CHAPTER 1
Introduction
1.1 GENERAL REMARKS
In steel construction, there are two main families of structural members. One is the familiar group of hot-rolled shapes and members built up of plates. The other, less familiar but of growing importance, is composed of sections cold formed from steel sheet, strip, plate, or flat bar in roll-forming machines or by press brake or bending brake operations.1.1,1.2,1.3* These are cold-formed steel structural members. The thickness of steel sheet or strip generally used in cold-formed steel structural members ranges from 0.0149?in. (0.378?mm) to about . (6.35?mm). Steel plates and bars as thick as 1?in. (25.4?mm) can be cold formed successfully into structural shapes.1.1,1.4,1.314,1.336,1.345
Although cold-formed steel sections are used in car bodies, railway coaches, various types of equipment, storage racks, grain bins, highway products, transmission towers, transmission poles, drainage facilities, and bridge construction, the discussions included herein are primarily limited to applications in building construction. For structures other than buildings, allowances for dynamic effects, fatigue, and corrosion may be necessary.1.314,1.336,1.345,1.417
The use of cold-formed steel members in building construction began in about the 1850s in both the United States and Great Britain. However, such steel members were not widely used in buildings until around 1940. The early development of steel buildings has been reviewed by Winter.1.5-1.7
Since 1946 the use and the development of thin-walled cold-formed steel construction in the United States have been accelerated by the issuance of various editions of the "Specification for the Design of Cold-Formed Steel Structural Members" of the American Iron and Steel Institute (AISI).1.267,1.345 The earlier editions of the specification were based largely on the research sponsored by AISI at Cornell University under the direction of George Winter. It has been revised subsequently to reflect the technical developments and the results of continuing research.1.267,1.336,1.346,1.416,1.417
In general, cold-formed steel structural members provide the following advantages in building construction:
- As compared with thicker hot-rolled shapes, cold-formed light members can be manufactured for relatively light loads and/or short spans.
- Unusual sectional configurations can be produced economically by cold-forming operations (Fig. 1.1), and consequently favorable strength-to-weight ratios can be obtained.
Figure 1.1 Various shapes of cold-formed sections.1.1
- Nestable sections can be produced, allowing for compact packaging and shipping, as well as for developing efficient structural applications.
- Load-carrying panels and decks can provide useful surfaces for floor, roof, and wall construction, and in other cases they can also provide enclosed cells for electrical and other conduits.
- Load-carrying panels and decks not only withstand loads normal to their surfaces, but they can also act as shear diaphragms to resist force in their own planes if they are adequately interconnected to each other and to supporting members.
Compared with other materials such as timber and concrete, the following qualities can be realized for cold-formed steel structural members1.8,1.9:
- Lightness
- High strength and stiffness
- Ease of prefabrication and mass production
- Fast and easy erection and installation
- Substantial elimination of delays due to weather
- More accurate detailing
- Nonshrinking and noncreeping at ambient temperatures
- Formwork unneeded
- Termite proof and rot proof
- Uniform quality
- Economy in transportation and handling
- Noncombustibility
- Recyclable material
The combination of the above-mentioned advantages can result in cost savings in construction (www.buildsteel.org).
1.2 TYPES OF COLD-FORMED STEEL SECTIONS AND THEIR APPLICATIONS
Cold-formed steel structural members can be classified into two major types:
- Individual structural framing members
- Panels and decks
The design and the usage of each type of structural member have been reviewed and discussed in a number of publications.1.5-1.75,1.267-1.285,1.349,1.358,1.418
1.2.1 Individual Structural Framing Members
Figure 1.2 shows some of the cold-formed sections generally used in structural framing. The usual shapes are channels (C-sections), Z-sections, angles, hat sections, I-sections, T-sections, and tubular members. Previous studies have indicated that the sigma section (Fig. 1.2d) possesses several advantages, such as high load-carrying capacity, smaller blank size, less weight, and larger torsional rigidity as compared with standard channels.1.76
Figure 1.2 Cold-formed sections used in structural framing.1.6
In general, the depth of cold-formed individual framing structural members ranges from 2 to 16?in. (50.8 to 406?mm), and the thickness of material ranges from 0.0329 to 0.1180 in. (0.836 to 2.997?mm). In some cases, the depth of individual members may be up to 18?in. (457?mm), and the thickness of the member may be in. (12.7?mm) or thicker in transportation and building construction. Cold-formed steel plate sections in thicknesses of up to about or 1?in. (19.1 or 25.4?mm) have been used in steel plate structures, transmission poles, and highway sign support structures.
In view of the fact that the major function of this type of individual framing member is to carry load, structural strength and stiffness are the main considerations in design. Such sections have commonly been used as primary framing members in buildings having multiple stories in height.1.278 In 2000, the 165-unit Holiday Inn in Federal Way, Washington, utilized eight stories of axial load bearing cold-formed steel studs as the primary load-bearing system.1.357 Figure 1.3 shows a mid-rise construction building. Cold-formed steel for mid-rise construction has become popular for these buildings that typically may range from 4 to 12 stories high. Chapter 12 provides additional discussion of cold-formed steel applications for low- and mid-rise construction. Additional information may also be obtained at www.buildsteel.org. In tall multistory buildings the main framing is typically of heavy hot-rolled shapes and the secondary elements may be of cold-formed steel members such as steel joists, studs, decks, or panels (Figs. 1.4 and 1.5). In this case the heavy hot-rolled steel shapes and the cold-formed steel sections supplement each other.1.264
Figure 1.3 Building composed entirely of cold-formed steel sections.
Courtesy of Don Allen.
Figure 1.4 Composite truss-panel system prefabricated by Laclede Steel Company.
Figure 1.5 Cold-formed steel joists used together with hot-rolled shapes.
Courtesy of Stran-Steel Corporation.
As shown in Figs. 1.2 and 1.6-1.10, cold-formed sections are also used as chord and web members of open web steel joists, space frames, arches, and storage racks.
Figure 1.6 Cold-formed steel sections used in space frames.
Courtesy of Unistrut Corporation.
Figure 1.7 Cold-formed steel members used in space grid system.
Courtesy of Butler Manufacturing Company.
Figure 1.8 Cold-formed steel members used in a 100?×?220?×?30-ft (30.5?×?67.1?×?9.2-m)triodetic arch.
Courtesy of Butler Manufacturing Company.
Figure 1.9 Hangar-type arch structures using cold-formed steel sections.
Courtesy of Armco Steel Corporation.1.6
Figure 1.10 Rack structures.
Courtesy of Unarco Materials Storage.
1.2.2. Panels and Decks
Another category of cold-formed sections is shown in Fig. 1.11. Historically, these sections are generally used for roof decks, floor decks, wall panels, siding material, and bridge forms. Recently, profiled deck has been used for shear wall. Some deeper panels and decks are cold formed with web stiffeners.
Figure 1.11 Decks, panels, and corrugated sheets.
The depth of panels generally ranges from 9/16 to ?in. (14.2 to 191?mm), and the thickness of materials ranges from 0.018 to 0.075?in. (0.457 to 1.91?mm). This is not to suggest that in some cases the use of 0.012-in. (0.305-mm) steel-ribbed sections as load-carrying elements in roof and wall construction would be inappropriate.
Steel panels and decks not only provide structural strength to carry loads, but they also provide a surface on which flooring, roofing, or concrete fill can be applied, as shown in Fig. 1.12. They can also provide space for electrical conduits, or they can be perforated and combined with sound absorption material to form an acoustically conditioned ceiling. The cells of cellular panels are also used as ducts for heating and air conditioning.
Figure 1.12 Cellular floor panels.
Courtesy of H. H....
