In this study, ship hull girder ultimate strength considering biaxial compression in bottom stiffened plates is investigated. Firstly, elasto-plastic finite element analysis is carried out for the stiffened plates under biaxial compression by using the commercial FEM software, LS-DYNA. From the observation of collapse behavior of the stiffened plates, it is found that the decrease of the effective width of the stiffened plate under biaxial compression is more rapid compared with the formula in the Common Structure Rules (CSR). This is because the buckling in one half-wave mode occurs when the transverse compressive load becomes larger, while the buckling with multi half-waves mode usually occurs in longitudinal direction when only longitudinal compressive load is applied. Based on the observation, a new formula to estimate ultimate strength of stiffened plates considering biaxial compression is proposed in which three new parameters are introduced which can represent the decrease of the effective width. Next, a method to estimate ultimate strength of hull girder considering the biaxial compression of ship bottom is developed based on the conventional Smith method provided in the CSR, by introducing the above new formula to estimate the ultimate strength of stiffened plates, and by improving the conventional formula of the hard corner element in the Smith method. Finally, the hull girder strength of a container ship under biaxial compression estimated by the proposed method is compared with the result of FEM analysis to show validity of the method.

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