MPa/psi – compression of plywood on end?
by Aquaboy on Friday, April 2nd, 2010 | 3 Comments
What is the max load of 180″ of 3/4″ plywood standing on end (I had to modify an aquarium stand – which now has about 180″ of vertical plywood (divide by 4 for each corner) supporting the tank (300 gallon ~4k lbs with rock/glass/water).
What is the max load of 180″ of 3/4″ plywood standing on end (I had to modify an aquarium stand – which now has about 180″ of vertical plywood (divide by 4 for each corner and 30″ tall) supporting the tank (300 gallon ~4k lbs with rock/glass/water).
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I would be more worried about it collapsing then breaking outright. Like the joints give out and it just collapses sideways.
Plywood is not really designed to have end-on loads like that. If you insist on loading it that way, I would make some “plus signs” mounted inside the base to spread out the load and provide rigidity from falling sideways.
There’s another issue, plywood goes mushy when it gets wet. It’s really not a good choice and I have to urge you again to reconsider.
Something like 2x4s would be a MUCH better choice.
Note you can use 2x4s or 4×4 for the actual supports and then wrap the whole thing in plywood so it looks the way you like.
Well, 180″ * 3/4″ = 135 square inches
the compression strength of wood depends on the species and whether the loading is parallel or perpendicular to the grain, in plywood the area will be roughly equally divided between the two conditions. Since the strength parallel to the grain is higher, lets just take half of the 135in^2 and ignore the portion loaded in the weaker perpendicular to the grain direction.
4000 lb / 67.5in^2 = about 60PSI, I’m pretty sure the lightest balsa wood is stronger than that in compression parallel to the grain. The design values for most commercial lumber ranges between around 1000 and 2000PSI
The real concern is buckling, not compression failure, so bracing is an issue if the height of your stand is significant.
compressive strength = 4500psi (???)
Max load (compressive failure) = 607,500lbs
Max load (buckling failure) = 913lbs
So depending on how good your bracing is, your stand will hold somewhere between 913lbs and 300 tons. Realistically, if your stand does fail, it will probably be due to weak joints or soggy wood. I’d build it out of something sturdier and more reliable, like Unistrut:
Math:
Compressive failure:
180 x 3/4 * 4500psi = 607000lbs.
Buckling failure:
3.14^2 * 1350000 * 1/12 * 180 * .75 ^ 3 / (1 * 96)^2 = 913lbs