A simple method for nonlinear analysis of cross-braced multistorey frames with hysteretic dissipative devices, subjected to monotonically increasing horizontal loads (push-over analysis) is presented. It is based on a model able to provide the approximate tangent condensed flexibility matrix of the system by closed analytical formulae. The model takes the axial deformability of the frame into account, considering that it significantly influences both the fundamental period and the corresponding mode of vibration of the structure. It enables one to evaluate separately the contributions to the first mode shape due to both the flexural deformability of the frame beams and columns (shear type behaviour) and the axial extension of the frame columns, furnishing further insight for the seismic design of the structure. The proposed simplified method is validated by comparison with the results of a push-over analysis performed by a FEM computer program for some frames. Moreover, the effectiveness of the push-over analysis in assisting the design procedure of the structure is checked by evaluation of the time history of the seismic response.