Allowable bending stress of rectangular plate
I'm using the 9th edition of the AISC steel manual and am getting myself all confused. I have a situation where I'm trying to use a steel .25" x 2.5" steel plate to stiffen an aluminum "beam" in bending. I'm orienting the plate such that I'm loading it in bending along the strong axis, where my Mom of I = .325in^4. I can only laterally brace this piece at 12" intervals. I suspect that when this thing is bent about the strong axis that my compression edge(not sure I can call it a flange as I'm basically just analyzing a prismatic rectangle) is going to buckle out of plane. I'm confused as to what my allowable bending stress would be for 36ksi plate steel.
How do I look at this in regards to Table B5.1? Do I even need to? It seems to me that I need to look at the "Unstiffened elements simply supported along one edge......." which gives me a limiting width-to-thickness ratio of 76/Fy^.5. Is that right? If so, then what do I do with that then in Chapter F? There is no axial load carried by this piece.....only bending.
Thanks for your help.