Shear Strength of Exterior Beam–Column Joints Retrofitted with Externally Bonded Fiber-Reinforced Polymer Systems

Externally bonded fiber-reinforced polymer (FRP) systems provide a practical retrofit solution for enhancing the shear strength of poorly confined reinforced concrete (RC) beam–column joints, particularly when increasing a structure’s lateral stiffness to reduce global drift demands is not feasible. However, existing models for estimating the contribution of FRP strengthening systems to joint shear strength are often complex and poorly compatible with as-built joint shear strength models, thereby limiting their practical adoption. To address these gaps, using a database of 47 FRP-retrofitted and 24 companion control exterior RC beam–column joint specimens, this study develops a practical model to predict the shear strength of FRP-retrofitted beam–column joints. The model predicts the shear strength enhancement provided by FRP strengthening systems, defined as the ratio of the shear strength of retrofitted joints to that of their companion as-built joints, as a function of the principal tensile capacity of the strengthening system. The principal tensile capacity is defined, using the stirrup design analogy, as a function of the geometric and material properties and effective strain capacity of the FRP strengthening system. The effects of surface preparation and mechanical anchorage are incorporated using strength multipliers, calibrated using the developed database. The proposed model suggests that, on average, for every 0.1 MPa (14.5 psi) increase in the principal tensile strength provided by an FRP strengthening system, the joint shear strength of retrofitted exterior RC joints increases by 5%. Strength reduction factors for design are also proposed based on reliability analysis to achieve a target probability of exceedance. The formulation is explicitly developed to ensure seamless integration with state-of-the-art provisions for predicting the shear strength of as-built beam–column joints.