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Framing With Light-Gauge Steel, continued

Building Walls

LGS walls use track members for top and bottom plates. Studs fit inside the track and are held in place with two #10x3/4-inch self-drilling screws or pneumatically driven metal-to-metal nails. All fasteners should have a locally accepted code approval. In most cases, an ASTM or ICBO approval will be printed on the box and a manufacturer's logo will be imprinted on the head of the fastener. Metal framing can also be joined by clinching or welding, but those methods require special inspection and/or certification. Welding also burns off the galvanizing, so welds need to be recoated with cold-forming galvanizing paint.

Bottom track is cut and butted the same as wood bottom plates. Top tracks are different than top plates because they aren't doubled. Instead, they run the full length of the wall, and the flanges are notched to lap at the corners. We cut plates in the field, so I buy track in 20-foot lengths. LGS is rolled to order, so I could get 60-foot pieces, but it's hard to handle anything that long. For long walls, you can splice the top plate by putting a short section of stud inside the track and screwing in through the sides. Be sure to land the splice over a bay, so it won't interfere with the studs.

When you figure stud lengths, you need to account for the thickness of the track. I usually deduct 1/4 inch for the plates, so a 9-foot wall would be framed with 8-foot 11 3/4-inch studs.

Wood framers assemble walls on the deck and tilt them up. You can do this with LGS, but it's easier to build walls in the upright position (Figure 3).

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Figure 3.Wood frame carpenters are accustomed to framing walls flat and then standing them up. It's a little more work to do this with LGS because you have to flip the wall to fasten the other side (left). Traditionally, steel stud walls are assembled in the vertical position. The bottom track is fastened to the deck, the top track is propped up in the air, and the studs go in last (center and right). In this case, the supplier provided curved track for the curved walls.

That's because you have to fasten studs through both sides of the track. If you assemble the wall flat, you have to flip it halfway through to fasten the other side. LGS is so straight that it's easier to brace up the corner studs and run the top track between them. The rest of the studs can be installed afterward.

Studs (including joists and rafters) are punched 2 feet on-center to provide openings for pipes and wire. Rule number one is that the punch holes should line up when you look down the wall. This will happen if you follow rule number two, which is to install studs with the cut end up. That way the lowest hole will always be 2 feet off the floor (Figure 4).

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Figure 4.Studs, joists, and rafters are punched 2 inches on-center to accommodate pipes and wire. You should install them so the holes line up.

Slab Foundations

Track is much more flexible than wood, so a wall that lands on a wavy slab-on-grade foundation will telegraph the lumps and dips to the ceiling and floor above. This means the foundation needs to be very flat — plus or minus 1/8 inch. There are a couple of fixes for a wavy foundation. You can set the top plate level and custom cut each stud, or you can use metal shims. I find that it's faster to cut. Slight variations in a foundation wall cause fewer problems with floors, because the track that functions as a rim joist does have some ability to span minor gaps, and you can shim the floor system as needed.

Anchor bolts. Because track is only 1/8 inch thick, anchor bolts should be set about an inch up from the slab surface. If you set the bolts too high, there won't be enough thread and you'll have to put a stack of washers under the nut. Pay attention to the wall thickness when you set the bolts, or else they'll be off center. A 2x6 is only 5 1/2 inches wide, but a 6-inch metal stud is exactly 6 inches wide.

Joists and Rafters

LGS joists and rafters are basically studs with wider webs; they're anywhere from 6 inches to 14 inches wide. Rim joists are typically wide pieces of track and are of lighter gauge than the joists. Steel rafters don't get seat cuts; instead, they're attached to the top track with L clips. Clips are also used to fasten rafters to hips and ridges. An alternate method is to cut 2 inches off the flanges, bend the web sideways, and screw through the tab that remains (Figure 5).

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Figure 5.Metal L clips can be used for a variety of connections. Here they're being used to fasten rafters to a bearing ridge, but they can also be used to attach joists to beams, beams to beams, and rafters to plates.

Web stiffeners required. Horizontal bearing members (joists, rafters, beams, and headers) require web stiffeners similar to those used in wood I-joist construction (Figure 6). Without web stiffeners, concentrated loads will cause the web section of the member to bow.

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Figure 6.Web stiffeners are required at the bearing points of joists, rafters, beams, and headers. In this photo, the carpenter is using a short section of stud to stiffen the end of a rafter.

A stiffener is simply a short piece of stud or L metal screwed vertically to the side or fit between the flanges of the member that's being stiffened. Web stiffeners are required where rafters bear on walls, walls bear on joists, and joists bear on foundations or walls below. They're also required at the ends of beams and headers, and anywhere else you put a concentrated load on a horizontal member.

