LOAD CRITERIA FOR SHIP STRUCTURAL DESIGN

1 MEMBERAGENCIES:UN1lkDSTATESCOASTGUARDNAV A1S141 PSYSTEMSCOMMANDMI{ , , &i &.,RearAdmiral, ,ShipStructureCommittee .. FinsSSC-240 Technica ReportonProjectSR-198, LoadCriteria , ; , indicatedbypossiblemodesOFstructuraldama geand/orfailure . ;[reishulls,asItisbyfatigue, ,whichisparticularlydifficulttodealwithb utcanbeminimizedbycontrolofmaterialquali tyanduseofthecustomary fail-safe , ,anultimateloadcriterionissetupinvolving thefol-lowingbendingmoments:Quasi-static wave-induced, ,includingeffectofship ,includingslamming,whipping, , ,il1expressedinprobabilityterms, ,involvingthepre-dictionoftheexpectednum b?rofcombinedloadsofdifferentlevels, ,calculationsofloadsarecarriedoutforatyp icalcargoship, : FOUNDERING (126).. , ,..67 NORTHATLANTICCALCULATEDCHANGESINSTRESSAN OEQUIVALENTBENDINGMOMENT.]}

2 ~ 12345678910111213;:161718;:2122232425262 72829 TYPICALVOYAGEVARIATIONOFMIOSHIPVERTICAL8 ENDINGSTRESS,.. ,LOAOEDCONOITION(3).TYPICALVOYAGEVARIATI ONOFMIDSHIPVERTICAL8 ENOINGSTRESS,.. (3)TYPICALRECOROOFMIDSHIPVERTICAL8 ENi)INGSTRESS,WITHSLAMMIiiG,.. , , ,ORECARRIER,FO! , (1972)REQUIRINGNOADDITIONTOSECTIONi100 ULUSCOMPARISONOFWAVESTATISTICS:08 SERVEDPERIODSANOHEIGHTSPEAK-TO-PEAKSLAMS TRESSOISTR18 UTIONSINOIFFERENTWEATHERCONDITIONS, !4!7 WTYPICALRECORO OFMIDSHIPSTRESSVARIATION, , RESSES(3)INVOLVED , ;;.. , (108 CYCLES) .KJTIONSOFSTILL-WATERBENDINGMOMENTS,..(1 Z8) , $l01 STR18 UTIONOFBENOINGMOMENT,LIGHT-LOADiONOiTiOi :::::LONG-TERMDISTRIBUTIONOF8 ENDINGMOMENT,FULL-LOADCOINOITIONLONG-TER M01 STR18 UTIONSOFcON81 NEDBENDINGMOMENTS:WAVE8 ENOiN(jj:::(VERTICALANDLATERAL)ANOSTILL- WATERBENOINGCYCLICLOADING SPECTRA.

3 ~ ,41919192031394042424343::46646B70;:80-v -SHIPSTRUCTURECOMMITTEETheSHIPSTRUCTUREC OMMITTEE isconstitutedtoprosecutearesearchprogram toimprovethehullstructuresofshipsbyanext entionofknowledgepertainingtodesign, ,III,USCG,ChairmanChief, , , ,USNVicePresidentMaintenanceandRepairOff icerPmericanBureauofShippingMilitarySeal iftCommandSHIPSTRUCTURESUBCOMMITTEETheSH IPSTRUCTURESUBCOMMITTEE actsfortheShipStructureCommitteeontechni calmattersbyprovidingtechnicalcoordinati onforthedeterminationofgoalsandobjective softheprogram,andbyevaluatingandinterpre tingtheresultsintermsofshipstructuraldes ign, , , , Formanyyearsthegoaloftrulyrationaldesign ofshipstructureshasbeendiscussed, ~,forexample,inanearlyplanningdocumentof theShipStructureCommittee(1),andsincethe establishmentoftheInter-nationalShipStru cturesCongress( )

