Lateral Torsional Buckling Bracing...

[GalileoG]

This is something that has bothered me for a long time, but couldn't ask because I thought it was something that was really trivial.

How can we determine that said beam is sufficiently braced somewhere along its length so that we can take L smaller than the beam length? For example, secondary beams spanning into the primary beam (without any bracing.) How can we determine that the secondary beams will brace the primary beam enough to cause an inflection of the buckled profile? Also, what criteria do we use to determine that a plate floor or a concrete slab braces the compression flange of the supporting steel beam sufficiently enough to allow us to only use Mr = SFy without using the lateral torsional buckling equation. I have a feeling that we neglect any beneficial restraint from floors, but I don't know why. Seems like floors can offer excellent restraint for lateral torsional buckling of beams.

Thanks! Would also appreciate links to reference for further reading,

Clansman

 

[271828]

Check out the new AISC Manual Appendix 6. It has a section on beam bracing that covers this question.

As for a slab on the compression flange, that will most always provide LTB restraint. It's hard to think of a time that it wouldn't.

 

[271828]

Oh yeah, other reading.

Most offices have at least one copy of the Yura/Helwig AISC bracing seminar notes. That's the best source of guidance.

The SSRC Guide to Stability Design Criteria has a chapter also.

Finally, there are several papers, probably the best place to start is the AISC EJ paper by Yura called "Fundamentals of Beam Bracing."

 

[teleBob]

Please check out the discussion on a contemporary thread "Web Stiffeners for Flange Bracing?". I found that Appendix 6 is quite helpful, but does not allow any calculation for continuous lateral compression flange bracing.

Your question is appropriate in the example of a roof beam with metal roof decking. Do puddle welds constitute adequate lateral bracing for the top flange? 2% of the flange force (for nodal lateral bracing) or even 0.8% (for relative bracing) is usually a pretty substantial number. Another example is a non-composite beam (no headed anchor studs) with metal decking and concrete floor slab.

In your example of secondary beams framing into a girder, I believe they will provide TORSIONAL bracing support per Appendix 6, especially if full fitted stiffeners are used, the connections will support the required torsional bracing moment, and the secondary beams have adequate stiffness.

 

[mlt55]

The old BS449 Clause 26 states that the restraining beams or slab must be capable of resisting 2.5% of the maximum flange force to provide effective lateral restraint.

 

[kelowna]

I have heard about the 2.5%, although in the past I have used 5% instead as a more 'conservative' approach. The lateral restraint required was surprisingly low

 

[oneintheeye]

code in UK specifies 2.5% of factored forces in compression flange as stated earlier. It is also worth noting that it is not satisfactory to simply brace a primary beam by connecting the braced beam to another unrestained beam so that they are mutually dependent. I agree with earlier statment that a concrete floor would certainly provide lateral restraint in pretty much all cases.

 

[samdamon]

There is a free webinar sponsored by the AISC called "Steel Design after College" which discusses the latest research on what constitutes bracing and how it affects LTB, see

http://www.aisc.org.

It costs nothing to view.

There are twelve sections in the webinar but I believe the pertinent discussion occurs in the first and second one.

 

[swearingen]

Do the newer BS standards include a stiffness requirement? AISC recently added an explicit one and I was curious if the British standards had done so as well...



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