土木工程英文翻译资料.docx

土木工程英文翻译资料.docx

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土木工程英文翻译资料

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（validforst