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 :)
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
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
Thanks so much for your input, though!
RE: compression and flexure in CS beams
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
No bridge designer here that could help out or discuss such problem?
RE: compression and flexure in CS beams
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
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.