Hot Innovations in Cold-Formed Steel Framing

By Administrator Cold-formed steel framing (CFSF) began as a kind of alternative lumber, but after decades of positive performance it has finally 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… Continue reading Hot Innovations in Cold-Formed Steel Framing
By Administrator

Cold-formed steel framing (CFSF) began as a kind of alternative lumber, but after decades of positive performance it has finally 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. Every special structural element, such as a rough opening, had to be detailed separately by the engineer of record. Contractors didn’t always follow those project specific details and may have done it another way. In any case, there has been considerable variation in the quality of field 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 specific needs at both the design and construction phases. Standardized, code-approved specialty elements solve many challenges in a manner consistent wit

h 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 inspections, 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 first 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 fire resistance; not susceptible 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 flanges. Tracks are horizontal framing members, 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 flange, 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 accommodate the overall width of studs. Until recently, more substantial members were built up in the field out of these two types of elements, stud and track. The exact configuration was often left to the contractor and could vary considerably within even one project. However, experience with CFSF over several decades has led designers 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 during 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 infiltrates the completed wall and collects due to condensation, leaks or spills.

The solution is a specialized track with drainage 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 “walking,” resulting in cleaner connections and more consistent finishes. 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 adequate for a simple wall with no rough openings. Loads-including the weight of the wall itself, attached finishes and equipment, wind and, in the case of a load bearing wall, dead and live loads from the roof or floors 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 vertical 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 operation. 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 beefier, stronger members. A typical box header is built by screwing and/or welding together five 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 header is made with only four pieces, two studs and two tracks. Another is made with three, two tracks and a stud. The exact method of fabrication of these assemblies is not standardized, but varies from contractor to contractor and even worker to worker.

Built-up fabrication is well established in the industry, even though it causes multiple problems. It incurs high cost at the engineering phase because there are no standards, so rough openings have to be individually designed and detailed. On-site cutting and construction of these labor-intensive assemblies also raise costs, waste material, increase construction-site waste and add job-site safety risks.

On-site cutting and construction also create quality and consistency issues that should be of special concern to design professionals. They tend to lower the consistency, quality and reliability of the framing, and can compromise the quality of the drywall finish. (For an example of these problems, see sidebar, “Bad Connection.”)


Header Systems

Connecting built-up headers to studs can also cause aesthetic problems. Metal-over-metal overlaps created by tabs of built-up headers compromise wall finishes. Interior wall gypsum board or exterior sheathing may not lay flat over metal tabs with screw heads protruding from them. Bulges in wall surfaces either result in an uneven finish or extra remedial work to conceal the unevenness.

One solution for the connection issue is a pre-fabricated clip that is attached to the jamb stud and then receives the header. This approach standardizes the connection. It eliminates inconsistencies caused by field fabrication-including metal overlapping metal and protruding screwheads on the wall surface-which improves the finish of the wall. It also halves installation labor. In the past, one worker had to hold the header level while another screwed it in place. With a clip system, one worker installs the clips and then snaps the header into place. The clip is generally made as part of a complete, pre-engineered header system.

The reason for building up several pieces of bent metal into a header is to provide something stronger than a single piece of track to support the wall above the opening. Since bends stiffen the metal against buckling-effectively creating mini beams within the larger planes of the member-the same result can be achieved using a single piece of metal with a greater number of bends in it.

This principle can be easily understood using a piece of paper. Hold your hands about 6 inches apart and have someone lay a sheet of paper across them. The paper buckles in the middle and slips between your hands. Now fold the paper once lengthwise, and open the fold so the paper forms a V-shaped channel. If you lay it across your hands now, it retains its shape better and does not buckle and slip through as readily. In fact, the more folds you put in it, the stiffer it gets within certain limits.

Multi-bend technology takes advantage of this effect by adding folded grooves, channels and returns into the overall shape. Direct Strength Design-a new analysis method that is only practical using computers-has replaced traditional Effective Width Design and has made possible the evolution of simple shapes into related, higher-performing configurations that get more strength from the steel. This trend can be seen in many CFSF systems currently available. Paired with higher-strength steel-57 ksi instead of the formerly industry standard 36 ksi-it increases the overall performance of a member without significantly changing its size, weight or steel thickness.

