Behavior of square concrete column reinforced with GFRP bars under reversed cyclic eccentric loading: Experimental and analytical study

In recent decades, fiber-reinforced polymer (FRP) bars have emerged as an alternative to steel reinforcement due to their advantage, such as corrosion resistance, light weight, and high tensile strength. Despite these advantages, current design codes and guidelines still face significant limitations, particularly regarding the use of FRP bars in compressive members under earthquake conditions, where issues such as buckling and low ductility arise. This study investigates the behavior of square concrete columns subjected to reversed cyclic eccentric loading (RCEL). Eight square specimens, each with cross-sectional dimensions of 160 mm and a height of 1100 mm, were tested in two groups reinforced with either steel or GFRP bars. The columns were subjected to cyclic eccentricities of 0, 30, 60, and 90 mm (eccentricity-to-width ratio of 0, 0.2, 0.4, and 0.6, respectively). Experimental results were compared between the GFRP- and steel-reinforced groups, focusing on failure mode, load-carrying capacity, load-moment interaction, stiffness degradation, energy dissipation, and ductility index. The findings revealed that increasing the eccentricity-to-width ratio from 0 to 0.6 under RCEL conditions reduced load-carrying capacity in both reinforcement groups, with a more pronounced decrease in GFRP-reinforced columns (82% versus 75%). At zero eccentricity, GFRP bars demonstrated a compressive strength equal to only 20% of their tensile strength, contributing approximately 10% to the column’s overall load capacity, which is substantially lower than the 28.8% contribution observed for steel-reinforced columns. All steel-reinforced columns exhibited ductile behavior, whereas GFRP-reinforced columns demonstrated brittle, non-ductile behavior at low eccentricity-to-width ratios (0 to 0.2). Flexural stiffness decreased with increasing eccentricity in both reinforcement types, but the reduction was more pronounced in GFRP-reinforced columns (94% versus 85%). In the steel-reinforced columns, ductility and energy dissipation decreased as eccentricity increased, whereas in the GFRP-reinforced columns, both parameters increased. Furthermore, an analytical approach was adopted to validate the experimental findings and provide deeper insight into the structural behavior of columns under varying eccentricities.