Study on the Hysteretic Behavior of Post-Tensioned Unbonded Prestressed Concrete Beam–Column Joints with Two-Stage Energy Dissipation

To address the shortcomings of rapid stiffness degradation and single energy dissipation mechanisms in conventional post-tensioned unbonded prestressed concrete beam–column joints, this paper proposes a beam–column joint configuration that integrates a “friction-bending” two-stage energy dissipation mechanism. By adopting a low-prestress strategy, the joint enhances the energy dissipation ratio. Furthermore, energy dissipation bars with a secondary activation function in the energy dissipater form a stable third stiffness, thereby improving the hysteretic performance under large structural deformations. To clarify the influence of key design parameters on the hysteretic performance of the joint, this study established a refined finite element model using OpenSees3.3.0. A systematic parametric analysis was subsequently conducted, covering the number of prestressing tendons, initial prestress force, friction force, diameter of energy dissipation bars, and activation displacement ratio. The results indicate that the number of prestressing tendons only regulates the second stiffness and bearing capacity of the joint, with no significant effect on the energy dissipation capacity. The initial prestress force has limited influence on the joint stiffness and absolute energy dissipation; reducing the prestress can increase the equivalent viscous damping ratio by approximately 32%. The friction force is linearly and positively correlated with the activation force, enabling independent control of the joint’s energy dissipation capacity. Increasing the friction force can enhance the energy dissipation per cycle by 21.7%, without affecting the stiffness at each stage. The third stiffness is dominated by the compression-bearing mechanism of the energy dissipation bars. Enhancing the third stiffness can increase the peak loading capacity of the joint by 28.7%, while slightly improving the ultimate energy dissipation capacity. The research finding can provide a theoretical basis for the collaborative optimization design that achieves “low prestress for efficiency enhancement, friction dissipation for guaranteed energy absorption, and third stiffness for safety assurance.”