Torsional Bracing Requirement in 13th edition
Torsional Bracing Requirement in 13th edition
(OP)
The 13th edition now addresses bracing strength (and stiffness) for points along columns and beams. For beam bracing, criteria for lateral bracing and torsional bracing are given (as opposed to using some of the old rules of thumb (i.e. 2% of the compressive force, etc.)). My question is: If my section (a doubly symmetric I-beam) cannot fail by flexural-torsional buckling (only by flexural buckling), does torsional bracing need to be considered? Or just lateral restraint for the compression flange?
Most I-beams (in the steel manual) are controlled by flexural buckling (at the typical lengths used in design). So I was uncertain if they meant this criteria primarily for singly symmetric shapes, or was it intended to take care of initial twist imperfections in the beam.
Any thoughts are appreciated.
Most I-beams (in the steel manual) are controlled by flexural buckling (at the typical lengths used in design). So I was uncertain if they meant this criteria primarily for singly symmetric shapes, or was it intended to take care of initial twist imperfections in the beam.
Any thoughts are appreciated.






RE: Torsional Bracing Requirement in 13th edition
Flexural-torsional buckling is something that happens to singly-symm columns.
If you're referring to lateral-torsional buckling, then Appendix 6 indicates that you can consider a location along your beam braced if the compression flange can't translate OR if the entire section at that location can't twist.
RE: Torsional Bracing Requirement in 13th edition
Sorry to nit-pick your terminology. Considering that App 6 has column and beam stuff, it needs to be clear which we're talking about.
RE: Torsional Bracing Requirement in 13th edition
I guess what is confusing me is: what torsion am I bracing it against? Initial imperfections? Flexural torsional buckling? (Not a mode of failure for doubly symetric I-beams in the manual, except at short lengths.) Torsion caused by external loading (not likely)?
RE: Torsional Bracing Requirement in 13th edition
Nothing special, just a regular unbraced steel beam subjected to flexure only. Doesn't matter if it's doubly-symm--the section still applies. Actually, the equations were derived for doubly-symm beams, so try applying them to mono-symm ones if you want to really have fun, LOL.
You can say your beam is braced at a point if that point is restrained from twisting or if the compression flange is restrained from translation. 6.3.2 is for the former--restraint against twist.
A beam doesn't have to have any initial imperfection to buckle. Neither does a column.
Real members do have initial imperfections, though, and those go into the derivations of the equations in Appendix 6.
If you want to say your beam is braced against twist at a section, then you have to have something bracing it there, whether it be another beam framing in, kickers, or whatever. This section just gives you a way to calculate how strong and stiff the bracing element(s) must be.
This is, by far, the most confusing set of provisions in the AISC Spec. The deeper you look into it, the less sense it makes also. If you want to research the background, look at the Fundamentals of Beam Bracing paper by Yura. There are various seminar notes floating around out there too.
Warning, though: Yura & Helwig have made this stuff so confusing that many people, steel gurus included, pretty much throw up their hands when trying to dig into the background.
RE: Torsional Bracing Requirement in 13th edition
What I suspected from looking at this spec was that it applied mainly to flexural-torsional buckling, thanks for confirming that for me.
The thing about it is: typically, I am dealing with a floor system (consisting of nothing but girders and intermediate beams) that does not have much of a restraint for the top flange (i.e. grating). I’ve always made it a point to tie my beam lines back to a lateral force resisting system (some times using in-plane bracing or a concrete slab) to give it the necessary stiffness.
Anyway, thanks again.
RE: Torsional Bracing Requirement in 13th edition
Hate to nit-pick again, but flexural-torsional buckling is something that happens to columns. So is flexural buckling.
In this case, we have lateral-torsional buckling.
RE: Torsional Bracing Requirement in 13th edition
RE: Torsional Bracing Requirement in 13th edition
What prompted this question was the language of section 6.3 (p. 16.1-193):
“At points of support for beams, girders and trusses, restrain against rotation about their longitudinal axis shall be provided. Beam bracing shall prevent the relative displacement of the top and bottom flanges, in other words, the twist of the section. Lateral stability of beams shall be provided by lateral bracing, torsional bracing or a combination of the two.”
The way that read, I was not sure if the first sentence meant that torsional bracing was MANDATORY. (And bracing girders for torsion is difficult where you just have beams with simple shear connections going from girder to girder.) And the last sentence was somewhat ambiguous (at least to me) as well. I was not sure if they meant that lateral bracing could be provided by either one of those methods or not. The code is a legal document; so interpretation of it has to be taken with care.
RE: Torsional Bracing Requirement in 13th edition
For example, if you're designing a singly-symm I-shaped beam using the green book, then you use equations that were derived specifically for doubly-symm shapes. The 2005 Spec. has specific sections for singly-symm I-shapes. For those with the need and/or inclination to dig in and use the most modern info, the 2005 Spec. is greatly preferable.
Also, I really don't think it's as bad as it looks. Take Chapter F for example. Sure it looks horrible, but if one only deals with compact W-shapes, then F1 & F2 are the only applicable sections. Compare the simple format of F2 to the spaghetti-like format of the green book's Section F1!!
RE: Torsional Bracing Requirement in 13th edition
"Lateral stability of beams shall be provided by lateral bracing, torsional bracing or a combination of the two.”
You can read the Fundamentals of Beam Bracing Paper or seminar notes to get further backup on this. There are lots of examples in teh seminar notes, some that show lateral bracing only and some that show torsional bracing only.
RE: Torsional Bracing Requirement in 13th edition
I attended Yura’s seminar on this (years ago) and looking back at the notes, it was clear HOW to provide each type of bracing, just not which one is supposed to be used.