12 Light Gauge Steel Frame Construction • The Concept of Light Gauge • Light Gauge Steel Framing Steel Construction and the Building Codes CONSIDERATIONS OF SUSTAINABILITY IN • Finishes for Light Gauge LIGHT GAUGE STEEL FRAMING Steel Framing METALS IN ARCHITECTURE • Framing Procedures FROM CONCEPT TO REALITY • Other Common Uses of Camera Obscura at Mitchell Park, Light Gauge Steel Framing Greenport, New York FOR PRELIMINARY DESIGN OF A LIGHT GAUGE STEEL FRAME STRUCTURE • Advantages and Disadvantages of Light Gauge Steel Framing Driving self-drilling, self-tapping screws with electric screw guns, framers add diagonal bracing straps to a wall frame made from light gauge steel studs and runner channels.indd 489 10/30/08 4:30:34 AM To manufacture the members used in light gauge steel frame cee members are punched at the fac- tory to provide holes at 2-foot (600- construction, sheet steel is fed from continuous coils through mm) intervals; these are designed to machines at room temperature that cold-work the metal (see allow wiring, piping, and bracing to Chapter 11) and fold it into efficient structural shapes, producing pass through studs and joists without linear members that are stiff and strong. Thus, these members the necessity of drilling holes on the are referred to as cold-formed metal framing to differentiate them construction site. For top and bot- from the much heavier hot-rolled shapes that are used in struc- tom wall plates and for joist headers, channel sections are used. The strength tural steel framing.
The term “light gauge” refers to the relative and stiffness of a member depend thinness (gauge) of the steel sheet from which the members are on the shape and depth of the sec- made. tion and the gauge (thickness) of the steel sheet from which it is made. A standard range of depths and gauges The Concept of frame building may be sheathed, in- is available from each manufacturer. Light Gauge Steel sulated, wired, and Þnished inside Commonly used metal thicknesses Construction and out in the same manner as a for loadbearing members range from wood light frame building.84 mm) Light gauge steel construction is the The steel used in light gauge and are as thin as 0.45 noncombustible equivalent of wood members is manufactured to ASTM mm) for nonloadbearing members light frame construction.
The exter- standard A1003 and is metallic- (Figure 12. nal dimensions of the standard sizes coated with zinc or aluminum-zinc At least one manufacturer pro- of light gauge members correspond alloy to provide long-term protection duces nonloadbearing light gauge closely to the dimensions of the stan- against corrosion. The thickness of steel members by passing steel sheet dard sizes of nominal 2-inch (38-mm) the metallic coating can be varied, through rollers with mated patterned framing lumber. These steel members depending on the severity of the en- surfaces, producing a dense array of are used in framing as closely spaced vironment in which the members will dimples in the metal of the formed studs, joists, and rafters in much the be placed.
For studs, joists, and raf- members. The additional cold work- same way as wood light frame mem- ters, the steel is formed into C-shaped ing of the metal that occurs during bers are used, and a light gauge steel cee sections (Figure 12. The webs of the forming process and the Þnished Figure 12.1 Typical light gauge steel framing members. To the left are the common sizes of cee studs and joists.
In the center are channel studs. To the right are runner channels.indd 490 10/30/08 4:30:36 AM The Concept of Light Gauge Steel Construction / 491 CONSIDER ATIONS OF SUSTAINABILITY IN LIGHT GAUGE STEEL FRAMING In addition to the sustainability issues raised in the previ- Even with insulating sheathing, careful attention ous chapter, which also apply here, the largest issue con- must be given to avoid undesired thermal bridges. For cerning the sustainability of light gauge steel construction example, on a building with a sloped roof, a signiÞcant is the high thermal conductivity of the framing members. thermal bridge may remain through the ceiling joist-rafter If a dwelling framed with light gauge steel members is connections, as seen in Figure 12.
Foam sheathing on framed, insulated, and Þnished as if it were framed with the inside wall and ceiling surfaces is one possible way to wood, it will lose heat in winter at about double the rate avoid this condition, but adding insulation to the inside of the equivalent wood structure. To overcome this limita- of the metal framing exposes the studs and stud cavities tion, energy codes now require light gauge steel framed to greater temperature extremes and increases the risk of buildings constructed in cold regions, including most of condensation. It also still allows thermal bridging through the continental United States, to be sheathed with plastic the screws used to fasten interior gypsum wallboard to the foam insulation panels in order to eliminate the extensive framing. Though small in area, these thermal bridges can thermal bridging that can otherwise occur through the readily conduct heat and result in spots of condensation steel framing members.
on interior Þnish surfaces in very cold weather. patterned surface result in members and miscellaneous shapes are manu- to suit a full range of connection situ- made from thinner sheet stock that are factured as accessories for light gauge ations. Welding is often employed to equal in and strength and stiffness to steel construction (Figure 12. assemble panels of light gauge steel conventionally formed members pro- Light gauge steel members are framing that are prefabricated in a duced from heavier gauge material.
usually joined with self-drilling, self- factory, and it is sometimes used on For large projects, members may tapping screws, which drill their own the building site where particularly be manufactured precisely to the re- holes and form helical threads in the strong connections are needed. Oth- quired lengths. Otherwise, they are holes as they are driven. Driven rap- er fastening techniques that are wide- furnished in standard lengths.
