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

-56-at the deck and keel. This stress should be assumed to taper to 6.7J KSI atthe neutral axis for the hull envelope and its framing, and tO ze~o foT O~h@rinternal structure to account for stresses at an angle of heel.Limitations on -platepanel deflections are no% considered necessary, sincethe increase in plate thickness in converting from mild steel to aluminum willgenerally provide a sufficient increase in inertia to offset the effects ofdifferences in elastic moduli.Design Criteria for Stiffeners - The design procedu~e for converting mildsteel stiffener scantlings to aluminum consists of increasing the sectionmodulus of the steel mem~er by the relative strength ratio noted previouslyfor plahes:Equation (4): Malum .WIteel 97,000( )Y+u/Where Y and U are asnoted previously for Equation (1).Corrosion allowances arebut are neglected to providedtechnically applicable to the above equation,an additional margin for member stiffness andhigh residual stresses in way of end connections. The additional weightresulting from this simplification is negligible, since the added areagenerally contributes to hull girder section modulus for longitudinallyframed ships.It is noted that longitudinal stiffeners on the shell and deck aresubjected to a combination of axial load from hull girder bending andsecondary bending from normal loads. Fortunately, however, the ratio ofhull girder primary design stress to the quantity (Y + U) of steel andaluminum bulk carriers are essentially identical at about 0.20, so thatthis combined loading condition affects both materials similarly, andEquation (4) remains valid.In addition to the foregoing, stiffeners should be checked for columnbuckling strength under the effects of longitudinal bending loads, andagainst local instability of flange and web as discussed in the previoussection. Ii is suggested that the L/r ratio of the plate-stiffener combinationbe sufficiently low that the safety fackoT on column buckling failurewould be 1 .67, and that the web and flange proportions would permit developmentof full welded yield stresses in the member without local instability.The deflection of aluminum stiffeners should be kept within reasonablelimits, so that vibration problems and secondary bending effects are minimized.However, it is difficult to establish a specific deflection limitation, sincethis is a somewhat arbitrary decision, with little technical justification.Until a valid technical foundation for such a limitation can be developed,it is proposed to limit deflections of Primarv framinp members (girders. webframes, ~tch end beams, etc.) to l-1/2-times-’thatof-the equiva~ent st~elsection, with no limits on the deflection of secondary stiffening. Equation(s) specifies the inertia required for primary hull framing members:Equation (~): Ialum = 2 Isieel (primary members only)