Beams, Posts, and Headers

One of the most labor-intensive tasks on a steel framing job is fabricating beams and headers. The most common assemblies are box beams and back-to-back headers.

Box beams are made by ganging two or more joists between a pair of tracks (Figure 7). The pieces are fastened through the top and bottom track at 12 inches on-center with #10 self-drilling screws. The track is there only to hold the joists together, so it doesn't have to run the full length. I've installed beams built up from as many as five joists.

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Figure 7.Box beams are made by putting two or more unpunched studs between a top and a bottom track. Here, joists have been flush-framed to a box beam in the floor system.

Back-to-back headers are made by putting a pair of joists back to back and screwing through the webs. Headers can also be boxed, but the top piece of track is replaced by the top plate of the wall. The lower header track is installed open side down so there's something to attach the cripples to. We don't use jacks or trimmers. Instead, we cut the header studs 6 inches long and create web extensions by cutting back the flanges. The extensions lap the king studs and are attached with a specified number of screws (Figure 8). It's important to remove the rolled corner when you cut back the flanges; otherwise, there will be a bump in the drywall. You should also notch 1 3/4 inches off the top of the web extension so the upper track will fit over the king studs.

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Figure 8.Jack studs aren't necessary in an LGS frame. In this case, the studs that make up the sides of the header are notched and the assembly is attached by screwing through the web.

The span charts in the prescriptive code apply to beams and headers made from unpunched material. It's possible to construct them from punched material, but you'll need an engineer's approval. In addition, stiffeners need to be installed wherever there is a concentrated load on the beam. The L clips that hold the beam or header to the structure often function as stiffeners, but in some cases you'll have to stiffen members with short sections of stud.

Layout

The biggest difference between framing with wood and framing with LGS is how you do the layout. With steel, you need to be much more careful about load paths. The top track can't support loads between studs, so in most cases, each rafter, truss, and floor joist needs to land directly on a stud (Figure 9).

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Figure 9.Wall track (plates) will not support vertical loads between studs, which is why studs have to be in line with joists and rafters unless there's a beam between them.

Steel studs have a hard side and a soft side: The web side is hard, the open side soft. We like to build walls with the open sides all facing the same direction. A few might face the other way, but only at openings, intersections, and the ends of walls. Ideally, the hard side of the stud will be in line with the hard side of the joist below and the rafter above. However, it's permissible to offset these planes as much as 3/4 inch by facing the joist one way and the stud the other. But they still need to stack.

When needed, however, there are ways to get around this. You can turn the rim joist into a beam or build the wall with an oversized top track (6 inches or taller) that's designed to function as a continuous header. If you size these members correctly, the wall won't need headers and the studs and joists won't have to align.

Fastening Trim

It's more difficult to attach trim to a building that's framed with steel, but there are ways to do it. Senco makes hardened 15-gauge finish nails that will penetrate 14-gauge steel. I personally like to put wood bucks in door openings to make it easier to hang and case the doors. For large crown, I will screw a wood nailer onto the drywall. I attach baseboard with construction adhesive and cross nail into the drywall to hold it till the glue sets.

Thermal Conductivity

One of the toughest issues facing the LGS industry is how to build an energy-efficient structure. Steel is a great conductor of heat — not a good thing when you're trying to conserve energy. LGS is extremely popular in Hawaii because the outside temperature doesn't vary much at all. I build in a coastal climate zone, where it's relatively easy to meet energy code requirements.

In this area, tract builders create a thermal break by putting foam insulation on the outside of 3 1/2-inch exterior walls. I build custom homes and meet the requirements by building 5 1/2-inch exterior walls, sheathing the entire surface, and completely filling the cavity with insulation. This is more insulation than we need, but it's a way to compensate for the added conductivity of steel. In colder parts of the country, you need to be a lot more serious about installing thermal breaks. One possible way to deal with this issue is to use less conductive proprietary studs, such as the ones made by Tri-Chord Steel Systems, Inc.


Matt Macarewichis a general contractor and the co-owner of Steel Professionals in Southern California (www.steelpros.com).

Sources

California Steel Framing Alliance

888/952-2732

www.casfa.org

Hawaii Pacific Steel Framing Alliance

808/485-1400

www.hawaiisteel.com

Light Gauge Steel Engineers Association

866/465-4732

www.lgsea.com

Steel Framing Alliance

202/785-2022

www.steelframingalliance.com

Steel Stud Manufacturers Association

312/456-5590

www.ssma.com

Tri-Chord Steel Systems, Inc.

602/426-8700

www.tri-chordsteelsystems.com