Cyclic response characteristics of RC interior beam–column joints with unintended variations in material properties

Beam–column joints are critical components of reinforced concrete (RC) moment-resisting frames, where seismic performance depends on strength, stiffness degradation, energy dissipation, damping and ductility. In this study, the influence of unintended material-property variations on three full-scale RC interior beam–column joints subjected to quasi-static reversed cyclic loading under constant axial compression was investigated. The specimens tested were one control joint with a higher concrete compressive strength (fc = 28 MPa) (BCJ-28) and two joints with lower concrete strength (fc = 21 MPa) (BCJ-21 and BCJ-1.5). BCJ-1.5 had a higher column longitudinal reinforcement ratio of 1.5%. Material testing revealed lower than specified fc and higher than anticipated reinforcement yield strength, reducing the intended strong-column–weak-beam and joint shear strength ratios. All specimens exhibited stable hysteretic behaviour up to drift ratios of 2.4–2.9%, followed by progressive deterioration governed by joint shear cracking, bond deterioration, reinforcement slip and concrete crushing. The stiffness decreased by 30–45% after cracking, while the residual stiffness converged to approximately 19% of the initial value. Higher fc improved stiffness retention, lateral resistance and energy dissipation, whereas increased column reinforcement provided modest benefits. Existing damping–ductility relationships overestimated the measured damping owing to hysteretic pinching. The findings highlight the effects of deviations in material properties on the seismic performance of RC joints.