AIA Credits: Elco Construction Products an Infastech company isRegistered Provider with American Institute of ArchitectsContinuing Education Systems. Credit earned on completion of thisprogram will be reported to CES Records for AIA members.Certificates of Completion for non-AIA members available onrequest.This program is registered with the AIA/CES for continuingprofessional education. As such, it does not include content thatmay be deemed or construed to be an approval or endorsement bythe AIA of any material of construction or any method or manner ofhandling, using, distributing, or dealing in any material or product.Questions related to specific materials, methods, and services willbe addressed at the conclusion of this presentation.
Learning ObjectivesIdentify where A/E must specify fastenersinstead of leaving product selection to contractor orsupplier.Learn techniques for reducing fastener rust andgalvanic corrosion damage.Be able to explain causes of hydrogenembrittlement and HASCC.Be able to specify high-performance, HASCC-resistant fasteners.Explain role of fasteners in building sustainability.
PART ONESPECIFYING FASTENERSRisks and Responsibilities
For want of a fastener…“For want of a nail, the shoe was lost. For want of a shoe, the horse was lost. For want of a horse, the rider was lost.” “God is in the details.” Mies van der Rohe - 20th Century Architect
Your reputation hangs on specifying the right fasteners. Fastener specification are critical to project success. Standard types and grades of fasteners are sufficient in many construction applications. In these instances, fastener selection can usually be based upon industry standards, building code requirements, and design loads. It may even be acceptable in some cases to leave fastener selection to theThe Petronas Towers in Kuala discretion of contractors or buildingLumpur, the world’s tallest buildingwhen constructed, required careful product fabricators, allowing them toengineering to assure that themillions of fasteners used to install select fasteners based on cost,cladding would not be the convenience, and trade practices.installation’s weakest link.
Your reputation hangs on specifying the right fasteners. Fastener specification are critical to project success. At other times, however, fastener selection can be one of the most crucial aspects of building design. Professional judgment is required to specify fasteners that: Perform as intended Are dependable and economicalThe Petronas Towers in KualaLumpur, the world’s tallest buildingwhen constructed, required careful Last the life of the buildingengineering to assure that themillions of fasteners used to installcladding would not be theinstallation’s weakest link.
A big job for such a small part Structural capacity: Static and dynamic load. Pull-out and pull-over. Shear on fastener and attached materials. Corrosion resistance. Seal against liquid or gas. Constructability, practicality, and inspectability. Design intent and aesthetics. Removability and other special requirements. Service life exceeds that of attached materials.Specifying fasteners is part of designing safe, durable,and practical connections between components.
HASCC Another Threat to FastenersHASCC = Hydrogen-Assisted StressCorrosion Cracking.HASCC has only been identified as afastener failure mechanism within thepast few decades.“Best Industry Practices” now requireconsideration of HASCC when specifyingfasteners.
PART TWO VISIBLE CORROSION Rust and Galvanic ActionBefore considering HASCC, let’s reviewsome basics of ordinary fastener corrosion: Oxidation (Rusting) Galvanic Corrosion
Must it Rust?Most architectural fasteners are made from grades or alloys of steel that willreact with oxygen to create ferrous oxide, commonly know as rust. Rust is atype of corrosion that weakens and deteriorates steel. Rusting isaccelerated when steel is also exposed to moisture, especially if themoisture contains chlorides (salts), a condition that is common in marine,industrial, and urban atmospheres.When designing steel-to-steel connections,both the fasteners and the items beingjoined must be protected against rusting.Steel building panels, for example, can beprotected by using zinc galvanizing and ahigh-performance coating.As a general guideline, fasteners shouldhave greater corrosion resistance than Once rust starts, it canitems being joined so that fasteners do not spread rapidly to adjacentbecome weakest link in connection. exposed steel surfaces.
Corrosion-Resistant Plating How they work: 1. Barrier against air and moisture. 2. Sacrificial, galvanic protection. Available Types: Hot-dipped galvanizing: Thick zinc clogs threads. Plating is not Electroplated Zinc: Insufficient protection foracceptable protection exterior or high performance fasteners.for high performance fasteners. Cadmium and Hexavalent Chromates: Greater protection, but environmental concerns. Plating is contraindicated for case- hardened fasteners.
Corrosion-Resistant Coatings Better protection for high- performance fasteners. Base Coat: Baked-on, zinc- rich, cross-linked polymer. Top-Coat: Baked-on, aluminum-rich, cross-linked polymer. Pigments optional.Colors provide product Eco-friendly: Minimize or identification and aesthetics. eliminate heavy metals. Lubricate threads: Eases installations
Corrosion-Resistant Stainless Steel 300-series stainless steel provides greater corrosion resistance. Use for critical structural connections, exposed exteriors, and in locations with corrosive environments. Can require coating for protection against galvanic corrosion of materials they are attaching.
Galvanic Corrosion Occurs when dissimilar metals are in presence of an electrolyte (an electrically conductive medium) and form a galvanic cell. Like an electric battery, except two pieces of metal form a circuit. H+ H+ H+(electrolyte)
Galvanic SeriesMetals conduct electricity because they have a tendency to giveup electrons easily. Some metals give up electrons more easilythan others. They can be listed in order of their potential to yieldelectrons, in a table known as a Galvanic Series. Cathode (Least Active) More “noble” Gold (protected and Silver less corroded) Aluminum is more active Stainless Steel Bronze (anodic) than steel or Copper stainless steel. Where Brass aluminum and steel form a Nickel galvanic cell, aluminum Lead will corrode, sacrificing Steel & Iron Less “noble” Aluminum electrons that will deposit (sacrificial and more Zinc onto and protect the steel. corroded) Magnesium Anode (Most Active)
Galvanic Corrosion This aluminum plate received two screws made from 300-series stainless steel, a grade that is highly resistant to oxidation and rusting. It was then exposed to 1000 hours of salt-water spray that acted as an electrolyte to form a galvanic cell between the aluminum plate and stainless steel fasteners. The screws were them removed to allow examination of the plate. Stainless Steel Stainless Steel Screw Screw: The With Anti-Corrosionaluminum sacrificed Coating: The coating electrons to the prevent formation of a steel. The surface galvanic cell and of the aluminum is prevented corrosion ofvisibly deteriorated. the aluminum plate. Using ordinary steel fasteners in architectural aluminum elements (such as windows, curtain walls, or wall cladding) can also create galvanic corrosion of the aluminum, weakening the connection until it fails at well below design loads. Avoid this problem by specifying fasteners with high-quality anti-corrosive coatings.
Galvanic CorrosionDissimilar metal combinations arecommon in construction. For example: Aluminum framed glazing units andcladding get attached to structural steelframing using steel, self-drilling, self-tapping screws. Aluminum parts are often attached toconcrete or masonry with self-tappingscrews made of steel. Even steel-to-steel connections caninvolve dissimilar metals, if either of thesteel parts, or the fasteners joiningthem, has been coated with zinc. Thin veneers can be attached with self-drilling fasteners in the support structure.
Galvanic CorrosionIt is difficult to prevent electrolytes from contact with fasteners.Moisture can enter construction due to: Rain or dew during Construction Condensation inside a wall or roof Leaks in building envelope Water from building maintenance or operations Plumbing failures Flooding and spills Perspiration from workers who installed the part Air pollutants can make atmospheric moisture more conductiveIf dissimilar metals are in contact, assume that anelectrolyte will also be in contact with the fasteners.Specifying fasteners with adequate corrosion resistance protectsagainst catastrophic failure, loss of use, injury, death, and liability.
PART THREE HASCC“The Invisible Corrosion”A secondary effect of the galvanic reaction canalso lead to fastener failures. Hydrogen, a by-product of galvanic corrosion, can weakenstandard, hardened fasteners and cause them tofail. It produces a type of corrosion that is notreadily apparent…until it is too late.
Construction Scene Investigation20 years after installation, self-drilling screws begansnapping for no apparent reason.Screws complied with code requirements and industrystandards.They had been exposed to weather due to roof damage,but were not visibly corroded.Scanning electron micrographsrevealed fractured fastenersurfaces and separated grainboundaries in steel.Screws had also lost ductility.The Verdict: HASCC
Hydrogen & Embrittlement HASCC starts with hydrogen. Hydrogen embrittlement is associated with galvanic action. However, steel fasteners are not necessarily weakened by galvanic corrosion itself. Rather, hydrogen generated by galvanic action attacks the steel. Even if steel is protected from galvanic corrosion, hydrogen can attack it rapidly.Screws attacked byhydrogen lose ductility Specialized fasteners have beenand become embrittled developed to avoid this risk.They typically show novisible corrosion.
Source of Hydrogen Galvanic action creates electrical current. Water in electrolyte separates into oxygen and hydrogen. Oxygen bonds with anode and oxidizes metal. Since oxides have little structural strength, anode weakens and corrodes. Hydrogen is attracted to cathode and penetrates into metal.Process is similar to laboratory procedure for separatingwater into hydrogen and oxygen by passing electrical currentthrough it, a process called electrolysis.
Hydrogen Diffuses into SteelHydrogen, the smallest atom,penetrates voids in steel’s crystalstructure.Hydrogen atoms are unstable andbond with other hydrogen atoms.The larger H2 molecules putpressure on the surroundingsteel structure.Internal tension in steel enlargesgrain boundaries and reducesductility. Scanning electron microscope image of steelSteel can no longer bear design shows enlarged grainload, and the fastener can fail. boundaries indicative of hydrogen embrittlement.
The Role of Case Hardening Hydrogen has little effect onCaseRockwell “mild” (soft) steel.Hardness Many building fasteners areHRC 52 min. case hardened, In case hardening, low- carbon steel is heated in a high-carbon environment toCore infuse extra carbon intoRockwellHardness metal’s surface.HRC 32-40 This hardens “case,” but makes it brittle. HASCC affects steel of Rockwell hardness ≥ HRC 35. The harder the Case hardened self-drilling steel, the more susceptible is it to and self-tapping screws HASCC. are hard enough to cut into structural steel or concrete.
Hydrogen Embrittlement & StressIn case-hardened fasteners, the hard outer layer is mostvulnerable to HASCC. Cross-section of failed case-hardened fastener: • HASCC Failure Mode • Strength loss due to Ductile and Embrittlement Mode • Ductile failure due to reduced cross-section of fastener.
Stress ConcentrationStress increases steel’s susceptibility to embrittlement.Stress concentration occurs at screw heads for 3 reasons: 1. MANUFACTURE Deformations required to form steel rod into screw heads induces stress into metal.
Stress Concentration 2. CLAMPING FORCES When screws are tightened, their heads bear on the surface of the object being attached. This places the area of the shank immediately under the head into tension, inducing stress.
Stress Concentration3. OUT OF ALIGNMENTScrews are rarely perfectlyperpendicular to surfacesthey are attaching.When tightened, unevenpressure is put on head byoblique angle at which it is A test for resistance to hydrogeninstalled. embrittlement closely mimics this real-world situation. The fastener is screwed throughThis puts additional stress on two plates of dissimilar metals. A shim is placed under one edge of the top plate,one side of fastener at creating an angle between the two plates and placing the screw at an oblique angle withjuncture of head and shank. respect to one of them.
Plating and Hydrogen Embrittlement Case hardened fasteners must still be protected against rust and galvanic corrosion. Standard plating of case hardened fasteners generates hydrogen that could contribute to “Hydrogen Embrittlement”. This is unlikely, because platers bake hydrogen out of fasteners @ 400+ for several hours after treatment. But you cannot bake-out Hydrogen from continuous Galvanic reactions on the job. In low stress, completely dry areas, zinc or other plated, case hardened fasteners, may work well, or, if exposed to moisture it can pose a risk for delayed failures. Case hardened fasteners can be protected with high- performance anti-corrosion coating. But it does not stop hydrogen diffusion into hardened steel. However, new innovations – described in next section – will.
HASCC RecapGalvanic action between dissimilar metals generateshydrogen.Hydrogen penetrates hardened steel and creates internalstresses that weaken fasteners.This process occurs primarily in high hardness/casehardened parts.Stress concentrations initiate micro-cracking that canpropagate across fastener.HASCC can occur days, months or years after fastenerinstallation if connection is exposed to moisture.Failure is often sudden and without warning. No rust isseen.
PART FOUR HASCC SOLUTIONSFor Self-Drilling Screws
Benefits of Self Drilling ScrewsSelf-drilling screws require just seconds ofinstallation time and labor VS that required fornuts-and-bolts, rivets, and other fasteners requiringpre-drilled holes.Fasteners install in single, fast operation using apower driver.They are much faster than two-handed bolt-and-nut Dril-Flex.exeinstallations, and more practical where the backside is not accessible for installing a nut.With hundreds of thousands of fasteners used inlarge buildings, this represents an enormoussavings in labor.Use to install windows, cladding, curtain wall,framing, anchors, equipment, fixtures, and otherbuilding components.
Design of Self Drilling Screws Select head style for easy installation,Tip drills pilot hole. acceptable profile, and pull-over resistanceLead threads tapthreads into Select thread stylesubstrate, so to hold in substratesubstrate acts as nut. Lead threads cutAvailable in threads into (tap) substrate for pull-outconfigurations to suit resistancea wide range of Tip drills hole and removes shavingsconstructionapplications.
The Remedy for HASCCSelf-drilling fasteners have to be case hardened inorder to drill and tap substrates – but casehardening makes them vulnerable to failure.Fortunately, there is a solution to fastener HASCChazards.
HASCC-Resistant Fasteners Vir tua to lly Im HA m SC un C e
A Metallurgical MarvelThere are two ways to achieve this performance:• Selectively Hardened Fasteners• Bi-Metal Fasteners Hardened for Ductile for structural drilling and performance and tapping HASCC-resistance.
1. Selectively Hardened Fasteners Made from special, high- carbon steel, they do not have to be placed in a high- DUCTILE WHERE NEEDED carbon environment during Grade 5 heat treat heat treatment. in load-bearing area of fastener Tip of screw is passed through electrical induction HARDENED WHERE coil that heats and hardens NEEDED Selectively hardened drill-point and lead-threads point and tapping without affecting rest of threads HRC 52 shank.
2. Bi-Metal FastenersUse where stainless steel is requiredfor increased resistance tocorrosive environments.A high-carbon steel tip is fusedonto stainless shank. 300 Series StainlessHigh carbon tip is then selectively Steel Headhardened using induction-coil heating. and Shank High-Carbon Steel Tip, Selectively Hardened Recommended for exposed fasteners.
Corrosion-Resistant CoatingsUse corrosion-resistant coatings on both selectivelyhardened and bi-metal fasteners.Coating provides galvanic protection and lubricatesthreads.Coating can be color matched as required. Bi-Metal Fastener: Before coating, above. After coating, below.
PROOF Self-drilling fasteners joining steel and aluminum plates, exposed to salt-spray test. Conventional fasteners fail due to stress concentration at screw heads that accelerates embrittlement and HASCC. Neither selectively-hardened nor bi-metal fasteners fail. HASCC- resistant fasteners are not affected Standardcase-hardenedfasteners failed
PROOFHASCC-resistantscrews were used tosecure crown at top ofbuilding and buildingenvelope.Exposed to marine andurban atmosphere,high winds, andearthquakes.Over two decades ofservice. US Bank Building/Zero failures. Los Angeles, CA Pei Cobb Freed Partners
Best Industry PracticesWhere dissimilar metals will bein contact, specify: selectively-hardened self-drillingfasteners to resist HASCC. Where stainless steel is required for protection against visible corrosion or extreme environments, specify: bi-metal self-drilling fasteners with a selectively-hardened tip to resist HASCC.
