土木工程英文翻译.docx
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土木工程英文翻译.docx
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土木工程英文翻译
TheFireResistanceofSteelStructures
E.GEHRI
FederalInstituteofTechnology,Zurich,Switzerland
INTRODUCTION
STEELISanoncombustiblematerial,butthemechanicalpropertiesofstructuralsteelareaffectedbyheat.Thefireresistanceormoreexactly,thefireenduranceofasteelelementvariesgreatlyand,therefore,weneedabetterunderstandingoftheconditionswhichaffectfireresistance.
Fireresistanceisunderstoodasthetimeduringwhichthestructuralelementcanwithstandthestandardprovisionsofafiretest.Forstructuralelementstheonlycriteriontobeconsideredisthestructuralcollapseoftheelement.Othercriterialiketemperaturetransmissionmaybedisregarded.
ForasteelcolumnthestandardprovisionsforafiretestaregiveninFigure1.
Wemustnotethatthestandardtestprescribestheuseofaconstantloadcorrespondingtothemaximumdesignvalue.Thethermallongitudinaloraxialexpansionofthecolumnwillbecompensatedduringthetest,i.e.thereisnorestraint,thecolumncanmovefreelyintheaxialdirection.
Bytestswecanfindtheultimatestrengthofthecolumnfordifferentfireendurances.Usingthesametypeofcolumnandsamedimensionsweobtainaninteractiondiagram(Figure2).
Thereductioninstrengthwiththepassingoftimedependsonthetemperatureincreaseinthespecimentested.Thetemperatureincreaseisafunctionoftheso-calledsectionfactorandoftheinsulationapplied.Thescatteroftestresultsisquitenormal,sincethecolumnstestedarenotidenticalduetothedimensionaltolerancesandthevariationinstrengthofthesteel.Moreorlessthesamescatterisfoundwhentestingsteelcolumnsatroomtemperature.
NowintroducingthedesignvalueintoFigure2,wecandirectlyfindthefireresistance.Generally,themeanvalueofthetestresultswillbehigherthanthefireresistancebasedonthenominalguaranteedstrengthofthesteel.
FromFigure2itcanalsoeasilybeseen,thatacolumnwhichissubjectedtoalowerstresslevel--i.e.,toaneffectiveloadlowerthanthedesignload--willhavehigherfireresistance.
Finally,wemustnotethattheconditionsduringthefiretestdonotcorrespondtothoseinpractice.Besidesdifferentheatingratesinanaturalfire,wehavetoconsidertherealboundaryconditions.Supportconditionsandthelongitudinalrestraintmayresultinsubstantiallydifferentfireresistances.
FIRERESISTANCEOFCONCRETE
Thefireresistanceofconcretemembersismuchhigherthanthatofnoinsulatedsteelmembers.Howeverthetendencytousehigherstrengthconcreteandsteelreinforcementsleadstosmallersectionsandthereforetolowerfireresistance.Furthermore,manysteelconstructionsincludeconcreteelementstoalargeextentorhaveso-calledcompositesteel-concreteelements.Wethereforeneedageneralproceduretoanalyzethefireresistanceofdifferentstructuralmaterialsandofcompositeelements.Wecouldalsotreatthereinforcedconcreteasacompositematerial,whichitis.
FIRERESISTANCE---ASPECIALASPECTOFFIREPROTECTION
Thefireresistanceofsteelstructuresisonlyasmallpartofthefireprotectionproblem.Wehavetostartbyreducingthefireload,byusingsmallcompartments,bytryingtoremovetheheatasquicklyaspossible,andfinally,bypreventingheattransfertothestructuralmembers.
Thecalculationoffireresistanceisthelastresort,whichdoesnotmeanthatfireresistancehasnoimportance.Butbeforecalculatingweshoulddetermineifthefireresistanceproblemcannotbeminimizedbyacombinationofdifferentsubdivisionsofthevolume,orbyplacingthemainstructuralpartsinlessexposedareasorbyremovingtheheatthroughopenings.
Furthermore,wehavetoacknowledgethattheactualfireconditionsaredifficulttoknow,whichmeansthatwehave,ontheso-calledloadside,greatuncertainties.
STRUCTURALPERFORMANCEREQUIRED
FIRERESISTANCE--COLLAPSELOAD
Intheintroduction,fireresistanceisdefinedasthetimeafterwhichacollapseofthestructuralelementoccurs,assumingariseoftemperatureinthefurnaceaccordingtoISO834.
Weknowfromexperiencethattestspecimensmayalsocollapseafterthetestduringthecoolingphase.Thismaybeduetocontinuinglossofstrengthandduetoredistributionoftemperatureinthespecimen.Forsteel,ingeneral,wedonotneedtoconsiderthatcase,butitshowsthedifficultyofaproperdefinitionofthefireendurancebasedonthecollapseload.
FIRERESISTANCE--DAMAGELEVEL
Theassumptionofcollapseasacriteriondoesnotmeanthatweexpect--eveninaseverefire--thetotalcollapseofthestructure.Itseemsfurthermore,usefultoconsiderdifferentcriteriaformainstructuralandforsecondarystructuralelements.
