W@M Beam unbraced lengths >Lu questions.

[drewinde]

My question is concerning unbraced lengths greater than Lu when using W or M beams. I have seen in past forums on this website discussions that have stated that if you are using a W-beam in a trolley situation or a condition where all of the loading is on the bottom flange of the beam and not the top flange, then you can essentially take Lu as double what is shown in the Steel Construction Manual. My question is about the allowable moment graphs. If Lu can be doubled for a situation where loading is on the lower flange only, then does that imply that an unbraced length shown on the allowable moment graphs can be doubled for the given total allowable moment shown there as long as all loading is on the lower flange of the beam?

Thanks so much for any insight on this issue.

 

[Lion06]

I would not double Lu, I would use a modified Cb value as given in Structural Stability of Steel by Galambos.