Allowable Rotation in a Roof Beam?

[waytsh]

I have a situation where it appears that when a building was constructed the steel roof beam was not plumbed up properly. The roof beam is 30" deep and the bottom flange is kicked to the side by about 1.25". I am trying to find a reference that addresses this allowable tolerance for rotation. The closest thing I can find is the allowable sweeps that are mentioned in MBMA but that would give you a different allowable angle of rotation depending on the depth of the roof beam so this does not seem appropriate to me. The flanges braces are all still in place and everything seems secure so it looks like any further rotation would be adequately restrained.

Any suggestions?

 

[Lion06]

1.25" seems like a lot to me. With a 30" deep beam, I am assuming that it is taking a substantial load. That being said, that 'misalignment' could be putting a relatively significant amount of torsion (since W shapes have a poor response to torsion) into your beam, which was probably not accounted for in design.
This may not be an issue depending on the spacing of your lateral braces, but I would definitely check the beam using an e=0.75" for all loads.
Just to make sure I am thinking of this correctly, I am picturing a beam spanning 30' with beams framing in on one side and the bottom of the supporting beam is 'kicked out' by 1.25" to the side opposite where the beams are framing into it.

 

[waytsh]

MYerges,

Thank you for the response. It is actually a 120' long PEMB rafter with "Z" purlins running continuous over the top of the rafter. It is a double slope frame so the eave to ridge distance is 60'. Flange brace angles run down from the purlins at ~45 degrees and brace the bottom flange of the rafter. It is possible that the flange braces were bolted to the incorrect pre-punched holes on the purlins thus pulling the bottom flange to the side. I agree that a torsional analysis would be in order if the beam can not be brought plumb.

 

[structuralaggie]

No fat in those things. Is it possible that the bracing was not installed until after the beam buckled? Maybe there was not enough bracing in the first place?

I think theoretically if there were enough bracing, the beam could be analyzed by calculating new section properties based on its rotated section, which in this case wouldn't be much different than the non-rotated shape. So I think adding bracing would be the way to go just to make sure.