ML18031A105

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Final Deficiency Rept on spot-welded Struts
ML18031A105
Person / Time
Site: Susquehanna  Talen Energy icon.png
Issue date: 12/29/1978
From:
PENNSYLVANIA POWER & LIGHT CO.
To:
Shared Package
ML18031A103 List:
References
P23-9, NUDOCS 7903010285
Download: ML18031A105 (22)


Text

Revision1JN4'~'-'0'8012FINALREPORTONSPOT-WELDED STRUTSFORSUSQUEHANNA STEAMELECTRICSTATIONUNITS1AND2Preparedby:Checkedby:Approvedby:BECHTELPOWERCORPORATION SanFrancisco, California September 29,1978December29,197U(Revision 1)(P23-9)o80102$

0le' Revision1~JN4>"-">OJSG'32TABLEOFCONTENTSSectionTitle~Pae1.02.0PurposeSpot-welded Struts3~0Background 4'Deficiency anditsSafetyImplications 5'Immediate Action6.0TestProgram7'Technical Evaluation ofDeficiency

8.0 Corrective

Action109.0RepairMethods10'Conclusion 14ATTACHMENTS (P23-9)TestReportby'Pittsburgh TestingLaboratory'est ResultsSpecifications

  • TheAttachments werepreviously transmitted viaPLA-295datedOctober9,1978andarenotforwarded withthisreport.

Revision11'PURPOSEThepurposeofthisreportistoprovidefinaldataand.information asrequiredby10CFR50.55(e)(3) subsequent tothenotification ofareportable deficiency.

Thesubjectdeficiency isassociated withspot/resistance weldinginstrutmaterial.

2.0SPOT-WELDED STRUTS2.1StrutsBasicstrutsectionsarelightgage(thickness varyingfrom0.105to0.109inch)channelsmanufactured bycoldformingmildsteelstrip.Thesechannelsec-Vtionsareconnected toeachotherinvariousconfigura-tionssuchasback-to-back, back-to-side, orside-to-side byusingaweldingprocesscommercially knownasspot-weldingorresistance welding.Thechannelsectionsandbuilt-upsectionsusedonSusquehanna projectareeithermill-galvanized orhot-dipgalvanized afterspot-welding andaresuppliedbyUnistrutCorporation, Wayne,Michigan; PowerStrut,DivisionofVanHuffelTubeCorporation, Warren,Ohio;andB-LineSystemsIncorporated,

Highland, Illinois.

Thesewereprocuredasstandardoff-the-shelf, itemswithnoformalQualityAssurance.

Variousconfigurations usedonSusquehanna aregiveninFigureI.Strutmembersareusedinfieldfabricated supportsforelectrical

raceways, HVACductsandinstrumentation

>e4r-'69BQj2..

lines.Thegoverning documents aredrawing8856-E-53 forelectrical

raceways, Specification 8856-M-323-C forHVACductsandDrawing8856-JG-16 forinstrumenta-tion.Thesupportsaredesignedinaccordance with'Specifi-cationfortheDesignofCold-formed SteelStructural Members'1968 Edition),

published byAmericanIronandSteelInstitute.

Thespot/resistance weldingprocessconsistsofpassinghighcurrentthroughthethicknesses ofadjoining platesresulting inmetal-to-metal fusion.Thequalityofaspotweldisdependent onmanyvariables suchaspres-sureontheelectrode tips,finishofthematerial, presenceofimpurities onthematerial, build-upofzincandothercontaminents ontheweldingelectrodes, contactbetweenjoiningsurfaces, voltageandamperage.

Therefore, unlesstheseparameters arecloselymoni-toredandcontrolled, theresultmaybeinadequate fusion.3.0BACKGROUND SincetheuseofstrutmaterialbeganontheSusquehanna Pro-ject,therewerethreeisolatedinstances whentheadjoining Fchannelsseparated duringhandlingorassembling.

Thesememberlengthswererejected.

(P23-9) evision1.)(z-r=.E8612However,inrecentmonthstherehavebeensignificant in-stancesonothernuclearprojectswhereinadequate fusionwasobservedattheweld.spots.Asaresultofth's',onMarch25and26,1978,Bechtelfieldengineering personnel performed aninspection onUnistrutmemberP-1004A,Power-strutmemberPS-3022andB-LinememberB22-X.Thetestedmemberswerepartofinstalled electrical racewayandHVACductsupports.

