Unbraced Length of Beam for Uplift

[Steel2Steel]

I have a condition where i have a roof that consists of perimeter steel beams with timber trusses bearing on these beams. For gravity loads, a W16x26 works for strength and deflection. When this beam goes into uplift, the bottom flange is completely unbraced for the entire length of the member. I original was going to provide a 2x kicker at midspan that attached to the truss and to blocking bolted into the web of the beam. I decided not to do this because i didn't want to count on the timber to brace the steel beam and have to worry about having the truss designed to take this additional point load on the bottom chord. The beam is approximate 28'-0" long. According to the design beam moment chart on page 5-99 or LRFD 3rd edition, a W12x53 is fine for this unbraced length. So my question is, do I bump the size of the beam up so the unbraced length is within the recommendations of the beam design tables. Second question, in the beam design tables, I am assuming that when the line for a particular beam ends, that would be the maximum length for this member to be unbraced. Sorry for the long post!

 

[271828]

jmiec, I speculate that it was a simple oversight. For example, I think one would be wrong to NOT include the destabilizing effect, if it exists, even though the Spec. doesn't give the equation. Now going the other way--less conservative. That's the one that requires a little more confidence (I guess that' sone word, LOL) to do.

I know for a fact that it has been requested for the next Spec. to allow Cb to be calculated "by rational analysis," so it will be interesting to see if this is allowed. If I was a betting man, I'd bet that it will be allowed.

 

[miecz]

271828-
I can't believe it was an oversight. The provisions relating to stability come straight from the Structural Stability Research Council's "Guide to Stability Design Criteria for Metal Structures." The effect of load location has been treated extensively in the Guide for many years. I have to think the Code Committee left it out of the Code on purpose.

This would be a tough call for me as I hate being overly conservative. I really think this provision (to calculate C[sub]b[/sub] by rational analysis) belongs in the code, but I don't believe that it is.