A few basic theory questions about buckling
A few basic theory questions about buckling
(OP)
I'm sure these will be trivial to many of you, but...
1. Will buckling occur in slender columns (steel is probably easier to think about for me as an example, but I guess infinitely well confined concrete or something may work too) even if the column itself is ideal (no imperfections, is plumb, etc...) and the load is applied ideally through the centroid? Or will it just shorten elastically then plastically undergoing poisson effects?
The second one relates more toward visualizing postbuckling strength (but also buckled sections in general).
2. If a plate girder's web buckles, is the stress that was being applied before buckling still acting on the web, or is it assumed that all/some of it is shed to the compression flange increasing the stress there? I guess specifically one thing I'm thinking about is in relation to the Dc term in AASHTO and what the depth of the web in compression would be if we assumed it buckled.
Sorry if these are terrible or somewhat nonsensical questions, these aren't for any projects or anything I'm working on as an EIT, just stuff I've been thinking about that I've never heard discussed or felt fully confident about. If you think it may help me to ask tangential questions that may improve my understanding, that'd be great too. Thanks.
1. Will buckling occur in slender columns (steel is probably easier to think about for me as an example, but I guess infinitely well confined concrete or something may work too) even if the column itself is ideal (no imperfections, is plumb, etc...) and the load is applied ideally through the centroid? Or will it just shorten elastically then plastically undergoing poisson effects?
The second one relates more toward visualizing postbuckling strength (but also buckled sections in general).
2. If a plate girder's web buckles, is the stress that was being applied before buckling still acting on the web, or is it assumed that all/some of it is shed to the compression flange increasing the stress there? I guess specifically one thing I'm thinking about is in relation to the Dc term in AASHTO and what the depth of the web in compression would be if we assumed it buckled.
Sorry if these are terrible or somewhat nonsensical questions, these aren't for any projects or anything I'm working on as an EIT, just stuff I've been thinking about that I've never heard discussed or felt fully confident about. If you think it may help me to ask tangential questions that may improve my understanding, that'd be great too. Thanks.






RE: A few basic theory questions about buckling
2. I'd be very interested to hear others' opinions on post-buckling behavior and strength. It would make sense to me that a column that buckles elastically has greater post buckling strength than a column that buckles inelastically simply because the E hasn't begun to reduce yet as a result of softening from yielding of the section and therefore is more stiff (flexurally).
RE: A few basic theory questions about buckling
Every time I've seen the buckling equation derived it's been with an assumed eccentricity which in turn I've always assumed was due to practical material or geometric/fabrication imperfections, but I don't think the cause (perhaps -- a cause) for the eccentricity was ever mentioned in classes I've taken. It was more or less just, "let's find the axial load required to hold this column at some deflection y."
RE: A few basic theory questions about buckling
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: A few basic theory questions about buckling
Dik
RE: A few basic theory questions about buckling
In practice it is found that a sufficiently good slender column will buckle at a load similar to that critical load.
Cheers
Greg Locock
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RE: A few basic theory questions about buckling
for 2) i'd say the web is stabilizing the compression flange ... if the web buckles 1st, i'd expect the compression flange would probably collapse (or cripple). of all things are possible, say a very thick compression flange and a very thin web ... the web could buckle (read up on on-set of elastic buckling) and the additional load would be carried by the heacy stable flange (the web would continue it buckling load)
RE: A few basic theory questions about buckling
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Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: A few basic theory questions about buckling
"If a plate girder's web buckles, is the stress that was being applied before buckling still acting on the web" Is it buckled under a load or at the support, I don't think there is a generic answer.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: A few basic theory questions about buckling
His mathematical solutions to problems of the real world are 'real'...
Dik
RE: A few basic theory questions about buckling
kL = Npi (which is they typical buckling behavior), but it can also be satisfied with delta = 0 (this is a trivial case, obviously, but it is the perfectly ideal case as questioned above with no imperfection whatsoever in member or load application). The delta = 0 solution is in unstable equilibrium, but equilibrium nonetheless.
RE: A few basic theory questions about buckling
RE: A few basic theory questions about buckling
I guess I was thinking about the web buckling under a load and about the ability of the section to still carry load if the web buckles.
I also agree with StrEIT's assessment of my first question and after spending some downtime today reading about the historical evolution of buckling calculations it definitely seems like one of the assumptions required is that at some point an unideal factor is introduced (if anyone is interested the paper was an ASCE J. Struct. Engr. paper called "Column Buckling Theory: Historic Highlights" by Bruce Johnston -- I'm not sure on the rules with respect to posting it).
As a side note I just wanted to say thank you to everyone that posts here (including many that don't appear in this thread like JAE, BAretired, and others). I have been sporadically reading this forum for a few months now and as a young engineer who just started working within the last month or so I find many of these topics (even though I'm presently working for a bridge design firm) really interesting / thought provoking.
RE: A few basic theory questions about buckling
now do we ... i mean they can bite ya ;)
RE: A few basic theory questions about buckling
Like any unstable structure, a slender column will fail when gravity load reaches the critical buckling load. Even if the column is presumed ideal, the environment is not. A slight air movement is enough to cause lateral deflection. So is the pull of the moon or other heavenly bodies.
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BA