Chapter One
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, plates, or flat bars in roll-forming machines or by press brake or bending brake operations. These are cold-formed steel structural members. The thickness of steel sheets or strip generally used in cold-formed steel structural members ranges from 0.0149 in. (0.4 mm) to about 1/4 in. (6.4 mm). Steel plates and bars as thick as 1 in. (25 mm) can be cold-formed successfully into structural shapes.
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.
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.
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). The earlier editions of the specification were based largely on the research sponsored by AISI at Cornell University under the direction of George Winter since 1939. It has been revised subsequently to reflect the technical developments and the results of continuing research.
In general, cold-formed steel structural members provide the following advantages in building construction:
1. As compared with thicker hot-rolled shapes, cold-formed light members can be manufactured for relatively light loads and/or short spans.
2. Unusual sectional configurations can be produced economically by cold-forming operations (Fig. 1.1), and consequently favorable strength-to-weight ratios can be obtained.
3. Nestable sections can be produced, allowing for compact packaging and shipping.
4. 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.
5. 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 members.
1. Lightness
2. High strength and stiffness
3. Ease of prefabrication and mass production
4. Fast and easy erection and installation
5. Substantial elimination of delays due to weather
6. More accurate detailing
7. Nonshrinking and noncreeping at ambient temperatures
8. Formwork unneeded
9. Termite-proof and rotproof
10. Uniform quality
11. Economy in transportation and handling
12. Noncombustibility
13. Recyclable material
The combination of the above-mentioned advantages can result in cost saving in construction.
1.2 TYPES OF COLD-FORMED STEEL SECTIONS AND THEIR APPLICATIONS
Cold-formed steel structural members can be classified into two major types:
1. Individual structural framing members
2. Panels and decks
The design and the usage of each type of structural members have been reviewed and discussed in a number of publications.
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.
In general, the depth of cold-formed individual framing members ranges from 2 to 12 in. (51 to 305 mm), and the thickness of material ranges from 0.048 to about 1/4 in. (1.2 to about 6.4 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 1/2 in. (13 mm) or thicker in transportation and building construction. Cold-formed steel plate sections in thicknesses of up to about 3/4 or 1 in. (19 or 25 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 can be used as primary framing members in buildings up to six stories in height. Figure 1.3 shows a two-story building. 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, 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.
As shown in Figs. 1.2 and 1.6 through 1.10, cold-formed sections are also used as chord and web members of open web steel joists, space frames, arches, and storage racks.
1.2.2 Panels and Decks
Another category of cold-formed sections is shown in Fig. 1.11. These sections are generally used for roof decks, floor decks, wall panels, siding material, and bridge forms. Some deeper panels and decks are cold-formed with web stiffeners.
The depth of panels generally ranges from 1 1/2 to 7 1/2 in. (38 to 191 mm), and the thickness of materials ranges from 0.018 to 0.075 in. (0.5 to 1.9 mm). This is not to suggest that in some cases the use of 0.012 in. (0.3 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.
In the past, steel roof decks were successfully used in folded-plate and hyperbolic paraboloid roof construction, as shown in Figs. 1.13 and 1.14. The world''s largest light-gage steel primary structure using steel decking for hyperbolic paraboloids, designed by Lev Zetlin Associates, is shown in Fig. 1.15. In many cases, roof decks are curved to fit the shape of an arched roof without difficulty. Some roof decks are shipped to the field in straight sections and curved to the radius of an arched roof at the job site (Fig. 1.16). In other buildings, roof decks have been designed as the top chord of prefabricated open web steel joists or roof trusses (Fig. 1.17). In Europe, TRP 200 decking (206 mm deep by 750 mm pitch) has been used widely. In the United States, the standing seam metal roof has an established track record in new construction and replacement for built-up and single ply systems in many low-rise buildings.
Figure 1.11 also shows corrugated sheets which are often used as roof or wall panels and in drainage structures. The use of corrugated sheets as exterior curtain wall panels is illustrated in Fig. 1.18a. It has been demonstrated that corrugated sheets can be used effectively in the arched roofs of underground shelters and drainage structures.
The pitch of corrugations usually ranges from 1 1/4 to 3 in. (32 to 76 mm), and the corrugation depth varies from 1/4 to 1 in. (6.4 to 25 mm). The thickness of corrugated steel sheets usually ranges from 0.0135 to 0.164 in. (0.3 to 4.2 mm). However, corrugations with a pitch of up to 6 in. (152 mm) and a depth of up to 2 in. (51 mm) are also available. See Chap. 10 for the design of corrugated steel sheets based on the AISI publications. Unusually deep corrugated panels have been used in frameless stressed-skin construction, as shown in Fig. 1.18b. The self-framing corrugated steel panel building proved to be an effective blast-resistant structure in the Nevada tests conducted in 1955.
Figure 1.19 shows the application of standing seam roof systems. The design of beams having one flange fastened to a standing seam roof system is discussed in Chap. 4.
In the past four decades, cold-formed steel deck has been successfully used not only as formwork, but also as reinforcement of composite concrete floor and roof slabs. The floor systems of this type of composite steel deck-reinforced concrete slab are discussed in Chap. 11.
1.3 STANDARDIZED METAL BUILDINGS AND INDUSTRIALIZED HOUSING
Standardized single-story metal buildings have been widely used in industrial, commercial, and agricultural applications. Metal building systems have also been used for community facilities such as recreation buildings, schools, and churches because standardized metal building provides the following major advantages:
1. Attractive appearance
2. Fast construction
3. Low maintenance
4. Easy extension
5. Lower long-term cost
In general, small buildings can be made entirely of cold-formed sections (Fig. 1.20), and relatively large buildings are often made of welded steel plate rigid frames with cold-formed sections used for girts, purlins, roofs, and walls (Fig. 1.21).
