By Nader Elhajj, P.E., NAHB Research Center
The primary objective of this article is to offer residential (and light commercial) designers some practical design advice and technical resources when considering the use of cold-formed steel (also called light gauge steel). A basic overview is also provided for those who may be completely unfamiliar with this increasingly popular building technology (see photo of steel house, Fig. 1). Future articles will give greater details on specific topics of interest.
What is Cold-Formed Steel?
Cold-formed steel (CFS) members are made from structural quality sheet steel that are formed into shape either through press-braking blanks sheared from sheets or coils, or more commonly, by roll forming the steel through a series of dies. No heat is required to form the shapes (unlike hot-rolled steel), and thus the name cold-formed steel. Cold-formed steel members and other products are thinner, lighter, easier to produce, and typically cost less than their hot-rolled counterparts. A variety of steel thickness is available to meet a wide range of structural and non-structural applications. Figure 2 shows typical cold-formed steel shapes.
Why consider CFS?
Cold-formed steel framing has gained some consumer appeal and market share in recent years for several reasons. In fact, over 50% of housing starts in Hawaii are CFS homes. There are several reasons for this increased interest in steel framing. CFS framing for floors and interior walls are very competitive with lumber and engineered wood products. Several time-and-motion studies have shown that cost premiums for experienced CFS users are typically in the neighborhood of 2 to 5 percent of the total house cost, depending on the style of the house. CFS also offers tight dimensional tolerances. For those in termite-prone areas, it offers a solution to the termite threat. There is no reason to suggest that there should be a design cost premium with the resources that are now available to assist designers, including direct manufacturer technical support. Second, CFS framing provides builder and consumers flexibility in design option that can not be economically accommodated using traditional framing materials (i.e., larger open space, longer spans, higher ceilings, arched ceilings and doorways).
Advantages of CFS Framing
Some of the unique qualities and benefits of steel framing are:
- Availability: Cold-formed steel members are readily available at local distributors or drywall supply house.
- Price Stability: Steel prices have been relatively consistent over the past two decades.
- Consistent Quality. Steel does not contain knots, twists, or warps that are commonly found in lumber. It is always dimensionally correct and manufactured to very strict tolerances.
- Code Compliance. Cold-formed steel framing is code approved (IRC, IBC, CABO and ICC). Its non-combustibility and high strength allow its use in buildings up to 6-story high.
- Design Flexibility. CFS members come in a variety of sizes enabling the designer to meet specific load requirements economically and achieve longer spans.
- Light Weight. Steel members weigh as much as 40% less than conventional lumber, reducing total building and seismic loads.
- Ease of Installation. Holes are preformed simplifying the installation of plumbing, electrical, and mechanical lines and components.
- Fewer Call- Backs. CFS members are fastened with screws, which virtually eliminate nail pop-outs.
- Common Appearance. Once exterior and interior finishes are installed, a wood and a steel building are indistinguishable from each other.
- Resistance to Termites and Pests. Cold-formed steel members are impervious to termites and other wood-destroying insects.
- Noncombustible. Steel does not burn and prevents the spread of fire. Steel framing can be easily designed to meet code fire rating requirements.
When specifying CFS framing members, the universal designator system is typically used. The "STUF" designation identifies any common CFS member using:
- Web Depth (D), expressed in 1/100th inches,
- Flange width (B), expressed in 1/100th inches,
- Minimum Base Metal Thickness (t), expressed in mils (1/1000th inches), and the following designators:
Example: Designation for a 5-½"-16 gauge C-shape stud with 1-5/8" flanges: 550S162-54
CFS members are typically labeled with manufacturer's identification or logo, minimum uncoated steel thickness, minimum yield strength and coating designation (if other than minimum) at a minimum spacing of 48 inches along the length of the member. For users that are accustomed to the old "gage" system of steel thickness, the following conversions may be useful.
Mil to Gauge Conversion
Do Steel Homes Corrode?
