Seismic performance of reinforced concrete beam-column joints with diagonal bars wrapped by steel tubes: experimental, numerical and analytical study

Diagonal bars can be used as joint reinforcement to address the congestion problem, yet a certain number of stirrups are still required for joint confinement. In this study, diagonal bars wrapped by steel tubes are proposed as joint reinforcement to fully replace the stirrups. Three interior beam-column joint specimens reinforced by conventional stirrups, debonded diagonal bars wrapped with plastic or steel tubes, are tested. The results indicate that replacing the plastic tubes with steel tubes can prevent shear failure in the joint without stirrups, increase the ductility, and reduce the joint distortion. However, it decreases the deformation capacity and increases the peak beam curvature. Compared to the stirrup-reinforced specimen, the specimen reinforced by the steel tube-wrapped diagonal bars has higher loading capacity and lower peak beam curvature without compromising the energy dissipation, stiffness, and joint distortion. Results from finite element analysis indicate that increasing the column axial load ratio to 0.4 leads to higher early stiffness and slower degradation of loading capacity. It should be noted that a sufficient area of steel tubes is required to avoid joint shear failure and sudden loss of loading capacity. The anchorage segments are recommended to be installed at the same depth as the beam longitudinal reinforcement to maximize their efficiency. Finally, an analytical model for predicting the failure mode and loading capacity is also developed, and a good agreement between predicted and tested results is obtained.