Beam Lateral Bracing

[DCBII]

I have continuous beam (negative bending moment) with the top flange (tension) braced by a diaphragm. Can I consider the bottom flange braced for compression if I run stiffeners from the top flange to the bottom flange?

 

[SteelPE]

I usually don't consider web stiffeners to brace a flange against LTB.

 

[DCBII]

SteelPE,

My first instincts were that it's not braced but, a collegue is telling me that if the tension flange can't move and the compression flange is tied to the tension flange by stiffeners, then the compression flange can't move either.

 

[steellion]

In my opinion, stiffeners are not sufficient to consider the flange to be braced against LTB. Stiffeners will restrict the flanges from rotating independently, but the entire beam can still roll over.

 

[woodman88]

The use of stiffeners does tie the tension and compression flanges together. But the buckling force in the compression flange is now creating a moment force, usually a small one, in the beam. So it depends on whether the tension flange connection to the diaphragm will transfer this moment and also if the diaphragm will resist the moment.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[PMR06]

I don't think the stiffeners brace the bottom flange either.

Here's how I think about it: The top flange is braced against lateral movement. The bottom flange buckling to the side will be rotation about the top flange though. The top flange is probably not braced for that rotation.

 

[JAE]

I would agree with what woodman88 suggests - it depends on the attachment of the top flange to the diaphragm and the diaphragms stiffness.

AISC Appendix 6 provides strength and stiffness requirements for bracing and these would have to be applied to see if the stiffener-diaphragm mechanism could do the job.

Usually decks are only screwed or spot welded to the beams so an adequate force couple may not be developed enough to do the job.

 

[hokie66]

As others have suggested, the top flange needs to be restrained for both translation and rotation in order for full depth stiffeners to brace the bottom flange. Roof purlins bolted to cleats on the top flange with two bolts have been shown by testing to provide the translational support required, but I would not rely on steel deck to do it.

 

[slickdeals]

@hokie:
A sidebar question.....

Assuming you have simply supported girders spanning to columns and roof purlin (say a Channel or WF section) is bearing on the girder (bolted with 4 bolts or welded parallel to flange).

If a stiffener is welded to the girder at the locations of these purlins, can the girder be assumed to be braced at purlin locations for uplift?

It’s no trick to get the answers when you have all the data. The trick is to get the answers when you only have half the data and half that is wrong and you don’t know which half - LORD KELVIN

 

[hokie66]

I would say yes in most cases. The bolts on both side of the girder flange will provide a substantial amount of rotational resistance. It probably depends on the depth of the girder, though, and I don't know how to quantify that.

 

[DST148]

Agree with hokie66.
Beam bracing must control twist of the section, but need not prevent lateral displacement.The LTB of the bottom flange may be idealized as due to two vertical equal and opposite forces at the two edges of the bottom flange and not as a horizontal force at the bottom flange since it would cause twist + bending about the minor axis. The full depth stiffeners would carry this push / pull to the top flange where further load path would depend on the adequacy of the fastening of the deck to the flange and flexural capacity of the deck as woodman88 suggested.
For the subject continuous beam, neither the deck at the top by itself can prevent twist of the beam, nor the stiffeners by themselves can prevent twist, though stiffeners would increase the effective radius of gyration about y-axis and hence increase Lr. However, an adequate diaphragm in combination with a couple of stiffeners at the inflection points can control twist of the section and the beam can be considered braced for LTB purposes.

 

[hokie66]

DST148,
I am happy to have you agree with me, but I don't believe I agree with you. To prevent lateral movement of the bottom flange (caused by a presumptive horizontal force), the stiffener acts as a cantilevered bending member. The moment in the stiffener has to be resisted at the top by a transverse bending member, and that resistance can't be provided by a mere diaphragm.

 

[DST148]

@hokie66 - There will be lateral movement of the bottom flange but caused not by a presumptive horizontal force but a combination of equal and opposite vertical forces acting at the edges of the flange. This simplification of the forces would produce twist of the beam without subjecting it to bending about the minor axis. The twist in the beam would be resisted by bending of the diaphragm.
In my last post I have said adequate diaphragm - by that I meant adequacy of the fastening of the deck to the flange and flexural strength of the diaphragm. It may or may not be enough in every case.

 

[hokie66]

Maybe we should define diaphragm. I imagined the OP was talking about a metal deck, perhaps puddle welded to the beam along the beam centreline. If that is the case, the rotational restraint would not be adequate. If the diaphragm is a substantial concrete slab, then possibly OK.

 

[BAretired]

The criteria for the strength and stiffness of a lateral brace is given in Canadian code CSA S16 and from previous discussions, I believe a similar requirement exists in AISC.

When web stiffeners are used as a brace, they must be developed in bending as mentioned by Hokie. If the web stiffener and the member to which it is connected are strong enough and stiff enough, then it qualifies as a brace. Otherwise it does not. In other words, it is covered by the structural steel code.

BA

 

[DST148]

@BAretired - My interpretation was wrong. In case of LTB, the lateral movement of the bottom (compression) flange would be caused by a presumptive horizontal force at the bottom and web stiffeners must be developed in bending like Hokie66 mentioned in his earlier posts.
Thanks to you both, BAretired and Hokie66.