Experimental and numerical investigation of the pure torsional behavior of GFRP-reinforced concrete beams with non-uniform longitudinal reinforcement

In engineering practice, longitudinal reinforcement in concrete members is often non-uniformly distributed, which may influence torsional behavior and the reliability of existing design methods. However, the torsional response of glass fiber-reinforced polymer–reinforced concrete (GFRP–RC) members with non-uniform longitudinal reinforcement remains insufficiently understood. This study combines experimental testing and finite element modeling to investigate this behavior. Experimentally, four full-scale GFRP–RC beams with non-uniform longitudinal reinforcement (top-to-bottom reinforcement ratio, R = 0.14), comprising two specimens without stirrups and two with stirrups, were tested under pure torsion. The experimentally observed failure modes were similar to those typically reported for uniformly reinforced members, including concrete splitting, stirrup rupture, and concrete strut crushing. However, the non-uniform reinforcement arrangement resulted in differences of up to 28.80% in diagonal crack inclinations between the top and bottom faces, accompanied by longitudinal strain variations of up to 53.49% and top-to-bottom stirrup strain differences of up to 66.37%. To investigate a broader range of reinforcement ratios ( R ), a numerical parametric study was conducted for R = 0.14, 0.50, and 1.00. The numerical results indicate that increasing reinforcement non-uniformity (i.e., decreasing R ) can reduce the torsional capacity and post-cracking stiffness by up to 9.29% and 32.10%, respectively. Finally, based on the available database, existing design provisions were evaluated for FRP–RC members with both uniform and non-uniform longitudinal reinforcement arrangements.