y limit, the tangent modulus Et becomes progressively smaller than the initial modulus of elasticity. For cold-formed steel design, the shear modulus is taken as G = 11,300 ksi (77.9 GPa or 0.794 x 106 kg/cm2) according to the AISI Specification.

6.4.4 Ductility

According to the AISI Specification, the ratio Fu/Fy for the steels used for structural framing members should not be less than 1.08, and the total elongation should not be less than 10% for a 2-in. (50.8 mm) gage length. If these requirements cannot be met, the following limitations should be satisfied when such a material is used for purlins and girts: (1) local elongation in a 2-in. (12.7 mm) gage length across the fracture should not be less than 20% and (2) uniform elongation outside the fracture should not be less than 3%. It should be noted that the required ductility for cold-formed steel structural members depends mainly on the type of application and the suitability of the material. The same amount of ductility that is considered necessary for individual framing members may not be needed for roof panels, siding, and similar applications. For this reason, even though Structural Grade 80 of ASTM A653, A875, and 1008 steel, and Grade 80 of A792 steel do not meet the AISI requirements for the Fu/Fy ratio and the elongation, these steels can be used for roofing, siding, and similar applications provided that (1) the yield strength, Fy, used for design is taken as 75% of the specified minimum yield point or 60 ksi (414 MPa or 4218 kg/cm2), whichever is less, and (2) the tensile strength, Fu, used for design is taken as 75% of the specified minimum tensile stress or 62 ksi (427 MPa or 4359 kg/cm2). For multiple web configurations a reduced yield point is permitted by the AISI Specification [7] for determining the nominal flexural strength of the section on the basis of the initiation of yielding.

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