Understanding the impact of spiral reinforcement on GFRP-RC beams under combined shear and torsion loading

To date, the combined shear and torsional behavior of reinforced concrete (RC) beams reinforced with continuous glass-fiber reinforced polymer (GFRP) spiral stirrups has not been thoroughly investigated. Accordingly, this paper investigates the behavior of RC beams reinforced with GFRP bars and rectangular continuous spirals under combined shear and torsion. The experimental program involved testing six RC beams, each measuring 3000 mm in length, 200 mm in width, and 400 mm in depth. The test parameters focused on the configuration of transverse reinforcement (GFRP spirals versus GFRP tie stirrups) and varying transverse reinforcement ratios. Four specimens were reinforced with GFRP spirals at different reinforcement ratios to assess the effect of transverse reinforcement on shear and torsional capacities. One specimen was reinforced with GFRP tie stirrups to investigate the influence of stirrup configuration, and one specimen was without web reinforcement (control) to evaluate the impact of concrete strength on the capacities. Experimental results showed that beams with GFRP spirals or tie stirrups failed due to the progressive widening of diagonal tension cracks and subsequent rupture of the GFRP reinforcement at bent portions, while the control beam failed due to concrete splitting. The results demonstrated that reducing the spacing of GFRP spirals significantly enhanced shear and torsional capacities. The experimental findings were compared with existing design codes and a newly proposed model based on the space truss analogy.