Glass and Metal Curtain Wall Systems.docx
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Glass and Metal Curtain Wall Systems.docx
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GlassandMetalCurtainWallSystems
GlassandMetalCurtainWallSystems
ThispaperisareprintedversionofBuildingPracticeNoteNo.37,entitled"BuildingEnvelopeDesignUsingMetalandGlassCurtainWallSystems".Notwithstandingsomeminordifferencesintheoralpresentation,itcontainstheessenceofthetalkpresentedattheBuildingScienceForum'82.Thereare,however,afewadditionalexamplesofinterfaceconnectiondetailsbetweenatypicalglassandmetalcurtainwallandotherpartsofthebuildingenclosure.
InadditiontothepaperthatfollowsthereareseveralquestionsandanswersrelatingtotheuseofcurtainwallsystemswhichweretakenfromtheafternoonforumdiscussionsduringtheBuildingScienceForum'82.Thesequestionsandanswerswillbefoundinthelastsectionoftheseproceedings.
Introduction
Metalandglasscurtainwallsystemshavefoundgrowingfavourinmodernarchitecture.Theyareeasilydistinguishedfromothertypesofcladdingsbytheirthinmullionsofhorizontalandverticalmetallicbarssurroundinganallglassormetalpanel.Thecurtainwallsystemhasevolvedrapidlyoverthelasttwodecades,especiallywithrespecttoweathercontrolperformance.Theearlysystemspresentedfrequentrainpenetrationproblems;largeicicleswouldformontheoutsidehorizontalbarsorcondensationontheinsidemullionsurfaces;glazingsealsweresometimespumpedoutoftherabbetofsealeddoubleglazingwindowunits.However,mostofthesedifficultieswereeventuallyovercomewithimproveddetaildesignofthesystemcomponents.Today,mostcurtainwallmanufacturersofferaqualityproductlineofcomponentswhichcanbe.usedtocreateoneofthebestoverallexteriorwallsystems.
Somedifficultyremains,however,inobtainingequalorevenadequateperformancefromthejointsattheinterfacebetweenthecurtainwallsystemandtheremainderofthebuilding.Numerousweathercontrolproblemsareencounteredatparapets,soffits,cornerdetailsandgradeconnections.Inadditiontorainwaterpenetrationandthefreeze-thawactionofconcealedcondensation,coldairseepsintosoffits,causingdrainpipes,steampipes,orsprinklersystemstofreezeup.Largeicicleswhichformalongtheroofedgeareoftenapotentialhazardtopedestriansbelow.
Recentfieldinvestigationsandstudiesofconstructiondetaildesigndrawingshaveindicatedsomepracticeswhichcontinuetocausetheseproblems.Designandconstructionrequirementsdiscussedinthisnotecanminimizeundesirableconditionsattheinterfaceconnectionofcurtainwallsystemstootherpartsofthebuilding.
WhatisaCurtainWall?
Acurtainwallsystemisalightweightexteriorcladdingwhichishungonthebuildingstructure,usuallyfromfloortofloor.Itcanprovideavarietyofexteriorappearancesbutischaracterizedbynarrowlyspacedverticalandhorizontalcapswithglassormetalinfilpanels.Thesesystemsprovideafinishedexteriorappearanceandmostoftenasemi-finishedinterioraswell.Theyarealsodesignedtoaccommodatestructuraldeflections,controlwind-drivenrainandairleakage,minimizetheeffectsofsolarradiationandprovideformaintenance-freelongtermperformance.Mostoftoday'smetalcurtainwallsystemsareconstructedoflightweightaluminum,althoughsomemaybeofsteel.
Sincecurtainwallsystemsareagoodexampleofbuildingscienceprinciplesappliedtowalldesign,itmaybeusefultoreviewsomebasicprinciplesthroughthedetailsofatypicalcurtainwallsystem.
DesignPrinciplesforExteriorWalls
Abuildingenclosuremaybebroadlydefinedasasetofinterconnectingelementswhichseparatetheoutsidefromtheinside.Theseelementswouldincludeexteriorwalls,aroof,othercomponentssuchaswindowsanddoors,andsometimesexposedfloors.Thefunctionofabuildingenclosureistocontrolthepenetrationofsnow,wind,rainandsuntotheinsideandtocontainthedesiredindoorclimate.Theenclosuremustmeetmanyindividualrequirementsbutforthepurposeofthispapertheyarelimitedtothefollowingsix:
1.controlofairflow,
2.controlofheatflow,
3.controlovertheentryofrainandsnow,
4.controlofsunlightandotherformsofradiantenergy,
5.controlofwatervapourdiffusion,
6.accommodationofbuildingmovements.