Mem- idly by hand-held electric or pneu- ly used include hand-held clinching bers may be cut to length on the matic tools, these screws are plated devices that join members without construction job site with power saws with cadmium or zinc to resist cor- screws or welds and pneumatically or special shears. A variety of sheet rosion, and they are available in an driven pins that penetrate the mem- metal angles, straps, plates, channels, assortment of diameters and lengths bers and hold by friction.2 Minimum Thickness of Steel Sheet Minimum thicknesses of base sheet Loadbearing Light Nonloadbearing Light metal (not including the metallic coating) Gauge Gauge Steel Framing Gauge Steel Framing for light gauge steel framing members.46 mm) Traditional gauge designations are also 14 0.73 mm) included (note how lower gauge numbers 16 0.37 mm) correspond to greater metal thickness).09 mm) Gauge numbers are no longer recom- 20 0.75 mm) mended for specification of sheet metal 22 0.69 mm) thickness due to lack of a uniform stan- 25 0.45 mm) dard for the translation between these numbers and actual metal thickness. Sheet metal thickness may also be speci- fied in mils, or thousandths of an inch. For example, a thickness of 0.033 inch can be expressed as 33 mils.indd 491 10/30/08 4:30:39 AM 492 / Chapter 12 • Light Gauge Steel Frame Construction Figure 12.3 Standard accessories for light gauge steel framing.
End clips are used to join members that meet at right angles. Foun- dation clips attach the ground-floor plat- form to anchor bolts embedded in the END CLIPS foundation. Joist hangers connect joists to headers and trimmers around open- WEB STIFFENER ings. The web stiffener is a two-piece assembly that is inserted inside a joist and screwed to its vertical web to help transmit wall loads vertically through the joist.
The remaining accessories are used for bracing. FOUNDATION CLIP V-BRACING FLAT STRAP BRACING JOIST HANGER 1 1/2" COLD ROLLED CHANNEL Framing Procedures to make wall frames. The wall frames possible, in fact, to frame any build- are sheathed either with wood panels ing with light gauge steel members The sequence of construction for or, for noncombustible construction, that can be framed with nominal a building that is framed entirely with gypsum sheathing panels, which are 2-inch (38-mm) wood members. with light gauge steel members is similar to gypsum wallboard but with To achieve a more Þre-resistive essentially the same as that described glass mat faces and a water-resistant construction type under the build- in Chapter 5 for a building framed core formulation.
The wall frames are ing code, ßoors of corrugated steel with nominal 2-inch (38-mm) wood tilted up, screwed down to the ßoor decking with a concrete topping members (Figure 12. Framing is frame, and braced. The upper-ßoor are sometimes substituted for wood usually constructed platform fash- platform is framed, then the upper- panel subßooring. ion: The ground ßoor is framed with ßoor walls.
Finally, the ceiling and roof Openings in ßoors and walls are steel joists. Mastic adhesive is applied are framed in much the same way as in framed analogously to openings in to the upper edges of the joists, and a wood-framed house. Prefabricated wood light frame construction, with wood panel subßooring is laid down trusses of light gauge steel members doubled members around each open- and fastened to the upper ßanges of that are screwed or welded together ing and strong headers over doors the joists with screws. Steel studs are are often used to frame ceilings and and windows (Figures 12.
laid ßat on the subßoor and joined roofs (Figures 12. It is Joist hangers and right-angle clips of JWBK274_Ch12.indd 492 10/30/08 4:30:40 AM Framing Procedures / 493 joist roof r Steel afters End clip Ridge beam— nested steel joists Anchor clip Steel joist roof rafter Steel joist soffit framing B EAVE A RIDGE Stud Figure 12.4 Runner Typical light gauge framing details. Each detail is keyed Continuous bead of by letter to a circle on the whole-building diagram in the adhesive center of the next page to show its location in the frame. (a) A pair of nested joists makes a boxlike ridge board or Web stiffener ridge beam.
(b) Anchor clips are sandwiched between the ceiling joists and rafters to hold the roof framing down to the wall. (c) A web stiffener helps transmit vertical forces from each stud through the end of the joist to the stud in the floor below. Mastic adhesive cushions the joint between the subfloor and the steel framing. (d) Foundation clips anchor the entire frame to the foundation.
(e) At interior joist bearings, joists are overlapped back to back and a web Closure channel stiffener is inserted. (continued) C-runner C JOIST BEARING AT UPPER FLOOR Runner—fasten through plywood into closure Plywood subfloor Web stiffener Steel joists Grout and shim as required Web stiffener Foundation clip Steel stud or beam D JOIST BEARING AT FOUNDATION E INTERIOR JOIST BEARING JWBK274_Ch12.indd 493 10/30/08 4:30:40 AM 494 / Chapter 12 • Light Gauge Steel Frame Construction Ceiling joists Rafter Steel stud A H GABLE END FRAMING Closure B channel or joist section H End tabs C 1 1/2" x 20- gauge bracing strap E D G F G JOIST PARALLEL TO END WALL Closure channel or joist section Figure 12.4 (continued) ( f, g) Short crosspieces brace the last joist at the end of the building and help transmit stud forces through to the wall below. (h) Like all these details, the gable end framing is directly analogous to the corresponding detail for a wood light frame building as shown in Chapter 5. F JOIST PARALLEL TO FOUNDATION JWBK274_Ch12.indd 494 10/30/08 4:30:40 AM Framing Procedures / 495 Opening Figure 12.5 Headers and trimmers for floor openings Joist hanger are doubled and nested to create a strong, Double joist header stable box member.