.Themethodofinspection consisted of'sounding'he memberswithaballpeenhammertodetectseparated spotweldsandverification oftheseparation byinsertion ofacardbetweenthemembersatthespotwelds.Theinspection indicated ahighincidence ofspot-welds withinadequate ornofusion.Therefore, ProjectQualityAssurance (QA)issuedManagement Corrective ActionReport(MCAR)1-23onMarch28,1978.4.0DEFICIENCY ANDITSSAFETYIMPLICATIONS Basedupontheresul'tsoftheinspection asdescribed inSection3.0,itwasdetermined that:a.Thedeficiency isrelatedtospot-welding technique and/orprocedure.

b.Thequalityofthespot-welding isindeterminate, withoutfurtherinvestigation.

Variouscombinations andconfigurations ofchannelstrutshavebeenusedinthesupportsystems.Individual members(P23-9) vision10.8012havebeendesignedascomposite sectionsforwhichthecon-nection(spot-welding usedinthisinstance) betweenadjoin-ingchannelsisreliedupontocarrythepostulated loadswithinestablished designmargins.Thus,inadequate fusionatthespot-welding mayresultininadequate

strength, andmayadversely affectthesafeoperation oftheplantunderdesignloadingconditions.

5.0 IMMEDIATE

ACTIONA'Hold'asimposedonfurtherinstallation ofmemberP-1004Aoritsequivalent unlessthememberwasstitch-weldedtodevelopequivalent designstrengthofthespot-welding.6.0TESTPROGRAM6.1General.Althoughitwasdetermined thatadeficiency existedinsomespot-welded members,theextentofthede-ficiencywasunknown.Sothefirststepwastoes-tablishthescopeoftheproblem.Sincethereisnopractical nondestructive methodforexamining thesound-Inessofspotweldsintheerectedmaterial, itwasde-cidedtoinitiateadestructive testprogram.6.2BasisofTestProramTheunderlying approachisdescribed below.(P23-9)

Resion1Jw4'r-I,:-"Q.Q'lR a.Obtainrepresentative samples,selectedatrandom,fromthe'nstalled andstockmaterialforallshapesandmanufacturers.

b.Performdestructive sheartestonthesamplestoobtainfailureloads.c.Analyzethetestresultsstatistically foreachshapeandmanufacturer tocomputecorresponding expectedstrengthperspot-weld atacertainconfidence level.6.3~samlesSampleswereobtainedbyBechtelfieldengineering personnel andsenttoarecognized testinglaboratory.

Samplesweretypically 6"longcontaining twospot-welds.

Forthestockmaterial, twosampleswereobtainedfromeach20'-0"lengthandonlyonesamplefromanygiveninstalled memberselectedatrandom.Forthcinstalled

material, generally onesamplewasobtainedforevery100feetoftheinstalled quantity.

6.4~TestisTestingwasdonebyPittsburgh TestingLaboratory (PTL)inPittsburgh, Pennsylvania.

Testmethodandprocedures arefullydescribed inPTL'sreport.(SeeAttachment A).(P,23-9)

Revision16.5Statistical Analsis>e4-isgasp:g.pBaseduponthetestresults,histograms wereplottedforeachshapeandmanufacturer.

Meanandstandarddeviation werecomputedforeachcase.Thetestresultsgenerally followthenormaldistribution.

Theexpectedfailureloadperspotweldwasdetermined byusingthemeanofthefailureloadbaseduponthetestresultsandsubtracting oneandahalftimesthestandarddeviation.

Thisapproachprovidesmorethan90%confidence levelfortheexpectedfailureload.Baseduponabovecriteria, theexpectedfailureloadswerecomputedandaregiveninTableI.7.0TECHNICAL EVALUATION OFDEFICIENCY 7.1GeneralThetechnical evaluation inthissectionislimitedtoallstrutmembersinstalled and/oratthejobsitewhicharenotfabricated.

7.2DesinCriteriaandTheoretical Considerations AISIspecification specifies allowable shearstrengthperspot-weld tobeequalto1.65kipswithafactorofsafetyof2.5for0.109inchthickness.

Thisal-lowableshearisbasedupon"Recommended Practices (923-9)

Revision1Jgg-(cg,gg.Q'j'f forResistance Welding,"

AWSC1.1,byAmericanWeldingSociety.Basedonthiscriteria, thefailureloadforaspotshouldbeover4,000pounds.However,inlightoftheproblemassociated withspot-welding, itisnecessary toassessbasesofcoderequirements andevaluatestructuraladequacyoftheinstalled materialwithoutcompromising thebasicdesignphilosophy.

Theconnection providedbyspotweldsbetweenadjoin-ingchannelsectionsisreliedupontomaintaintheintegrity ofthebuilt-upsections.

