Small Details, Big Consequences
How tiny a detail is worth getting right? Does the selection of something as seemingly insignificant as a self-drilling screw merit an engineer’s attention?
If use of the wrong fastener could predictably lead to failure, possibly causing loss of life and/or property, most engineers would consider it a big detail, even though it pertains to a small object. If there were two fasteners of the same size and load rating, but one could be predicted to fail, any design professional would want to specify the other one.
Over 50,000 self-drilling, self-tapping screws were used to hold slabs of Sardinian granite onto the exterior of the massive Adrienne Arsht Center for the Performing Arts in Miami, FL. The cladding system for the complex was designed by CDC Curtainwall Design Consulting, Inc, Dallas, TX, who also worked extensively coordinating the different trades during construction. A leading specialist in cladding systems, CDC deemed this detail important enough to specify a specialized fastener – a selectively hardened self-drilling screw - instead of the conventional case-hardened fasteners that are widely used in the industry. The reasons for this selection might be surprising.
The Adrienne Arsht Center for the Performing Arts
Designed by renowned architectural firm Pelli Clarke Pelli, the Arsht Center (formerly known as the Carnival Performing Arts Center) is the second largest performing arts center in the US, a 570,000 sf complex that took five years to construct. Its two main structures, the Sanford and Dolores Ziff Ballet Opera House and the John S. and James L. Knight Concert Hall include a 2400-seat theater, a 2200-seat concert hall, and a 200-seat studio theater.
Pelli Clarke Pelli describes these buildings as “composed of a series of stepped masses clad in light-colored Sardinian granite. The forms, a modern interpretation of ancient stone architecture, project a sense of both permanence and excitement.”
That appearance of ancient stone architecture is actually two layers of very modern design, a backing wall of concrete masonry units (CMUs) and a decorative veneer of granite just 1.4 inches thick. The stone slabs, which averaged 2-feet,6-inches x 4 feet, were made from three different kinds of granite, each requiring slightly different treatment because of variations in thickness and hardness. The wind loads reach 140 psf near the roofline of the structures. And of course, Miami is hurricane country, located in one of only two High Velocity Hurricane Zones under Florida’s stringent building codes.
Many of the stone slabs on the Arsht Center exterior, weighing approximately 200 lbs each, are held in place by six ¼-inch structural self–drilling screws.
Hard Facts About Hardening
Screws capable of self-drilling and self-tapping in structural steel are a relatively recent invention, dating back to the 1970’s. They rapidly replaced slower attachment technologies such as bolt-and-nut, welding, or riveting. Self-drilling fasteners can be installed in a few seconds by a single worker with a power driver. They also simplify attachment because they eliminate the need for access to the opposite side of the object they are attaching too (a requirement with bolted nuts). They drill their own holes and tap their own threads, making the substrate they are attaching onto act as the nut that secures the fastener. They are used by the hundreds of millions around the world every year. Attachment of cladding systems is one of their major uses, but they are also specified in a wide variety of structural applications.
In order to drill into metal, fasteners need an integral drill-point that is harder than the metal being penetrated. This is achieved by first, forming the fastener, and then, hardening it.
Case-hardening has been the most common method used. Low-carbon steel fasteners are heated in a high-carbon environment, infusing carbon into the outer layer of the steel and creating a hardened shell, or “case,” that is hard enough to drill and tap soft steel. The inner core of the fastener remains softer and more ductile.
The chief weakness of these fasteners can be, ironically, their hardness. Only the drill tip and the first few threads need to be hardened for self-drilling. The part of a screw used for load-bearing, however, is the main section of the shank, behind the tapping-threads and up to the head. That section has no need of surface hardening, but unfortunately, case-hardening is not selective, and the entire fastener is treated.
Hardened steel can account for about 25% of the total diameter of the screw. The hardened metal is brittle, not ductile. The effect is that the cross-sectional area of ductile steel can be reduced to 75% or less. In certain situations, as we shall see, that ductility is highly desirable.
Selectively hardened self-drilling fasteners were developed about 20 years ago. They perform the same function as case-hardened units, but avoid surface hardening of the load-bearing portion of the fastener. In selective hardening, only the drill-tip and tapping threads of the unit are hardened. The entire head and the main length of the shank remain softer and more ductile.
