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Old 6th Aug 2009, 16:20   #1 (permalink)
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Join Date: Sep 2006
Location: Los Angeles, USA
Age: 46
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Calculating size of beam

After extensive searching on the net until I'm now blue in the face, I need a pointer.

Where can I find simple to understand formulas for load bearing in beams etc (doesn't have to be wood specifically).

I know my load (force).
I know the distance from the fulcrum (moment).
I know the tensile and compressive strength of my material (MoR and MoE).

What I want to calculate is how thick my beam needs to be using the above info.

Should be a simple thing, one thinks. And I used to study this in engineering school, but I've forgotten all and have none of the books left. And as I said, I can't seem to find any info on the net after extensive searching.

Even a straight forward construction formula for house construction with cantilevered beams would do. You have a beam, you load it at the end with this much = how thick does the beam have to be?

Please help. Thanks.
AdamFrisch is offline   Reply With Quote
Old 6th Aug 2009, 17:43   #2 (permalink)
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I don't know what's your favourite search engine, maybe it's the words your using, but put "beam deflection formula" into Google and there's 1.4million hits, add "+wood" (all without the speach marks) and it's reduced to 137,00 and the ones at the top of of the first page should help.

Happy hunting!
SincoTC is offline   Reply With Quote
Old 7th Aug 2009, 10:24   #3 (permalink)
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If you have abandoned your tech library, you probably don't want to lash out big dollars for new texts.

Try your local library where you will find several useful books in both the reference section and via Dewey look for texts on Mechanics of Materials.
john_tullamarine is online now   Reply With Quote
Old 7th Aug 2009, 17:05   #4 (permalink)
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"Machinery's Handbook" provides a wonderful reference for most questions like this.

After an excellent landing you can use the airplane again!
Flash2001 is offline   Reply With Quote
Old 9th Aug 2009, 07:21   #5 (permalink)
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Depends on what is the maximum stress you're going to allow your beam to carry.

Since I don't know what material you're dealing with, I can't recommend stress values.

stress = Mc/I, where M is maximum moment across the beam, c is distance from bending centroid to the most outer face of beam, and I is the moment of inertia.

Plug in the desired max stress and solve for your c or I, and make/construct your beam to match.

Another method to find beam size is to do it by knowing the maximum deflection in the beam you're willing to have. But this is another animal entirely, and there's no "simple formula" you're looking for.

Above method will work with timber and, in a simplified way, steel. Hopefully you're not using concrete. Concrete is a non-linear material and you'll need an engineer to do it.

Have fun!

A CivEng from US.
William Wonka is offline   Reply With Quote
Old 9th Aug 2009, 16:11   #6 (permalink)
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There's a useful index for various cases at:
Beam Deflection Calculator - Engineers Edge

You also need to know material properties for the wood - which you can look up online.

Then it's a case of choosing a trial balloon - pick a beam size, calculate stress and deflection, and see if you're within the material limits (including factor of safety).

If that's undersize, or significantly oversize, try another beam size and repeat the job.

Don't forget to include the dead weight of the beam.
barit1 is offline   Reply With Quote

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