This paper reports on a collaborative activity developed as part of the DPC-ReLUIS Research Project, year 2017. It aims at comparing the results obtained by considering alternative options in the definition of the nonlinear FEM model employed in pushover analyses for seismic assessment of existing RC frames. Specifically, the RC frame structure of the De Gasperi-Battaglia school building located in Norcia, Italy, is considered as a relevant case-study. This structure has been designed in the ‘60s of the past century according to the seismic code of the time and, hence, without taking into account the principles of Capacity Design. Although the building was actually retrofitted before the 2016 Central Italian earthquake, in this paper its original configuration has been considered. The nonlinear behaviour of the frame structure has been modelled by both following alternative approaches and employing different analysis codes. Therefore, this paper proposes an overview about how different the simulation output can be as a result of different modelling and analysis choices. In doing that, the work can be relevant to practitioners, as they may be warned about the consequences of those choices in terms of seismic vulnerability evaluation.