The first type of selectively hardened fasteners were made of a single piece of special, high-carbon steel alloy. Since carbon is already present in the metal, enrichment in a high carbon chamber is unnecessary. Instead, the fastener tip is passed through a high-voltage induction coil to heat it, hardening it to approximately Rockwell hardness HRC 52, while leaving the rest of the fastener unaffected at Rockwell hardness in the range of HRC 28-34.
More recently, bi-metallic fasteners have been perfected. These highly corrosion-resistant units are made by fusing a high-carbon steel drill-tip and tapping threads onto a 300 series (18-8) stainless steel screw shank. The tip is then selectively hardened. These bi-metallic fasteners are emerging as the fastener of choice for exteriors and other aggressive environments.
Under extreme loading situations such as hurricanes, tornadoes, earthquakes, or explosions, the ductility of fasteners becomes a significant issue. These loads are applied impulsively, like a hammer-blow, which can produce a different type of response from gradual or continuous loading in a structural material. Brittle materials tend to shatter easily under impulsive loading. However, the energy of these loads can be dissipated by ductile structural elements that deform without failing. (This is, in general, the leading strategy in design of blast-resistant buildings.)
With a case-hardened screw, the ductile steel can be diminished to 75% or less of the full cross-sectional area. This makes the fastener more vulnerable to failure under an impulsive load.
In a selectively hardened screw, the full diameter remains ductile, more likely to stretch than to break under extreme loading. This makes selectively hardened fasteners a good choice for building enclosure systems, structures in seismic zones, severe weather regions, buildings that require high security or are considered likely targets for violent attack, or buildings adjacent to potential attack targets.
Surprisingly, however, hurricane-rated fasteners were not required for the cladding on the Arsht Center. The granite slabs were considered a sacrificial veneer that could break and fall off without compromising the weather-proof “skin” of the building.
According to Tom Wallace, Senior Project Manager for CDC, selectively hardened fasteners were chosen “because of all the dissimilar metals we were using.”
He is alluding to a little-known but significant issue, hydrogen assisted stress corrosion cracking (HASCC), also known as delayed hydrogen embrittlement. It only affects steel hardened above certain levels. It can cause the heads of standard quality, properly load-rated, code-approved fasteners to pop off without warning, potentially causing failure of the cladding system or other structural connection. Under test conditions, it can occur in as little as 24 hours. It can also occur in a fastener that has been in service, under load, for 20 years, if moisture is introduced. (See sidebar The Hydrogen ‘Bomb’)
When a galvanic reaction occurs and causes HASCC, the hardened case of the fastener can form micro-cracks right down to the inner, ductile core. Since many of these fasteners have a hardness up to HRC 42, even the core is hard enough to be vulnerable to embrittlement, and micro-cracking can continue inwards. This can compromise not only the hardened case but a considerable part of the softer core, leaving as little as 25% uncompromised metal. The design load may then exceed the capacity of the screw, causing the head to pop off.
Selectively hardened fasteners are HRC 34 or less (Grade 5 strength) in the load-bearing portion of the shank and head, and therefore immune to HASCC.
The cladding system of the Arsht Center involved aluminum, stainless steel, galvanized steel, and carbon steel. Given so many possibilities for galvanic reactions and HASCC, with so many metal combinations in a humid environment, selectively hardened fasteners were chosen.
The Arsht Center granite was attached using aluminum anchor clips that fit in kerf-slots on the edges of the stone slabs. Top and bottom courses of stone were attached with running aluminum extrusions the full width of the stone. Intermediate courses have smaller clips, ranging from 8 to 12 inches long.
The anchor clips are two-piece aluminum assemblies. One L-shaped piece is anchored to the CMU backing-wall with 5/8-inch stainless steel wedge-bolts, painted with yellow chromate to prevent corrosion. This piece is anchored to the wall previous to lifting the stone slab into place.
The second piece is screwed to the first with three ¼-inch selectively hardened screws. This clip-piece has a downward-facing lip that mates with the kerf-slot in the top edge of the stone below it.
The fasteners have a corrosion-preventive coating. “Corrosion protection is a requirement for long term structural integrity of the stone anchor,” explains Wallace. If moisture infiltrated the clip assembly, it could set up a galvanic cell between the aluminum clip and an unprotected steel screw. Even though the fastener would be immune to HASCC, galvanic action would cause accelerated corrosion of the aluminum, weakening it at the connection point and possibly resulting in pull-through.
