In a previous paper, two of the authors have developed an effective method for the ultimate strength analysis of large size structures. The method is named as “Idealized Structural Unit Method”. In the method, a large size element with idealized nonlinear character was necessary and an example element, “Girder Element”, was developed. In this paper, this method is applied to double bottom structures. The state of two-dimensional stress in the tank top and the bottom shell plating is considered. Conditions for buckling of these plates are established and its post buckling stiffness is determined. The condition for their ultimate strength and stiffness at the ultimate strength are determined in connection with the girder element. An example structure is analysed and the results of the analysis are presented.