design considerations for aluminum hull structures - Ship Structure ...

-88-The design of the midship s~ction of the aluminum bulk carrier has beencarried out h two steps. The first design was developed h accordance withthe criteria developed previously with the hull girder section modulus basedupon the relative short-term or static strengths per Equation (l). The deckand bottom scantlings were then upgraded to suit the fatigue strength criteria(See Figure 13) to determine the relative effects of this more severe requirement.The resultant midship sections are shown in Figures 16 and 17.These midship sections are based upon the welded properties of 5083 alloyas delineated im Table 4. Built-up sections have been used for the stiffenersat the extreme fibers of the hull girder so that the somewhat higher strengthof the plate temper, H321, would govern the design- Btrusions have beenspecified for stiffeners in portions of the hull closer to the neutral axis.The tank top and lower wing bulkhead plating reflect an allowance of about0.80 inch for impact and abrasion in the hold. This is an increase of about4 times the 0.20 inch allowances applied by ABS for steel bulk carriers, inaccordance with Equation (3). As noted previously, this approach is considerdto be more desirable and less costly than installing mechanically fastenedsteel chafing strips or steel doubler plates. In deriving the hull girdersection modulus of the aluminum ships, a somewhat thinner “effective” thictiesswas used for this plating, based upon increasing the minimum required aluminumplating thiclmess by a factor equivalent to the addition of 0.20 inches to theminimum steel thiclumss for abrasion.Throughout the design of the aluminum midship sections, the maximumplating thickness has been restricted to 1-1/2 inches, since the propetiiesof thicker plate am less. This has created some difficulty in obtainingsufficient deck area for the midship section in Figure 17. For this study,the additional area has been included in the deck longit,udtials,sheer strakeand hatchside girder. Several alternatives are available:(a) Use aCelkkT deck structure, with two skins about 1-1/4 inchesthick separated by about 3-1/2 feet, tied together with longitudinalwebs, about 3 feet apart. This structure might be more difficultto fabricate, but would provide a safety factor on cracktig in thata crack initiating in the upper skin would probably not extend downto the lower skin except under extreme circumstances.(b)The deck and longitudinal stiffener thicknesses could be reducedby installing a doubler on the deck. However, this could lead topossible crevice corrosion problems.In accordance with the discussion of crack arresting in Section IIC.,a mechanically fastened sesm has been indicated at the lower edge of the sheerstrake. However, the two additional seams per side incorporated at the deckand bilge of the steel ship have been omitted.Several additional Iongitudimal stiffene~s have been added to the bottomshell outboard and lower wing bulkhead to increase buckling strength.Table 16 presents a comparison of the three midship sections shown inFigure Is through 17. As indicated therein, the use of static strength designcriteria results in a weight pe~ foot ratio of approximately 0.47, while themore stringent fatigue strength criteria increases this factor to 0.62. hterms of overall hull structural weight, this fatigue strength criteria isexpected to add about 43s,tons or 15 per cent to the hull structural weight,