Relocating plastic hinges in reinforced concrete beam-column joints by mechanically anchored diagonal bars

The implementation of strong column-weak beam concept in reinforced concrete (RC) moment-resisting frames results in the formation of plastic hinges at the beam ends, which could lead to the strain penetration of beam longitudinal reinforcement into beam-column joint (BCJ) cores. This causes high shear forces inside joint cores, bonding deterioration of longitudinal reinforcement and consequently jeopardizes the seismic behaviour of BCJs. Plastic hinge relocation by mechanically anchored diagonal bars has been proposed to mitigate such potential damage to BCJ cores. This paper investigates the effect of relocating distance of plastic hinge on the seismic performance of BCJs with mechanically anchored diagonal bars. Five interior BCJ specimens including one control specimen reinforced with conventional stirrups and four others reinforced with mechanically anchored diagonal bars were tested under cyclic load. Test results indicate that the plastic hinges can be generally relocated by different distances to the anchorage ends of diagonal bars. Increasing the relocating distance can enhance the loading capacity and stiffness of BCJs, but also increase the potential of joint shear failure. The ductility, energy dissipation and joint shear deformation of BCJs are first improved then reduced as the relocating distance increases. Although the strain penetration of beam longitudinal reinforcement into the joint cores can be gradually alleviated as the plastic hinges are moved farther away from the joints, the induced bending moments at the beam ends are amplified and subsequently increase the joint shear forces. Moreover, increasing the length of anchorage segments increases the forces sustained by the mechanically anchored diagonal bars. Therefore, a lower bound of the relocating distance of plastic hinges is recommended to take advantage of the benefits from reduced strain penetration, while an upper limit needs to be specified to avoid joint shear failure. In addition, an analytical model is developed to predict the failure modes and loading capacities of BCJs with the plastic hinges relocated by different distances. A satisfactory agreement between the prediction and the experimental results is obtained.