Welding of CFS has gained popularity as taller structures lend themselves to thicker members and pre-panelized construction. this built-up column will receive zinc-rich paint before sheathing is installed. Don Wheeler, Wheeler Construction Co.

Cold-formed steel (CFS) framing is gaining wider acceptance in many applications, especially in mid-rise construction from four to nine stories. With this broader usage, connection design has become even more critical, as applications and innovative details stretch the limits of both materials and connectors. This article will cover some of the common and uncommon methods of fastening CFS framing, as well as offer resources for more information.

The American Iron and Steel Institute’s (AISI) North American Specification for the Design of Cold-Formed Steel Structural Members (S100) gives information in Chapter E on connections for CFS. This goes beyond just framing: The equations in S100 apply to all CFS, including metal buildings, racks, metal siding and decking, and other applications with thin sheets (material up to 1 inch thick). With respect to framing, the most common fastening type is screw attachment; with welding second, and bolting third. These three are the only connection types listed in S100; however, others are included in this article.

Screws
The screw connection requirements are fairly straightforward: capacity depends upon the thickness and tensile strength of the sheet steel, and the diameter of the screw. For pullover capacity, screw head diameter or washer diameter is also an issue. The ultimate strength of the screw may also be a limiting factor and should be checked for connection of thicker steels, but typically does not govern in framing up to 54 mils thick (0.0566 inch design thickness). Many engineers have developed their own spreadsheets for calculation capacities based on commonly specified screws. Manufacturers’ data may also be used if a specific fastener is to be used. Note that the tensile strengths most common for CFS framing are 45 kips per square inch (ksi) for minimum 33-ksi framing, and 65 ksi for minimum 50-ksi framing. These tensile strengths come from ASTM A1003, which gives requirements for the sheet steel used for the framing members.

Welding
Welding of CFS framing is becoming more common as more framing is shop-fabricated. With field welding, and especially with connection of CFS to structural steel, melt-through of the steel sheet can be a problem. However, if lap joints as opposed to T-joints, corner joints, and butt joints are specified, it is easier making a successful weld from CFS to thicker, structural members. Note that both the procedures and the qualification of the individuals performing the welding are specified in American Welding Society (AWS) D1.3: Structural Welding Code – Sheet Steel. D1.3 should be included in specifications and/or general notes for CFS welding. Some common concerns for CFS are the effects of the zinc coating on the weld, the minimum thickness required to prevent melt-through, and corrosion protection after welding. The thin coating of zinc typical on CFS, between 0.40 ounces (oz.) per square foot and 0.90 oz. per square foot (G40 to G90) is often not enough to affect the weld capacity. Still, welding must be done in well-ventilated areas due to the health hazard of vaporized zinc. Although the code does not specify a minimum thickness for welding, and expert welders using appropriate equipment can weld to almost anything, Table 1 has been used successfully by some specifiers.

The minimums presented in Table 1 are merely suggestions, based on experiences of some structural engineers and field and shop welders. Results may vary by market, training, and levels of expertise. Note that several of the prequalified joints in chapter 3 of AWS D1.3 show a minimum of 18 gage (43 mil). The gage thicknesses shown in the appendix of AWS D1.3 differ slightly from the minimum thicknesses specified by AISI and SSMA; make sure to use appropriate AISI design thicknesses. For corrosion protection of welds, the most common method is application of a zinc-rich paint after the weld has cooled to ambient temperature. Some specifications include a reference to ASTM A780 Standard Practice for Repair of Damaged and Uncoated Areas of Hot-Dip Galvanized Coatings.

Bolting
Bolting is also part of the chapter E requirements in AISI S100. Bolted connections are not that common in CFS framing, although the bolt bearing equations have often been used for other round-shank steel connectors through CFS to approximate bearing failure of the CFS sheet. Some truss configurations use bolted connections. When designing trusses, make sure to consider and specify AISI Standard for Cold-Formed Steel Framing – Truss Design (S214), which has truss-specific requirements for connections.

The most common fastening methods beyond those specified in S100 include pneumatically driven pins, powder-actuated fasteners, concrete anchors, clinching, crimping, and rivets.

Pneumatic pins have some of the widest applications with CFS. Very similar to nails in a nail gun, pneumatic pins have a point hardness and shank surface specifically tested for CFS applications. These applications include CFS to CFS, wood (and other fiber-based sheathing) to CFS, CFS to concrete, and even CFS to fiberglass-faced exterior gypsum panels. Note that pneumatic tools typically do not have the consistency for connection of finished gypsum panels to CFS; screws or screws with adhesives are still more appropriate. Several pneumatic pin manufacturers have research reports for shearwalls and diaphragms using their products to attach wood or other materials to CFS.

Other connections
Powder-actuated fasteners are most common for attaching CFS to other substrates: structural steel or concrete. Not designed typically for CFS-to-CFS connections, powder-driven pins are excellent for attachment of top and bottom runner tracks to slabs and beams for support of non-structural walls and ceilings. Note that in certain seismic applications, the use of powder-driven pins (and some other power-driven fasteners) in applications loaded in withdrawal are not permitted by code. However, threaded or expansion anchors may be used in similar applications: There are several types of excellent concrete anchors that are typically limited by the connection to the concrete and not the connection to the CFS. Bolt-bearing equations may be appropriate when these connections are loaded in pure shear.

