Theoretical Analysis of Behavior of FRP-Strengthened Reinforced Concrete Members Subjected to Combined Torsion and Biaxial Bending

Abstract

Reinforced concrete (RC) structural members, such as bridge girders and columns, flanged and spandrel beams, and edge frame beams, may be subjected to combined actions in the general case of loading. Despite extensive research on the behavior of RC members strengthened with fiber-reinforced polymer (FRP) composites under pure actions, questions have been raised about the strengthened RC members subjected to combined actions, particularly actions including torsion. As a contribution to this demanding field of research, the first theoretical model capable of predicting the full load–deformation response of RC members strengthened with conventional FRP-strengthening configurations, and subjected to torsion combined with biaxial bending, was developed based on the principles of the well-known combined-actions softened truss model (CA-STM). The proposed model considered the influence of various parameters, such as FRP stiffness, on the constitutive laws of materials. In addition, the computational operations and duration were significantly reduced, and the numerical stability of the solution procedure was enhanced by extending the refined efficient solution algorithm for FRP-strengthened RC members for the first time. Furthermore, the efficiency and contribution of the FRP system and the governing failure mode was accurately predicted. Three categories of equations, including equilibrium equations, compatibility equations, and the constitutive relationships of materials, were incorporated into the proposed model, then a system of 16 nonlinear equations, containing 16 unknown variables, was solved to obtain the full load–deformation response of an FRP-strengthened member subjected to combined actions, including torsion. A detailed comparison conducted between the test results of a database with various strengthening patterns collected from the literature and the predictions of the proposed model showed that the model was able to estimate the ultimate strength, with a mean value and coefficient of variation of 1.11% and 13%, respectively, in addition to predicting the overall load–displacement response well. This research highlights the importance of considering the combined effects of bending and torsion on the capacity of fiber-reinforced polymer (FRP)-strengthened reinforced concrete (RC) members, particularly in members where torsion is critical. The addition of flexural moment in certain situations can lead to a reduction in the torsional capacity of the member’s cross-section. While in specific situations, where the strengthening system is asymmetrically applied or the internal steel arrangement of the section is asymmetrical, the torsional capacity can even increase compared to the case when subjected to pure torsion. Therefore, the arrangement of the strengthening system plays a crucial role in determining the failure mode and strength of the member. The model proposed in this study, along with its resulting findings, will provide valuable assistance to engineers in determining the optimal percentage of an FRP system and its pattern in these situations. This will allow them to achieve the highest possible effectiveness of the strengthening system in cases involving combined actions, including torsion and flexure.