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compression and flexure in CS beams

compression and flexure in CS beams

compression and flexure in CS beams

(OP)
Hello everyone,
I'm having a bit of difficulties with designing a composite girder that's subjected to both compression and flexure. Now, my take on it was to calculate plastic moment resistance of the composite section and plastic resistance of the composite section to axial compression. I was to treat it separately at first- not sure if that's a good idea- My issue is with the axial force- I've never really dealt with axial forces in composite sections. How do I go about determining the position of the PNA in such situation? When an axial force is present, the location of the plastic neutral axis should differ from the location of the PNA in a section where only sagging moment is present, right? Could anyone guide me or present me with a worked example of (at least) a composite beam subjected to axial force? Would appreciate any input :)

RE: compression and flexure in CS beams

By "composite girder", I presume you mean concrete & steel deck over a steel W (or other shape) beam, is that correct?

This won't fully answer your question, but my here is my initial thought: if your axial load is not extreme, perhaps you could just check your steel member to take all the compression load. This is certainly conservative as it ignores the concrete contribution, but this would seem valid and save you lots of time. Presuming your concrete deck spans to adjacent beams, I believe it's also valid to consider this a continuous bracing element for the sake of KL/r (and in effect, your KL/r would become 0 because your L is 0). And then you get the highest possible allowable compressive stress which should help your capacity.

If that still leaves you short of your required axial capacity, then you're back to square one. And I don't believe I've done the type of detailed section analysis for axial loads like you've requested.

RE: compression and flexure in CS beams

(OP)
Sorry, I meant composite deck not a composite girder. And yes, it's a composite slab (concrete + steel sheet) over a steel beam. However I'm also working on a bridge that doesn't have any profiled sheets- and it's the same issue here- axial forces acting on the beam. Now, I would consider ignoring concrete in that matter, however compression is tad bit high. I'm just really curious how structural engineers deal with that. It's a common situation when designing composite decks of bridges, yet I have found very limited information in that regard.
Thanks so much for your input, though!

RE: compression and flexure in CS beams

Got an AISC seismic manual? I'm pretty sure that there's an example in there someplace although I'm unable to recall the details from memory.

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: compression and flexure in CS beams

(OP)
Thank you, KootK. I've flipped through the manual quickly but have not found a design example of a concrete deck (w/ or w/o profiled sheets) over a steel beam. It does have an example of concrete filled sections however I found the example not really addressing the issue of behaviour of the composite section under both compression and flexure. No principles I could apply to my case.
No bridge designer here that could help out or discuss such problem?

RE: compression and flexure in CS beams

Again some more information would be useful, i.e. is it a steel-composite plate girder bridge? Is it a two-span continuous, or simply supported etc.

To answer the question there are ways yes, but:

a) It can get rather very complicated quickly, because you will need to consider combined bending, flexure, torsion etc (for a curved bridge it can/will become even more complicated).
b) In practice the webs of composite girders will often be class 3 or 4 (i.e., the girder can only realise its elastic capacity particular over supports).

There is an example in Designer's Guide to EN 1993-2 by Chris Hendy and Murphy that goes into more detail. I will also say the nice thing about class 3 sections is that there isn't much you can't verify using the Von Mises criteria, except when there is some buckling mechanism e.g. bottom flange in compression over the supports.

I'm not sure what locale you're from, but if you're using the Eurocodes (like me) then that reference should be very helpful.

At the end of the day, for me its the choice between pouring over a long complicated textbook in overtime, or knocking it off on-time and having a pint to celebrate. After 5-6 years its the latter for me these days!

ukbridge.

RE: compression and flexure in CS beams

This document addresses the issue from a North American, buildings perspective: Link. It's basically Norcal's recommendation: in your interaction equation, go composite for flexure and steel only for axial. If additional capacity is needed, you could evaluate the axial as constrained axis buckling about the top flange. There's definitely an example of that in the AISC seismic manual.

The complexity seems to be mostly a function of addressing torsional buckling when there is axial load in the mix. If your concrete slab can be relied upon to provide continuous torsional restraint then I'd think the problem much simpler. Maybe just lower your PNA so that your compression zone is sufficient to resist both the axial force and the flexural compression force.

In typical building applications, deck slab is considered solid top flange bracing but usually not convincing torsional bracing. I suspect that, in reality, it's often both.

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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