Whetherastructuralelementispartofthemainstructureorsecondarystructureisnotalwaysobvious.Aguidelinetofollowcanbe:
ifthetotalcollapseofanelementdoesnotseriouslyaffectthestructuralbehaviorasawhole,itcanbeconsideredsecondary.Thereforelowerrequirements–evennone--maybelaiddownforsuchstructuralelements.
Weshould,therefore,discussmainstructuralorsecondarystructuralelements,totalcollapseorpartialorlocalcollapse,degreesofdamageandpossibilitiesofrepair.
Structuralmaterialslikesteel,timberorconcrete,showdifferentreactionstofire.Afterafire,concreteandtimbershowalossofresistancewhereasprotectedsteelstructuresmayhavetheoriginalstrengthaftercooling.
Repairpossibilitiesandrepaircostsmaythereforebeverydifferent.Inextremecasesareplacementofaconcretestructurewillberequired,whereasonlyasimplereapplicationoffireprotectionsurfacestothesteelelementswillbenecessary.
Dependingonthestructuralperformancerequiredduringandafterafire,wemustestablishreasonableandrealisticcriteria.Thepresentorientationbasedonlyonthecollapseofanindividualisolatedelementcannotbetheentirebasisoffireresistanceclassification.
FIRERESISTANCECLASSIFICATION
STANDARDFURNACETESTS
Thewell-knownproceduretoestablishfire-resistancetimeneedsnoexplanation.IcanreferheretoISO834whereacceptanceconditionsarespecified.Thereisacertaincriticismaboutthoseconditions,namelyabouttherequirementtotestthespecimen,asfaraspossible,undersupportingandrestrainingconditionsthataresimilartothoseinservice.
Theseconditionsareseldomfulfilledinstandardfurnacetests,butIseenoprobleminitaslongastheactualboundaryconditionsaregiveninconnectionwiththeobservedfireresistance.Thesameconditionholdsfortestinginsulatedsteelmembers,where,inadditiontotheinsulation,thesectionfactororthesteelelementhastobeintroduced(seeFigures3and4).
Ifappropriatedesignloadsareapplied(useofthesamesafetyfactoratroomtemperature}thefireresistanceofthethreecolumnsinFigure3willbenearlythesame,ifthereisnoaxialrestraint.
Withfullknowledgeoftestingconditionsandtestlimitations{onlysimpleelementslikecolumnsandsimplegirders;dimensionallimitationsinsectionandinlength}testsmadeinaccordancewithISO834andfireresistanceclassificationremainavaluabletool.
ANALYTICALPROCEDURES
Apartfromthelimitationsmentioned,thefurnacetestshavethemajordisadvantagesofbeinglaboriousandcostly.Thiswasthereasonfordevelopingdifferentanalyticalproceduresinthelast10years.
Forsteelelementstheanalyticalprocedureconsistsof(seealsoFigure5)
•Acalculationofthetemperaturelevelinthesteelelement,assumingaheatingprocesscorrespondingtostandardfiretests.
•Acalculationoftheeffectofthetemperatureontheloadbearing"capacityoftheelement.
Withsuchananalyticalprocedureitshouldbepossibletoreproducetheresultsoffiretestsinfurnaces.Iftheanalyticalprocedureiscorrect,thecalculatedtimeoffailureshouldbeidenticalwiththefire-resistancetimeobtainedinatest,assumingstructuralelementsofthesamemechanicalcharacteristicsandthesameloadlevel.However,somedissimilaritymustbeexpected,sincealsoatroomtemperature,predictionsofultimatestrengthshowacertainscatter.
Oftheexistinganalyticalproceduresitispreferabletousethosewhichareeasytohandlebutstillhaveanappropriatelevelofaccuracy.Thisleadstosimplificationsandasaresult,therangeofvaliditymaybenarrower.
Anideaoftheappropriatelevelofaccuracymaybeseenfromthefollowingchart.
Mostanalyticalproceduresaretoogood.Theyarealsotoolaboriousandpronetoerrors.Wethereforeneedasimpletooltoestimatewithreasonable.Precisionthefireresistanceofasteelelement.SuchatoolwasrecentlypublishedbytheEuropeanConventionforConstructionalSteelworkunderthetitle"CalculationoftheFireResistanceofLoadBearingElementsandStructuralAssembliesExposedtotheStandardFire."
Thefollowingcalculationprocedureisbasedontheabovepublication,butintroducessomesmallmodificationswhichallowaverysimpleandquickdeterminationofthefireresistanceofnoinsulatedandinsulatedsteelelements.Themethodallowsusalsototakeadvantageofalowerstresslevelthantheallowabledesignvalue.Wehaveseenbeforethatalowerstresslevelleadstohigherfireresistanceofthemember.
PRACTICALAPPROACH
ASSUMPTIONS
Thefollowingassumptionsaremade:
•TemperatureinfurnaceaccordingtoISO834
•Theheatingofthesteelelementisuniform(largeandcompactsteelsectionsdonothoweverShowauniformtemperature)
•Thereductionofallstrengthandstiffnesscharacteristicsisproportionaltothereductionoftheyieldpoint(validforstru
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