Italian existing steel buildings are often characterized by joints with semi-rigid and partial-strength behaviour, which result in poor global performance, particularly under seismic loads. This issue is due to the wrong design assumption of fully rigid or ideally pinned restraints, widespread in the usual practice of the past and acquired by national technical codes, which neglected the assumption of semi-continuous behaviour of frames. This paper presents a numerical study on the evaluation of the effects of semi-rigid and partial-strength joints on the seismic behaviour of real steel structures. For this purpose, single-story steel buildings were considered, whose column bases and beam-to-column joints were found to be semi-rigid and partial strength through the component method application. Nonlinear dynamic analyses were performed on finite element models of buildings and fragility curves were constructed, taking into account different levels of joint strength, stiffness, and ductility, to assess the seismic vulnerability of the semi-rigid frames.