Homeowners expect their homes to last for a lifetime or more. Therefore, it is critical that framing materials have the proper protection to provide this longevity. With steel, this protection comes in the form of galvanizing. Steel members located in an indoor atmosphere (such as wall and floor framing) have a very low rate of corrosion. Studies showed that typical G40 zinc coated steels, in such an environment should last for more than 100 years. All structural-framing members have a minimum metallic coating of G60 (or equivalent). Non-structural framing members have a minimum metallic coating of G40 (or equivalent). It is essential not to install CFS joists in contact with copper. CFS members do not react with dry wood, drywall, insulation products, or dry plaster or mortar. In particularly severe climates, such as coastal, G90 coating should be specified for protected framing members. as is common in Hawaii where corrosion, termites, earthquakes, typhoons and tsunamis are significant concerns.
The C-shape section is the most commonly used shape in CFS framing. A typical C-shaped member consists of a web, a flange, and a lip and comes with or without holes (punchouts). Flat straps, angles, plates, furring channels, and hat channels are also available. Member thickness is specified in mils (1 mil=1/1000 inch), although the gauge designation is still widely used (the higher the gauge the thinner the thickness. Steel has the largest strength-to-weight ratio among construction materials.
CFS framing members are cold-formed to shape from structural quality sheet steel complying with the requirements of one of the following:
- ASTM A 653: Grades 33, 37, 40, & 50 (Class 1 and 3); or
- ASTM A 792: Grades 33, 37, 40, & 50A; or
- ASTM A 875: Grades 33, 37, 40, & 50 (Class 1 and 3); or
- Steels that comply with ASTM A 653 except for tensile and elongation requirements shall be permitted provided the ratio of tensile strength to yield point is at least 1.08 and the total elongation is at least 10 percent for a two-inch gage length or 7 percent for an eight-inch gauge length.
Availability and Handling.
CFS is generally supplied through steel framing distributors, drywall supply houses or steel-framing manufacturers. NASFA publication NT5-99 provides a comprehensive list of steel suppliers. Steel components come in bundles or on pallets. A forklift can handle most unloading jobs quite easily. Planning your cut list carefully can simplify the job. The SteelXpert software is an excellent tool for planning a cut list efficiently. It is also recommended that you order at least 10% additional material as is customarily. To get a copy of NT5-99 or the SteelXpert program, please visit the Steel Framing Alliance's website at www.SteelFramingAlliance.com.
Fasteners for CFS Members
Self-drilling, tapping screws (or self-piercing screws) are the most prevalent fasteners. Other connection technologies, such as pneumatically driven fasteners, powder-actuated fasteners and crimping (fastenerless) can also be used.
- Steel-to-Steel Connections. No.8½-inch pan head screws are commonly used to connect studs/joists and tracks together. No.10-3/4" hex-head, self-drilling screws are used on thicker steel materials. Self-drilling screws with low profile heads are used when rigid sheathing or finishing materials are installed on top of the screw head. Screws should be installed with a minimum edge distance and center to center spacing of ½ inch.
- Wood-to-Steel Connections. No.8 self-drilling screws with pilot points are generally used to fasten wood sheathing to steel members. Self-drilling screws with bugle heads are designed to countersink slightly in the wood surface without crushing or tearing the material. Wafer head screws are typically used to connect soft materials to steel joists. Flat head screws are designed to countersink and sit flush without causing finishes to splinter or split. A sheathing screw should penetrate the sheathing and steel framing first before engaging the threads to pull the sheathing material tight towards the steel.
- Gypsum Board-to-Steel Connections. Gypsum board is fastened with No.6, sharp point, bugle head screws (known as drywall screws) Use a depth-setting nosepiece to avoid damaging the gypsum board. For steel thicker than 43 mils, self-drilling bugle head screws should be used.
National and regional manufacturers, roll-formers, suppliers, and distributors of cold-formed steel can be found on the www.steelframingalliance.com website by the Steel Framing Alliance. Manufacturers can offer detailed product information, including options, dimensions, design tables, and specifications. The Steel Stud Manufacturers Association (SSMA) provides a unified and comprehensive catalog of most of the residential and light commercial steel products available today. A copy of this catalog can be obtained from a local distributor.
Choosing a manufacturer or a roll-former that provides the technical support and a product with the desired attributes (such as specialty shapes for unique architectural or structural features) is an important issue. There are also issues that are important to construction efficiency that must be considered, though this factor is usually resolved through personal experience or by discussions with a knowledgeable contractor. It should be recognized that there is a "learning curve" that must be overcome in becoming efficient with any new technology. But, with a good approach, the proficient adoption of a viable new technology can pay great dividends in business profits and diversity.
