In this paper the method of forecasting the seismic response of metal structures of gantry and portal cranes is proposed. This simplified method allows to evaluate possible damage of steel structure of the portal. The foregoing procedure is based on the modeling of the structure of the gantry crane using nonlinear dynamic system with element of plasticity. The physically nonlinear element of dynamic system simulates the possible damage of steel structure. The comparison of the results of the seismic analysis using the proposed method and finite element method shows that results match to each other with accuracy higher than 13%. Proposed method may be used for gantry and portal cranes at the early stages of design process.