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

-26-(c)Members with partial or continuous longitudinal welds maybedesigned on the basis of an average stress, where the annealedor O temper values apply to material within a 1-1/2 inch radiusof the weld and the prime values apply elsewhere:AannealedFc =Fc-—Fc - Fcavg prime ‘prime prime annealed()where Aamea~ed and Apr~e are the respective cross sectionalareas.Similar considerations could be applied to the design of aluminumplate panels subject to compressive buckling loads. In general, weldingdoes not affect the buckling characteristics of plate panels, since thereis seldom welding in the middle of a panel. However, for welded aluminumbstructure, the cut-off panel strength for low k ~ ratios should be basedupon the ‘JweldediJstrength.One aspect of buckling which is often overlooked in the design ofship structures relates to local instability of fr~ing members. Becauseof the low modulus of elasticity of al~inuj norm~ ProPOr~iO~of beams and flanged plate girders which are commofiy used for steel~shipstructures are unacceptable for aluminum sections, since local instabilityof th~ flange or web may result. In the selection of framing memberscantlingsj the following limitations should be.met if the member is to becapable of resisting stresses approaching the yield strength of thematerial:(a) ~~~~$ness for[ 1(b)Flange widthFlange thickness[1forwhere E is the modulus of elasticity (10 x 106 PSI)I?yis the unwelded yield strength of the alloyFor structural sections meeting themaximum span between supports should notprevent over-all lateral instability:foregoing requirements, theexceed the following limits toT 57.283 ~~1 +0.2 (d/bF) - 0.128 (bF/d)2where bF = flange width for tees;twice the flange width for anglesand flanged platesd = depth of sec”hionThis equation is derived fromDDS 9110-4. If this distance is exshouldbe provided to prevent tripping ofceeded, an intermediate chockthe section..—