ssc - 419 supplemental commercial design guidance for fatigue ship ...

Structural Design of the Ship Hull GirderHSLA-80 steel. The section modulus at all points along the hull must be sufficient so that theallowable hull girder bending stress is not exceeded.In the design of longitudinal members, the stress computed from the local loading isadded to the stress from hull girder bending using an interaction formula. For longitudinalstiffeners, the interaction formula is for combined compressive and bending load is:where:fFbbfc+K Fsc≤1.0f b is the compressive bending stress in the member computed using the designload.F b is the allowable bending stress, equal to 27 ksi for medium steel ships, 40 ksifor high strength steel, and 55 ksi for HY-80 or HSLA-80 steel.f c is the assumed hull girder compressive bending stress, equal to the allowablestress increased by a margin of 1.0 tsi. This stress is taken as the maximumat the strength deck and keel, and for shell plating, tapered to one-half themaximum at the neutral axis.K s is a slenderness coefficientF c is the buckling strength of the member.The U.S. Navy approach is nearly a “first principles” approach, except that the designloads are less than the maximum loads and a high factor of safety is used to compensate for thereduced loads. It was estimated (Sikora et al., 1983) that the standard bending moments will beless than the maximum lifetime hull girder bending moments by a factor ranging from 0.430 to0.916, with an average value of 0.73. On that basis, the allowable hull girder bending stress foran average medium steel ship would be 7.5 ÷ 0.73 = 10.3 tsi., with a range between 8.2 and 17.4tsi. The design wave loads on the side of the ship are similarly less than the lifetime maximumloads. Therefore, the allowable bending stress for stiffeners is significantly less than the yieldstrength.One of the greatest changes in U.S. Navy design practice since the writing of the designmanual has been the use of the finite element method. In 1976, this method of structural analysiswas only beginning to be used in ship structural design, but it has since become standardpractice, particularly for the design of transverse members. In finite element analysis, thestandard loads are still used in conjunction with the standard design allowable stresses. Thejustification for this approach is that the former methods of stress analysis did attempt toreplicate the exact response of the structure to a given load. For example, when a longitudinalstiffener is supported by uniformly spaced transverse frames and subject to a uniform load, thebending moments will be the same as for a fixed end beam, which was used in analysis.Likewise, approximate methods of analysis of transverse frames, such as the Hardy-Crossmoment distribution method were also used to estimate bending moments and shears.Recently, combatant loads, such as hull girder whipping moments and fatigue effectshave been used for design of U.S. Navy ships, particularly the LPD-17 Class (Sieve et al., 1997).That analysis was conducted assuming 40 years of operations in the NATO North Atlantic sea2-20