The 2003 Tokachi-Oki earthquake caused severe damage to oil storage tanks due to liquid sloshing. Seven single-deck floating roofs had experienced sinking failures in large diameter tanks at a refinery in Tomakomai, Japan. The pontoons of the floating roofs might be buckled due to bending load during the sloshing. The content in the tank was spilled on the floating roof from small failures which were caused in the welding joints of pontoon bottom plate by the buckling. Then the floating roof began to lose buoyancy and sank into the content slowly. The elastic buckling of the pontoon is important from the viewpoint of the single-deck floating roof sinking. The authors had reported the buckling strength of the pontoons subjected to bending and compressive loads in the published literatures. The axisymmetric shell finite element method for linear elastic bifurcation buckling was used in the analysis. The buckling characteristics of the pontoon both with and without ring stiffeners were investigated. The initial geometrical imperfection may diminish the buckling load. This paper presents the lower bound buckling load according to the reduced stiffness method proposed by Croll and Yamada. The lower bound buckling loads of the pontoon subjected to circumferential bending load are evaluated from the axisymmetric finite element analysis which includes the reduced stiffness method.

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