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

-11o-(a)(b)(c)(d)Inerting system for cargo holds and other unmanned spaces asrequired, including type of gas, dispersion of gas throughout thecargo, methods of gas-freeing the spaces and extent to which gasfreeingis required for human safety.Optimum insulation system(s) for vertical and horizontal surfacesin the Engine Room, accommodations and other working or livingspace.Composition of deck covering to limit surface temperature to therequired 400 degrees F.Discussions with U. S. Coast Guard to determine the extent offire protection required in the deckhouse. The present study isbased upon full compliance with the intent of current U. S.Coast Guard requirements. However, a lesser degree of protectionhas been accepted in previous aluminum deckhouses, though thesewere installed on steel hulls.DESIGN CRITERIASeveral factors entering into the establishment of design criteriarequire further clarification, including the following:(a)(b)(c)The question of design stresses for welded structures is notfully clarified. At present, a ‘Iwelded’lyield based upon the0.2 per cent offset in a 10 inch gage length is proposed,rather than the prime or O-temper values. However, thisaverage design stress may not adequately account for thestructural response in way of the heat-affected zone, whichis the weak link in the structural system, since the use of a10 inch gage length in lieu of a 2 inch gage length tends todiminish the apparent effects of this degradation.The relative importance of yield and ultimate strengths in convertingfrom steel to aluminum requires further consideration.In specific cases, the equal ranking used in this study may notbe optimum.The question of safety factors should be considered when thevariability in structural performance due to the human elementin fabrication is better understood. The use of identical safetyfactors for aluminum and steel designs implies that the conversionof raw materials into a fabricated product produces identicalstress concentration and residual stress effects which may not betrue. The entire question of residual stress levels must beinvestigated.DEFLECTIONSThese studies indicate that hull deflection should not be a limitingfactor in itself as long as the hull length/depth ratio conforms to ~reseutstandards, and stresses are kept reasonable low. However, the quest>on ofallowableextensiveshould behull girder deflecti~ns deserves” further study-in via of thebody of opinion among Regulato~ Agencies that such limitationsestablished.