Thecalculated shearinaspotweldinamemberdependsuponmanyvariables suchasloading,sectional properties, endconditions andifthememberisusedasabeam,brace,columnortie.Theallowable shearof1.65kips/spot specified bythecodeistheupperlimitfordesigning purpose.However,fromtheevaluation pointofview,itismorerealistic toconsideractualmaximumdesignshearcal-culatedindividually asrequiredforeachshape.Sec-ondly,forthestrutmaterial(Fy=33k.s.i.)theallowable bendingstress(Fg)percodeis20k.s.i.for0.105inchthickness.

Therefore, thefactorofsafetyforthebendingstressisconsidered tobe1.65whileforthespot-welds, itis2.5,whichisratherhigh.Reasonforthiscouldbeattributed tomanyvariable, w7(P23-9)

Revision1Ju4ra098012affecting theweldstrength.

Forthematerialinques-tiononSusquehanna, anextensive testprogramhasbeencarriedoutandexpectedfailureloadsforeachshapearewelldefined.Therefore, itisreasonable toas-sumethatafactorofsafetyof2.0forspot-welds, whichisstillhigherthanthefactoryofsafetyforbendingofthestrutmaterialwouldstillbeadequateandconsistent withthebasicdesignphilosophy.

73Thedesignofframingmembersinthesupportsystemisbaseduponallowable shearof1650lbs.perspotweld.Therefore, tomaintainaminimumsafetyfactorequalto2.0,theexpectedfailureloadmustbe3300lbs.orgreater.7.3.1Thestrutsectionswiththeexpectedfailureloadequaltoorgreaterthan3300lbs/spotweld,areconsidered tobestructurally adequate.

7.3.2However,fortheothersections, furtherevaluation isnecessary.

Designloadsonthesupportsystemsaregovernedbymanyconsiderations suchastypeofsupport,structure, elevation, etc.Forindividual mem-bersinasupport,designshearforcesaddition-allydependuponifthemembersareloadedflex-urallyoraxially.Etisnotedthatforflexuralmembers,shearforcesaregenerally high.

Revision1JhNg<cg>GOjpFortheearlierinvestigation, a5'-0"longmember'sup-portinguniformly distributed loadwasconsidered tobearepresentative caseforvariousloadingconditions, andmembershapes,andcomparison wasmadebetweencom-puteddesignshearsandexpectedfailureloads.However,inthefinalanalysis, actualdesignshearswerecomputedinindividual membersforallsupport~typesinvariousbuildings andatdifferent eleva-tions.Theseshearvaluesvaryconsiderably; therefore, theexistingmembers,whichmaybeadequateatacertainlocation, mayneedstrengthening elsewhere depending ontheaforementioned variables.'n anycase,afac-torofsafetyof2.0hasbeenmaintained.

7.4~SummarBaseduponafactorofsafety,of2.0,allshapesaregroupedinthreecategories.

CateorIUnistrut:

P-5501,P-1001C,P-3301,P-1001AP-1001C3Powerstrut:

PS-151,PS-202Theseshapesarestructurally adequateandneednorepair.

CateorIIUnistrut:P-1001,P-5001Powerstrut:

PS-201,PS-101,PS-204PS-3080B-Line:.B-12AThesesectionsareconsidered deficient atcertainlocations andneedtoberepairedasrequired.

CateorIIIUnistrut:P-1004APowerstrut:

PS-3022B-LineB-22X,B-11AAllmembersofaboveshapesaretoberepaired/

strengthened toprovideshearconnection capacityequaltotwicethedesignstrengthrequired.

8.0 CORRECTIVE

ACTION8.1GeneralCorrective actionsaregroupedinseveralcategories.

Eachcategoryandcorresponding actionisdescribed below.8.2Installed andStockMaterial8.2.1Powerstrut PS-3022,IUnistrutP-1004AB-LineB-22X.Basedupontheinspection byBechtelfieldengineering personnel, allmaterialofthis-10-(P23-9) evisionlconfiguration wasdeemedtobedeficient, andwillbecompletely repairedwithouttakinganycreditfortheexistingspotwelds.8.2.2B-LineB-11A.Installed andstockquantityforthisshapewassmall;therefore,.

nosampleswereobtained.

Intheabsenceofanytestdataontheexistingspotwelds,thissectionwillberepairedtodeveloprequireddesigncapacitytomeetthedesigncriteria.

8.2.3Powerstrut:

PS-151,PS-202Unistrut:

P-5501,P1001G,P-3301,P-1001A,IP-1001C3Baseduponthetechnical evaluation inSection7.0,existingspot-welding isadequateandwillperformsatisfactorily underdesignloads.Therefore,nofurtheractionisdeemedneces-sary.8.2.4Powerstrut; PS-201,PS-101,PS-204,PS-3080Unistrut:

P-1001,P-5001IB-Line:B12A-11-(P23-1/1)

Revision1igu7.0912Thesemembersdonotmeetthedesigncriteria, therefore,adetailedre-analysis andredesignhasbeenperformed tocomputerequiredadditional strengthanddesigndrawingshavebeenpreparedtoshowthenecessary repairdetails.There-quiredrepairwillbeperformed ontheinstalled andstockmaterial.

