Analysis and strengthening of RC exterior beam-column joints considering column axial force variation

In this study, an analytical method is developed to predict the behavior of deficient reinforced concrete beam-column joints after strengthening. Deficient joint is a joint without shear reinforcement. In this analytical method, different types of failure are estimated using principal stress-strain relationships in the joint and by determining the final capacity of beam and column. The use of relations between principal stresses in the joint accounts for the variation of the axial stress of the column in equations and joint capacities. The equations are obtained by considering the strength and deformation limit states in the joint, and a numerical method is employed to solve them. The proposed analytical method provides useful information regarding the strengthened joint capacity as well as how different types of strengthened methods work. This information can be used to select the strengthening method of the deficient joints. In the study, the calculation sequence of beam-column joints is illustrated using column axial force versus flexural capacity relationship of the joint members, employed as a criterion to determine the method and the amount of strengthening materials. In the experimental part of the study, 3 specimens with axial load variation and 3 specimens with constant axial load were conducted. The specimens were strengthened using either near-surface mounted steel reinforcement (NSM) or externally bonded reinforcement (EBR) with CFRP sheets. The experimental results indicated that the behavior of joints was significantly affected by considering the column axial force. Additionally, strengthening the joints by NSM method with steel bars rather than EBR method led to more ductile behavior. Finally, the proposed analytical method was compared with the results of the experimental specimens from other studies and 6 experimental specimens conducted in this study. Comparison of the experimental results with the values of the analytical method demonstrated that the proposed analytical method could adequately predict the behavior of beam-column joints.