Curtain Wall Engineering

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CurtainWallEngineering CE426:FinalReport JackGreenberg May6,2011

JackGreenberg

5/6/2011

CurtainWallEngineering Thecurtainwallisanimportantdevelopmentofthe20thcenturythatallowedfortaller, economicalandefficientconstructionofbuildings.Concisely,acurtainwallisanexteriornon loadbearingwallsupportedbythestructuralformworkofabuilding. 1 Acurtainwall,which connectstothestructureofthebuilding,isconstructedmerelytosupportitsowndeadweight, transferringverticalgravityandhorizontalwindloadstothebuildingstructure.Curtainwalls havebecomethenormforbuildingerection,findingapplicationinmostbuildingsthatare constructedtoday.However,curtainwallconstructionisnotactuallyanewinvention;rather,it hadexistedunnamedforcenturies.TheGreeksandRomanshaddevelopedarudimentary versionofmoderncurtainwallsusingnonbearingfillssuchasbrick,stoneorconcreteinside postandlintelconstructedframes.Likewise,medievalcathedralsemployedsimilar construction,withcolumnssupportingarchesthatcontainedcurtainwallsofstainedglass. 2 Onawidespreadlevel,curtainwallsystemsreplacedtheconventionalbuilding technologyofbearingwalls.Asopposedtocurtainwalls,bearingwallsarestructuralelements ofbuildings,supportingtheactualweightofthebuilding.Theinventionofsteelandconcrete framingallowedforthedevelopmentofcurtainwalltechnologybecausebuildingmaterials becamestrongenoughtocarrytheentireloadofthebuilding,nolongerneedingexterior bearingwallsforsupport.Theoldmasonrybearingwallsusedtoconstructolderbuildings1 2

Donaldson,Barry.ExteriorWallSystems:GlassandConcreteTechnology,Design,andConstruction. Hunt,WilliamDudley.TheContemporaryCurtainWall.

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JackGreenberg

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neededtobeenormoustosupporttheloadsofthefloorsabove,especiallyfortallbuildings wheresomewallscouldbeaswideassixfeetormore.Therefore,curtainwallshaveprovided animportantalternativetobearingwallconstructionbecauseitenabledthereductionofthe sizeoftheexteriorwalltoamerethreeinches,savingtime,spaceandmoneyfordevelopers andallowingtheconstructionofhighrisebuildings. Thisrelativelynewwallsystemlendsitselfverywelltomodernpracticesof prefabricationandmassproductionallowingforeconomyandflexibilityinthedesign.Themain drawbackofconstructingacurtainwallisthedifficultyinconnectingthewalltothestructural elementsofthebuildingwhichiseasilyachievedinbearingwalldesignsincethewallisactually partofthestructure.Nevertheless,thecurtainwallsadvantagesalsoincludeareducedweight, customizableappearance,andfaster,moreefficienterectionmakingitthemostviableoption forbuildings. Thecurtainwallfunctionsasafilterfortheelements,fire,people,animals,sounds odors,andanythingelsethatmightpassintooroutofabuilding.Thedifficultyindesigninga curtainwallisintegratingthesometimesconflictingindividualfunctionsthewallmustprovide forthebuilding.Forexample,controllingthecondensationthatappearsupontheglazing usuallycomplicatestheprotectionagainstprecipitation.Therefore,throughouttheentire processofdesign,carefulthoughtandconsiderationgoesintotheformationofeachelementin thecurtainwall. Curtainwallsaregenerallycomposedofseveralbasicelements,notlimitedtobut includingthefillorpanel,joints,vaporbarrier,backingandinsulation.However,somecurtain3

JackGreenberg

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wallsdontcontainalloftheseelementssuchasaluminumframedcurtainwallsthatdont containaconcretemasonryunitbacking.Additionally,curtainwallelementsarecomposedof differentmaterials,varyinginshapeandsize. Probablythemostdifficultelementsofthecurtainwalltodesignarethejointsthat connectthecurtainwall.Oneofthethingsthatacurtainwallismostsusceptibletoiswater leakage.Porousmaterialssuchasstoneandmasonry,allowwatertobesiphonedthroughtheir surfacestotheinside. 3 Nevertheless,thecriticallocationofwaterentrythoughcurtainwallsis atthejointsofthefaade.Thedesignofthejointiscomplicatedforcurtainwallssincethejoint invariablybecomestheflexiblelinkinthewall.Itmustbeweathertight,yetfreetovaryits dimensionswithchangesintemperature.Itmustprotectagainstrainwhileallowing condensationtobleedoff.Therearevastnumbersofjointsinthefaademakingitaneven greaterdesignchallengeforengineers. Besidesforprecipitationprotection,controlagainstcondensationisimportantfor maintainingclearwindowsandformaintainingtheintegrityoftheinsulationofthewall.A vaporbarrierisconstructedtoallowthelittlewaterthatinevitablyentersthebuildingtoreturn theatmosphere.Inmanycurtainwalldesigns,thisprovisionconsistsofopeningorweepsholes inthebaseoftheunit.Weepholesaredifficulttodesignbecausetheycanactuallyhave negativeeffectsonthebuildingifitisnotdesignedproperly.Iftheweepholesarenotlarge enoughtheycanbecomeineffectivesincegrimeanddirtcouldpotentiallyclogthem. Additionally,iftheweepholesaretoolargeorareplacedtofrequentlythroughoutthefaade, watercanenterthebuildingmoreeasily,defeatingthepurposeoftheholes.Similarly,the3

Hunt,WilliamDudley.