Oncompletion oftherepair,thestrutmaterialwillsatisfythedesigncri-teria.8.3StrutMaterialtobeReceivedintheFutureUntilnow,thestrutmaterialwasprocuredasastan-dard'offtheshelf'tem.

However,inlightoftheproblemexperienced withthespot-welding, anewspeci-.fication8856-C-92 (seeAttachment C)wasdeveloped forprocurement of,allspot-welded strutsinthefuture.TheresultsofthetestingbyPTLpositively indicatethatthespot-welds onaplainmaterialhavemuchhigherstrengththanspot-welds onmill-galvanized material.

Therefore, thespecification stipulates thatstrutmaterialtobecoatedafterspotwelding.

Inadditiontothis,toprovideareasonable assuranceofattaining thedesignstrength, thesupplierisre-quiredtointroduce adestructive testingprogramontheproductionsamplesandnomaterialwillbeshippedtoSusquehanna jobsiteunlessitmeetstheacceptance

.criteria.

Thisactionshouldpreventtherecurrenceofthisprobleminthefuture.(P23-1/2)

Revision19.0REPAIRMETHODSN4'Kid+Q3,+r,'.

1GeneralConstruction hasbeenprovidedtheoptiontoselectanyoneofthefollowing methodsonacase-by-case basis.9.Thismethodistoreplacethedeficient materialwithnewmaterialtobeprocuredperSpecification 8856-C-92 (seeAttachment C).9.3Mechanical Fasteners Secondmethodistoprovide1/4"dia.self-drilling andself-tapping metalscrewstoobtainrequiredshearstrength.

These,screwswillbeprocuredperSpecification 8856-C-91 (seeAttachment C).Inordertoassurestrengthrequirements forthescrews,itisrequiredtoperformadestructive sheartestonsamplesandeachlotmustmeettheacceptance criteria.Inadditiontoself-drilling andself-tapping screws,othermechanical fasteners arebeingconsidered suchaspop-rivets orclamps.Ifthesearedetermined tobetechnically acceptable, theywillbeusedasalternate tothescrews.-13-(P23-1/3)

'

ision19.II~VeldknThirdapproachistoprovideintermittent

.stitchweldingatthejointbetweenchannelsand/orchan-nelandsideplate.

10.0CONCLUSION

Oncompletion oftherequiredrepair/replacement oftheexistingspot-welded strutsdetermined tobedeficient, theexistingsupportsystemswillbestructurally ade-quatetosatisfythedesignrequirements.

Secondly, pro-curementofnewspot-welded strutmaterialpernewspec-ification (seeAttachment C),willprovideadequateas-suranceofpreventing therecurrence ofthisprobleminthefuture..-14-(F23-1/4)

Hevislon1TABLE1EXPECTEDFAILURELOADS~FE82'80C99480StandardExpectedFailureLoadMeanDeviation

=A-1.5BMeuufactur erMember(1bs)(1bs)(lbs)RemarksUnistrut(A)(B)P-10014140-1213P-550154991229P-50014357-,1072P-1001C53861033P-3301'4651025P-1001A4192554P-1001C3,5186 629232136563837392833614243Cat.IICat.ICat.IICat.ICat.ICat.ICat.I(Backtoback)P-1001C351264584439Cat.IPowerstrut (Backtoside)PS-2014418,1194PS-1515533892PS-1013855782PS-20441502233PS-308041261252262741952682801Cat.IICat.ICat.IICat.IIICat.II(Backtoback).PS-30804666(Backtoside)PS-2025783B12A356515111317240039501590Cat.IICat,ICateIINote'.1.Abovenumbersinthetablearebasedupononespotveld.2.Forexplanation ofRemarks"column,refertosection7.4.3.ForshapesinFigure1andnotintableabove(i.e.thoseinCategoryIII),refertosection7.4.(P23-10)

FIGURE1STRUTSHAPESANDIDENTIFICATION Revision1~Shae0UnistrutP-5001Powerstrut PS-101B-LineB-11A~ShaeRUnistrutP-5501Powerstrut PS-151B-LineB-12A~ShaeKUnistrutP-1001Powerstrut PS-201.~ShaeNUnistrutP-1004APowerstrut PS-3022B-Line,B-22X3~haeNUnistrutP-1001C3Powerstrut PS-3080~ShaeLUnistrutP-1001CPowerstrut PG-204~Shae0UnistrutP-1001APowerstrut PS-202ShauePUuiattutP-3301