Effects of square spiral pitch and tie overlap on the behavior of GFRP-reinforced concrete columns under concentric and eccentric loads

The structural behavior of glass fiber-reinforced polymer (GFRP)-reinforced concrete (RC) columns is studied to evaluate the effects of square spirals and ties under concentric and eccentric loading. An experimental program was conducted on fifteen specimens with varied configurations, including spiral pitches, tie spacing and overlap lengths, and reinforcement ratio, to evaluate their impact on confinement, load-bearing capacity, and deformability. Results indicated that GFRP bars contributed between 7 % and 15 % of load-bearing capacity, while steel reinforcement contributed approximately 30 %. Under concentric loading, a denser spiral pitch increased the peak load by about 6 %, though spiral pitch had minimal effect on peak loads under eccentric loading. In both loading types, columns with denser spirals exhibited a more gradual post-peak load decline. Whereas under eccentric loading, larger spiral pitches led to a more brittle failure pattern, often resulting in the crushing of compressive bars. In specimens with ties, increasing the tie overlap length by 40 % above the code minimum shifted the failure mode from sudden, crack-free failure to gradual post-peak failure. An analytical model was developed and validated using data from the experiment in this study and from the literature. The model accurately predicted load-displacement and strain behavior across various eccentricities.