On 2/13/2020 9:36 AM, Leon wrote:
On 2/11/2020 5:45 PM, dpb wrote:
On 2/11/2020 3:41 PM, Leon wrote:
On 2/11/2020 1:21 PM, whit3rd wrote:
On Tuesday, February 11, 2020 at 6:39:07 AM UTC-8, Scott Lurndal wrote:
whit3rd writes:

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

Which actually makes it stronger.

Tougher, resistant to splitting, yes.Â*Â* 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,
Â* backed by a thicker layer with the grain running the wrong way.


Followed by grain running "in the right direction".Â* In fact there is
more "in the right direction" plies then the wrong.Â* If that means
anything.
...

Only that it's somewhat more rigid in the direction of the
longitudinal plies than the other...but it's still less than solid
wood longitudinal of the same species.

Â*From US FPL Handbook Chap 12 on mechanical properties a summary table
shows Doug fir modulus of elasticity as 1.98x10^6 lb/insq whereas
plywood is 1.01-1.24 or only about half.Â* Since difference in
deflection of two pieces of same size and species is only the effect
of the different E as the geometrical factor is the same, the computed
sag is directly proportional to the inverse of the E.Â* IOW, the sag
for those two is almost 2X for ply vis a vis solid of same dimension.

https://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr190/chapter_12.pdf

Sagulator gave 0.11" for "Plywood, fir" and while Doug fir wasn't one
of the firs it gives specifically, they all were less (altho not by
factor of 2 which does seem somewhat excessive by common experience).
It doesn't have any other plywood to compare against.


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The sagulator determines sag, not strength.
Yes this discussion is mostly about what is best for shelving and it
went in two different directions.Â* What is stronger and what sags less.

The statement I responded to, near the top of this response, was that
plywood was weaker than solid wood, that is not necessarily true
regardless of what the sagulator says.

Considering shelving intended to carry a lot of weight both solid wood
and plywood will bend to some extent and solid wood will likely bow less
along its length, but it will bend.

So how do you get rid of bow along the length when using plywood or
solid wood?Â* You add support.Â* Typically along the front edge and often
along the back edge.Â* Lets say you use equal steel beams to reinforce
the length of the shelves so that there is no bow at all along the front
and back edge

Add weight to the center until one fails.Â* My money is on the plywood
coming out the winner.Â* The hardwood is likely to split/fail along the
length, with the grain.

Have I seen solid wood shelving split along its length?Â* Yes, especially
as it get older.Â* And that was not necessarily with a lot of weight. And
the deeper the shelving the more likely the solid wood will fail over
plywood.

Again, I'm not saying plywood will sag less than solid wood when used
for shelving, I'm saying that plywood is stronger than like sized wood.

Well, the poster of the above comment was used the word strength in an
imprecise manner and really was referring to the ability to resist
bending moment which is, for a fixed geometry, directly proportional to
the modulus of elasticity.

What you're describing also is not really "strength" per se, but the
effect of the very high anisotropic nature of wood with and across
grain. Certainly if you don't load it or constrain/support it across
grain, solid wood will failure much sooner in that direction. Plywood
definitely has the advantage in that regards and in dimensional
stability, yes.

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