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

Structural Design of the Ship Hull GirderWave loading on the side and bottom plating is the greatest of the following:• a 5 meter (16.4 foot) minimum head• the head to the design waterline plus 0.3 √L (L in meters)• a head to 1.1 √L at the forward perpendicular, tapering to the design waterlineamidships.(L in meters)The allowable hull girder bending stress is a function of the yield strength of the materialand the breadth-to-thickness ratio of plating in the strength deck and shell, as shown in Table 2.2Table 2.2. UK MOD Allowable Hull Girder Stress (percent of yield strength)Allowable StressYield StrengthAllowable Stress forMild Steel (tsi)Breadth/thickness (b/t) Strength Bottom Strength BottomDeckDeckb/t < 60 0.65 0.54 10.1 8.490 > b/t > 60 0.57 0.43 8.9 6.7b/t > 90 0.43 - 6.7 -Note that it has been the British practice to use only mild steel for hull structure. Criticalareas, such as crack arrestor strakes, use tougher materials, but higher strength steel is not used toreduce weight because of concerns for buckling and fatigue.The allowable primary hull girder stress is greater for the strength deck than for thebottom structure because the effects of pressure loading are included when assessing strength. Inthe design of deck plating and longitudinals, only the calculated primary hull girder bendingstress is considered. The maximum bending moment is divided by the actual hull girder sectionmodulus to determine the compressive stress in the deck. Various methods are given in SSCP23for computing the strength of the members, including load-shortening curves and grillagestrength methods to determine the collapse strength of the structure. The recommendation ismade that in later stages of design finite element analyses should be conducted to calculate thestrength of the structure more accurately. Because a maximum lifetime load is used, no factor ofsafety should be taken.2.5.2 Side and Bottom ShellFor the design of stiffeners in the bottom shell, the computed primary bending stress isadded to the secondary stress from stiffener bending. This combined stress is used in thebuckling calculations without a factor of safety for assessing structural adequacy. Likewise,longitudinals on the side shell are designed similar to those on the bottom, with the primary hullgirder bending stress reduced based on the distance from the neutral axis. Again, no factor ofsafety is used in computing structural adequacy.2-22