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LTB slenderness in singly-symmetric girders

LTB slenderness in singly-symmetric girders

LTB slenderness in singly-symmetric girders

(OP)
Maybe it's a silly question but I was wondering about the assessment of LTB slenderness in bridge girders. I'm working on a composite deck that's connected via cables to an arch- I'm determining the slenderness of singly-symmetric plated girders at the construction stage- manually determing critical moment is way too complex; I don't think in my case LTBeam software would provide correct values. Now, I don't personally think there's a huge risk of the beams buckling- they're laterally restrained every 5m (with the main span being 150m) and the cross-section being quite large and not too slender. But I was wondering- why not use clause 6.3.2.4 in EN-1993-1-1 (simplified assesment method that considers the length of the compression flange between effective restraints and treats it as a compression member to derive slenderness )

Any thoughts?

RE: LTB slenderness in singly-symmetric girders

I think that it sounds like a reasonable approach in general but I'd really like more detail on your structure before attempting to give definitive advice. I'm imagining a 150m, simple span girder that is hung from the top by a cable every 5m and, in the construction case, that cable is what is providing restraint to the top flange. Is that accurate?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: LTB slenderness in singly-symmetric girders

Without a sketch or general arrangement of some sort, theres no way of giving you a half-decent answer.

I expect its a "no" though, and you'll need to do some type of elastic critical buckling analysis using shell elements. Even for the simplest of buckling problems e.g. plate girder bridge I tend to do a buckling analysis instead of the old BS5400 formulae.

http://www.steelconstruction.info/File:SCI_ED008.p...

RE: LTB slenderness in singly-symmetric girders

(OP)
@Kootk Yes, you are very much correct. The problem is that only 2 of the 4 girders are connected via cables-- however, there's bracing just like in the attached pic (thought that would provide some restraint?)



RE: LTB slenderness in singly-symmetric girders

I'm not at all familiar with the European codes, but if you have plate girders connected with cross frames as shown in your picture at 5m on center, that is definitely sufficient to consider each plate girder locally braced at that 5m interval.

Now, there's the possibility that a girder pair could buckle globally -- I believe Yura published some information on that.

I still don't understand how the cables are playing into this. If your cables are purely vertical, I don't think those do much to restrain buckling -- they may take vertical load and reduce your vertical bending moment, but they'll have almost no stiffness in the lateral direction. You could look at Helwig's recent work on picked girders to get a little boost in capacity.

----
The name is a long story -- just call me Lo.

RE: LTB slenderness in singly-symmetric girders

I agree with Lomarandil. With the diaphragm bracing in place, you ought to be plenty stable. In fact , with the full width diaphragm bracing every few bays, it almost looks as though someone has already considered the global buckling that Lomarandil mentioned.

A hanging connection has some inherent stability when self weight is the only concern. That, simply by virtue of the fact that the load is half a beam depth lower than the the point of rotation at the supports. If the beam will not LTB between cables, it's pretty unlikely that it's just going to roll the entire length. That would imply the load moving upwards and further from the earth which is a gravity load buckling contradiction of sorts.

I certainly prefer more positive -- and effective -- bracing of course. And trying to asses the cable mechanism's ability to brace the non-cable supported beams would be a sketchy proposition for us non-Galambos/Yura types.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: LTB slenderness in singly-symmetric girders

"they're laterally restrained every 5m" - This isn't quite correct, as the bracing does not provide fully effective lateral restraint to the compression flange.

What you have is torsional bracing; simply put, your "Le" for LTB is not simply equal to that of the spacing of the bracing. You will need to check for any potential global buckling modes, using either the empirical BS5400 formulae or revert to an eigenvalue buckling analysis of some kind. There's a very good paper on this issue here http://www.atkinsglobal.com/~/media/Files/A/Atkins...

That being said, I will admit that the bracing is very closely spaced relatively to the length of the span (150m). I don't really have the experience for that kind of span to say thats enough to restrain the effective length to the spacing of the bracing. In any event there will be enough issues to contend with for that order of span such as construction sequence and the like, so I imagine you'll have to go down the FE avenue anyway.

good luck





RE: LTB slenderness in singly-symmetric girders

Great paper ukbridge. Thanks for that.

When it's sufficiently stiff and strong, I believe that torsional LTB bracing is actually superior to compression flange bracing, even if it does not prevent lateral translation of the compression flange. It is, after all, beam rotation that we're trying to prevent with LTB bracing, not compression flange lateral buckling per se. I know that uk codes lean towards a treating LTB prevention as compression flange bracing but I feel that to be an unnecessary, and occasionally misleading, simplification.

For the design of the individual girders, I also believe that Le would be equal to the brace spacing here. I agree that multi girder global buckling modes would require evaluation but I consider those to be additional tiers in the design process that consider the performance of what are effectively different members (tandem girder systems).

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

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