Choosing The Wrong Fastener Can Have Catastrophic

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s, Inc.Marwin Co.Medallion Entry SystemsMegaMet IndustriesMesker Door-DesignHardwareMidwest Wholesale HardwareMIFABMillennium DoorMMF IndustriesMohawk Flush Doors, Inc.National Custom Hollow MetalDoors & FramesNational Guard Products, Inc.Ohio Valley Door Corp.OSG (Opening SolutionsGroup)Overly Door CompanyPDQ IndustriesPioneer Industries, Inc.Premier Products Inc.Protective Door IndustriesPyrophobic Systems Ltd.Red River Door Co.Reese Enterprises Inc.Republic Doors & FramesRyan Transportation ServicesInc.Samuel Heath & Sons, plcSecuritech Group, Inc.Security Metal Products/Security AcousticsSecurity Door ControlsSELECT Products Ltd.Simon Door Co. LLCSoftware for HardwareStier Steel Corp.Strek-O Doors LLCSupa Doors Inc.Tanner Bolt & Nut Corp.Tempo Hardware, Inc.Timely Prefinished SteelDoor FramesTop Notch Distributors Inc.Total Opening ApplicationDevelopersTownsteel Inc.Trans Atlantic CompanyTRIMCOUnderwriters LaboratoriesInc.Unison ArchitecturalHardwareVT Industries, Inc.Warminster FiberglassCompanyWikk Industries Inc.Woodfold Mfg., Inc.Woodtech TradingCompany, Inc.Zero International, Inc.

910Hydrogen generated throughgalvanic reaction can invade thesestress points, embrittling themetal. Micro-cracking appears,creating an attractive site forstress-corrosion. In an elementunder load, stress corrosion isprogressive: the more the crackedsite corrodes, the less metal there isto bear the tensile load, the greaterthe stress build-up at that point.Failure MechanismsPhoto Courtesy of Elco Construction Products,an Acument Global Technologies companyPhoto Courtesy of Elco Construction Products,an Acument Global Technologies company9. Self-drilling fasteners that are resistantto hydrogen assisted stress corrosioncracking have three zones. Section Ahas to be hard enough to drill into steeland tap threads while Section B has tobe strong and ductile enough to carrythe intended load. The third section, thehead, is marked with an embossment toallow visual identification of an installedfastener’s type.10. For applications where high corrosionresistance is required, such asinstallations regularly exposed tomoisture, a bi-metal fastener is used sothat 300 series stainless steel is exposedto the elements.Galvanic reaction can occur even insteel-to-steel connections if one of theparts is zinc-coated. One of the steelmembers may be galvanized steel,(steel coated with zinc to preventrust), or two similar pieces of steelmay be connected by a zinc-coatedfastener. All that is required is for thetwo dissimilar metals to be in contactin the presence of an electrolyte.Under these conditions, hardenedsteel fasteners are vulnerable toinfiltration by hydrogen and subsequent embrittlement at stress points.Stress and SteelOne of the most common stresspoints in a self-drilling screw isits head. This is due to both themanufacturing process and the34DOORS & HARDWARE AUGUST 2009way in which the screw is typically used. The shape of the screwis made by deforming the metalcomprising the head to a diameteras much as 500% wider, causingit to retain residual stress.Application-induced stress alsoconcentrates at the junction betweenthe screw shank and head. Whenthe screw is driven all the way intothe hole, the head is the part thatresists further progress, putting it ina constant state of tension.A third source of stress resultsfrom the practical fact that self-drilling screws are rarely installed at aperfect perpendicular to the surfacethat they bear on. When the screw isdriven at a slight diagonal, one edgeengages first, placing uneven stresson the head.HASCC attacks the hard shell ofthe screw, but a crack need not goall the way through the fastener tocause failure. Corrosion throughthe hardened outer layer effectivelyreduces the diameter of steel available to bear the load. When thetensile load exceeds the tensilestrength of the diminished crosssection, the fastener fails. In the casenoted above, the complete ductilefailure of the central 20% of thecross-section indicated that whatwas left of the fastener core, afterthe hardened case cracked, simplycould not accommodate the workingload of the original design. In someinstances, fasteners broke at theconnection point; at other locations,the screw head simply popped offdue to stress concentration.Preventing FailureTheoretically, eliminating theelectrolyte could prevent problems.If the fastener is completely sealedfrom any possible moisture contact,corrosion ought to be avoided.However, this is often not a practical possibility. Joint sealants, forexample, can fail without notice. Inthe case of the facility mentionedearlier, the building was stable for

