Slenderness effect on GFRP-RC columns with square spirals under concentric and eccentric loading: Experimental and analytical study

This study investigates the behavior of slender glass fiber-reinforced polymer-reinforced concrete (GFRP-RC) columns, specifically those reinforced with GFRP bars and square spirals. Nine column specimens were tested, with varying slenderness ratios of 20, 40, and 60, and load eccentricities of 0 %, 15 %, and 30 % of the section height. Under concentric loading, increased slenderness led to a 16–31 % decrease in column strength, primarily due to amplified bending moments and lateral deformations, worsened by the asymmetry of the square spirals. Under eccentric loading, spiral rupture was observed only at a slenderness ratio of 20 and 15 % eccentricity. Spirals provided critical stability for shorter columns under moderate loads, but their effectiveness diminished in more slender columns and under higher eccentricities. As slenderness increased, the spirals’ contribution to confinement and load-bearing capacity decreased; for instance, at 15 % eccentricity, increasing slenderness from 20 to 60 reduced load capacity by 13 % and 32 %, respectively. A transition in failure mode from compression-dominated to bending-dominated failure was observed as eccentricity increased. An analytical model, assuming sine-shaped deformation for single-curvature bending, was developed and validated against experimental data. Based on this, a slenderness limit of 14 for GFRP-RC columns with 2–4 % longitudinal reinforcement ratio was recommended. • Slender GFRP-RC columns with square spirals were tested, and an analytical model was developed and validated. • Tests included concentric and eccentric loading (15%, 30%) with slenderness ratios of 20, 40, and 60. • A slenderness limit of 14 is recommended for GFRP-RC columns with longitudinal reinforcement ratios ranging from 2 % to 4 %.