Triscuit theorycrafting

[Stoickthelesser]

I'm trying to wrap my head around some finer details of the physics of stressed skin.

Do both plywood skins have to be the same thickness? It feels like they should, because the skins having different deflection strengths would seem to undermine the strength of the construction, but would I actually not be screwing myself by having one face be 3/4" and the other 1/2", for example?

How much difference does density of support make vs. thickness of skin? Would a 4'x6' triscuit made with 1/2" ply skins and with studwalls at 0, 36(center) and 72" be able to take more or less load than a 4'x4' with 3/4" skins and studwalls at 0 and 48"? My suspicion is less, but how much less?

If it's gotta be 1x3 sandwiched between 3/4" ply, then so be it, but I'm curious as to the implications of modifying the formula.

 

[BillConnerFASTC]

The engineering is available at APA. You can vary all of those things. Can be good reasons to use different thicknesses, but cantilevers and reversing the decks can make it pointless.

You mention "deflection strengths" but the skins are primarily in tension or compression. There bending properties are not unimportant - for deflection between framing - but generally not the controlling factor.

I've seen 16' spans - one with 1/4" skins and 1 x 3s or 4s - walks on only for set change. And one with 2 x 6s and heavier ply on top - for a storage loft. Might have been 20' - not sure.

So the formulas are simple for a standard set of stock" platforms than anyone can build. But consider the Spruce Goose (which was made of birch). Same principles.