For medium size container ships, curved bilge strakes should be stiffened with angle bars. However, ultimate strength design formulations for those structures are not available. Design equations for flat stiffened plates are employed in design without considering the effects of curvature of shells. in this study non-linear finite element analyses were performed and the curvature effects were quantified. According to the analysis results the effects of curvature can not be neglected when the curvature is larger than a certain amount. Therefore design formulations should be developed for curved stiffened shells, which is presently designed using the formulations for flat stiffened plates.