4 , , , ,or factorsofignorance, de-mand uponand capability ,insteadofinsuringthatasimplecalculatedd esignstressisbelowtheultimatestrengthoft hematerialbyanarbitraryfactorofsafety, ,thisapproachrequiresadefinitionoffailur e,whichmaybeaseriousbuckle,amajorcrack,c ompletecollapse,oratensilefailure(Chapte rII). , , , ,withthecooperationoftheshipstructuralde signer,itisanticipatedthatarationaldesig nprocedurewillevolve(2).* , ,fasterships,unusualhullconfig rations(suchasthecatamaran), , sef , ,thereareresidualstressesintheplatingand locked-instressesduetowelding, ,sincetheytendtobeeliminatedby shakedown , ,b aysbecalculated,b tthemidshipstresswillprobablynotcorrespo ndexactlytothiscalculatedvaluebecauseofp ossiblebuilt-inhogorsagreaid alstresses, , ,shears,andperhapsthetor-sionalmomentsas cargoisdischargedandloaded,fueloilandsto resaretakenaboard, , (3).

5 Finally,attheendofvoyagechangesresulting fromcargodischargingandloading,pluspossi blefueloilandballastchanges,willagainmod ifythebendingmoments, ]IIII1I1 IIIIIII,1!,,,,,,,,,,,,,,,,,,/,;1,,,,,,,, ,,,,,,,,,, ,,,,;6,11,~1,I11 IIII11 -4-TIIIIIIIIIIIII<1 SVU3A18W69/91b)W,S8M83,,36,/9/,1,0<Tl,Y31N3691 SlbI,V8WI>V1d38uo~vnb]8s083I (,,,,,,,IIH0009403dV1 SSVdI69/22/, s -,08,,111834lWd3069/L/fTIIIIIIIIIIIIIIII IIIIIN(lSdX] , Thiscapabilityisnecessavtopre-ventthedyn amicstressrangefromexceedingthelimitsoft heinstrumentationSystsm (4).Whentheshipgetsunderwaytogotosea,the firstnewbullloadingtobeexperienced--espe ciallyiftheshipisahigh-speedvessel--isth esaggingbendingmomentinducedbytheship sownwavetrain. l%islongitudinalbendingmomentisafunction ofshipspeed,andwillbesuperimposedwithlit tlechangeontootherbendingmoments(5).)

6 Anotherloadvariationresultsfromdiurnalch angesinairtemperature* ,whichcanperhapsbeconsideredasthe loads. Ingeneral,ifabeamissubjecttoheatin& ,ifthegradientisnotuniform, ,thetemper-atureofallthesteelincontactwi ththewaterwillbeatthenearly uniformwatertemperature, (extentofcloudiness,durationofsunlight,a ltitudeofsunatnoon).Inrespecttothelatter factor, ,especiallyonthesunnysideoftheship, ( ),Thisloadusuallyvariesinanirregularfash ionwithanaverageperiodof5-10seconds, , ,ofcourse, , ,whodevelopedthenecessarystatisticaltech niquesfaranotherpurpose. hewrote(6). Atonetimethewavesranhigh,fleckedwithpqtc hesoffoam,whileatanother,theywerebarelyn oticeabler~ IntimeWienerevolvedhismathematicaltool,s pectrumanalysis--ameansofbreak-ingdownco mplexpatternsintoalargenumberofmeasurabl e, , (4)(7) (8)thattwodif-ferentmathematicalmodelsca nbeusedtoextrapolatesuchresultstomuchlon ger*Gagesweretemperaturecompensated.

7 :- -t - ;I ---------vi_- .. ~1_= ._ ..a- . -. -. -___ _.. _.._- _ IFigure3-Typical,RecordofMidshipYertical BendingStresswithSlamming, 14.*.FOTIl& ,ithasbeenshownthatusingthesamemathemati -calmodels--combinedwithmodeltestsinregu larwavesandoceanwavespectra--short-term( 9)andlong-termtrends(10)canbepredictedwi thaprecisionthatdependsonlyonthereliabil it!~ ,computerprogramshavebeendevelopedforap~ ,oceangoingshipsexperiencedynamicloads,t hemosttroublesomeofwhichresultfromimpact (slamming)andthevibratorvresponse(whippi ng)thatfollowsit(Fig,3). (11)andtheoreticalstudies(12)(13)havebee ncarriedoutonslammingandwhipping, ,springing,whichundercertainconditionsse emstobeexcitedmore-or-lesscontinuouslyin flexible-hulledships, (14). (15) ,usuallyinvolvingnaturalforces,onemustma keastatisticalanaly-sisandendeavortodesi gnonthebasisoftheexpectedloads,withnOlim itationonthestructureoritsoperation.

