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

-54-000000000Bottom Shell PlateSide Shell PlateDeck PlateTank Top PlateWing Bulkhead Plate (Upper and Lower)Inner Bottom Floor and Girder PlatesBottom LongitudinalDeck LongitudinalTank Top Longitudinal00Other Hull FramingStanchionsMembersIn general, — these criteria will establish minimum scantlings to resist combinationsof primary and secondary stresses, local loads, impact, abrasion, slamming, etc.,with consideration given to vibration and buckling problems. Tt will often benecessary to increase these minimum scantlings to suit hull section modulusrequirements.Design Criteria for Plates - In gene~al, the approach to converting steelplate thicknesses to equivalent aluminum thicknesses requires the derivationof an “effective” steel thickness by deducting all corrosion or abrasion allowances,then increasing this thickness by a function of the relative strengthratios, and adding back any required corrosion or abrasion allowances.The corrosion allowance to be deducted from steel will depend upon itsanticipated exposure to salt water. An allowance of I/8 inch or 10 per centof the thickness, whichever is less, is proposed for the hull envelope (deck,side and bottom plate) with a 1/16 inch allowance for the internal plates.If the owner or Regulatory Bodies have added an additional margin for abrasion,such as on the flat of bottom or on the bottom of the hold, this should alsobe deducted.The factor by which the “effectivelythickness is to be modified is basedupon the ratio of the sum of the welded yield and ultimate tensile strengthsof the materials as in.Equation (1) previously. For plates loaded primarilyin shea~, tension or compression, the full ~atio should be used. However, forplates which are loaded primarily in tertiary bending (bending betweer stiffnersdue to applied normal load) the square root of this ratio should be used, sincethe section modulus of an element.of plate is a function of (thickness)2. Forplates subjected to a combination of tertiary bendfng and tension, compressionor shears an average factor should be used.The allowance for abrasion to be added back to the resultant aluminumthiclmess is somewhat arbitrary. Howeverj the previous discussion of aluminumalloy abrasion resistance indicates that aluminum will abraid about 4 timesas fast as steel in a similar environment. Thus, for equal life, the steelallowance should be multiplied by four, unless a detailed economic analysis