This paper investigates the seismic impact of modeling knee-beam staircases in an existing six-story reinforced concrete (RC) building designed for gravity loads based on 1970s Italian code. Nonlinear dynamic analyses are used to generate fragility curves for different configurations of the case study building, including bare frames and frames with centrally or corner-located staircases, with and without infills. Distributed plasticity models with fiber sections are used for the RC elements and equivalent strut-and-tie models are used for the infills. The results show that neglecting the knee-beam staircases may significantly affect the predicted seismic response and vulnerability, emphasizing the importance of their inclusion in seismic performance assessments.