In this paper, a numerical study on the response of multi-storey steel moment resisting frames subjected to seismic actions is presented. Two real cases of study have been considered for the purpose. The examined buildings are characterized by beam-to-column joints and column bases with semi-rigid and partial strength behaviour, although they were improperly assumed as rigid and full-strength in the original design process of the frames they belong. In order to appraise how this wrong approach influenced the building safety, finite element models have been setup by modelling the joints according to both the design assumptions and including the real stiffness and strength. Then, nonlinear dynamic analyses were performed and fragility curves were extracted for different limit states, considering the record-to-record variability as main source of uncertainty.