Beam-column joints are critical components influencing the seismic performance of reinforced concrete (RC) frames, especially in regions with moderate to high seismic activity. A significant proportion of Algeria’s building stock was constructed before 1980, prior to the adoption of the Algerian seismic regulations, resulting in inadequate seismic detailing and making these joints especially vulnerable to earthquakes. This study examines the impact of initial cycles of lateral load on the seismic performance of non-conforming external reinforced concrete beam-column joints, focusing on early stiffness degradation and energy dissipation processes. A three-dimensional nonlinear finite element model was created in ANSYS and compared with experimental data from the literature, confirming the accuracy of the numerical method. The originality of this work resides in the infrequently studied correlation between stiffness loss and energy dissipation, illustrated by a normalized dual-curve representation. This approach helps the identification of a pivotal intersection point that signifies the threshold of post-elastic behavior. A hybrid index (DHybrid) is proposed, incorporating these two parameters into a singular synthetic indicator for the preliminary evaluation of seismic performance. A detailed parametric analysis was performed to investigate the impact of critical variables such as concrete compressive strength, beam longitudinal reinforcement ratio, and joint core confinement. The findings indicate that the early loading cycles are crucial in initiating stiffness degradation and cumulative energy dissipation, determining the beginning of nonlinear behavior, and substantially affecting the seismic response of nonconforming joints.
