On Tuesday, February 11, 2020 at 6:39:07 AM UTC-8, Scott Lurndal wrote:
whit3rd writes:

Plywood
is weaker than solid wood (half the grain runs the wrong way).

Which actually makes it stronger.

Tougher, resistant to splitting, yes. Stronger in the shelf-sag sense, no.
Sagulator gets this right.

Shelves need compressive strength in the top surface, and tensile strength
in the bottom surface (knots on top are less troublesome than on bottom, for instance).
Plywood has, on bottom surface, a very thin veneer of good high-tensile strength wood,
backed by a thicker layer with the grain running the wrong way.