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Photo 2005 Sherman Sham, used courtesy of Elco Construction Products, an Acument Global Technologies companyalmost 20 years. Then, investigatorssurmise, wind damage allowedmoisture entry. A year later, thefasteners failed. In other situations, exposure to moisture duringconstruction before a building isenclosed, and even common humidity levels and condensation withina building assembly, have beenidentified as the source of moisture.36DOORS & HARDWARE AUGUST 2009A more reliable solution is to usefasteners that are invulnerable toHASCC. Two methods of fastenermanufacture have been developedspecifically to resist HASCC.A fastener can be selectivelyheat-treated, not case-hardened, sothat only the drilling tip and leadthreads are hardened. The majorityof the shank is softer steel that isnot prone to HASCC. This processuses Grade 5 steel, an alloy that hasenough carbon already present tobe hardened simply by heating. Thetip of the part is passed throughan induction coil that heats it andhardens it locally without affectingthe rest of the shank.When a selectively heat-treatedfastener is installed, the hardened tippenetrates the anchoring metal andcuts threads in it, but then movespast the connection area as the screwis driven to full depth. The relativelysofter steel portion of the shank,which is not vulnerable to HASCC,bears the load. The fastener performsas would a ductile Grade 5 bolt, thesame type of bolt used for structuralsteel connections, assuring a highdegree of reliability.Another type of HASCC-resistantfastener is a bi-metal screw, ahigh-performance part suitable forapplications where greater corrosionresistance is desirable. In this case,a high-carbon steel is used, but onlyfor the drill point and lead threads.The headed shank is made of300-series stainless steel (also knownas 18-8 stainless steel). The twopieces are friction-welded together.Then, the point and lead threads areselectively hardened by inductionheating, similarly to the process ofheat-treating a selectively hardenedcarbon steel fastener.An additional treatment is appliedto some fasteners to enhance theirresistance to galvanic reaction. Bycoating the fastener with a multilayer, aluminum-filled, cross-linkedbaked-on polymer, the outer surfaceof the fastener acquires high aluminum content. This corrosion-resistantfinish makes the fastener morecompatible with aluminum partsand is less prone to galvanic reaction.

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1113Photo 1998 Wes Thompson, used courtesy of Elco Construction Products,an Acument Global Technologies companyPhoto 1998 Wes Thompson, used courtesy of Elco Construction Products,an Acument Global Technologies company11. Cladding, including glazing units and stone facades, often contain 13. The 88-story Petronas Towers in Kuala Lumpur, Malaysia in 1998.aluminum in the framework that attaches the unit to the building’sThe façade contains literally millions of 300 series self-drillingsteel superstructure. This contact of aluminum and steel sets up thefasteners with multi-layer zinc rich and aluminum-filled coating,risk of galvanic reactions that generate hydrogen and precipitatechosen to prevent embrittlement from galvanic reaction betweenhydrogen assisted stress corrosion cracking. To preserve thesteel and aluminum and to mitigate the potential for sacrificialintegrity of the cladding’s attachment to the building, fastenersloss of aluminum in the cladding frame system.that resist hydrogen embrittlement are recommended.12. Screws resistant to HASCC are available in a wide variety ofconfigurations for applications that include self-drilling and selftapping in structural steel or in concrete.Expecting the Unexpected12Photo 1998 Wes Thompson, used courtesy of Elco Construction Products,an Acument Global Technologies company38DOORS & HARDWARE AUGUST 2009Extreme loading is a condition where loads far inexcess of the normal design load are temporarilyapplied to a structure, sometimes only for a few milliseconds. The source of the extreme load might be a blastshockwave, an earthquake, or a hurricane-driven gustof wind.In places where the occurrence of an extreme loadcan be predicted, it must be designed for. Seismicallyactive regions and locales subject to frequent hurricanesor tornadoes are well identified. In recent years, buildings that can be considered likely targets for attack, orare adjacent to likely targets, are also receiving specialdesign consideration.

Current design strategy for extreme loading is tomake the building absorb the load by deforming butremaining stable long enough to allow evacuationand the preservation of life safety. For threats suchas blast shockwave or wind impact, the designertypically seeks to transfer load from the verticalelements of the building shell to the building framework, often using the horizontal floor diaphragmsto spread the load over more of the structure. Thismeans it is vital for the elements of the buildingenvelope to stay in place and the fasteners that attachthem to transfer load to the frame without failing.Extreme loads are often impulsive in nature,that is, they hit like a hammer, applying the fullforce virtually instantly rather than gradually. Thisimpulsive load can make materials perform in anatypical fashion. It can fracture a brittle material,such as the outer shell of a hardened steel fastener.As noted above, when the hardened outercase of a fastener cracks, the load on the fastenermust be born by the narrower ductile innercore. If a significant percentage of the fastener’scross-section is disabled, there is effectivelya smaller fastener left to bear the load.A fastener that is not case-hardened, such asbi-metallic or selectively-hardened fasteners, hasfull ductility through the entire cross-section ofthe shank where the fastener bears its load. Underextreme loading, it is far more likely to deform butremain in service, as compared to a more brittle part.When designing for extreme loading, it is importantto have a realistic design-threat as the baseline fordetermining loads. Expert consultants may be useful.Consideration of connections that must transfer load inthis situation should include ductile specialty fastenersthat can handle impulsive loading more effectively.Footnotes1. Society of Automotive Engineers SAE J78—SteelSelf-Drilling Tapping Screws.ABOUT THE AUTHORS: Gregg Melvin is a Senior Applications Engineerwith Elco Construction Products, an Acument Global Technologies company. He has been in the fastener industry for over 20 years and worksclosely with design professionals, system manufacturers and contractorsto create safer and more effective fastening systems for construction. Hecan be reached at gmelvin@acument.com or www.elcoconstruction.com.Michael Chusid, RA, FCSI, is an architect and a Fellow of theConstruction Specifications Institute (CSI). His company, ChusidAssociates, provides marketing and technical consulting services for innovative building products. He can be reached at www.chusid.com.Albany Hardwarestocks Rixson closers,pivots and stops.Complete units andreplacement parts.Rixson Model 27Offset Floor CloserFeaturing.Rixson Model 117-1/2Offset Pivottoll–free 1-888-680-9990 I fax 1-888-680-1998www.albanyhardware.comAUGUST 2009 DOORS & HARDWARE39

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