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

-121-APPENDIXAReferences:(Al)DETERMINATIoN OF LONG-TERM BENDING MOMENTS FORALUMINUM BULK CARRIERLewis, E.V. llpredi~tingLong Term Distribution ofWave-Induced Bending Moments on Ships Hulls”, SNAITE,196T(A2) fiA~lysis and Interpretationof Full Scale Data onMidship Bending Stresses of Dry Cargo Ships” ShipStructure Committee Report SSC-196, June 19691. The stresses occurring in a critical po~tion of the shipshull girder, such as the deck edge, are function of the basic still waterbending moment and randomly varying wave-induced vertical and horizontalbending moments and similar~ varying inclinations of the shipls principalaxes. The still water bending moment is easily established. Statisticalanalysis methods can be used to predict long term distribution trends ofrandom variables, yielding expected stress or bending moment levels versusnumber of encounters at such levels during a shipls lifetime. This is alengthy p~ocess when considering a single variable, and would be prohibitiveati without proven precedent for phase-combined effects of’severalrandom variables. Accordingly, since vertical bending moments account forthe major portions of actual stresses, and have been the most thoroughlyresearched, the bulk carrier design will be based upon expected stillwater bending moments and predicted long-term distribution of waveinducedvertical bending moments and resulting stresses.2. The dimensions of the bulk carrier CHALLENGER fit wellwith those for a Se~ieg 60 tanker for which long-term probability data isreported in Reference (Al). Figure 17 of Reference (Al) gives Xj vsQ kg 10 (X~Xj) Plots of bendi~ moment coefficients in head seas ofvarious sigmif~cant heights for a 600 foot ship with L/B = 7.0, L/d = ly.~CB = .8 andV/m= .3b. Probabilities of occurrence of such seas inthe designls intended service may be combined with Figure 17 values toproduce a single curve of expected bending moment coefficient for theanticipated distribution of weather according toNQ(X~Xj) z piQi(x>xj) ~.i=lMinor adjustment of this curve to account for a V/m of the design,higher than 0.34, may be based upon proportiofi~ the ordinates accordingto variations in bending moment with ship speed for the 600 feet,CB = .8o ship shown inFigwe 12 Of Reference (Al). ‘TheXj vs Q log 10(X~Xj) (and corresponding Xj swN) plot will indicate the steepness ofstatic trochoidalwaves which would produce bending moments equivalent tothe levels of dynamic bending moments expected to be induced by perturbationfrom the static condition. These moments are superimposed on thestill water bending moment.3* A standard bending moment calculation for a wave of agiven height combines the bending moment due to the differing weight andbuoyancy distribution at the still waterline with the moment due to