Abstarct: Abstract Moment-resisting frxames, as one of the primary seismic-resistant structural systems, heavily depend on the proper performance of beam-column joints, which are responsible for transferring bending moments and shear forces. The presence of cold joints in these critical connections, caused by interruptions in concrete casting, results in reduced bond strength, load-bearing capacity, stiffness, and ductility, ultimately weakening the structural performance under seismic loads. In this dissertation, the effects of cold joints in reinforced concrete structures were investigated through experimental testing and numerical simulations. In the experimental phase, three reinforced concrete frxames were designed baxsed on ACI 318-19 and scaled to a 2:3 ratio. These frxames were subjected to gravity and cyclic loading, including a control frxame without cold joints, a frxame with cold joints, and a frxame with cold joints strengthened using FRP sheets. In the numerical study, a beam-column connection with a cold joint was initially modeled in Abaqus and validated against experimental data. Subsequently, 20 numerical models of a single-span, single-story concrete frxame were analyzed under cyclic and gravity loading, incorporating proposed mitigation techniques for cold joint effects, including shear keys, additional reinforcement, and steel box reinforcements. The findings of this study demonstrate that cold joints significantly impair the seismic performance of reinforced concrete frxames. In specimens containing cold joints, initial cracks formed precisely at the joint location, and ultimate failure also occurred in the same region. In contrast, frxames without cold joints exhibited more uniform behavior, with fewer and shallower cracks. Additionally, the critical relative displacement required for the onset of significant cracking was greater than 2% in frxames without cold joints, whereas in frxames with cold joints, it was approximately 1%, indicating reduced deformation resistance under seismic loading. Hysteresis curves further revealed that the presence of cold joints led to an 18% reduction in lateral load-bearing capacity, a 25% decrease in ductility, and a 10% decline in cumulative energy dissipation, reflecting a weaker energy absorption and dissipation capacity under seismic excitations. Regarding strengthening techniques, the application of FRP sheets resulted in a 54% increase in lateral load-bearing capacity and a 33% improvement in energy dissipation capacity. Among the numerical models, it was observed that steel box reinforcements with a thickness of 5 mm placed at cold joint locations improved lateral load-bearing capacity by 30%, while shear keys measuring 200 × 200 mm enhanced the capacity by 15% compared to frxames with cold joints. These findings highlight the imselecting appropriate strengthening methods, demonstrating that proper reinforcement techniques can significantly enhance the performance of cold joint-affected structures and reduce their seismic vulnerability.