Published byNoelle Duty Modified over 8 years ago
Unit 45 Metal Framing Industry and Code Regulations • Light-gauge Steel Framing Members • Fasteners • Framing Tools • Metal Framing Safety • Light-gauge Steel Construction Methods • Structural Steel Framing • Building Envelope and Insulation
The use of light-gauge steel framing continues to increase in residential and commercial construction. Many metal framing methods are employed today. Light-gauge steel construction is often referred to as “steel for stick” construction since each wood framing member in a typical assembly is replaced with a metal framing member. See Figure Another metal framing method, similar to light-gauge steel construction, involves the use of heavier-gauge light-gauge steel studs spaced 4′ OC, with lighter gauge members installed between the studs.
C-shapes, tracks, U-channels, and furring channels are the most common light-gauge steel framing member shapes. Light-gauge steel framing members are available in various shapes, thicknesses, and strengths. Common light-gauge steel framing member shapes include the C-shape, track, U-channel, and furring channel. See Figure The C-shape is the most common shape produced for light-gauge steel framing, and is used for studs and joists. C-shape members consist of a web, flange, and lip. Web depths range from 1 5/8″ (1.625″) to 12″. The flanges stiffen the web and provide surfaces for attaching sheathing and gypsum board. The lips extend from the flanges on the open side and stiffen the flanges.
Light-gauge steel thickness is expressed as gauge, mils, and millimeters.
The term gauge traditionally has been the unit of measurement for identifying sheet steel thickness. The higher the gauge number, the thinner the steel. Steel thicknesses also have traditionally been given on prints as mils. A mil is equal to one thousandth of an inch (1 mil = .001″). Metric dimensions (in millimeters) are increasingly being used to express steel thickness. One millimeter is equal to .001 meter (1 mm = .001 m). The table in Figure 45-3 provides gauge dimensions and their decimal inch, mil, and SI metric equivalents.
The Right S-T-U-F system identifies the web depth, shape, flange width, and steel thickness.
When using The Right S-T-U-F system, the web depth and flange width are expressed in hundredths (1/100) of an inch. For example, the web depth for a stud with a 6″ depth is expressed as 600 (6″ = 1/100 × 600). The flange width for a stud with 1 5/8″ flanges is expressed as 162 (1 5/8″ = 1.625″; 1.62 = 1/100 × 162). Steel thickness of light-gauge steel framing members is the thickness of the base metal (in mils) before being galvanized. See Figure 45-4.
Self-drilling and self-piercing screws are used to connect light-gauge steel members.
Self-tapping screws are used to fasten metal framing members to each other and to fasten other materials to metal framing members. As the name implies, self-tapping screws cut their own threads as they are being driven into the metal framing members. Self-tapping screws include self-drilling and self-piercing screws. See Figure 45-5.
A variety of screw head types are available for metal framing operations. Pan, hex washer, and pancake heads are commonly used for light-gauge steel framing operations. A variety of screw head types are available for different metal framing operations. See Figure Screws with pan, hex washer, or pancake heads are recommended for fastening steel to steel.
No. 8 screws are used most often as light-gauge steel fasteners.
Figure 45-7 provides a chart of fasteners and their applications.
Drive pins are held in place by the compressive force of metal framing members.
Drive pins are made of heat-treated high-carbon steel that produces a very hard fastener. Drive pins have spiral grooves or knurls on the shanks to draw the pins tight when driven. As a pin is driven, the point penetrates the sheathing and bores through the steel. This action pushes the steel outward away from the pin. The steel then compresses and grips the pin. See Figure 45-8.
Welding must be performed by a certified welder.
Welding can be used to prefabricate wall sections. Most often, wall sections are welded in a shop, rather than on the job site. See Figure Welding must be performed by certified welders and according to American Iron and Steel Institute (AISI) specifications.
Spot clinching is a method for joining two metal framing members and provides a strong connection.
Spot clinching is a method for joining two metal framing members and provides a strong connection without the use of mechanical fasteners. In the spot clinching process, a pneumatic clincher is used to press a section of one framing member into the adjoining member, leaving a button or stitch indentation. See Figure Spot clinching is currently approved by the International Code of Building Officials (ICBO), but is not yet approved by other code organizations.
A drywall screwdriver is commonly used in metal framing operations.
A drywall screwdriver is used to fasten metal framing members together and to fasten panel sheathing or gypsum board to metal framing members. See Figure The nose piece on a drywall screwdriver controls the depth to which the screw is driven and prevents screws from going through the panel or gypsum board.
Pneumatic steel framing tools are used to install drive pins.
Pneumatic steel framing tools, or pin guns, are used to install drive pins. See Figure Pneumatic steel framing tools operate on air pressure ranging from 90 psi to 120 psi. The pressure required to install drive pins depends on the steel thickness. Pneumatic steel framing tools have a safety contact to prevent the gun from firing unless the trigger is pressed and the tool is firmly pressed against a work surface. An overdrive control mechanism prevents overdriving of the pins. In addition, the air compressor line pressure must be adjusted to the proper setting for the steel to be fastened.
When attaching wood to steel, a pin must be driven so its head is flush with the surface of the wood. A drive pin should be driven so its head is flush with the panel surface. See Figure
An abrasive cutoff saw is used to cut heavy-gauge framing members.
An abrasive cutoff saw, or chop saw, is a power tool used to cut metal framing members. Unlike power miter saws, abrasive cutoff saw blades cannot be rotated from side to side for angled cuts. Rather, the saw fence is adjusted from side to side for angled cuts. An abrasive cutoff saw is equipped with an abrasive blade. See Figure
Power shears can cut metal up to a thickness of 68 mils.
Hand-held power shears can cut metal up to a thickness of 68 mils. See Figure Various models of power shears are available.
Plasma arc cutting is commonly used when prefabricating metal-framed panels in a shop.
Plasma arc cutting is an electric arc cutting process in which metal is cut by melting a portion of the metal and blowing the molten metal away with a stream of heated, high-velocity ionized gas. See Figure Plasma arc cutting is used when a large number of metal framing members must be cut.
A locking C-clamp secures metal framing members tightly together while they are being fastened.
A locking C-clamp is convenient for holding metal framing members tightly together while they are being fastened. See Figure Small bar clamps may also be used to hold members together, but are used less frequently than locking C-clamps.
Joist thickness and size are determined by the live and dead loads the floor must support and its unsupported span. Load-bearing C-shape steel joists are available in sizes comparable to wood joists and range from 2 × 6s to 2 × 14s. Joist thickness and size are determined by the live and dead loads the floor must support and its unsupported span. See Figure Openings are provided in the webs for utilities such as electrical wiring and plumbing. Floor joists should be positioned so the openings are at least 12″ from any bearing points. The outer ends of the joists are attached by metal angles to a rim track that serves the same purpose as a header joist in wood-frame construction.
The main components of a metal-framed floor unit are the joists, rim track, cross bridging, and blocking. Ensure framing components are installed in the same direction so cutouts align to allow easy installation of utilities. Figure shows a framed floor unit over a full-basement foundation. The rim track is directly fastened to the foundation with clip angles. One part of the clip angle is secured to the foundation wall using an anchor bolt and the other part is secured to the rim track with screws. Bearing stiffeners tie together the ends of the joists and the rim track.
Holddowns tie the wall studs and track together and fasten the assembly to the foundation.
In hurricane and seismic areas, holddowns may be required to tie the wall studs and track together and fasten the assembly to the foundation. See Figure
Similar to wood-framed floor openings, additional support must be provided around floor openings in metal-framed buildings. The floor opening for a stairway is framed with headers tied to trimmer joists. See Figure A trimmer joist is constructed by fitting a C-shape joist member inside a track member of equal size and securing the two members together with #8 screws 24″ OC. Clip angles tie the headers to the floor joists.
In-line framing requires that the joists, studs, and roof rafters be in a direct line ± 3/4″.
In-line framing (stacking) is used for light-gauge steel construction. In-line framing requires that all joists, studs, and roof rafters are in a direct line with one another. See Figure A maximum variance of 3/4″ on either side of the centerline is allowed. Unlike the double top plates of a wood-framed building, the horizontal tracks of a metal-framed wall cannot adequately support weight between the spans of the studs or joists.
Studs fit into the top and bottom tracks and are secured with one #8 screw in each flange. Studs should be oriented the same way to ensure utility cutouts align properly. Top and bottom tracks are comparable to the top and bottom plates in wood-frame construction. Tracks consist of a web and two flanges. The web width of a track must be equal to the web width of the studs. See Figure Track flanges may be bent slightly inward to ensure a tight fit between the track and studs. The studs are placed at an angle between the track flanges and turned so they are perpendicular to the track.
The bottom track of a wall is secured to the foundation with anchor bolts. Bottom tracks must be reinforced to ensure a proper connection. Anchor bolts are commonly used to secure the bottom tracks of load-bearing exterior walls to concrete slabs or foundations. Bottom tracks must be reinforced with a washer, hold-down bracket, or plate to ensure a proper connection. See Figure Anchor straps and mushroom spikes may also be used to secure the bottom tracks in position. Powder-actuated fasteners should not be used to permanently anchor bottom plates to slabs or foundation walls.
The main components of an exterior wall placed over the floor unit include top and bottom tracks, studs, diagonal tension straps, horizontal bracing, strap stud bracing, and corner posts. A first-floor exterior wall placed over the floor joists and subfloor is shown in Figure Note the in-line framing of the studs directly above the joists. The wall includes a window opening with a header, sill plate, and top and bottom window cripple studs. Diagonal braces, or tension straps, are used to brace studs against lateral movement. An outside corner post and stud-to-track connections are identified. Exterior walls are sheathed with APA-rated structural panels (OSB or plywood).
Reinforced headers are required in load-bearing walls
Reinforced headers are required in load-bearing walls. Box beam, back-to-back, or L-headers may be used. Similar to wood-frame construction, headers are installed above wall openings in exterior walls and interior load-bearing walls. Headers are formed with two equal-size C-shape framing members (box beam header or back- to-back header), or may be constructed with one or two angle pieces that fit over the top track (L-header). See Figure
An inside corner post provides support for interior wall finish materials.
An inside corner post is required for proper attachment of interior wall finish material. An interior corner post can be constructed in several ways. Figure shows two methods for constructing inside corner posts. Where a non-load-bearing wall intersects with a load-bearing wall, a 6″ or larger stud can be installed in the exterior wall and a smaller stud is fastened back-to-back with the larger stud. Another method, similar to constructing inside corners for wood-framed buildings, involves fastening two equal-size studs back-to-back.
A short piece of stud material is commonly used to splice tracks.
Wall studs can be precut to the proper length and delivered to the job site, or they can be cut on the job site with a chop saw. The walls are framed on the subfloor or a separate panel table. A common wall-framing procedure is as follows: 1. Lay out the walls on the subfloor or panel table and snap lines. 2. Cut the bottom and top tracks. Where it is necessary to splice the tracks, insert a short piece of stud material and fasten it to the tracks where they butt together. See Figure 3. Position the tracks on edge next to each other. Use a black felt-tip marker to lay out the studs, usually 24″ OC. Mark the tracks using standard conventions as shown in Unit 43. …see complete procedure for wall framing on page 432.
After a wall has been framed and squared, it is raised into position
After a wall has been framed and squared, it is raised into position. Temporary diagonal bracing remains in place until structural sheathing is fastened to the wall. A common procedure for raising and placing the wall is as follows: 1. Measure the locations of the foundation anchor bolts. Transfer the measurements to the bottom tracks and drill holes so the tracks fit over the bolts. 2. Raise the wall and position it over the anchor bolts. See Figure 3. Attach braces to the ends of the wall and approximately every 8′ to 12′ between the ends. Studs about 12′ long work well for braces. Nail wood blocks to the subfloor next to the lower ends of the intermediate braces. …see complete procedure for raising and placing the wall on page 433.
A continuous strap beneath the joists and solid bridging 12′ OC are attached to the ceiling joists to provide rigidity. Ceiling joists are placed after the walls below have been plumbed, aligned, and braced. For a multistory building, the ceiling joists also serve as floor joists. The framing procedure, fasteners, and components are the same as those used for the first-floor joists. Ceiling joist bracing is fastened to the bottoms of the joists where spans exceed 12′-0″. See Figure
When joists are lapped over a load-bearing wall, studs should be placed directly below the lapped ends. A bearing stiffener provides additional support. Ceiling joists must be placed directly above and in line with the studs below. When joists are lapped over an interior load-bearing wall, the first-floor wall studs will be directly under one of the lapped ends. See Figure A bearing stiffener is used to secure the lapped ends in position.
Where a wall below runs parallel with the joists, blocking is placed a maximum of 48″ OC between the joists for fastening the tops of the wall to the ceiling. If continuous-span joists supported by a load-bearing wall at the midpoint are used, the stud should be placed directly underneath. Where walls below run parallel with the ceiling joists, blocking is placed between the joists a maximum of 48″ OC to provide anchorage for fastening the top of the walls to the ceiling. See Figure
A light-gauge steel roof framework requires considerable bracing.
For rafters covering longer spans, rafter support braces extending from the ceiling joists to each of the rafters are installed. See Figure Rafter support braces are typically 2 × 4 × 33 mil C-shape members which are fastened to each ceiling joist and rafter with four #10 screws at each end. The rafter support braces and rafters are stiffened by lateral braces. Lateral support braces for rafter support bracing are C-shape or track members spaced 4′ OC. Lateral support braces for rafters are fastened to the bottom flange of the rafters, and could be flat straps, C-shape members, or track members.
Light-gauge steel trusses are very frequently installed over steel-framed walls.
Metal roof trusses are frequently used to construct roofs over metal-framed buildings. See Figure The design, shapes, and engineering principles of metal roofs are similar to wood trusses (discussed in Unit 50). Temporary and permanent bracing methods for metal trusses are also similar to those for wood trusses.
Light-gauge steel trusses are commonly prefabricated in a shop.
Metal roof trusses can be constructed on the job site. Most often, however, metal roof trusses are prefabricated in a shop. See Figure The trusses may be constructed on a level surface, such as the shop floor or subfloor, or on an elevated jig platform. Many manufacturers design the entire roof structure and furnish shop drawings and specifications to a general contractor.
Seismic and hurricane ties should be used to tie metal-framed walls to metal trusses in areas prone to earthquakes and severe winds. Seismic and hurricane ties should be used with metal rafters and trusses in areas prone to earthquakes and severe winds. Figure shows metal connectors used to tie metal-framed walls to metal-framed roofs.
Structural steel framing is on the increase for residential and light-commercial buildings. Large open spaces can be created within a structure. Structural steel framing, also referred to as red iron steel framing because of the characteristic color of the framing members, involves the use of bolted or welded connections to form the framework of a structure. The framing members are painted with a durable red oxide coating that resists corrosion after being manufactured to specified sizes. The use of structural steel framing is on the increase for residential and light-commercial buildings up to two stories in height. See Figure
Bolted connections are commonly used for structural steel framing.
Depending on the size of the framing members, structural steel framing members may be joined on the ground at the job site and erected using a crane, or the framing components may be erected individually and joined together. The assemblies or individual structural steel framing members are erected using a crane or other hoisting equipment. Columns are fastened to the foundation or concrete slab using anchor bolts. The assemblies and individual framing members are joined using bolts and/or welding. See Figure
Spaces between structural steel framing members are filled with heavy-gauge steel framing members spaced 16″ to 24″ OC. Due to the size of the structural steel framing members, larger open spaces can be created within a building, offering greater flexibility in room layout. Structural steel framing members are commonly spaced 8′-0″ OC, and heavy-gauge steel framing members spaced 16″ to 24″ OC are used to fill the spaces between the structural steel members. See Figure
Window and door openings are framed with wood or heavy-gauge steel framing members.
Door and window openings are framed using wood or heavy-gauge steel framing members. See Figure Heavy-gauge steel is also used for ceiling joists and roof purlins.
Oriented strand board or plywood is typically used as wall sheathing on steel-framed buildings.
Screws or drive pins are installed 3/8″ from the panel edge and typically spaced 6″ OC along the edge and 12″ OC at intermediate studs. See Figure Increased strength is obtained by applying a continuous bead of construction adhesive along the studs and rafters before fastening the panels in place.
When installing a brick veneer wall, the wall ties are fastened to the sheathing and metal studs and embedded in the mortar between bricks. When masonry is used as an exterior finish, wall ties fastened to the studs are embedded between the masonry courses. Several wall tie designs are available. One example is shown in Figure
Exterior insulation and finish systems are commonly installed over metal-framed buildings.
Exterior insulation and finish systems (EIFS) are common as exterior finish on metal-framed buildings. See Figure One of the biggest advantages of using EIFS with metal-framed buildings is the elimination of cold bridging. Cold bridging occurs when construction components (usually structural members or door or window frames), which have greater thermal conductivity than the rest of the building, extend from the interior face to the exterior face of the building. Since structural members do not extend across the faces, cold bridging in EIFS construction is eliminated. In addition, there are no fasteners in contact with the exterior to transfer the cold to the interior of the building.
Insulation for steel-framed buildings can be rigid foam insulation, fiberglass blankets or batts, or a combination of both. Metal studs conduct heat flow faster than wood; therefore, adequate insulation takes on added importance for metal-framed structures. Types of insulation similar to those used for wood-framed buildings are used for metal-framed buildings. See Figure
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