Torsion in concrete beams reinforced with GFRP spirals

Current FRP guidelines do not have enough evidence on the torsional performance of beams reinforced with glass fiber-reinforced polymer (GFRP) reinforcement due to the lack of research and complexity of the problem. A review of the past work was presented in the state-of-the-art section to stand on mile-stone prospective considering the variety and development of GFRP manufacturing. This study examined the rectangular spirals and rectilinear stirrups to reinforce concrete beams in the transverse direction under pure torsion loading. The test parameters included four levels of transverse reinforcement ratio and the type of transverse reinforcement in terms of rectilinear stirrups vs. spirals. The test results demonstrated that rectangular GFRP spirals provided superior tensile resistance required to develop high ultimate torsional moments after cracking and hence can be effectively used. The failure of the tested beams was attributed to the rupture of GFRP rectangular spirals at bent portions. The test results of the spiraled specimens were discussed and compared with predictions from a developed analytical model and based on CSA equations. Two values of failure plane angle and stress levels were considered. The 45-degree inclination angle provided safe predictions with an average experimental-to-practiced result of 1.16 compared to 0.99 using the 37.5-degree. Design equations were proposed and validated for the ACI code for a probable future incorporation.