Unbraced Length of Beam for Uplift
Unbraced Length of Beam for Uplift
(OP)
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!






RE: Unbraced Length of Beam for Uplift
RE: Unbraced Length of Beam for Uplift
I have not calculated this yet b/c we are in preliminary stages of design. I do know the beam will go into uplift. Estimate at approximate 700 lbs ea truss and trusses are spaced at 2 feet OC
RE: Unbraced Length of Beam for Uplift
if you have selected a large beam based on gravity loading, then you check uplift....which will hopefully be a much smaller moment, yet you have a larger Lb, you're not going to find the exact info you want in the chart, you'll need to use the specifications, which is simpler than it appears since most of the info is already provided for you (i.e. Lp, Lr, BF, etc)
RE: Unbraced Length of Beam for Uplift
He did say he doesn't want to brace the bottom flange for negative bending, therefore his Cb will be =1 since the maximum moment is occuring away from a brace point.
DeFrazie-
I do agree that you should just use the spec for your allowable. It is not a hard calc. If your size isn't quite working, bump up the weight on the W16, dropping to a W12 will cause the weight to go up a lot more.
RE: Unbraced Length of Beam for Uplift
DaveAtkins
RE: Unbraced Length of Beam for Uplift
RE: Unbraced Length of Beam for Uplift
So i ran the numbers for a 28'-0" full unbraced W16x26 for for 700 lbs of uplift at each truss (350 P/F). A W16x40 can handle this load fully unbraced and the deflection is fine.
I guess i just wanted to know is this looked down upon since the beam design tables stop at a certain unbraced length (for W16x40 say 19'-0").
Blake, not to sound like an idiot but on you comment about the span to depth ratio of 30. Lets look at a W14x22. The chart stops at 12'-6". Where do you get a ratio of 30 for this case??
RE: Unbraced Length of Beam for Uplift
This is a reasonably common occurance in light framed buildings with wood or cold-formed steel trusses bearing on W shaped beams.
I basically run two load cases for beams like this. Case 1 with maximum downwards loads and a fully braced compression flange, and Case 2 with net uplift forces and an unbraced bottom (compression) flange. Take the best section that fits both criteria.
RE: Unbraced Length of Beam for Uplift
RE: Unbraced Length of Beam for Uplift
RE: Unbraced Length of Beam for Uplift
DaveAtkins
RE: Unbraced Length of Beam for Uplift
RE: Unbraced Length of Beam for Uplift
RE: Unbraced Length of Beam for Uplift
RE: Unbraced Length of Beam for Uplift
RE: Unbraced Length of Beam for Uplift
The AISC Spec. doesn't say that Cb can be determined by rational analysis, so it's not clear whether it's ok to use the higher Cb. I'd use it, but that's my personal opinion only.
RE: Unbraced Length of Beam for Uplift
RE: Unbraced Length of Beam for Uplift
As it sounds like the OP is new to handling this sort of scenario, my first inclination would be to see what the office standard is in the situation.
RE: Unbraced Length of Beam for Uplift
RE: Unbraced Length of Beam for Uplift
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.
RE: Unbraced Length of Beam for Uplift
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 Cb by rational analysis) belongs in the code, but I don't believe that it is.