"Jim" writes:
"blueman" wrote in message
...
Ray Mandeville writes:

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


Interesting article.
From a quick scan of the math, it seems like to first approximation in
a multiply laminate (e.g., plywood) that the MOE (modulus of
elasticity) is only about 50% of the modulus for a similar thickness
of pure hardwood of the same species oriented longitudinally. The
logic being that the transverse-oriented plys contribute only
minimally to stiffness.
Your understanding is faulty. The modulus of elascticity is gives the
relationship between stress and strain.

Please explain. For a shelf, the modulus of elasticity (MOE)
determines the relationship between the geometry of the shelf, the
weight applied, and the amount of resulting deflection. For plywood,
the total effective MOE of the entire sheet is a weighted average of
the MOE's of each individual ply (where the moments of inertia of each
ply are the weighting factors). To a first approximation, since the
moments of intertia of each ply are essentially the same, the
effective MOE is equal to the sum of the MOEs of the transverse and
longitudinal plies. Since the tranverse plies have a MOE of only 4-6%
of the longitudinal plies, it seemed pretty clear from the article
that the Modulus of Elasticity is really only driven by the
longitudinal plies and hence is about 1/2 as high as pure
hardwood. Again, please clarify how I have misread the several
formulas in the article.

What the article should have said is that the calculations apply to
computing the moment of inertia. As the article indicated, cross plies
don't contribute to the moment of intertia hence the stresses are higher
with plywood that with wood of the same thickness.
Jim

Well that really gives the same effect. You can look at it this
way. The overall moment of inertia of the shelf is independent of the
plies and is governed only by the shape and density of the
materials. However, the moment of inertia is only effectively resisted
by the longitudinal plies. So, the same force is applied to
effectively half the bending resistance. So, macroscopically and to a
reasonable approximation the plywood shelf is equivalent to a
similarly shaped hardwood shelf made of a material with about half the
modulus of elasticity. Again, my physics is a bit old and rusty, but
I believe I am directionally correct in my understanding and
summary. If not, please explain..