HASCC Recap• Galvanic action in dissimilar metals generates hydrogen.• Hydrogen penetrates case hardened steel and creates internal stresses that weaken fasteners.• Stress concentrations initiate micro-cracking that propagate across fastener and lead to failure.• HASCC can occur days, months or years after fastener installation if connection is exposed to moisture.• Failure is often sudden and without warning.• Failures can be avoided by using selectively-hardened or bi-metal self-drilling screws with corrosion-resistant coating.
PART FIVESPECIAL FASTENERS
Extreme LoadsStructure is subjected to brief loads far higher thannormal operating loads.Risks include:• Hurricane, tornado, and windblown missile impact• Earthquake, tsunami, landslide, avalanche• Industrial accidents and vehicular collisions• Load redistribution due to failure of building elements• Explosion, including boiler, natural gas leaks, attack Located near possible targetsIf there is reason to suspect a risk, design for it. Determination Estimate Risk-Resistant of Risk of Risk Loads Design
Extreme Loading of FastenersIf a structure is designed towithstand extreme load, load istransferred to fasteners thatconnect it to other building QuickTime™ and a decompressor are needed to see this picture.elements.Ductile fasteners can deform toabsorb some load whilemaintaining connection.If fasteners are brittle, extreme Extreme loads canloading may cause them to fail. affect all building components, not justUsing ductile, HASCC-resistant structural members.fasteners helps assure full designstrength of fasteners can be usedto resist extreme loads.
Where to SpecifyBlast-resistant windowsCurtain wall framingBuilding equipmentCritical life safety andcommunicationsequipmentTo assure continuousservice of essentialfacilities
Concrete and Masonry Screws Choice ofhead styles Specifying concrete and masonry Choice of head anchors also styles requires Grade 5 hardness for consideration of strength and ductility dissimilar metals and corrosion resistance. Hi-lo thread A variety of special taps into masonry Higher designs are hardness for tapping available to satisfy threads most construction requirements. Concrete Masonry
Pressure-Relief Fasteners In buildings with potential for interior explosions, special panels can be designed to blow off and relieve pressure, a built in safety valve. Examples include places where highly volatile liquids are in use, or storage for extremely fine dry, materials subject to dust-explosion.
Pressure-Relief Fasteners panelPanels have an holes larger separates from framingthan fastener heads. panel member centeringA special aluminum washer deviceretains the panel in position. framingUnder explosive pressure,washers fail and allow aluminum washerpanels to blow off, venting pressure- relieving collapsesthe pressure. washer Normal ExplosionPanel is tethered to keep itfrom becoming a dangerousmissile.
Threaded Rod Anchors 1/4-20 & 3/8-16 Threaded rods are used to support fire-sprinklers and other essential buildingservices. These fasteners simplify installation while protecting against potential failures of other fastening methods that are not positively engaged in the structural materials.
Sealing WashersSpecial head design plus high performance washermaterial help prevent leaks through building envelope.
SUMMARY Manage Your RiskFasteners are critical components of buildings. A/Es must be aware of their professional responsibility tospecify fasteners that fulfill design intent and provide safe and durable connections.
EconomyThe lowest cost fastener may not be themost economical when labor and service lifeare considered.Specialty fasteners can often save moneyby providing lower installed cost.Fasteners are typically less than .0025%per dollar of project cost, but specifyinginadequate fasteners can cause up to 100%of construction defect costs.
Sustainability Most metal fasteners have recycled-material content and are recyclable. New corrosion-resistant finisheseliminate toxic heavy metals plating. Durability over life of structure is themost important measure of sustainability.
Fasteners are critical building components A/Es have professional responsibility to specify fasteners that fulfill design intent and provide safe and durable connections. It is not safe to assume that contractors will be aware of these issues and will select anything other than standard fasteners. Specify accordingly.
For More Information Elco Construction Products Dril-Flex® and Bi-Flex™ HASCC-Resistant Fasteners www.elcoconstruction.com Gregg Melvin (815) 979-3249 email@example.comAsk for reprint of article fromConstruction Specifier.