Research on flexural behavior of square and circular cross-section timber beams strengthened with externally bonded and near-surface-mounted hybrid FRP plates

Hybrid Fiber Reinforced Polymer (HFRP) offers comprehensive performance and cost efficiency. However, existing studies primarily concentrated on timber beams strengthened with single-FRP material, the research on mechanical performance of square and circular timber beams strengthened with HFRP remains severely insufficient. In this study, HFRP plates are innovatively utilized to reinforce square and circular timber beams, and the strengthening effects are validated through experimental and theoretical research. Considering the effects of fiber hybrid ratios, reinforcement methods, and cross-sectional forms, the flexural behavior of timber beams strengthened with carbon-aramid-glass HFRP plates was investigated. Results show that the NSM-reinforced timber beams with circular cross-sections exhibit a significantly greater flexural improvement than square cross-sections. Specimens reinforced with HFRP-I plates (i.e., carbon-aramid-glass volume ratio of 2:1:1) display the most notable enhancement in ultimate bearing capacity, with an increase ranging from 39.1 % to 104.5%, while also providing a good improvement of initial stiffness and ductility. Considering its comprehensive improvement effects, the HFRP-I plate offers advantages in both flexural reinforcement and cost-effectiveness. The flexural capacity calculation model, suitable for NSM HFRP plates, was innovatively established for circular timber beams and further verified by numerical analysis. The deviation between the theoretical and experimental ultimate bearing capacities is within 16.5%, and within 10% compared to numerical results, demonstrating that the proposed formula is of certain value for engineering applications.