Glass and Metal Curtain Wall Systems.docx

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