Technical Information Resources
Technical information on cold-formed steel framing can be found on the North American Steel Framing Alliance website, www.steelframingalliance.com, which also links to other sites such as the Light Gauge Steel Engineers Association website, www.LGSEA.com. Both sites contain helpful publications for designers, builders, and code officials who are interested in learning more about CFS. Cold-formed steel framing is also recognized with detailed prescriptive design and construction provisions in the International Residential Building Code, 2000 Edition, published by the International Code Council.
Regional Steel Alliances
Eight regional steel alliances were set-up (in addition to the national one) to enable and encourage the widespread, practical, and economic use of CFS in residential and light commercial construction. The alliances also help educate builders, contractors and designers on steel framing. Many of these alliances have their own web page, newsletter and conduct regular meetings. Regional alliances can be a great place to meet local suppliers, contractors, builders and designers. The Mid-Atlantic Steel Framing Alliance (MASFA) for example, covers the states of Maryland, Virginia, Delaware, Pennsylvania, New Jersey, and the District of Columbia. It has over 27 members representing various industries, a quarterly newsletter, and a website, www.MASFA.com. To join MASFA, call (410) 787-7020 or (301) 430-6281.
Designing a home with CFS Framing
There are basically three resources available to guide the design of a typical CFS home application:
- Prescriptive Method for Residential Cold-Formed Steel Framing, 2000 Edition;
- Sections R505, R603 and R804 of the International Residential Code, 2000 Edition; and,
- Manufacturer technical data.
Other publications and technical bulletins are also available for specific components or design methodology (www.steelframingalliance.com and www.LGSEA.com). For example:
- Design Guide for Cold-Formed Steel Trusses
- Shearwall Design Guide
- Low-Rise Residential Construction Details
- Durability of Cold-Formed Steel Members
- Builder's Stud Guide
- L-Shaped Header Field Guide
- Builder's Guide to Residential Steel Floors
- Inspection Checklist for Cold-Formed Steel Framing
- Shear Transfer at Top Plate: Drag Strut Design
- Vertical Lateral Force Resisting System: Boundary Elements
- Lateral Load Resisting Element: Diaphragm Design Values
- Screw Fastener Selection for Light Gauge Steel Frame Construction
For unique conditions, it may be necessary to use the AISI Specification for the Design of Cold-Formed Steel Structural Members.
Armed with the above information, most design decisions regarding CFS can be easily resolved by following prescriptive span tables and straightforward connection details. However, there are a few "design tips" that may be valuable to the first-timer or novice user:
- A team approach involving the
contractor-builder-manufacturer-designer-owner is encouraged. This means that
the entire team should be included in the process as soon as possible and
during the design concept stage if possible.
- Make sure that the contractor understands appropriate
steel framing methods and practices. A poorly planned construction endeavor
can reflect badly on the CFS technology as well as the designer.
- Always follow applicable Occupational Safety and
Health Administration (OSHA) guidelines and safety requirements.
- All structural members shall be aligned vertically
(in-line framing) to transfer all loads to the foundation, unless horizontal
load distribution members are specified.
- When connecting steel to steel members, drilling
tools should drive screws at low speeds of 2500 rpm to prevent improper
- Keep plumbing in the slab and route through floor and
interior wall cavities to the greatest extent possible.
- Avoid locating plumbing in exterior walls because of
potential interruption of structural members and a reduction of the thermal
value of the wall.
- Don't forget web stiffeners for bearing points of
floor joists. These are essential components. Stiffeners can be installed on
either side of the joist's web.
- Consult the Thermal Design Guide (www.steelframingalliance.com) for the proper
- Snap-in plastic insulators, grommets, conduits, or
other approved wire protection methods should always be used to protect the
plastic sheathing on electrical cables and wires when passing through holes in
- Ensure that copper pipes are always separated from
the steel framing by using plastic insulators or other approved methods.
- Fire rated steel assembly details can be found in the UL's Fire Resistance Directory, the Gypsum Association's Fire Resistance Design Manual or
- All load-bearing studs shall be seated in the tracks with a maximum gap of 1/8 inch between the end of the stud and the web of the track.
Finally, there is no substitute for doing your own homeworkexplore the resources mentioned in this article and become informed in the various attributes of CFS systems.
Figure 1. Photos of a Steel Houses
Figure 2. Basic Shapes of CFS