In cold-forming steel, another factor comes into play. Cold working of steel, such as making the bends, alters the properties of the steel itself. The portions of the steel that are worked gain in yield strength and in ultimate strength, and see a reduction in ductility. The portions that are worked the most gain the most. Advances in rollforming have led to tighter bends, meaning that steel laying closest to the bent edge is worked more than in the older rollforming processes. The more bends and the tighter they are, the more of the steel within the member that is strengthened by cold working, raising the overall strength of the member.

A conventional U-shaped track has two bends. C-shaped studs have four bends. A pre-engineered header in a modified W shape has 14 bends, located to maximize the portions of the metal that are actively engaged in resisting load. A single piece in this configuration can be the entire header in a doorframe.

For very wide openings-more than approximately 2 m (7 feet)-or situations with high loads, multi-bend headers can be stiffened even further by incorporating a mating W-shaped insert. It adds more metal and 14 more bends, increasing the total number of bends in the overall shape to 28. The insert is placed inside the multi-bend header with the W inverted, so the two Ws together form a rough X-shape.

The legs of the W function as the top track of the header. They receive the cripple studs above the rough opening, which are attached with screws. This works with or without the stiffening insert in place.

The main benefits of such a pre-fabricated header/clip system are speed, consistency and an improved finish. By selecting a manufactured header system that is code-approved, the designer can specify components according to load and wall type fire requirements, and avoid having to design and detail each individual opening, saving time and resources. It ensures that the rough openings will be built as designed and have consistent structural soundness and quality, without variations introduced by field-cutting and assembly. Consistency of installation is increased, too, since clips have pre-drilled screw holes that leave no doubt about the number and location of connections to the jamb stud. Metal-overlapping-metal connections on the wall plane are eliminated, improving the flatness of the gypsum board finish and avoiding bulges.

Systems like this can be an environmental plus, too. A single-piece header can use 40 percent less steel than built-up elements. It requires no welding and, therefore, eliminates toxic gas emissions associated with welding galvanized steel or special inspections for welding.


Sidebar: Bad Connection

The most important part of a rough opening assembly is the connection between the header and jamb studs. Ironically, it is the element that tends to vary the most in construction method and quality. A built-up header assembly is connected to the jamb studs by cutting the ends of the stud and track members into tabs that extend past the end of the header and over the jamb studs. One method is to cut the legs of the track sections long to develop dog-ear tabs. These tabs get screwed to jamb studs.

Employing non-standardized, field cut built-up fabrication, the quality and structural value of these connections depends entirely on the skill and consistency of the worker cutting and assembling the headers. That worker is, in effect, designing the connection, determining the size and shape of the tab that holds up everything. To save money, contractors often assign this task to the lowest-paid (i.e. least experienced) member of the framing team.

If the header assembly’s screw/weld patterns are not clearly detailed, they, too, will be determined by the worker making the assemblies, who will decide how many screws or welds to use and where to place them. The result can be extremely inconsistent, even within a single project. This can potentially cause delays or rejections during inspections, and could compromise the integrity of the project. On-site welding can bring other complications, including toxic fumes from welding galvanized parts.

Clearly, standardizing this common structural configuration would be a benefit to both designer and owner. Pre-engineered, manufactured members with code approvals eliminate much of the individual detailing of openings and avoid the as-built variations from the details and inconsistencies of construction.


Todd Brady is president of Sacramento, Calif.- based Brady Construction Innovations, makers of the Pro-X Rough Opening system and Slip-Track head of wall solutions. He is a metal stud framing expert with 30 years field and contracting experience and a passion for efficient, code approved building methods. Brady Construction Innovations was founded in 1989 in an effort to bring trade-friendly products to the industry.

Steven H. Miller is an award-winning writer and photographer specializing in issues of the construction industry. He is creative director of Chusid Associates, a Tarzana, Calif.-based consulting firm providing marketing and technical services to building products manufacturers.