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
I am trying to choose now between Baltic Birch (13 plys) vs. Natural
Birch with poplar core. Baltic Birch is almost 2x the cost.
- 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?
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.
For a comprehensive understanding of plywood modulus determination, and
plywood stiffness determinations this is a very good resource:
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.
While somewhat intuitive, I would have thought with all the glues and
resins that you would get some benefit that would make the stiffness
more than just 50% of the equivalent pure hardwood.
Interestingly, one of the web deflection calculators gives fir plywood
about 2/3 the MOE of equivalent thickness douglas fir. So maybe there
is some benefit to the other laminations and resins...
In any case, it still would be good to get some general specs on the
typical MOE for Birch plywood.
Your understanding is faulty. The modulus of elascticity is gives the
relationship between stress and strain.
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.
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.
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..
For isotropic materials, the moment of inertia calculations need only
consider the crossectional geometry. When the crossection is comprised
of dissimilar materials, the Modulus of each material must be included
in the calculation. For plywood, the cross laminations actually
represent different materials due to the fact that the longitudinal
versus transverse properties of the wood laminations differ. So, in
reality, the moment of inertia and or section modulus is different.
Try this for some Baltic birch properties. Be careful to distinguish
between the terms 'modulus of elasticity' and 'section modulus'.
from a Google search on 'plywood birch "modulus of elasticity"'
Incidentally, glues have low elastic modulus also and, like the cross plies,
contribute very little to the overall stiffness, most of which comes from
the contribution of the surface plies. For an extreme example of this
behavior try a search on "sandwich panel" wherein a relatively thin surface
skin is bonded to a foam or honeycomb core, as in hollow core interior
The practical solution to your shelf application is to bond a solid wood
strip (wider than the plywood thickness) to the front of your plywood shelf
where it will serve the dual purpose of stiffening the shelf and of hiding
the cut edge of the plywood. Perhaps add a mid-span support (cleat, clip,
pin) under the back of the shelf. Or, use a solid wood shelf. Note that
minimal initial deflection, while important, is not the whole story. Highly
stressed wood will take a set over time (engineers call this 'creep') which
shows up as a sag in the shelf. Either build a shelf stiff enough to resist
this or plan to turn the shelf upside-down occasionally. I have to do the
latter every few years on a commercial bookcase with 3-foot shelves of teak
veneer over particle board -- heavily loaded with my wife's collection of
Southern Living annual cookbooks.
Based upon looking at some calculators and skimming the analytical
paper cited in another response, it seems like Baltic Birch plywood
would have an MOE of 1/2 to 2/3 that of the Baltic hardwood number
that you quoted.
Also, if I am understanding you correctly, then I think you are
incorrect in saying that the construction of the plywood doesn't
effect the MOE -- in fact, per the article mentioned in another
response, the transvers-oriented plys contribute only minimally to the
stiffness. So much so that when there are only a small number of plys
there is a significant difference in MOE between plywoods with the
face grains oriented parallel vs. orthogonal.
Even if I assume that the plywood has only 50% of the MOE of pure
hardwood, then using the equation for deflection of a shelf with
uniformly distributed load and supported ends (like adjustable
shelving), I get that the shelf could support about 280lbs and still
deflect only 1/32 inch per foot which is reportedly the approximate
limit for being noticeable to the average human eye. If I assume that
the MOE is more like 2/3 of the hardwood equivalent, then I get a
maximum support without visible deflection of about 380 lbs.
Given that I need to only generously support 200 lbs (and probably
more like 150), I should be OK, even taking the worst case and
allowing for additional sagging with age.
IIRC the OP said that he was using a double thickness of 3/4 ply for
the shelves as well as the sides of this bookcase. Even with the 46"
shelf span I think it would take some serious weight to noticeably bow
I would use the regular birch shelving. Baltic birch is nice stuff to
work with and probably stronger, but I think the poplar cored 5 or 7
ply is more than up to the task. And I wouldn't worry much about the
shelf pin problem either. Drill the holes for a good, snug fit. If
you're like most once the shelf is assembled and loaded with books you
probably won't adjust the shelves again. I've come to believe that
adjustable shelves are overrated, and the last few bookcases I made
(and one of them was 9 ft tall & 4 feet wide) I used fixed shelves.
No dumb questions, just dumb answers.
Larry Wasserman - Baltimore, Maryland - firstname.lastname@example.org
Don't confuse initial deflection with sag over time.
I have a whole wall of bookcases with 3/4 oak plywood shelves. They
didn't deflect much when I first loaded 'em up; barely visible, I
would say. Over time they eventually sag about 1/2 inch (they are
30" wide) at which point it bothers me enough to take the books off
and flip them over (time to dust them anyway!). It probably takes 6
months or so to sag that far.
The shelves with paperbacks don't sag at all; but the ones with
hardbacks all do eventually.
I'd add a stiffener along the edge if I could afford to give up the
extra space, but I don't have enough shelf space as it is.
Forget the science, Baltic Birch is stronger than standard Birch ply. If
this is a really heavy duty shelving unit, BB would be better.
What are you planning on storing on this unit that qualifies it as heavy
I don't have a clue. What I do know is that the design and construction have
much more to do with performance than pushing the pencil or slide rule with
all the stuff you are asking about. Rather than do all of that math, add
stiffness with construction. I couple of simple support under the shelf, or
on the front edge will do more than adding thickness of shelving. Just take
a look at bridge design.
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