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DesigningAgainstFireOfBuilding
文献、资料来源:
国道数据库
文献、资料发表(出版)日期:
2008.3.25
院(部):
土木工程学院
专业:
土木工程
班级:
土木辅修091
***********
学号:
**********
指导教师:
周学军、李相云
翻译日期:
20012.6.1
外文文献:
DesigningAgainstFireOfBuliding
JohnLynch
ABSTRACT:
Thispaperconsidersthedesignofbuildingsforfiresafety.Itisfoundthatfireandtheassoci-atedeffectsonbuildingsissignificantlydifferenttootherformsofloadingsuchasgravityliveloads,windandearthquakesandtheirrespectiveeffectsonthebuildingstructure.Fireeventsarederivedfromthehumanactivitieswithinbuildingsorfromthemalfunctionofmechanicalandelectricalequipmentprovidedwithinbuildingstoachieveaserviceableenvironment.Itisthereforepossibletodirectlyinfluencetherateoffirestartswithinbuildingsbychanginghumanbehaviour,improvedmaintenanceandimproveddesignofmechanicalandelectricalsystems.Furthermore,shouldafiredevelops,itispossibletodirectlyinfluencetheresultingfireseveritybytheincorporationoffiresafetysystemssuchassprinklersandtoprovidemeasureswithinthebuildingtoenablesaferegressfromthebuilding.Theabilitytoinfluencetherateoffirestartsandtheresultingfireseverityisuniquetotheconsiderationoffirewithinbuildingssinceotherloadssuchaswindandearthquakesaredirectlyafunctionofnature.Thepossibleapproachesfordesigningabuildingforfiresafetyarepresentedusinganexampleofamulti-storeybuildingconstructedoverarailwayline.Thedesignofboththetransferstructuresupportingthebuildingovertherailwayandthelevelsabovethetransferstructureareconsideredinthecontextofcurrentregulatoryrequirements.Theprinciplesandassumptionsassoci-atedwithvariousapproachesarediscussed.
1INTRODUCTION
Otherpaperspresentedinthisseriesconsiderthedesignofbuildingsforgravityloads,windandearthquakes.Thedesignofbuildingsagainstsuchloadeffectsistoalargeextentcoveredbyengineeringbasedstandardsreferencedbythebuildingregulations.Thisisnotthecase,tonearlythesameextent,inthecaseoffire.Rather,itisbuildingregulationssuchastheBuildingCodeofAustralia(BCA)thatdirectlyspecifymostoftherequirementsforfiresafetyofbuildingswithreferencebeingmadetoStandardssuchasAS3600orAS4100formethodsfordeterminingthefireresistanceofstructuralelements.
Thepurposeofthispaperistoconsiderthedesignofbuildingsforfiresafetyfromanengineeringperspective(asiscurrentlydoneforotherloadssuchaswindorearthquakes),whilstatthesametime,puttingsuchapproachesinthecontextofthecurrentregulatoryrequirements.Attheoutset,itneedstobenotedthatdesigningabuildingforfiresafetyisfarmorethansimplyconsideringthebuildingstructureandwhetherithassufficientstructuraladequacy.Thisisbecausefirescanhaveadirectinfluenceonoccupantsviasmokeandheatandcangrowinsizeandseverityunlikeothereffectsimposedonthebuilding.Notwithstandingthesecomments,thefocusofthispaperwillbelargelyondesignissuesassociatedwiththebuildingstructure.
Twosituationsassociatedwithabuildingareusedforthepurposeofdiscussion.Themulti-storeyofficebuildingshowninFigure1issupportedbyatransferstructurethatspansoverasetofrailwaytracks.Itisassumedthatawiderangeofrailtrafficutilisesthesetracksincludingfreightanddiesellocomotives.Thefirstsituationtobeconsideredfromafiresafetyperspectiveisthetransferstructure.ThisistermedSituation1andthekeyquestionsare:
whatleveloffireresistanceisrequiredforthistransferstructureandhowcanthisbedetermined?
Thissituationhasbeenchosensinceitclearlyfallsoutsidethenormalregulatoryscopeofmostbuild-ingregulations.Anengineeringsolution,ratherthanaprescriptiveoneisrequired.Thesecondfiresituation(termedSituation2)correspondstoafirewithintheofficelevelsofthebuildingandiscoveredbybuildingregulations.Thissituationischosenbecauseitwillenableadiscussionofengineeringapproachesandhowtheseinterfacewiththebuildingregulations–sincebothengineeringandprescriptivesolutionsarepossible.
2UNIQUENESSOFFIRE
2.1Introduction
Windandearthquakescanbeconsideredtobe“natural”phenomenaoverwhichdesignershavenocontrolexceptperhapstochoosethelocationofbuildingsmorecarefullyonthebasisofhistoricalrecordsandtodesignbuildingtoresistsufficientlyhighloadsoraccelerationsfortheparticularlocation.Deadandliveloadsinbuildingsaretheresultofgravity.Alloftheseloadsarevariableanditispossible(althoughgenerallyunlikely)thattheloadsmayexceedtheresistanceofthecriticalstructuralmembersresultinginstructuralfailure.
