Deflection in beams
Deflection in beams
(OP)
What could be the causes for a difference in measured and calculated values of deflection, other than human error, material impurities etc?
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RE: Deflection in beams
How well does your model correspond to the actual measurement conditions, loading conditions, and the actual beam properties?
TTFN
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RE: Deflection in beams
Ive also ignored the fact that it is a UDL as i zeroed the DTI prior to loading.
My measured value came out at 0.07mm and my calculated at 0.91mm, i have gone over my method a hundred times and still obtain a similar result.
RE: Deflection in beams
RE: Deflection in beams
RE: Deflection in beams
fixity in general will affect your results. Knowing how to apply actual restraint conditions can be difficult.
You should post a sketch of your frame/beam.
RE: Deflection in beams
RE: Deflection in beams
Fe
RE: Deflection in beams
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Deflection in beams
jt
RE: Deflection in beams
RE: Deflection in beams
Double integration method is only accurate for small deflections. One of my mechanics of materials books states that it is good up to deflections of 10% of beam length. How long is your beam?
Also, how accurate is the thickness of your beam? Your beam stiffness and deflection are a function of thickness cubed. Any thickness error will be multiplied. Beam length is another value that gets cubed.
RE: Deflection in beams
you're looking as max deflection, yes? instead of repeating the same calc (potentially repeating the same error), what does Roark (or some other handbook) say the deflection should be ??
RE: Deflection in beams
More knowledgeable people than me suggested to pay attention to boundary condition assumptions – makes sense to me.
Some possible ways to explore further:
1 – take multiple measurements to see if the shape is as predicted by beam theory.... and examine any deviations as a clue to the behavior.
2 – Post a more detailed description of the physcial problem here. Then people can provide comments on validity of assumptions for that problem and also possibly double check the conclusion.
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RE: Deflection in beams
Well more information on your beam set up might help getting a better understanding of your problem and ultimately a better answer.
The difference between a point load and a uniformly distributed load, would result in a deflection of 1.6 times the deflection of a uniformly distributed load to get to the deflection of the equivalent point loaded beam, everything else being equal.
This clearly doesn't account for your difference in deflections so we need to look at other things.
desertfox
RE: Deflection in beams
TTFN
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RE: Deflection in beams
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Deflection in beams
As was mentioned, the assumptions for a cantilever beam don't work as well near the fixity, so for a very short beam, things can start to fall apart. Can you give the dimensions of your beam?
RE: Deflection in beams
RE: Deflection in beams
Fe
RE: Deflection in beams
Putting my Sherlock holmes hat on:
1) Macauleys tends to be taught at schools but not generally practised in industry.
2) the OPs name has "88" which i take it they are only around 22 years old.
3)I must be getting grouchy
RE: Deflection in beams
I have used double integration to solve stuff. Macauleys method is not completely accurate. Either I was not taught, or I did not remember this from school. It doesn't hurt to crack open the old textbooks once in a while.
88 could be a year of birth, it could be the numnber of Kierans on Eng-Tips. It could be the number of keys on a piano. It could be the size of a gun. It could be an Oldsmobile. It could be part of a famous rock and roll song.
It sounds like he has phyically tested this beam. At worst, we are not doing his homework, and it is not a dumb question.
RE: Deflection in beams
Fe
RE: Deflection in beams