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!

 

[blake989]

out of curiousity, what is the magnitude of uplift you're dealing with?

 

[Steel2Steel]

Blake,
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

 

[blake989]

a span to depth ratio of 30 is where the line ends in charts. Also the charts assume a Cb of 1.0, which is fine and I stick with almost always, however you can increase capacity dependent upon loading condition by applying an appropriate Cb factor.

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)

 

[Lion06]

blake-
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.

 

[DaveAtkins]

Yes, you need to design for uplift. If you go to a shallower beam, recheck deflection.

DaveAtkins

 

[blake989]

StructuralEIT, for Defrazie's case with somewhere around 12-13 point loads, he could use Cb = 1.14 via Table 5-1. I don't disagree with 1.0, but "in a pinch", that 14% could come in handy.

 

[Steel2Steel]

Ok,

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??

 

[lkjh345]

Agree with DaveAtkins. You definately need to check 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.

 

[Steel2Steel]

I know i have to check uplift on the beam, that isn't what I am unclear about. What im unclear about is it better to beef the beam up in size or brace the bottom flange of the beam with the truss bottom chord?

 

[Steel2Steel]

I mean with a kicker to the bottom chord. Then the truss manuf would need these loads and have to design his truss for these additional loads.

 

[DaveAtkins]

Typically I bump up the beam size--think about it this way: if you were the contractor, would you rather pay a bit more for the beam, or mess around with installing kickers?

DaveAtkins

 

[Lion06]

bump up the beam size. An extra 14 plf is nothing. Also, it is certainly not frowned upon to actually calc the allowable stress rather than using the charts. The charts are there simply for simplicity's sake.

 

[Steel2Steel]

Thanks for the replies! I will check the beam for the full unbraced length and forget about kickers...

 

[hokie66]

Just to add a bit to your discussion, I think if you are just considering one beam, increasing the weight to avoid bracing the bottom flange is reasonable. However, if this is a typical condition in your building involving a number of beams, I think your decision should be to brace.

 

[gwynn]

Not familiar with the American Code and nomenclature, but if the trusses are attached to the top flange then for the uplift condition this is effectively bottom flange loading which leads to significantly higher buckling loads. In other words, Cb of one could be overly conservative.

 

[271828]

The question is whether you follow mechanics and research or the Spec. There are equations for Cb for cases like this and they give numbers like 5, so ltb ends up not controlling if you use them. See the stability bracing notes by Yura and Helwig.

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.

 

[UcfSE]

I would size the beam as unbraced for uplift.

 

[gwynn]

I agree with 271828 here, that either using or ignoring the benifits of bottom flange loading could be justified. My personal preference is to take it into account.

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.

 

[miecz]

In AISC's 13th Edition, the commentary to Article F1 mentions the stabilizing/destabilizing effect of load location, but does not provide a means to modify C[sub]b[/sub] for "bottom" flange loading. Since the commentary gives a method for dealing with "top" flange destabilizing effect, I have to think that the code would have provided a stabilizing method here, it the committee wished to include one. I would stick with equation F1-1, keeping in mind that I have a whopping hidden safety factor. (According to figure 6.8 in my old SSRC Guide, the true C[sub]b[/sub] for uniform tension flange loading is around 2.6.)