This paper proposes the method to reduce the MDOF system to the equivalent SDOF system for low-rise steel structures with a flexible roof. The target structure has two braced frames and some moment resisting frames. Evaluation methods of the maximum seismic forces acting on frames are presented with dimensionless parameters. The envelope curve and the equivalent damping ratio of the SDOF system is formulated. It is confirmed that the roof deformation potentially magnify the maximum ductility factor at the side braced frame. Finally, the maximum elasto-plastic responses are predicted based on the response spectrum method.
