Ships in bad weather condition are likely to be subjected to accidental loads such as collision and grounding. Once she has damage on the hull, her ultimate strength will be reduced. This paper is to investigate the effect of the damage on ultimate strength of ship structure by using a series of collapse tests. For the experiment, 720mm×720mm in section and 900mm in length of five box-girder models with stiffeners were prepared. Of the five, one has no damage and four have an ellipse shaped damage which represents the shape of bulbous bow of colliding ship. The damage size is different between models. Among the damaged models, the damages of 3 of them were made by cutting the plate and one by pressing to represent collision damage. Experiments were carried out under pure bending load and load-deflection curves and ultimate bending moment were recorded. The ultimate strength was also calculated using LS-DYNA and compared with the experiment results. The ultimate strength is reduced as the damage size increase, as expected. The largest damage one has the damage size of 30% of depth and its ultimate strength is reduced by 19%. The pressed one has higher ultimate strength than cut one. This might be due to the fact that the plate around the pressed damage area contributes to the ultimate strength, whereas the cut one has no plate to contribute.
Recently, the International Maritime Organization (IMO) and the International Association of Classification Society (IACS) are making a standard and regulation of ships considering the ultimate strength of plate and stiffened plate in ship structural design. The final goal of Goal Based Standards (GBS) prepared by IMO is the safe of ship during her life.