Clinching is a fastenerless connection that resists load by the interlocking of one sheet of steel into another. Used for decades in the automotive and other industries, clinching is most often seen in shop applications, due to the large equipment requirements. Crimping is another fastenerless connection, but is typically only for non-structural drywall framing, and has a relatively small connection capacity. Riveting of CFS, although not that popular in U.S. construction, has been used successfully for years in Australian CFS construction, especially in roof truss connections. Rivets are pressed into both layers of sheet steel, and the legs of the rivet spread out to develop the connection capacity. The outer “head’ of the rivet is flush with the outer layer of steel, and a round bulge is visible on the back of the back layer.

Multiple applications for screwed connections with cFS framing: hex-washer-head for truss web-to-chord connection, low-profile head for strapping and hold-down connection below roof sheathing, and bugle-head screws for gypsum sheathing attachment. Don Wheeler, Wheeler Construction Co. Both shop and field welding can be performed with cFS to cFS and cFS to heavier steels. Lap joints (shown here for cFS) work better than butt, t, or corner joints when there is a large difference in material thickness. Don Wheeler, Wheeler Construction Co.

In addition to these connections, specific connectors have been developed for specific applications. One of the most common is the slip track. AISI Standard for Cold-Formed Steel Framing – Wall Stud Design (S211) now includes a methodology for single slip track design; Steel Stud Manufacturers Association (SSMA) Technical Note 3 gives details and tabulated values for capacities of double slip tracks. Some CFS and specialty product manufacturers now have CFS-specific connectors such as hold-down anchors, bypass slip connectors, joist hangers, and truss connectors. Most of these use screw, bolt, or welded connections, and have connection-specific capacities listed in manufacturers’ catalogs. Also, The Cold-Formed Steel Engineers Institute (CFSEI) of the Steel Framing Alliance (SFA) has a series of Technical Notes that give specific information about CFS connections. All notes are available through www.cfsei.org including the following:

Powder-actuated fasteners are common for attachment of cFS to both concrete and structural steel. designers must ensure there is enough clearance for the head and nosepiece of the tool for a successful fastening. Don Wheeler, Wheeler Construction Co. The most common use of bolts for CFS framing is at slab anchorage, followed by bolted truss connections and cFS bolted to structural steel or wood. Don Wheeler, Wheeler Construction Co.

• 560b-1: Welding Cold-Formed Steel;
• 560b-5: Fastener Corrosion;
• 560c: Clinch (Integral) Fastening of Cold-Formed Steel;
• 561b: Pneumatically Driven Pins for Wood-Based Panel Attachment;
• 561c: Diaphragm Design With Pneumatically Driven Pins;
• 562: Powder-Actuated Fasteners in Cold-Formed Steel Construction;
• 544: Design of By-Pass Slip Connectors in Cold-Formed Steel Construction;
• 565d: Screws For Attachment of Steel-To-Wood and Wood-Steel; and
• 565c: Screw Fastener Selection for Light Gauge Steel Framing.

Specific configurations of CFS have specific connection requirements. For example, window and door openings in walls have specific requirements for connections of jambs, headers, and sills; and shearwalls have specific requirements for connections of sheathing and anchorage of boundary elements.

In addition to configuration-specific CFSEI Technical Notes, both AISI and CFSEI have design guides that address these connections. Two of the most common are AISI D110-07 Cold-Formed Steel Framing Design Guide and CFSEI Cold-Formed Steel Framed Wood- or Steel-Sheathed Shear Wall Assemblies. Also, the framing standards listed in section 2210 of the International Building Code (IBC) give the specific requirements of both members and connections in these applications. For connection details, both SSMA and CFSEI have downloadable details for wall, roof and floor configurations available from their respective websites.

The best sources for practicing structural engineers are from CFSEI (www.cfsei.org) and their parent organization Steel Framing Alliance (www.steelframing.org). Both have some information as free downloads, some information as “members-only” resources, and some information for sale, such as AISI code-referenced standards. SSMA (www.ssma.com) also has a technical library of resources, all of which are free and downloadable. In addition, specific fastener manufacturers and distributers maintain technical information on their specific products, and many of them have Evaluation Service Reports, most through International Code Council Evaluation Service (ICC-ES), but some through other agencies providing similar services.

In conclusion, there are many different ways to connect CFS framing: screws, bolts, and welds addressed in AISI S100, and other methods addressed in manufacturers’ literature, industry technical notes, and design guides. Additional information is available from industry associations, fastener and framing manufacturers, as well as the Cold-Formed Steel Engineers Institute.

Brian Ide, S.E., P.E., is a principal at Allison-Ide Structural Engineers, LLC. He can be reached at 808-536-2108 or brianide@allisonide.com. Don Allen, P.E., SECB, is the technical director for the Steel Framing Alliance, Steel Stud Manufacturers Association, and Cold-Formed Steel Engineers Institute. He can be reached at 706-597-8076 or dallen@steelframing.org.