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Deflection in beams

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?

RE: Deflection in beams

How big a difference?  Small errors are par for the course.

How well does your model correspond to the actual measurement conditions, loading conditions, and the actual beam properties?

 

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RE: Deflection in beams

(OP)
I've used macauleys method to calculate a value for deflection and in doing so i have only assumed the value for youngs modulus.
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

end fixity is one cause.

RE: Deflection in beams

(OP)
Would end fixity still affect it if the supports are positioned in from the ends?

RE: Deflection in beams

trying to solve a frame with beam equations comes to mind also.  

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

Is the way you are measuring OK? 0.07 mm is almost negligible, not many instruments will measure that correctly, even 0.91 is very little. The accuracy of your measuring instrument could account for the difference.

RE: Deflection in beams

multiply your force by 100 and redo everything you did.  

peace
Fe

RE: Deflection in beams

Is this a homework / lab question or is there a real-world problem here?

jt

RE: Deflection in beams

Amen to Greg's suggestion.  Of all the structural materials, steel has the most consistent modulus of inertia.  So, for a pure bending exercise, measured deflections should match computed.  Support conditions can dramatically differ from assumed, and for short spans, members tend to act like tied arches rather than beams.

RE: Deflection in beams

Quote (Kieran88):

What could be the causes for a difference in measured and calculated values of deflection, other than human error, material impurities etc?

   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.

               JHG

RE: Deflection in beams

how ideal are the supports ? i imagine you analyzed them as pinned (or fully fixed) ?  how accurate is your measurment ? (0.07mm is pretty darn small) ...

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

If your measured was more than your actual, I'd ask if this were a short stubby beam where shear deformation needs to be included.   But your actual is more than calculated.

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

Hi Kieran88

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

For most beams, Bernoulli-Euler beam theory is going to give you results that are very nearly dead-on. I would recheck your calculations, material properties, and your measurements.

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

Until and unless the OP provides more information as requested, I suggest we stop speculating about his nebulous problem.

RE: Deflection in beams

....period smile

peace
Fe

RE: Deflection in beams

Methinks its a student question.
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

Quote (40818):

Methinks its a student question.

...

   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.   

               JHG

RE: Deflection in beams

It is common to put the yr of birth in the handle, good thing I didn't smile. Or did I? pipe

peace
Fe

RE: Deflection in beams

88 could be 96 in base 11

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