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时间:2018-09-29
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1、TheFireResistanceofSteelStructuresE.GEHRIFederalInstituteofTechnology,Zurich,SwitzerlandINTRODUCTIONSTEELISanoncombustiblematerial,butthemechanicalpropertiesofstructuralsteelareaffectedbyheat.Thefireresistanceormoreexactly,thefireenduranceofasteelelementvariesgreatlyand,therefore,weneedabette
2、runderstandingoftheconditionswhichaffectfireresistance.Fireresistanceisunderstoodasthetimeduringwhichthestructuralelementcanwithstandthestandardprovisionsofafiretest.Forstructuralelementstheonlycriteriontobeconsideredisthestructuralcollapseoftheelement.Othercriterialiketemperaturetransmission
3、maybedisregarded.ForasteelcolumnthestandardprovisionsforafiretestaregiveninFigure1.Wemustnotethatthestandardtestprescribestheuseofaconstantloadcorrespondingtothemaximumdesignvalue.Thethermallongitudinaloraxialexpansionofthecolumnwillbecompensatedduringthetest,i.e.thereisnorestraint,thecolumnc
4、anmovefreelyintheaxialdirection.Bytestswecanfindtheultimatestrengthofthecolumnfordifferentfireendurances.Usingthesametypeofcolumnandsamedimensionsweobtainaninteractiondiagram(Figure2).Thereductioninstrengthwiththepassingoftimedependsonthetemperatureincreaseinthespecimentested.Thetemperaturein
5、creaseisafunctionoftheso-calledsectionfactorandoftheinsulationapplied.Thescatteroftestresultsisquitenormal,sincethecolumnstestedarenotidenticalduetothedimensionaltolerancesandthevariationinstrengthofthesteel.Moreorlessthesamescatterisfoundwhentestingsteelcolumnsatroomtemperature.Nowintroducin
6、gthedesignvalueintoFigure2,wecandirectlyfindthefireresistance.Generally,themeanvalueofthetestresultswillbehigherthanthefireresistancebasedonthenominalguaranteedstrengthofthesteel.FromFigure2itcanalsoeasilybeseen,thatacolumnwhichissubjectedtoalowerstresslevel--i.e.,toaneffectiveloadlowerthanth
7、edesignload--willhavehigherfireresistance.Finally,wemustnotethattheconditionsduringthefiretestdonotcorrespondtothoseinpractice.Besidesdifferentheatingratesinanaturalfire,wehavetoconsidertherealboundaryconditions.Supportconditionsandthelongitu
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