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Beam Bracing Questions

Beam Bracing Questions

Beam Bracing Questions

(OP)
If you have a interior beam framing into a perimeter beam, and the interior beam is coped to be at the same level as the perimeter beam, will this be considered a point of bracing?  It seems like it wouldn't because it doesn't restrain either the top or bottom flange, like it would if it were sitting on top the perimeter beam.  

Also, can anyone describe or provide a link for some general methods of providing bracing for a beam when you are concerned about lateral-torsional buckling?  I hear people speak of providing bracing for beams, as if it is something easy to incorporate into the framing, I just have no visual idea of how this is accomplished.

I am sure these topics have been discussed before, I just couldn't find these specific answers when I did a search... thanks.

RE: Beam Bracing Questions

There are times when a full fitted stiffener is provided in the perimeter beam to which the interior beam frames into. This connects the top and bottom flanges together. However, this is a question for which the elders here might be able to provide better insight.

HTH

RE: Beam Bracing Questions

What cave did you crawl out of? (JOKING!  Just giving you a hard time. LOL)  Seriously, though, this has come up probably a half-dozen times in the last few months.

The 13th Ed. AISC Spec. Appendix 6 and its commentary are all about this subject.  For more info (and it will certainly be needed to really understand what's going on), dig up a copy of the AISC bracing seminar notes by Yura and Helwig.  They recycled this seminar about 4000 times, so most offices have at least one copy floating around.

For your specific case, there will usually be a diaphragm there, but let's assume there's not one.  Assuming the spandrel beam has only the top flange in compression, the filler beams will be connected close enough to the top flange to count as _lateral_ braces, to use Spec. terminology.  I'm sure there are weird cases in which they won't be sufficient to be lateral braces, though, but I think they'd be rare.

RE: Beam Bracing Questions

Greetings:

"271828" hit the nail right on the head.

    best regards registeredpe in AZ

RE: Beam Bracing Questions

“the interior beam is coped to be at the same level as the perimeter beam” could make the beam bracing ineffective. In accordance with AISC Spec. Appendix 6 and its commentary. “beam bracing must prevent twist of the section, not lateral displacement”. This means if the connection is through a shear plate near center of beam web and if the connection and the filler beam do not possess the stiffness as a torsional bracing, the filler beam doesn’t help the beam LTB strength Mr at all.

But I feel that this kind of filler beam is still helpful because beam buckles in a lateral-torsional mode. If the lateral displacement is restrained, Mr should be higher.

RE: Beam Bracing Questions

Does anyone have the Yura bracing papers in PDF Format?

RE: Beam Bracing Questions

I think that even if the beam won't provide torsional restraint and is coped, it still provided lateral stability for the top flange even though it is not directly connected to the top flange - provided the connection from the supported beam extends for most of the supporting beam web height.

RE: Beam Bracing Questions

StrlEIT, I agree as long as "most of the supporting beam web height" includes the part near the top flange.

RE: Beam Bracing Questions

J1D, I thought AISC said that a beam can be considered a brace if you prevent lateral translation of the flange AND/OR twisting of the cross section (emphasis on the OR.)

RE: Beam Bracing Questions

Clansman is absolutely right. A brace must provide either resistance to twist OR resistance to lateral translation.

Your beam is fine.

RE: Beam Bracing Questions

One caveat needs to be stated for folks new to this subject, Clansman and frv: Lateral braces MUST be attached near the compression flange, however.  I can "prevent translation" of a beam by putting a threaded rod through the centroid, but this wouldn't be a lateral brace for lateral-torsional buckling.

RE: Beam Bracing Questions

Clansman and frv, not saying that you two are the ones new to the subject.  Dang, where's the Edit option when I need it.

RE: Beam Bracing Questions

Does anyone have any info in PDF format?

as slickdeals asked
"Does anyone have the Yura bracing papers in PDF Format?"

RE: Beam Bracing Questions

Look in the 13th edition steel manual.

RE: Beam Bracing Questions

271828,

You are right. The lateral translation that must be resisted is at the compression flange. I believe that 1/3 of the depth of the cross section from the compression flange is considered adequate for bracing.

RE: Beam Bracing Questions

StructuralEIT, don't have 13th Edition.  I live in Canada, our newest edition is CISC 8th or 9th.

RE: Beam Bracing Questions

(OP)
frv, so just as long as the connector plate welded to the primary member to connect the secondary member is at least 1/3 of the member depth from the considered flange, the secondary  member should provide a brace point?

RE: Beam Bracing Questions

Not disputing it, but I'd be interested to know where 1/3 came from.  I don't remember seeing that in the App. 6.  It just says "near the top flange."  I don't remember seeing it in Yura's notes, but there are 27 versions of those floating around.  

For example, say you have a W36 girder with a filler beam shear connected to the web.  The top of connection plate is 12" from top of girder.  Top flange of girder in compression.  I wouldn't consider that filler beam to provide a lateral brace.

RE: Beam Bracing Questions

The specification only states that the brace must be attached "near" the compression flange.

I believe (although I'm not stating categorically) that yes, as long you connect your bracing member within 1/3 of the member depth from the compression flange, the brace is considered sufficiently "near" the compression flange.

RE: Beam Bracing Questions

Don't you normally have deck/slab attached to both beams anyway if it's a floor/roof?

RE: Beam Bracing Questions

To be honest, I can't find anything that backs up what I just posted, but I have it in my mind, so I'm fairly certain I read it somewhere (just don't know where-- wouldn't stand up in court)

RE: Beam Bracing Questions

frv, I think "within the top one-third" could be easily mis-interpreted.  Does that mean that any part of the connection can barely squeak into that 1/3 area?  The other option seems to be that the entire connection, or a large part of it, lies in that area.

Unless you or someone can point to some text, I wouldn't use the 1/3 depth guideline.  I'd go more subjectively, such as:

1. Girder, top flange in compression, filler beam attached near the top in the typical manner (i.e. T/STL same for both, or nearly so) = a brace point.  The connection will begin a very small distance down from the top flange.

2. Girder, top flange in compression, filler beam attached more or less in the middle or below != a brace point.

Of course, as JAE and others point out, a diaphragm will often be there.  I'm typing about cases in which the filler beam is providing the braced point.

We're also typing specifically about *lateral* braces.  Torsional braces are a whole different deal.  Web distortional stiffness is the usually the critical item that gets left outta discussions of those.

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