Ultimate Strength Assessment of Hull Girder under Cyclic Bending Based on Smith's Method

Ultimate strength is a critical and fundamental consideration in the design of ship and offshore structures. Accurate assessment of the (residual) ultimate strength of a ship hull is important not only for the initial design, but also for the operation, maintenance, repair, and even for disaster management, rescue, and salvage assessment of the ship hulls. In fact, under ocean wave conditions, the hull girder is likely to withstand extreme cyclic bending. In this article, a simplified method based on Smith's method is proposed to assess the ultimate strength of the ship hulls under extreme cyclic bending. To obtain the nonlinear average stress-average strain relationship (sigma-epsilon curve) of the basic structural element (consisting of a stiffener with its attaching plating) under cyclic in-plane compression and tension, systematic calculations are carried out by using a combination of the Beam-Column analysis and the nonlinear finite element method (NLFEM). By introducing the cyclic sigma-epsilon curves of basic structural elements to a simplified calculation program, which is developed by the authors and based on Smith's method, the ultimate strength of hull girders exposed to extreme cyclic bending is subsequently determined. The ultimate strength of three box girder models and an actual ship are obtained under cyclic bending by using the authors' developed procedures including the simplified calculation program and are compared with the corresponding results from NLFEM calculations and also the box girder experiments.