Axial load–moment interaction diagram of full-scale hollow square columns reinforced with GFRP bars and ties

The present study investigated the structural behavior of hollow square concrete columns reinforced with glass fiber-reinforced polymer (GFRP) bars and ties under eccentric loading. Eight columns with a square cross-section of 400 × 400 mm and 2000 mm in height with a 150 mm diameter hollow core were cast and tested. Four columns were reinforced with steel stirrups and bars, and the other four were reinforced with GFRP bars and stirrups. All the columns had the same reinforcement ratio (1.6%). This study considered different test variables such as reinforcement type (steel and GFRP) and load eccentricity levels (10%, 20%, 40%, and 80%). In addition, the effect of hollowness was assessed through comparative analysis. The test findings show that the GFRP-reinforced hollow columns exhibited similar failure modes and crack patterns to their steel-reinforced counterparts across all eccentricities. In addition, increasing eccentricity progressively reduced the peak load for both GFRP- and steel-reinforced hollow square concrete columns. The failure of the specimens tested at low and moderate eccentricity levels (10%, 20%, and 40%) was compression-controlled. In contrast, the specimens loaded at high eccentricity (80%) experienced excessive lateral displacement, extensive cracking, and large GFRP-bar strains, indicating tension-controlled failure. Furthermore, the failure envelope of the GFRP-reinforced specimens was marginally smaller than that of their steel-reinforced counterparts. Overall, eccentricity had a greater effect on column behavior than reinforcement type. Compression bars in the hollow square columns contributed more to axial-load capacity than those in solid columns, with their contribution increasing with eccentricity. Lastly, a parametric study was conducted to evaluate the effects of reinforcement type and ratio, concrete compressive strength, hollowness percentage, and compression reinforcement. The results show that neglecting the compressive contribution of longitudinal GFRP led to an underestimation of the axial and flexural capacities of the hollow square concrete columns. In addition, increasing the hollowness ratio reduced the axial and flexural capacities of the GFRP-reinforced HSCCs under concentric and low eccentric loads, with diminishing effect at higher eccentricities. • Structural behavior of hollow square concrete columns reinforced with GFRP bars and ties. • Failure mechanisms of hollow square columns reinforced with GFRP bars and ties. • Developing the P-M interaction diagram of GFRP reinforced hollow square columns. • Assessing the contribution of GFRP bars on the capacity of hollow square columns.