Cold-formed shapes are usually made from hot-rolled sheet or strip steel, which costs less per pound than cold-rolled steel. The latter, which has been cold-rolled to desired thickness, is used for thinner gages or where, for any reason, the surface finish, mechanical properties, or closer tolerances that result from cold-reducing is desired. Manufacture of cold-formed shapes from plates for use in building construction is possible but is done infrequently.
8.1.1 Plate, Sheet, or Strip
The commercial distinction between steel plates, sheet, and strip is principally a matter of thickness and width of material. In some sizes, however, classification depends on whether the material is furnished in flat form or in coils, whether it is carbon or alloy steel, and, particularly for cold-rolled material, on surface finish, type of edge, temper or heat treatment, chemical composition, and method of production.
Although the manufacturers classification of flat-rolled steel products by size is subject to change from time to time, that given in Table 8.1 for carbon steel is representative.
Carbon steel is generally used. High-strength, low-alloy steel, however, may be used where strength or corrosion resistance justify it, and stainless steel may be used for exposed work.
8.1.2 Mechanical Properties
Material to be used for structural purposes generally conforms to one of the standard specifications of ASTM. Table 8.2 lists the ASTM specifications for structuralquality carbon and low-alloy sheet and strip, and their principal mechanical properties.
8.1.3 Stainless-Steel Applications
Stainless-steel cold-formed shapes, although not ordinarily used in floor and roof framing, are widely used in exposed components, such as stairs, railings, and balustrades;
doors and windows; mullions, fascias; curtain walls and panel work; and other applications in which a maximum degree of corrosion resistance, retention of appearance and luster, and compatibility with other materials are primary considerations.
Stainless-steel sheet and strip are available in several types and grades, with different strength levels and different degrees of formability, and in a wide range of finishes.
Information useful in design of stainless-steel cold-formed members can be obtained from the Specification for the Design of Cold-Formed Stainless Steel Structural Members, American Society of Civil Engineers (ASCE), 1801 Alexander Bell Drive, Reston, VA 20191-4400. The specification is applicable to material covered by ASTM A666, Austenitic Stainless Steel, Sheet, Strip, Plate and Flat Bars for Structural Applications. It contains requirements for 201, 202, 301, 302, 304, and 316 types of stainless steels. Further information on these steels as well as steels covered by ASTM A176, A240, and A276 may be obtained from the American Iron and Steel Institute (AISI).
Material for cold-formed shapes may be either black (uncoated), galvanized, or aluminized. Because of their higher costs, metal-coated steels are used only where exposure conditions warrant paying more for the increased protection afforded against corrosion.
Low-carbon sheets suitable for coating with vitreous enamel are frequently used for facing purposes, but not as a rule to perform load-carrying functions in buildings.
8.1.5 Selection of Grade
The choice of a grade of material, within a given class or specification, usually depends on the severity of the forming operation required to make the required shape, strength desired, weldability requirements, and the economics involved.
Grade C of ASTM A611, with a specified minimum yield point of 33 ksi has long been popular for structural use. Some manufacturers, however, use higher-strength grades to good advantage.
8.1.6 Gage Numbers
Thickness of cold-formed shapes was formerly expressed as the manufacturers standard gage number of the material from which the shapes were formed. Use of millimeters or decimal parts of an inch, instead of gage numbers, is now the standard practice. However, for information, the relationships among gage number, weight, and thickness for uncoated and galvanized sheets are given in Table 8.3 for even gages.