infrastructure.