In recent decades, more and more attention has been paid to studying the mechanisms of resistance to the progressive collapse of various types of buildings and structures. Wherein, one of the most common types of structural systems of multi-storey residential and public buildings is a reinforced concrete frame or reinforced concrete frame-braced system. The scientific literature contains the following mechanisms of resistance of such structural systems to progressive collapse: arch, catenary or Vierendeel truss. However, currently there is not strict correspondence between the type of structural system, the nature of the accidental impact and the resistance mechanisms to progressive collapse. A similar situation exists in the field of developing effective ways to ensure the structural safety of such frames in case of accidental impacts. Therefore, a multi-storey reinforced concrete frame braced structural system with prestressed girders was selected as the object of study in this work. The purpose of the study is thus to establish the resistance mechanism of the reinforced concrete frame with prestressed girders at the failure of an outer column on the ground floor of the building. For the purpose of this study, using the decomposition method, the substructure in the form of two-story two-span reinforced concrete frame has been cut from the 3D model of the structure under consideration and has been performed a nonlinear quasi static analysis of the finite element model of this substructure. As a result of nonlinear numerical analysis, the diagrams of the axial forces and moments and schemes of destruction have been obtained for different values of prestressing in the girders. It has been established that over failed outer column the reinforced concrete frame under consideration transforms to Vierendeel truss. Change of the level of prestressing in the girders of the frame allows varying the stress-strain state and ensures load-bearing capacity of its elements under accidental impacts.