The slab joints are then sealed with a non-staining silicone to protect against wind and rain infiltration.
Surprisingly, selectively hardened fasteners can be less expensive to use than the more conventional case-hardened ones. Selectively hardened fasteners are usually more expensive on a per-unit basis. However, according to Paul Lavasseur - formerly Senior Project Manager for Titan Stone, the cladding contractor for the Arsht Center, and currently a consultant to CDC - the selectively hardened fasteners are often less expensive in practice.
“The drill-tips of conventional fasteners frequently snap off,” explains Lavasseur. “You waste a lot of screws. You also slam your knuckles into the wall, sometimes your whole arm. This does not make the installers happy. We don’t see the selectively hardened fasteners snapping off very much.”
When designing structural connections between dissimilar metals, attention must be paid to galvanic reaction. If hardened-steel parts such as self-drilling screws are used for the connection, HASCC is also a possibility. Selectively hardened fasteners with corrosion-preventative coating can avoid these dangers and protect the integrity of the structure. In exterior applications or aggressive interior environments, bi-metallic selectively hardened fasteners are recommended.
About the Authors:
Gregg Melvin is a senior applications engineer with Elco Construction Products. He has been in the fastener industry for more than 20 years and works closely with design professionals, system manufacturers, and contractors to create safer and more effective fastening systems for construction. He can be reached at 815-979-3249 or www.elcoconstruction.com.
Steven H. Miller, CDT, is an award-winning writer and photographer specializing in issues of the construction industry. He works with Chusid Associates, a technical and marketing consultant to makers of advanced building products. He can be reached via www.chusid.com.
Sidebar: The Hydrogen ‘Bomb’
It is well-known that when two dissimilar metals - such as steel and aluminum - are in contact, galvanic reaction is a possibility. If an electrolyte – such as wind-driven rain, condensation, or even sweat from the installer’s hands – is introduced, a galvanic cell can be formed, causing current to flow and electrons to migrate. One of the two metals corrodes at a much higher-than-normal rate, “sacrificing” itself to the other metal, (which is thus protected and corrodes little if at all). In an aluminum-steel connection, aluminum would be the sacrificial metal.
When hardened fasteners are involved, a second, little-known type of attack can also occur, with potentially devastating results. Hydrogen assisted stress corrosion cracking (HASCC), also known as hydrogen embrittlement, is a problem exclusive to hardened steel. It can cause hardened fasteners to fail without warning, the heads literally popping off.
HASCC can affect steel of Rockwell hardness HRC 35 or greater. In hardened fasteners, it is usually a by-product of galvanic reaction, caused by the hydrogen generated in the galvanic cell.
Hydrogen attacks hardened steel because the hydrogen atoms are so small, they migrate through steel’s crystalline structure. Inside, it is believed that individual hydrogen atoms link with other hydrogen atoms to form more stable H2 molecules. The molecules occupy more space than individual atoms, putting pressure on the crystal structure from within. The grain boundaries get enlarged, causing a loss of tensile strength and ductility.
This is a progressive problem. Hydrogen attacks stress-points most aggressively. As a stress-point begins to lose strength under attack, it bears less load, putting increased stress on adjacent metal, which then becomes more vulnerable to hydrogen attack and embrittlement.
The entire process can happen very quickly. Since there is no visible rust, it can also happen without warning. Instances have been known where fasteners have been in service for 20 years and then, due to a temporary condition of the structure that allowed moisture infiltration, HASCC suddenly began and the fasteners failed.
Case-hardened fasteners are typically Rockwell hardness HRC 52 on the case, and therefore vulnerable to HASCC. The most common attack point is at or near the screw-head. This stems both from the metal deformation that induces stress in the head during screw manufacture, and from the specific types of stress induced in a screw in service. When a screw is fully tightened, tensile stress is induced in interface of the screw shank and head because the head is resisting tensile forces created by the wedging action of the screw threads. In practice, this stress is often lopsided, because the screw is rarely driven perfectly perpendicular to the surface it is attaching. One edge of the head engages first, putting all the stress on one side of the screw. In fact, the test standard apparatus for HASCC emulates this condition, tightening test units onto a pair of dissimilar metal plates angled by a shim.