The unpredictable nature of earthquake direction and intensity causes complex responses and significantly undermines the integrity of affected structures. Structural damage assessment following an earthquake is essential for guiding necessary repairs and retrofitting. Previously, the evaluation of concrete damage has relied on visual inspection, which is subjective, hazardous, and prone to human error. In response, the author collected and analyzed 11 surface crack pattern images from 7 RC beam-column joints with diverse reinforcement details. The study aims to investigate how reinforcement detailing, such as stirrups, longitudinal reinforcement in beams and columns, affects the damage index estimation of RC joints under seismic loading. This article develops and validates a quantitative method for assessing the energy-based damage index (EDI) in reinforced concrete beam-column joints by employing fractal analysis of crack patterns alongside modified mathematical models. The results reveal that in addition to energy dissipation and crack patterns, several other factors derived from reinforcement detailing, such as concrete-reinforcement bond stress and stress-strain distribution, also determine the damage index estimation. Future research should concentrate on seismically detailed structural elements and consider adjacent damaged areas to ensure that the damage index works for post-earthquake assessments of existing buildings and as a tool for designing new resilient structures.