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JackGreenberg

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vaporbarrierneedstobewideenoughtoallowroomforthedeflectionofthepanelsspanning acrossthewallwithoutcommandeeringtoomuchofthespaceofthebuilding. 4 Thepanelsandglazingthatspanacrossthebuildingareelementsthatcoveramajority ofthebuildingenvelope.Thecriticaldesignchallengeintheseelementsisthelargesurface areatheypossess.Probablymostimportantly,thematerialchosenforthepanelandglazing greatlyinfluencesthethermalpropertiesofthecurtainwall.Amongthebiggestconcernsin curtainwalldesignisadequatetemperaturecontrol.Thevarioustypesofpanelsandglazing canhavelargeimplicationsinthemoneyspentoncoolingandheatinginthebuilding. Additionally,fireprotectioniscontrolledbytheeffectivenessofthepanelsandglazing.Thewall mustnotallowfirestospreadfromonebuildingtoanotherorallowfirestospreadtodifferent floorsofthebuilding.Masonrymaterialsslowdowntherateofheattransferbyusingalarge amountofheattoevaporatethewatercontainedintheirphysicalorchemicalcomposition, offeringanexcellentsourceoffireprotection.Largeamountsofheatcanbestoredinthemass ofmasonry,enablingittowithstandexposuretofireforalongerperiodoftimebeforethe oppositewallfacetemperaturerisesappreciably. 5 Panelsandglazingmustalsowithstandstressescausedbythebuildingmovement resultingfromexpansionorcontraction,orlateralloadingfromearthquakesandhurricanes. Thisismuchgreaterthanthedeadloadofthepanelswhichisusuallymuchsmallerin comparison.Nevertheless,structurally,thedesignofacurtainwallhingesontheeffectiveness ofthesealantandtheconnectiontothestructuralframing.4 5

Hunt,WilliamDudley. Hunt,WilliamDudley.

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JackGreenberg

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Theadhesiveusedtofixtheglassofcurtainwallsinplaceisasiliconeelastomeric sealant.Siliconesealantsusedinstructuralglazingvaryastotypeandusage,howeverthemain propertiesaregenerallythattheyarelittleaffectedbyultravioletlight,canbondreliablyto glassandaluminumandhaveasufficientelasticitytoaccommodatethermalexpansionand structuralmovements.Thestructuraladhesivejointdesignrequiresengineeringanalysis, consideringthestrengthofthesealantintensileadhesion,shearadhesion,cohesionand movementcapability,thegeometryofthesealantincontactwidthandthicknessandthe imposedloadsonthesealant.Thesiliconeadhesivemustbesizedtocarryallforcesand accommodateallmovementswithoutoverstressingit.Additionally,thesystemmustdrain waterfromthesealsandshouldbecapableofbeingretrofittedwithnewglazingforwhenthe glassbreaks.Atthestartoflargescalecurtainwallglazingconstruction,theuseofsealantwas anuncertainformanyengineerssinceallofsiliconesmaterialpropertieshadnotbeenfully researched.Safetyfactorsofgreaterthansixwerenotuncommonamongdesigns.However,as thematerialscienceandresearchdeveloped,creativeandlessoverdesignedtechniqueswere usedintheconstructionusingthesealant.Thus,developmentssuchasstructuralglazing spacersmitigatedthestructuralprogressivecollapsefailureofthecurtainwall. 6 Allstructureswhethersteelorconcretewilltransfermovementtothebuildingfaade duetothermalexpansionandcontractionofthematerialssupportingabuilding.Steel structuresaremoregenerallymoreflexiblethanconcreteandwillapplyforcestothefaadein itsmovement.However,concretestructuresalsoimpartmovementtotheexteriorwalldueto

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Donaldson,Barry.

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longtermshrinkage.Therefore,wind,earthquakeandotherlateralloadsimposedonbuildings makeitcriticalforengineerstocarefullydesignexteriorwallstohandlethesedynamicforces. Thereareseveraltypesofcurtainwallsystemsthatarecurrentlyemployedinthe constructionofbuildings.Themostcommonincludeconventionalmasonry,panelizedwall systemsandaluminumframedcurtainwalls.However,therearemanymoreanddifferent typesofexteriorwallsystemsthatareusedinbuildingsbecausecurtainwallsbydefinition includeanytypeofnonbearingwall.Thedifferenttechniquesusedtocladabuildingare usuallyarchitecturallydriven,eachgivingadifferentlookandfeeltothebuilding.Nevertheless, astheeconomy,energyandfunctionalityofbuildingsbecomesadominatingroleindesign, exteriorsystemsaremoreengineeringdriven,witheachtypeoffaadehavinguniquequalities thatisadvantageousfordifferentdesigncriteria. Conventionalmasonryexteriorwallsystemsconsistofabrickorstonefaade, supportedbyconcretemasonryunitbacking.Thesetypesofwallsystemsaretypicallywell suitedforbuildingsthatrequirevariousnonuniformwallopeningsthroughoutthefacade. Thereforeitsapplicationismostresonantwithresidentialdesignmethodssinceresidential buildingsaretypicallynotrepetitivehorizontally.Theusageofthefloorspaceinresidential buildingsvariesmuchmorethanfloorspaceinofficebuildingsrequiringvariedwindow openingsinthefaade.Masonryisidealforthistypeofbuildingbecauseitssmallelemental sizecanadapteasily,enablingtheseopeningsbetterthanaluminumorglasssystems.

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JackGreenberg

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Panelizedexteriorwallsystemsconsistofprefabricatedorfactorymanufacturedpanels thatformastructuralenvelope. 7 Thebuildingpanelsarebestandeconomicallymanufactured uniformly;therefore,unliketheconventionalmasonryfaade,theyareunsuitableforsmall variedopeningsinthefaade.However,panelizedw