A.Tim Garrison, a
professional engineer, the president of
ConstructionCalc.com, and the author of Cracks,
Sags, and Dimwits: Lessons to Build On,
responds: Wood achieves its greatest
compressive strength when it's oriented like a
post, so that loads are applied parallel to grain
and resisted by its long fibers.
But most shims are cut along the grain so that
they don't fall apart; they receive compression
loads perpendicular to grain, which tends to crush
the wood fibers.
Or, to put it another way, Doug fir's allowable
compression parallel to grain is around 1,500 psi;
its allowable compression perpendicular to grain is
around 625 psi.
Thus, the answer to your question depends a lot
on how well the shim is cut. A 3.5-inch-square Doug
fir shim at the top of a 4x4 post in a crawlspace
can take approximately 7,656 pounds of load (625 x
3.5 x 3.5 = 7,656 pounds) as long as the shim seats
perfectly. That's a lot of load — probably
more than will ever be applied by a floor beam.
The rub comes, of course, when shims are not cut
carefully and do not seat properly. Most shims are
not perfect, uniformly thick wafers. They're
wedges, hacked from a stray board by an overworked
framer toting a worm drive. If such a shim, cut
from a 2-by, is driven into the gap at the top of
the same 4x4 post and bears on half its length and
its complete width, the load capacity is only 1,640
pounds (625 x [3.5/2] x 1.5 = 1,640 pounds).
When this shim gets overloaded — during
the annual Thanksgiving gathering, say —
it will crush and the beam will settle a bit. Once
the feast is over and the in-laws go home, the
floor will either have a permanent dip (if the post
was at a beam end) or be bouncy or squeaky (if the
post was located midbeam).
Plywood may seem like a reasonable alternative,
but its allowable stress perpendicular to grain is
around 300 psi, making it only about half as strong
as Doug fir for this purpose. Unless the loads are
light and you are sure the shim is seated
completely, I wouldn't recommend making shims from
Steel-plate shims are the best choice, provided
they seat properly and are available in the right
thickness. The allowable compressive stress for
steel is on the order of 25,000 psi — far
more than for any wood.