8 ,Stillcalculationsof-7-waterloadsaregene rallycontrollableandwavetYPiCalCondition sofloadingindicatethat~X_ cessivestillwaterbendingmomentsmightoccu r, , , , , ,toguardagainstthepossibilityofsomeunfor eseenextremeloadcondition,itisrecommende dthatsuitablestressin-strumentationbepro videdasawarningdeviceforaddedsafety(16). Agreatdealofresearchhasbeendoneinrecenty earsontheshiphullload-ingsmentionedinthe previoussectio~muchofitintheShipStructur eCommittee(SSC) , ,theSocietyofNavalArchitectsandMarineEng ineersandtheAmericanBureauofShippinginth iscountry,andbyvariousorganizationsinGre atBritain,Nor-way,theNetherlands,andJapa n,asreportedtotheInternationalShipStruct uresCongress( ).Apartialbibliographyisgivenattheendoft hisreport(AppendixA).Sometypicalloadshav ereceivedmoreattentionthanothers,however , , , , , ,particularmentionmightbemadeoftheworkof Caldwell(17),Aertssen(18),Abrahamsen,Nor denstr6m,andR6ren(19), (20).

9 ,wemustask,whatarethecriti-calloadsandho wdotheycombine?Meanwhile, .Caldwell(17) ,itisclearthataconsiderablylessseveredam agewouldbeaseriousmatter;asindicatedbysu chfactorsasnecessityformajorrepairs, (17),,!In,je~igni~gamidshipsection,thede signershouldcOnsiderthevariouslevelsofda nagewhichahullgirdermayexperiencebetween thelimitsofinitialyieldandfinalcollapse, andshouldattempttorelateeachlevelofdamag etoanappliedbendingmoment. Hence, *Furtherstudyofthesubjectofcriticalloads duringthisprojecthasre-sultedinnobasicim provementinGerard sanalysisofspecificwaysinwhichthehullgir dercouldfail,asgivenin ALong-RangeResearchPrograminShipStructur alDesign (1).Heconsideredoveralldamagebycompressi vebuckling,overallten-sileyielding, , , *Thisissometimesreferredtoas collapse (20),butwefeelthatthistermconnotesbuckli ngfailuretotheexclusionoftensilefailureo rperma%nentsetandthereforeprefer failure.

10 -9-Althoughonlythelastisconsideredtobest ructuralfailure, , ,considerationshouldbegiventootherminore ffects, ~iceloading,suchasberthing,drydocking,an dgrounding,whichmayhavedirecteffectsonpr imaryhullgirderstructure, , flanges (deckorbottom)ineithercompressionortensi on, , , ,abasicques-tioniswhetherornotthesuperim poseddynamiceffectsofhighfrequency whipping following?slamand/orflareentryshouldbeco nsidered, , ,,,It~hOuldbeborneinmindthattheshorttime inwhichthe ave Om-entsduetoslammingdeveloptheirmaximumv alues,andtheentityofthetotalde-flectiont hatwouldbeconsequentonthem,maketheprobab ilityofitsrealizationextremelyscarce (21).Andinreferringtoplasticdeformation, Nibberingstates, ,especi-allywhenapartoftheloadisduetosla nnning (22).Thisisaprobleminstruc-turalmechanic snotwithinthescopeofthi6project,andthere foreweshallattempt= ,localloads(notduetolongitudinalbending) onwhichallofthezbovearesuperimposedm ,cargoloadsoninnerbottom,liquidpressures withintanks, ,invol ingextensiveplasticyieldingandforbucklin g, ,however, ,whichseldomconstitutesfail-~urebutisimp ortantfortworeasons:fatiguecrackscangrow tothepointthattheymustberepaired, ,!