Beam Bracing Questions

[abusementpark]

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.

 

[slickdeals]

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

 

[271828]

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.

 

[registeredpe]

Greetings:

"271828" hit the nail right on the head.

best regards registeredpe in AZ

 

[J1D]

“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.

 

[slickdeals]

Does anyone have the Yura bracing papers in PDF Format?

 

[Lion06]

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.

 

[271828]

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

 

[GalileoG]

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.)

 

[frv]

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

Your beam is fine.

 

[271828]

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.

 

[271828]

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.

 

[JrStructuralEng]

Does anyone have any info in PDF format?

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

 

[Lion06]

Look in the 13th edition steel manual.

 

[frv]

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.

 

[JrStructuralEng]

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

 

[abusementpark]

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?

 

[271828]

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.

 

[frv]

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.

 

[JAE]

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