I assume you are talking about hot rolled structural steel.
According to the current 2005 AISC Spec, the allowable strength is FyZ/1.67, where 1.67 is the safety factor and Z is the minor axis plastic section modulus. This has to be less than 1.6FyS where S is the weak axis section modulus. According to the AISC BAsic Design Value Cards, the weak axis allowable strength for channels can be taken as .9FyS, where S is the weak axis section modulus. The design cards use equations that have been simplified from the Spec.
If you are using the 9th Edition AISC (green book), it's not explicitly given. Logiacally, you would expect at least the same allowable stress assuming a compact section because you won't have lateral torsional buckling. However, if you read section F2, the text says that 0.75Fy only applies for doubly symetrical I and H shapes. It was the practice of my office when we used the green book to only use .6Fy as the allowable stress for weak axis bending of channels. This is very conservative, but we didn't feel that the spec supported a higher value, although from a technical standpoint we knew it was much higher.
According to the 2005 spec, yielding will control for most of the channels currently manufactured with 36 ksi or 50 ksi steel. Therefore, flange instability is not an issue so it doesn't matter if the flange is turned up or down. For cases where the flange would experience some local instability before yield, it seems that it would matter if the flanges where in compression though the spec doesn't seem to differentiate.
The ASD 9th edition does not address minor axis channel bending. I have to disagree with the reference to section F2 since that section deals with weak-axis bending of I-shapes and bars and plates, not channels.
I mentioned F2 from the green book to point out that the section on weak axis bending does not include channels, not to cite it as a reference for the allowable weak axis bending stress.
Can anyone point to an example, in practice, (and still standing) where channels were used in positive bending with their flanges upward to support a downward load?
Please state the geographical location, so I can run the other way...
Channels give me the creeps, whenever used in a non-traditional way (read: Hyatt Regency, although that was a completely different application)
