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dpb[_3_] dpb[_3_] is offline
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Default Wide shelving advice needed

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.

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. 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. 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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