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RE: Weak Axis Bending of Channels

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>Does anyone use 75% of Fy for channels bent about their weak
>axis as you can for I-shapes and doubly symmetric sections (Section
>F2)? Although it strictly appears you can't, it seems that you can get
>a plastic section modulus (Zy) that is almost 2 times the elastic
>section modulus (Sy) for channels. Thus it should be reasonable to
>use 75% of Fy for channels also 
 
In LRFD, you'd be able to use 0.9 times the nominal strength, which is FyZy <= Fy(1.5Sy). Since the shape factor for a channel in weak-axis bending is normally larger than 1.5, latter quantity will normally control. The allowable strength for an ASD solution based upon LRFD equations can be determined by dividing the nominal strength by an appropriate factor of safety. For beam bending, the normal factor of safety is 5/3. Thus, the ASD equivalent design for an LRFD solution is:
 
    1.5FySy/(5/3) = 0.9FySy
 
So the use of 0.75Fy on the elastic section in ASD appears conservative.

Charlie