The design of pre-engineered standardized metal buildings is often based on the Low Rise Building Systems Manual issued by the Metal Building Manufacturers Association. This document contains design practices, commentary, design examples, common industry practices, guide specifications, bibliography, glossary, and appendix. The design of single-story rigid frames is treated extensively by Lee et al. In Canada the design, fabrication, and erection of steel building systems are based on a standard of Canadian Sheet Steel Building Institute.
Industrialized housing can be subdivided conveniently into (1) panelized systems and (2) modular systems. In panelized systems, flat wall, floor, and roof sections are prefabricated in a production system, transported to the site, and assembled in place. In modular systems, three-dimensional housing-unit segments are factory-built, transported to the site, lifted into place, and fastened together.
In the 1960s, under the School Construction Systems Development Project of California, four modular systems of school construction were developed by Inland Steel Products Company (modular system as shown in Fig. 1.17), Macomber Incorporated (V-Lok modular component system as shown in Fig. 1.22), and Rheem/Dudley Buildings (flexible space system). These systems have been proven to be efficient structures at reduced cost. They are successful not only for schools but also for industrial and commercial buildings throughout the United States.
In 1970 Republic Steel Corporation was selected by the Department of Housing and Urban Development under the Operation Breakthrough Program to develop a modular system for housing. Panels consisting of steel facings with an insulated core were used in this system.
Building innovation also includes the construction of unitized boxes. These boxes are planned to be prefabricated of room size, fully furnished, and stacked in some manner to be a hotel, hospital, apartment, or office building. For multistory buildings these boxes can be supported by a main framing made of heavy steel shapes.
In the past, cold-formed steel structural components have been used increasingly in low-rise buildings and residential steel framing. Considerable research and development activities have been conducted continuously by numerous organizations and steel companies. In addition to the study of the load-carrying capacity of various structural components, recent research work has concentrated on (1) joining methods, (2) thermal and acoustical performance of wall panels and floor and roof systems, (3) vibrational response of steel decks and floor joists, (4) foundation wall panels, (5) trusses, and (6) energy considerations.
In Europe and other countries many design concepts and building systems have been developed. For details, see Refs. 1.25, 1.40-1.43, 1.117, 118, 1.268, 1.270, 1.271, 1.273, 1.275, 1.290, 1.293, and 1.297.
1.4 METHODS OF FORMING
Three methods are generally used in the manufacture of cold-formed sections such as illustrated in Fig. 1.1:
1. Cold roll forming
2. Press brake operation
3. Bending brake operation
1.4.1 Cold Roll Forming
The method of cold roll forming has been widely used for the production of building components such as individual structural members, as shown in Fig. 1.2, and some roof, floor, and wall panels and corrugated sheets, as shown in Fig. 1.11. It is also employed in the fabrication of partitions, frames of windows and doors, gutters, downspouts, pipes, agricultural equipment, trucks, trailers, containers, railway passenger and freight cars, household appliances, and other products. Sections made from strips up to 36 in. (915 mm) wide and from coils more than 3000 ft (915 m) long can be produced most economically by cold roll forming.
The machine used in cold roll forming consists of pairs of rolls (Fig. 1.23) which progressively form strips into the final required shape. A simple section may be produced by as few as six pairs of rolls. However, a complex section may require as many as 15 sets of rolls. Roll setup time may be several days.
The speed of the rolling process ranges from 20 to 300 ft /min (6 to 92 m/min). The usual speed is in the range of 75 to 150 ft /min (23 to 46 m/min). At the finish end, the completed section is usually cut to required lengths by an automatic cutoff tool without stopping the machine. Maximum cut lengths are usually between 20 and 40 ft (6 and 12 m).
As far as the limitations for thickness of material are concerned, carbon steel plate as thick as 3/4 in. (19 mm) can be roll-formed successfully, and 3-4 stainless steels have been roll-formed in thicknesses of 0.006 to 0.30 in. (0.2 to 7.6 mm). The size ranges of structural shapes that can be roll-formed on standard mill-type cold roll forming machines are shown in Fig. 1.24.
The tolerances in roll forming are usually affected by the section size, the product type, and the material thickness. The following limits are given by Kirkland as representative of commercial practice, but they are not necessarily universal:
Piece length, using automatic cutoff [+ or -] 1/64 to 1/8 in. (0.4 to 3.2 mm) Straightness and twist 1/64 to 1/4 in. (0.4 to 6.4 mm) in 10 ft (3 m) Cross-section dimensions Fractional [+ or -] 1/64 to 1/16 in. (0.4 to 1.6 mm) Decimal [+ or -] 0.005 to 0.015 in. (0.1 to 0.4 mm) Angles [+ or -] 1?1 to 2?
Table 1.1 gives the fabrication tolerances as specified by the Metal Building Manufacturers Association (MBMA) for cold-formed steel channels and Z-sections to be used in metal building systems. All symbols used in the table are defined in Fig. 1.25. The same tolerances are specified in the Standard of the Canadian Sheet Steel Building Institute. For additional information on roll forming see Ref. 1.119.
1.4.2 Press Brake
The press brake operation may be used under the following conditions:
1. The section is of simple configuration.
2. The required quantity is less than about 300 linear ft /min (91.5 m/min).
3. The section to be produced is relatively wide [usually more than 18 in. (457 mm)] such as roof sheets and decking units.
The equipment used in the press brake operation consists essentially of a moving top beam and a stationary bottom bed on which the dies applicable to the particular required product are mounted, as shown in Fig. 1.26.
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Excerpted from Cold-Formed Steel Designby Wei-Wen Yu Copyright © 2008 by Wei-Wen Yu. Excerpted by permission.
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