Thenatureandinfluenceoffiresinbuildingsarequitedifferenttothoseassociatedwithother“loads”towhichabuildingmaybesubjectedto.Theessentialdifferencesaredescribedinthefollowingsections.
2.2OriginofFire
Inmostsituations(ignoringbushfires),fireoriginatesfromhumanactivitieswithinthebuildingorthemalfunctionofequipmentplacedwithinthebuildingtoprovideaserviceableenvironment.Itfollowsthereforethatitispossibletoinfluencetherateoffirestartsbyinfluencinghumanbehaviour,limitingandmonitoringhumanbehaviourandimprovingthedesignofequipmentanditsmaintenance.Thisisnotthecasefortheusualloadsappliedtoabuilding.
2.3AbilitytoInfluence
Sincewindandearthquakearedirectlyfunctionsofnature,itisnotpossibletoinfluencesucheventstoanyextent.Onehastoanticipatethemanddesignaccordingly.Itmaybepossibletoinfluencethelevelofliveloadinabuildingbyconductingauditsandplacingrestrictionsoncontents.However,inthecaseofafirestart,therearemanyfactorsthatcanbebroughttobeartoinfluencetheultimatesizeofthefireanditseffectwithinthebuilding.Itisknownthatoccupantswithinabuildingwilloftendetectafireanddealwithitbeforeitreachesasig-nificantsize.Itisestimatedthatlessthanonefireinfive(Favre,1996)resultsinacalltothefirebrigadeandforfiresreportedtothefirebrigade,themajoritywillbelimitedtotheroomoffireorigin.Inoc-cupiedspaces,olfactorycues(smell)providepowerfulevidenceofthepresenceofevenasmallfire.Theadditionofafunctionalsmokedetectionsystemwillfurtherimprovethelikelihoodofdetectionandofactionbeingtakenbytheoccupants.
Firefightingequipment,suchasextinguishersandhosereels,isgenerallyprovidedwithinbuildingsfortheuseofoccupantsandmanyorganisationsprovidetrainingforstaffinrespectoftheuseofsuchequipment.
Thegrowthofafirecanalsobelimitedbyautomaticextinguishingsystemssuchassprinklers,whichcanbedesignedtohavehighlevelsofeffectiveness.Firescanalsobelimitedbythefirebrigadedependingonthesizeandlocationofthefireatthetimeofarrival.
2.4EffectsofFire
Thestructuralelementsinthevicinityofthefirewillexperiencetheeffectsofheat.Thetemperatureswithinthestructuralelementswillincreasewithtimeofexposuretothefire,therateoftemperaturerisebeingdictatedbythethermalresistanceofthestructuralelementandtheseverityofthefire.Theincreaseintemperatureswithinamemberwillresultinboththermalexpansionand,eventually,areductioninthestructuralresistanceofthemember.Differentialthermalexpansionwillleadtobowingofamember.Significantaxialexpansionwillbeaccommodatedinsteelmembersbyeitheroverallorlocalbucklingoryieldingoflocal-isedregions.Theseeffectswillbedetrimentalforcolumnsbutforbeamsformingpartofafloorsystemmayassistinthedevelopmentofotherloadresistingmechanisms(seeSection4.3.5).
Withtheexceptionofthedevelopmentofforcesduetorestraintofthermalexpansion,firedoesnotimposeloadsonthestructurebutratherreducesstiffnessandstrength.Sucheffectsarenotinstantaneousbutareafunctionoftimeandthisisdifferenttotheeffectsofloadssuchasearthquakeandwindthataremoreorlessinstantaneous.
Heatingeffectsassociatedwithafirewillnotbesignificantortherateoflossofcapacitywillbeslowedif:
(a)thefireisextinguished(e.g.aneffectivesprinklersystem)
(b)thefireisofinsufficientseverity–insufficientfuel,and/or
(c)thestructuralelementshavesufficientthermalmassand/orinsulationtoslowtheriseininternaltemperature
Fireprotectionmeasuressuchasprovidingsufficientaxisdistanceanddimensionsforconcreteelements,andsufficientinsulationthicknessforsteelelementsareexamplesof(c).TheseareillustratedinFigure2.
Thetwosituationsdescribedintheintroductionarenowconsidered.
3FIREWITHINBUILDINGS
3.1FireSafetyConsiderations
Theimplicationsoffirewithintheoccupiedpartsoftheofficebuilding(Figure1)(Situation2)arenowconsidered.Firestatisticsforofficebuildingsshowthataboutonefatalityisexpectedinanofficebuildingforevery1000firesreportedtothefirebrigade.Thisisanorderofmagnitudelessthanthefatalityrateassociatedwithapartmentbuildings.Morethantwothirdsoffiresoccurduringoccupiedhoursandthisisduetothegreaterhumanactivityandthegreateruseofserviceswithinthebuilding.Itistwiceaslikelythatafirethatcommencesoutofnormalworkinghourswillextendbeyondtheenclosureoffireorigin.
Arelativelysmallfirecangeneratelargequantitiesofsmokewithintheflooroffireorigin.Ifthefloorisofopen-planco