On Monday, April 11, 2016 at 7:51:10 PM UTC-4, dpb wrote:
On 04/11/2016 6:13 PM, dadiOH wrote:
...

glue a 1/4 x 36 x 18 piece of ply across a couple of 2x4s then clamp down
the 2x4s and stand on the ply, do you think the ply will bend? I don't.

Of _course_ it will; it doesn't have an infinite modulus of elasticity;
if it were 1/4" steel plate it would still bend; just not quite so much...

Let's see...from FPL Wood Handbook, for plywood

E -- 1.10-1.24x10E5 lb/sq-in

and for a rectangular solid beam the moment of inertia is bh^3/12 where
b=width, h=height so for the drawer that's

I=18*(1/4)^3/12 -- 18/(12*4*4*4) -- 3/(2*4*4*4) = 3/128 =0.02344 in^4

For a constrained beam at both ends, max deflection at midpoint is

ymax=Wl^3/192EI

For a 200 lb load that's

ymax=200*36^3/(192*1.1E6*0.023) = 1.88"

For comparison, E for steel ~27E6 so the deflection would be about 0.07"
instead.

...

Note that the panels would not have warped if the edges had been
constrained...

If you build a panel door that is fully and tightly constrained on all
sides with no expansion room, it'll fail sooner rather than later. Door
panels in exterior doors are _not_ fixed; they float.

--

I wasn't going to mention the fact that dadiOH wants me to secure the bottom
on 3 sides. Aside from all the other reasons that fix won't work, securing the panel
on 3 sides is probably a bad idea too.