For a comprehensive understanding of plywood modulus determination, and
plywood stiffness determinations this is a very good resource:

http://www.fpl.fs.fed.us/documnts/fp...d%20modulus%22

Jim wrote:
"blueman" wrote in message
...
blueman writes:
Still trying to minimize sag and tearout on my bookcase project.
Doubled-up 3/4" plywood sides and shelves with 46" inch span and shelf
pins.

I am trying to choose now between Baltic Birch (13 plys) vs. Natural
Birch with poplar core. Baltic Birch is almost 2x the cost.

Questions:
- Will Baltic Birch give significantly less deflection?
- Will Baltic Birch be less susceptible to tearout on the shelf pins
under heavy weight?

I am going to be painting the shelves.
Still any difference in terms of surface hardness or paintability?

Overall, is Baltic Birch worth the cost typically?

Thanks
Also, does anybody know the "typical" Modulus of Elasticity (in N/mm^2
or PSI) and density (in Kg/m^3 or lb/ft^3) for:
Baltic Birch plywood (assume 13 ply for 3/4" thickness)
Birch plywood with Poplar core (assume 7 ply for 3/4" thickness)
The modulus of elasticity would be about 2,000,000 psi. Sorry to say, my
manuals only cover hardwoods such as oak, hickory, and maple; thus I cannot
give you a better answer than this.. However, the construction of the
plywood has no influence on the modulus of elasticity. But the construction
certainly affects the moment of inertia, and therefore, the maximum stress
and the maximum deflection.
Jim