PRODUCT INSTALLATION FEATURE Hot Innovations in Cold-Formed Steel Framing By Todd Brady and Steven H. Miller, CDT
Cold-formed steel framing (CFSF) began as a kind of alternative lumber, but after decades of positive perfor- mance it has fi nally come into its own. From the beginning, steel framers cut and combined steel studs and track to build up other, more complex shapes, much as wood carpenters do, without any real standardization of assemblies or connections. Ev- ery special structural element, such as a rough open- ing, had to be detailed separately by the engineer of record. Contractors didn’t always follow those project specifi c details and may have done it another way. In any case, there has been considerable variation in the quality of fi eld assemblies. Eventually, familiarity bred discontent, and
discontent inspired innovation. Using advanced roll-forming methods, new framing elements— beyond standard C-shaped studs and U-shaped tracks—became not only possible, but were pre-engineered and pre-approved for specifi c needs at both the design and construction phases. Standardized, code-approved specialty elements solve many challenges in a manner consistent with better, more reliable performance. They simplify detailing and provide a solution that is more likely to be followed properly by the contractor. They also speed up construction and streamline inspec- tions, saving time and headaches. Finally, through less cutting, assembly, screwing and welding, they improve job-site safety.
Standard Practices without Standards CFSF has become such an accepted part of the
landscape that it is hard to imagine commercial or high-rise residential construction without it. The fi rst design standards for CFSF were
published in 1946 by the American Institute of Iron and Steel (AISI). The latest version—AISI S200-07: North American Standard for Cold-Formed Steel Framing—General Provisions—is now the standard for all of North America. The attraction of CFSF is easy to explain. As a
system, it is affordable; fast to construct; lightweight; non-combustible; versatile in the design of acoustics, thermal insulation, and fi re resistance; not suscep-
tible to rot, mold growth or termite infestation; and high in recycled content. As groundbreaking as the AISI standard is,
it does not codify everything. Much is still left to designers and contractors to decide. The CFSF system is founded on studs and
track. Steel studs, like wood studs, are vertical members. They are usually formed in a C-shaped cross-section, with the top and bottom of the C forming the narrow dimensions of the stud, or its fl anges. Tracks are horizontal framing mem- bers, such as sill plates and header members, that are designed in a U-shape to receive the studs. Studs are generally made in sizes similar to nominal “2-by” lumber. For example, a 41-mm by 89-mm steel stud equates to a 1 5/8 inch by 3 1/2-inch, or “2-by-4” wood stud. A 41-mm by 140-mm steel stud equates to a 1 5/8-inch x 5 1/2-inch or “2-by-6” wood stud. In these examples, the 41-mm dimension is
referred to as the fl ange, and the 89-mm or 140- mm dimension as the web, drawing on concepts familiar from hot-rolled steel and similar I-beam type members. The track is sized to accommo- date the overall width of studs. Until recently, more substantial members were built up in the fi eld out of these two types of elements, stud and track. The exact confi guration was often left to the contractor and could vary considerably within even one project. However, experience with CFSF over several decades has led design- ers to identify the limitations of these basic shapes and problems associated with them. For example, if water gets into the bottom
track of a framed wall—which can easily occur dur- ing construction when the framing is exposed—the water collects in the track. In the presence of saw dust, paper or other organic material, this can result in mold, or other moisture-related problems such as gypsum board deterioration or the attraction of pests, after the wall is enclosed. Similar problems can occur if water infi ltrates the completed wall and collects due to condensation, leaks or spills. The solution is a specialized track with drain- age holes punched into it. This is a small innovation
that can save big expenses and headaches and help meet indoor air quality standards. Stud designs are also being improved. They
feature innovations such as strategically placed ribs bent into the cross-section that increase stiffness. Textured stud surfaces prevent screws from “walk- ing,” resulting in cleaner connections and more consistent fi nishes. These small improvements, multiplied over tens of thousands of studs, can make a big difference on a project.
Beyond Studs and Track Conventional studs and track are generally ad-
equate for a simple wall with no rough openings. Loads—including the weight of the wall itself, at- tached fi nishes and equipment, wind and, in the case of a load bearing wall, dead and live loads from the roof or fl oors above—transfer from top track to studs to bottom track, and from there to the foundation or deck under the bottom track. If there is a rough opening in the wall—a door,
window or large HVAC duct, for example—the loads from above the opening must be transferred around it. A header must be strong enough to bear the load of one or more cripple studs above the header, and transfer it to jamb studs, the verti- cal members of the rough opening. Jamb studs must, likewise, be designed for greater loads than ordinary studs. For example, on interior walls the rough opening must be strong enough to carry the weight of the drywall above the opening—29 kg/ m2 (6 psf) for one-hour walls or 54 kg/m2 (11 psf) for two-hour rated walls—plus seismic loads and, often, the weight of a door and its inertia of opera- tion. On exterior walls, the rough opening must resist wind, seismic and similar loads. In traditional CFSF construction, headers and
jamb studs are fabricated on-site by building up combinations of standard studs and track into beefi er, stronger members. A typical box header is built by screwing and/or welding together fi ve pieces. Two studs are enclosed by two tracks, and third track is attached across the top, open side up, to receive the cripple studs above the opening (see Image #1). Another type of box
28 METAL CONSTRUCTION NEWS July 2012
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