Axial Compressive Behavior of Hybrid GFRP-Steel Reinforced Concrete Columns Confined by Spirals

Glass fiber-reinforced polymer (GFRP) composites offer a compelling solution to the durability degradation of reinforced concrete (RC) structures in harsh marine and de-icing environments. Hybridizing fiber-reinforced polymer (FRP) with conventional steel reinforcement synergizes the superior corrosion resistance of FRP with the high ductility of steel. However, the synergistic mechanisms of GFRP–steel hybrid reinforced columns confined by either GFRP or steel spiral stirrups under axial compression remain insufficiently quantified. This study systematically investigates the axial compressive performance of such structures through material testing, static axial compression tests on seven short column specimens, and advanced finite element (FE) modeling. The investigation focuses on the effects of the steel-to-GFRP area ratio and the spiral stirrup type. Experimental results reveal that spirally confined hybrid columns exhibit failure modes remarkably similar to conventional RC columns. The incorporation of GFRP bars significantly enhanced the ultimate load-bearing capacity, while the steel bars ensured the requisite ductility. Notably, a higher ultimate capacity was achieved at a steel-to-GFRP area ratio of 1:1 under steel spiral confinement, retaining a ductility index equivalent to 83.6% of a pure RC column. Furthermore, an ABAQUS-based FE model was developed and rigorously validated against experimental data, successfully capturing the failure progression and ultimate capacities across diverse parameters. Ultimately, based on the superposition principle, by quantifying the independent load-bearing contributions and synergistic interactions of the spalled concrete cover, confined core, and hybrid bars, this study derives a theoretical formula. The proposed model accurately predicts the axial compressive capacity of spirally confined hybrid columns, providing an analytical tool for resilient structural design.