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Calculating sag

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Deadeye

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Did I dream it or did someone post a lovely link to a webpage that calculated sag of different materials resting between two points with certain loads on them?
In my case the material is 2x9mm thickness of plywood (i.e. 18mm total but the sheets offset from each other) resting on parallel battens/joists.
I'm trying to work out how close the battens need to be to avoid getting waves once I put heavy things on it.
I can't do a grid because of, well, reasons.
 

colinc

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mmm, I could write a page or two on how to do it, but then the reliability of the answer depends upon how accurate the data is for the actual materials you use. For wood it is difficult, we all know it's strength and stiffness varies tremendously. These on-line calculators generally assume conditions that are too simplistic to be useful. That's why god invented Structural Engineers!

Gut feeling is usually quite accurate if you have the materials to hand.

Colin
 

ED65

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This^

IIRC the Sagulator is fairly limited as to plywood anyway, and on top of that the ply in the US isn't necessarily the exact equivalent of ours either.

Deadeye, if you don't want to just go with very conservative numbers, and given it's two thicknesses and not a single board of 18mm, maybe it would be simpler and more accurate to mock up the shelves on the benchtop, load 'em up with more weight than they'll see in service and see what deflections you get with various batten spacings?
 
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