Innovative Strengthening of Shear and Torsion Performance in Box-Section Concrete Beams with Near-Surface Mounted GFRP Rods

The study of reinforced concrete (RC) hollow box-section beams reinforced with glass fiber-reinforced polymer (GFRP) bars and stirrups under combined shear and torsional stresses is still in its early stages, with the mechanisms of torsional failure not yet well-defined. This research examines the impact of adding external transverse strengthening to box-section beams, specifically utilizing GFRP rope as near-surface-mounted (NSM) external stirrups to enhance their structural performance. A total of nine RC box-section beams, each 2200 mm long, 400 mm wide, and 600 mm high, were constructed and tested under simply supported conditions. The study evaluated three variables: GFRP bar diameter, inclination angle, and spacing. Nonlinear finite element analysis was conducted using ANSYS to compare the behavior of these beams, internally reinforced with GFRP bars and stirrups, against beams externally strengthened with GFRP ropes as NSM stirrups. Three-dimensional finite element models were developed, incorporating a smeared cracking approach for the concrete and 3D elements. The analysis showed strong alignment between numerical results and experimental data through both linear and nonlinear phases up to failure, confirming the reliability of the model for future investigations. The findings demonstrated that integrating GFRP shear reinforcement, whether internally or externally, increased the load-carrying capacity by up to 45% relative to the control beam, depending on the variables studied