Seismic performance and mechanism analysis of recycled aggregate concrete beam-column joints employing ultra-high strength steel bars in columns

To explore the influence of employing ultra-high strength steel bars (UHSB) in columns on the seismic performance of recycled aggregate concrete (RAC) beam-column joints, four beam-column joints were tested under constant axial load and cyclic lateral load. The results revealed that the shear failure at the end of beams occurred in all beam-column joints. And the crack widths of beam-column joints employing UHSB in columns satisfied the AIJ guidelines, indicating satisfactory resilience. Due to the weaker bonding between UHSB and RAC, the strain distribution of UHSB was more uniform than that of HRB600 grade steel bars in columns. Consequently, the finite element models of beam-column joints were established by setting nonlinear springs, ensuring accurate representation of the hysteretic behavior. The numerical results also confirmed that beam-column joints employing UHSB in columns had a slighter damage degree of RAC, and the strain of stirrups was smaller in the joint core. It was indicated that the shear force transferred from UHSB to joint cores was relatively smaller. In conclusion, beam-column joints employing UHSB in columns had good seismic performance and resilience, which can be applied to the resilient structures.