huangyhg

beam in compression
hi all,
let me preface i am far from being a structural engineer, which will explain the triviality of my post.
how to calculate deflections for a horizontal beam, both ends clamped, under distributed force (own weight) and compression?
i am looking at the basics, but as you can not superimpose the euler beam theory solution to a compressive case my repertoire is almost over.
there must be more than one approach for such a common case, i am hoping in your patience!
thank you and all the best
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a column is a "beam" loaded in compression, easily solved by euler.
a beam is a beam (der) loaded with transverse loads, easily solved by equations of equilibrium.
a "beam column" is the combination of the two. you're right you can't superimpose them ... the bending deflections due to the transverse loads exagerate the column loading. "advanced" structures books, like bruhn, solve this structure ... it isn't any more complicated than a redundant beam.
clear as mud ?
a solution is published in roark's formulas for stress and strain.
do you not have an analysis software?
seit,
that is not an appropriate suggestion. if he doesn't know how to do it manually, he shouldn't be using software to solve his problem.
a bit off topic, but, how is using software any less than using roark's??
personally, i would think a novice has a better chance of screwing up using roark's than use something like sap or ram.
just because you can write it on a piece of paper doesn't mean you know know it, ya know?
i suspect very few engineers actually know how to truly solve this problem (beam-column).
we use codes and approximations to come up with safe solutions . but to understand the true behavior of the
no no no! never design by black-box methods what you cannot solve to within 30% by hand!!!
sorry frv, love your postings, as well as respect your opinion from numerous other threads, but i have to counter-comment on your comment. i doubt there was anything self-righteous in hokie's comment... and even if it was, that wouldn't change the fact that our transplanted american friend is absolutely right. i do not accept that there is anything inappropriate about telling someone that they should not use software to solve a problem they aren't able to do by hand.
and it's true that beam-column solutions are complex, often requiring higher level analysis methods, however even then, with all of our high-end wizardry, we still cannot know the true state of stresses. wooten's third law still applies my friend.
with utmost respect and with the hopes that no one misunderstands my passion as being arrogance or disrespect,
ys
b.eng (carleton)
working in new zealand, thinking of my snow covered home...
youngstructural-
i gave you a star for disagreeing with me . primarily because you were so nice about it. is it the canadian thing?
i don't misunderstand. in fact, you have a valid point.
ironically, two of the posters i have come to consider most insightful are you and structuraleit. i say "ironically" because it would seem that at the core, you two disagree about this issue. our current political climate in the us notwithstanding, people can disagree, yet respect one another.
i also respect hokie66's opinion. it is obvious he has years of experience and is well versed in many aspects of what we do. if my post came off as anything other than respectful disagreement, it was inadvertent, and i again apologize for it.
i wasn't suggesting that someone with no knowledge of structural behavior attempt to model this. however, i do think that if you know how to properly model something, software can be an invaluable tool. therein lies the problem. how do you define "know how to properly model"? i think this is where our disagreement lies.
btw.. i think 30% is excessive. if your hand calculation is 30% off of a computer generated one, you probably didn't catch the second-order effects.

thanks frv; i was concerned i would be misinterpreted, and am greatly relieved that you have understood the heart of my concern.
yes, i suppose being polite is something that many canadian pride themselves upon. it tends to be a faut-pas to fail to be polite, however we have our fair share of rude & obnoxious people!
i suppose structuraleit and i are disagreeing here, althoug i too normally find his (her?) posts insightful; at the end of the day, for this case, you're probably right that we'll have to agree to disagree. however, that said, i think a lawyer would easily chop a designer up on the stand if they blindly followed a computer analysis and were not able to check the results by hand.
with regard to the level of accuracy for a check, 30% is my red flag, do it again until it checks out point. it's the no-go point for my analysis before it goes to the design phase. i normally find i can estimate structures to withing 15%, and am only really happy if my back of the envelope is out less than 10%.
cheers,
and thanks for the star,
ys
b.eng (carleton)
working in new zealand, thinking of my snow covered home...
all,
many thanks to your suggestions.
i found the formula in roark's book but i am reluctant tu use a formula which i do not the derivation of, hence significance of its assumptions.
the only software i could use is ansys, but i would be ashamed of using it for this problem..
right, the beam column, i could certainly solve a 4th order differential equation numerically (as the ei is varying) but what about buckling?
yesterday evening i went back to princioples and i am pretty confident that i have found the critical load for a beam in compresion and with one concentrated load in the middle (the maths was easier than the distributed load).
i will assume conservatively this case (will certainly give me a lower critical load) and start from here.
many thanks to all
atb
favollo

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