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

-53-hull with machinery amidships, the shaft bending stresses and bearing reactionswould increase roughly in proportion to the hull girder deflection ratio, sothat the question of shafting and bearing reactions would require seriousconsideration in this case.The effects of greater hull girder deflection on longitudinally orientedbilge and ballast systems shouldbe negligible, since the materials recommendedfor these systems have greatly reduced elastic modulus compared to conventionalsteel piping. Aluminum piping wouldbe stressed to only 1/s the level ofequivalent steel piping for equal hull deflection, while the stress ratio forfiberglass piping would be only 1/10 to 1/1S that of steel. Thus an increasein hull girder deflection could be accepted without overstressing the pipe.Again, this presents a relatively simple design consideration which canbereadily incorporated in the design of the piping system.For longitudinal runs of steel piping, such as fuel oil piping, an expansionloop cm be incorporated to absorb the additional deflection of the hull girder.Stless-Strain Ilelationship- Consideration of design stresses, includingfatigue, have been dealt with previously in detail, and it is these considerationswhich affect deflections rather than deflection considerations affecting stresses.Therefore, if previously established strength relationships have been satisfied,there is no apparent reason to impose a limit on hull girder deflection basedupon consideration of material properties. Tt is noted, however, that increasedhull deflection increases the strain energy in the post yield (plastic) range inway of stress concentrations.The only area in which excessive deflection would affect structural designis in the design of hull longitudinal, where the secondary bending due to hydrostaticor deadweight loading should be augmented by the moment resulting fromend loading being applied along an axis which is not in line with the neutralaxis of the deflected beam, i.e.:Additional secondary bending . ~[ 1Primary ‘%eamxAbeammoment at midspam‘ere Cprimar is the hull girder primary bending stress~eam =x the area of the beam, including hull plate supported‘beamiS the midsPan deflection of the beamThe stresses resulting from such secondary moments are usually negligible,but for an aluminum hull, where Abeam will be greater than with steel,this additional bending should be considered.Conclusion - It is concluded that no limits should be placed on thehull girder deflection of an aluminum bulk carrier, but that the affectsof the deflection resulting from normal structural design should be consideredin the areas”noted above.PROPOSED CRITERIA - PRIMARY HULL STRUCTUREIn this section, criteria are proposed for converting ABS steelscantlings to aluminum for application to the design of the primary hullstructure of an aluminum bulk carrier. The following structural elementsare considered: