10. LIGHT-GAUGE STEEL FRAMING

Transcription

1 COLLEGE OF ARCHITECTURE AND PLANNING Department of Architecture and Building Sciences ARCH 436 Contemporary Building Construction Methods 10. LIGHT-GAUGE STEEL FRAMING Dr. Mohammed Ghonim Lecture Objectives Upon completion of this lecture, the student will be able to: 1. Explain the concept of light-gauge frame construction. 2. Discuss the advantages and disadvantages of light-gauge frame construction. 3. List the types of light-gauge frame construction. 4. Describe the contraction process of light-gauge frame construction. 1

2 Lecture Content Introduction. Historical Brief. Applications of Light-gauge Steel Framing. Cold Forming Process. Steel Galvanization. Advantages & Disadvantages of Lightgauge Framing. Light-gauge Steel Members. Light-gauge Steel Joist Floors. Light-gauge Steel Stud Walls. Light-gauge Steel Rafter Roofs. Framing Process. Finishes for Light-gauge Steel Framing. Electrical & Plumbing Installations. Tools Used Construction. in Light-gauge Steel Introduction Light gauge steel construction is the noncombustible equivalent of wood light frame construction. The external dimensions of the standard sizes of light gauge members correspond closely to the dimensions of the standard sizes of nominal 2-inch (38-mm) framing lumber. These steel members are used in framing as closely spaced studs, joists, and rafters in much the same way as wood light frame members are used, and a light gauge steel frame building may be sheathed, insulated, wired, and finished inside and outside in the same manner as a wood light frame building. 2

3 Historical Background The use of cold-formed steel members in building construction began in the 1850s in both the United States and Great Britain. In the 1920s and 1930s, acceptance of cold-formed steel as a construction material was still limited because there was no adequate design standard and limited information on material use in building codes. One of the first documented uses of cold-formed steel as a building material is the Virginia Baptist Hospital, constructed around 1925 in Lynchburg, Virginia. The walls were load bearing masonry, but the floor system was framed with double back-to-back cold-formed steel lipped channels. In the 1940s, Lustron Homes Company, built and sold almost 2500 steel-framed homes in America, with the framing, finishes, cabinets and furniture made from cold-formed steel. Virginia Baptist Hospital Lustron Homes Applications of Light-gauge Steel Framing Light-gauge (Cold-formed) steel framing refers specifically to members in light-frame building construction that are made entirely of steel sheets, formed to various shapes at ambient temperatures. The most common shape for light-gauge steel is a lipped channel, although Z, C, tubular, hat and other shapes and variations have been used. The building elements that are most often framed with cold-formed steel are floors, roofs, and walls, although other building elements and both structural and decorative assemblies may be steel framed. In high-rise commercial and multi-family residential construction, light-gauge frames are typically used for interior partitions and support of exterior walls and cladding. However, in many mid-rise and low-rise applications, the entire structural system can be framed with light-gauge steel members. 3

4 Cold Forming Process In building construction there are basically two types of structural steel: hot-rolled steel shapes and cold-formed steel shapes. The hot rolled steel shapes are formed at elevated temperatures while the cold-formed steel shapes are formed at room temperature. Coldformed steel structural members are shapes commonly manufactured from steel plate, sheet metal or strip material. The manufacturing process involves forming the material by either press-braking or cold roll forming to achieve the desired shape. Cold Forming Process Cold-formed steel (CFS) is the common term for products made by rolling or pressing thin gauges of sheet steel into goods, cold-formed steel members are created by the working of steel sheets using stamping, rolling, or presses to deform the sheet into a usable product. Cold-formed steel products are commonly used in all areas of manufacturing of durable goods like appliances or automobiles but the phrase cold form steel is most prevalently used to described construction materials. Cold-formed steel construction materials differ from other steel construction materials known as hot-rolled steel. The manufacturing of cold-formed steel products occurs at room temperature using rolling or pressing. 4

5 Steel Galvanization Galvanization is the process of applying a protective zinc coating to steel or iron, to prevent rusting. The most common method is hot-dip galvanization, in which parts are submerged in a bath of molten zinc. Although galvanizing will prevent attack of the underlying steel, rusting will be expected, especially if exposed to the natural acidity of rain. For example, corrugated iron sheet roofing will start to degrade within a few years despite the protective action of the zinc coating. Marine and salty environments also lower the lifetime of galvanized steel because the high electrical conductivity of sea water increases the rate of corrosion primarily through converting the solid zinc to soluble zinc chloride which simply washes away. The Advantages & Disadvantages of Light-gauge Framing Advantages Disadvantages Like wood light framing: flexible, simple to construct, and accept the same interior and exterior finishes. Readily conducts heat and requires extra attention to eliminating thermal bridging effects. Noncombustible. Members require more frequent bracing. Lighter weight, so it is an advantage in seismic zones Although it is galvanized, it is subject to corrosion. Less waste and recyclable. More expensive, specially in the labor part. Steel is not subject to fungal decay or termite attack. Conducts electricity if not well isolated. 5

6 Light-gauge Steel Members For studs, joists, and rafters, the steel is formed into C-shaped sections. The webs of cee members are punched at the factory to provide holes at 2-foot (600-mm) intervals; these are designed to allow wiring, piping, and bracing to pass through studs and joists without the necessity of drilling holes on the construction site. For top and bottom wall plates and for joist headers, channel sections are used. The strength and stiffness of a member depend on the shape and depth of the section and the gauge (thickness) of the steel sheet from which it is made. A standard range of depths and gauges is available from each manufacturer, but commonly used metal thicknesses for loadbearing members range from 2.46 mm to 0.84 mm, and are as thin as 0.45 mm for nonloadbearing members. Light-gauge Steel Joist Floors Light-gauge steel joists are manufactured by cold-forming sheet or strip steel. The resulting steel joists are lighter, more dimensionally stable, and can span longer distances than their wood counterparts, but conduct more heat and require more energy to process and manufacture. The cold-formed steel joists can be easily cut and assembled with simple tools into a floor structure that is lightweight, noncombustible, and damp proof. 6

7 Light-gauge Steel Joist Floors As in wood light frame construction, the framing contains cavities for utilities and thermal insulation and accepts a wide range of finishes. Light-gauge Steel Joist Floors The web stiffener is an important piece in light-gauge steel floors that is inserted inside the joist and screwed to its vertical web to help transmit wall loads vertically through the joist. Web Stiffener 7

8 Light-gauge Steel Stud Walls Light-gauge steel studs are manufactured by cold-forming sheet or strip steel. The coldformed steel studs can be easily cut and assembled with simple tools into a wall structure that is lightweight, noncombustible, and damp proof. Metal stud walls may be used as nonloadbearing partitions or as bearing walls supporting light-gauge steel joists. As in wood light frame construction, the stud framing contains cavities for utilities and thermal insulation and accepts a wide range of finishes. Light-gauge Steel Stud Walls 8

9 Light-gauge Steel Stud Walls Typical detail of the anchorage of a cold-formed steel bottom track to the foundation. Light-gauge Steel Stud Walls Detailed view for a CFS stud wall Hold-downs Tilt-up construction is used for building stud walls Horizontal bracing Diagonal bracing 9

10 Light-gauge Steel Rafter Roofs Detail at ridge. Because assemblies are prefabricated, roof framing is in the form of trusses. Being lighter than corresponding wood trusses, cold-formed steel trusses can generally be hoisted into position without special lifting and hoisting equipment. Additionally, the joints between members of a cold-formed steel truss are more rigid, so that the trusses are easier to handle and more forgiving during their placement. Light-gauge Steel Rafter Roofs Typical eave detail The depth of light gauge steel roof trusses is usually based on the desired roof pitch. A typical depth is one quarter of the width (span) of the building. 10

11 Framing Process The sequence of construction for a building that is framed entirely with light gauge steel members is essentially the same as for a building framed with nominal 2-inch wood members. Framing is usually constructed with the platform method; the ground floor is framed with steel joists. Mastic adhesive is applied to the upper edges of the joists, and wood panel subflooring is laid down and fastened to the upper flanges of the joists with screws. Steel studs are laid flat on the subfloor and joined to make wall frames. The wall frames are sheathed either with wood panels or, for noncombustible construction, with gypsum sheathing panels, which are similar to gypsum wallboard but with glass mat faces and a waterresistant core formulation. The wall frames are tilted up, screwed down to the floor frame, and braced. The upper-floor platform is framed, then the upper floor walls. Finally, the ceiling and roof are framed in much the same way as in a wood-framed house. Prefabricated trusses of light gauge steel members that are screwed or welded together are often used to frame roofs. Finishes for Light-gauge Steel Framing Any exterior or interior finish material that is used in wood light frame construction may be applied to light gauge steel frame construction. Whereas finish materials are often fastened to a wood frame with nails, only screws may be used with a steel frame. Wood trim components are applied with special finish screws, analogous to finish nails, which have very small heads. 11

12 Electrical & Plumbing Installations The above pictures show the electric and plumbing installations for CFS buildings and the accessories used. Tools Used in Light-gauge Steel Construction Screw Gun Power Saw Torched Cutter Power Shear Tin Snips Power Riveting tool 12

13 Summary Light gauge steel construction is the noncombustible equivalent of wood light frame construction. Light-gauge (Cold-formed) steel framing refers specifically to members in lightframe building construction that are made entirely of steel sheets, formed to various shapes at ambient temperatures. CFS framing resembles wood light framing in many aspects as; it is flexible, simple to construct, and accept the same interior and exterior finishes. CFS framing is more advantageous than wood light framing in many aspects as; it is noncombustible, Lighter in weight, less in waste, recyclable, and it is not subject to fungal decay or termite attack. The sequence of construction for a building that is framed entirely with light gauge steel members is essentially the same as for a building framed with nominal 2-inch wood members. References Allen, Edward & Lano, Joseph (2009) Fundamentals of Building Construction: Materials and Methods, John Wiley & Sons. Ching, Francis (2008) Building Construction Illustrated, John Wiley & Sons. Mehta, Madan et. al. (2013) Building Construction, Pearson, USA

14 Lecture Activity Each student is required to prepare a brief but informative internet-based research about one of the topics related to this lecture (Light-gauge Steel Framing) Timber Construction); i.e., new developments, successive and local case studies or important details. The research should be presented in only one A4 sheet. 14