Ring-stiffened cylindrical shells are often used in offshore and submergible structures, it is important for the structural safety to estimate the buckling strength of those under extremely high water pressure. Although the buckling strength can be accurately calculated using the finite element analysis (FEA), the estimation by more easy way is required at an initial stage of the structural design. This paper proposes two formulas for the local shell and stiffener-tripping buckling strength of the ring-stiffened cylindrical shells under external pressure. One can estimate the tripping buckling strength only and is derived assuming the tripping buckling as buckling of only the flange-beam supported by springs and considering effects of the cylindrical shape and torsional stiffness which are not included in a conventional formula. The other can estimate both the shell and tripping buckling strength and is considered interaction of buckling deflection between a cylindrical shell and ring-stiffeners and influence of stresses acting on the web which are not included in conventional formulas. Based on the principle of minimum potential energy, the formula is derived using the functions to express the buckling deformation in the cylindrical shell and ring-stiffeners. The buckling strength estimated by the two proposed formulas is compared with that by existing conventional formulas and the finite element analysis, and this study discusses the influence of new considerations on the shell and tripping buckling strength. From the results, it is found that the second formula can deal with any buckling mode and has greatly high accuracy compared with other conventional formulas.
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