KL/R greater than 200 in a beam!

[KingKongdoor]

I'm designing a roof system that has horizontal x bracing to carry lateral loads from wind to braced frames. This puts compression in my roof beams. Economically my beams can not meet the KL/R limit state(compact shape doesn't work for roof deflection). I have bar joist bearing on the beams at 5' O.C. and will brace the bottom flange of my roof beams with angles on either side of columns and at 10' O.C. elsewhere. Is this kind of bracing sufficient so i can ignore kl/r over 200 on my beam? Also, metal deck is not large enough to develop shear strength required.

Is KL/r over 200 a big deal for a beam in compression from lateral loads? My calcs show that the beam design is sufficient even though it is over kl/r limit state. I never go over kl/r limit state for column design and this is the first time to run into this on beam design.

Thanks,

 

[JAE]

Under AISC the KL/r = 200 is a preference, not a mandate. If it works at the higher values then it works.
But for practical design we try to keep to the 200.

For roof beams, yes, joists and such do brace the beams providing the braces have a clear load path into the diaphragm.



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[KingKongdoor]

Thanks for your input JAE, I know the roof joist brace the beam flanges for lateral torsional buckling i just wasn't sure if it was sufficient for buckling from axial loads. The member by itself could handle axial load over the entire span but it would just be over the kl/r200 ratio. Which i didn't think was a big deal for a roof beam.