Therequirementforairtightnessandconsequentlyairleakagecontrolismetbymostcurtainwallsystemsbecausetheairbarrierofthewallisinherentinthestructuralpropertiesofglassandaluminumorsteeltubesthatcomprisethesystem.Thecontinuityoftheairbarrier(Figure1)isachievedbythecontinuityoftheglasspanelthroughtheairsealattheshoulderflangesofthetubularmullion,andthroughthealuminumsectiontotheotherflangesurface.Theairsealbetweenthelowershoulderflangeofthecurtainwallmullionandthemetalpanofthespandrelpanelprovidescontinuityofairtightnesstotheairbarriermetalpanandontothenextmullionconnection.Suchassembliesareregularlytestedusingairpressuretodeterminethestructuralpropertiesoftheglass,metal,andsealsandtodeterminetheequivalentleakagearea(ELA)thatremains.Inaddition,theArchitecturalAluminiumManufacturersAssociationimposesuponitsmembersmanyotherrequirementsincludingaspecificationthatthesystemmustnotleakmorethan.30L/sperm2(.06cfmperft2)ofwallatapressuredifferenceequivalenttoa40km/h(25mph)wind.
Figure1
ThermalInsulation(Controlofheatflow)
Thecontrolofheatflowisgenerallyachievedthroughtheuseofinsulation.Althoughitisnotapparentfromtheexterior,thecurtainwallsystemusesconsiderableinsulationusuallybehindspandrelglassoranyopaquepanels.Becauseofthematerialsusedinthestructure,i.e.,glassandmetal,whicharehighlyconductive,thesystemmustalsocontendwithpotentialcondensationontheinteriorsurfaces.Tocurtailthiseffect,mostcurtainwallsystemsincorporatetwodistinctfeatures:
first,asealeddoubleglazedwindoworaninsulatedmetalpanandsecond,athermallybrokenmullion,usuallywithaPVCplasticinsertandmorerecently,afoamed-in-placepolyurethaneconnection.Asealeddoubleglazedwindowunitcanaccommodateanindoorhumidityuptoabout35%atanoutdoortemperatureof-25°Cwithlittlecondensationappearingontheglass.Similarly,thethermalbreakinthealuminumorsteelmullionensuresthatthesurfacetemperatureofthestructuralmullionwillremainwellabovethedewpointtemperatureoftheairformostbuildingtypes,exceptforhighhumidityindoorenvironmentssuchasinswimmingpoolsorcomputercenters.Thethermalbreakalsoensuresthatthestructuralmullionisthermallystable,thatis,notsubjecttoextremesofexpansionandcontraction.
The"RainScreen"Principle(Controlofrainandsnowpenetration)
Tocontrolrainpenetrationthroughexteriorwallstheconventionalapproachistosealtheexteriorfaçadeofthebuilding.However,experiencehasshownthatitisunreasonabletoexpectperfectsealingofafaçade;mostsealingstrategiesrequirecontinuousattentionandmaintenance.
Studiesoftherainpenetrationproblemhaverevealedabettersolutionthanthefaçadesealingapproach.Iftheairthatleaksinandthroughcracksandcrevicesofafaçadeduringarainstormwerelimitedorstopped,mostofthewaterimpingingonthefaçadewouldmigratestraightdownthesurfaceandlittlewouldpenetratethewall.Thisistheessenceofthe"RainScreen"principle.Ifanairtightelementispositionedbehindafaçade,thecavityformedbetweentheexteriorcladdingandtheairtightelementmayreachthesameairpressurelevelasisexertedonthecladdingsurface,thusremovingtheforcewhichcausesairtoflowthroughanyfaçadeopening.The"RainScreenWall"isthereforecharacterizedbyacavitybehindtheexteriorsurfacethatisconnectedtotheexteriorbutsealedtightly,orastightlyasreasonablypossible,totheinterior.Theinnersurfaceofthechamberisusuallyreferredtoastheairbarrierofthewall.
Inmostcurtainwallsystemsthejointbetweentheinfilpanel(i.e.,windoworspandrelpanel)andthestructuralmullionisusuallydesignedtobepartofarainscreensystem(Figure2).Itcomprisesapressure-equalizedcavity,connectedtotheexteriorbythedrainholesintheexteriorcaps,andapressureequalizedraindeflectorsealbetweentheoutsidesurfaceoftheglassandthemullioncap.Thechamberportionofthecavityiscomposedoftheairsealsconnectingtheinsidefaceofthewindowglassandthespandrelpanelmetalpan,totheshoulderflangesofthestructuralmullionandotherpartsofthestructuralsection.Thusthesetofelementscomprisingthewindowglass,theairseals,andthealuminumsectionandmetalpanperformtheairbarrierfunctionforthiswallassembly.Thisdesignconfigurationforcurtainwallsectionshasprovensuccessfulandhasbecomewidelyaccepted.
Figure2
SolarRadiation(Controlofsunlightandotherformsofradiantenergy)
Solarradiationfallingonbuildingsurfacesmayhavetwodistincteffects:
thefirstistocauseasignificantchangeintemperatureofthefaçadeelementsandthesecondistheslowbutdestructiveeffectofultravioletradiationimpingingonallmaterials,particularlyorganic.Oncurtainwallsystemsthemostimportantconcernswithsolarradiationhavebeenthethermalexpansionandcontractionofcurtainwallcomponents,inparticularthoseformingtheoutsidecladding,andtheeffectsofsolarradiationontheglazingelements.Awarpingofglassoccursduetodifferencesintemperaturebetweentheinnerandouterpanes,whilepumpingresultsfromexpansionandcontractionoftheairinthecavit
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