64 Materials and Mechanical Properties

In the AISI Specification [7], 15 different steels are presently listed for the design of cold-formed steel members. Table 6.2 lists steel designations, American Society for Testing Materials (ASTM) designations, and yield points, tensile strengths, and elongations for these steels. From a structural standpoint, the most important properties of steel are as follows:

1. Yield point or yield strength, Fy

2. Tensile strength, Fu

3. Stress-strain relationship

4. Modulus of elasticity, tangent modulus, and shear modulus

5. Ductility

6. Weldability

7. Fatigue strength

In addition, formability, durability, and toughness are also important properties of cold-formed steel.

6.4.1 Yield Point, Tensile Strength, and Stress-Strain Relationship

As listed in Table 6.2, the yield points or yield strengths of all 15 different steels range from 24 to 80 ksi (166 to 552 MPa or 1687 to 5624 kg/cm2). The tensile strengths of the same steels range from 42 to 100 ksi (290 to 690 MPa or 2953 to 7030 kg/cm2). The ratios of the tensile strength to yield point vary from 1.08 to 1.88. As far as the stress-strain relationship is concerned, the stress-strain curve can be either the sharp-yielding type (Figure 6.5a) or the gradual-yielding type (Figure 6.5b).

6.4.2 Strength Increase from Cold Work of Forming

The mechanical properties (yield point, tensile strength, and ductility) of cold-formed steel sections, particularly at the corners, are sometimes substantially different from those of the flat steel sheet, strip, plate, or bar before forming. This is because the cold-forming operation increases the yield point and tensile strength and at the same time decreases the ductility. The effects of cold work on the mechanical properties of corners usually depend on several parameters. The ratios of tensile strength to yield point, Fu/Fy, and inside bend radius to thickness, R/t, are considered to be the most important factors to affect the change in mechanical properties of cold-formed steel sections. Design equations are given in the AISI

TABLE 6.2 Mechanical Properties of Steels Referred to in the AISI North American Specification"'13
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