Strength of 3/4" oak-veneer plywood...
 

All:

I'm trying to become better educated about the relative strengths of
different types of plywood, and right now I'm dealing with a 3/4" block
of seven-layer oak-veneer plywood.

I'm wondering what would you folks would consider to be the maximum
reasonable span that type of plywood should be expected to sustain for
a shelf holding a 27" tube TV? I'm presently planning a 36" span 22"
deep, secured to the sides of a cabinet via 1/4" dadoes in a solid oak
frame. The cabinet will be backed by 1/4" plywood, glued and screwed to
the shelf from the backside.

I've read some posts that say this type of plywood would be more than
adequate for even larger TV's, but a few others that say it will sag,
but none of the discussions I've seen have talked about how that sag
tendency varies with the number of layers in the plywood, so I have to
plead uncertainty.

Thanks for your kind help in advance.

David

wrote in message
oups.com...
All:

I'm trying to become better educated about the relative strengths of
different types of plywood, and right now I'm dealing with a 3/4" block
of seven-layer oak-veneer plywood.

I'm wondering what would you folks would consider to be the maximum
reasonable span that type of plywood should be expected to sustain for
a shelf holding a 27" tube TV? I'm presently planning a 36" span 22"
deep, secured to the sides of a cabinet via 1/4" dadoes in a solid oak
frame. The cabinet will be backed by 1/4" plywood, glued and screwed to
the shelf from the backside.

I've read some posts that say this type of plywood would be more than
adequate for even larger TV's, but a few others that say it will sag,
but none of the discussions I've seen have talked about how that sag
tendency varies with the number of layers in the plywood, so I have to
plead uncertainty.

Unless you reinforce front and back it is going to sag. I suggest double
thickness with a solid wood trim on the front and back edges.

David wrote: I'm trying to become better educated about the relative
strengths of
different types of plywood, and right now I'm dealing with a 3/4" block

of seven-layer oak-veneer plywood.
I'm wondering what would you folks would consider to be the maximum
reasonable span that type of plywood should be expected to sustain for
a shelf holding a 27" tube TV? I'm presently planning a 36" span 22"
deep, secured to the sides of a cabinet via 1/4" dadoes in a solid oak
frame. The cabinet will be backed by 1/4" plywood, glued and screwed to

the shelf from the backside.
I've read some posts that say this type of plywood would be more than
adequate for even larger TV's, but a few others that say it will sag,
but none of the discussions I've seen have talked about how that sag
tendency varies with the number of layers in the plywood, so I have to
plead uncertainty.
Thanks for your kind help in advance.


http://www.woodbin.com/calcs/sagulator.htm Tom

If your looking for education.... contact the plywoods manufacturer.... They
are the only ones who can supply you with specifications.. no guess work!




wrote in message
oups.com...
All:

I'm trying to become better educated about the relative strengths of
different types of plywood, and right now I'm dealing with a 3/4" block
of seven-layer oak-veneer plywood.

I'm wondering what would you folks would consider to be the maximum
reasonable span that type of plywood should be expected to sustain for
a shelf holding a 27" tube TV? I'm presently planning a 36" span 22"
deep, secured to the sides of a cabinet via 1/4" dadoes in a solid oak
frame. The cabinet will be backed by 1/4" plywood, glued and screwed to
the shelf from the backside.

I've read some posts that say this type of plywood would be more than
adequate for even larger TV's, but a few others that say it will sag,
but none of the discussions I've seen have talked about how that sag
tendency varies with the number of layers in the plywood, so I have to
plead uncertainty.

Thanks for your kind help in advance.

David

"HMFIC-1369" wrote in message
news:fiaze.6523$Gv3.3103@trnddc02...
If your looking for education.... contact the plywoods manufacturer....
They
are the only ones who can supply you with specifications.. no guess work!


Actually you can cut the piece of plywood to the size that you are going to
use, place the TV that you are going to use on top of the plywood elevated
on the ends, and measure the deflection.

Leon wrote:

"HMFIC-1369" wrote in message
news:fiaze.6523$Gv3.3103@trnddc02...
If your looking for education.... contact the plywoods manufacturer....
They
are the only ones who can supply you with specifications.. no guess work!

Actually you can cut the piece of plywood to the size that you are going to
use, place the TV that you are going to use on top of the plywood elevated
on the ends, and measure the deflection.

But that won't tell what the deflection will be down the road 10
years...

Hmmm then you wait a year across the spectrum of atmospheric conditions....


"Leon" wrote in message
. ..

"HMFIC-1369" wrote in message
news:fiaze.6523$Gv3.3103@trnddc02...
If your looking for education.... contact the plywoods manufacturer....
They
are the only ones who can supply you with specifications.. no guess
work!


Actually you can cut the piece of plywood to the size that you are going
to
use, place the TV that you are going to use on top of the plywood elevated
on the ends, and measure the deflection.

"HMFIC-1369" wrote in message
news:WFaze.6536$Gv3.3851@trnddc02...
Hmmm then you wait a year across the spectrum of atmospheric
conditions....


You have a point but I wonder if the manufacturer will also consider heat
radiation from a TV, Weight of a particular TV.

"Duane Bozarth" wrote in message
...


But that won't tell what the deflection will be down the road 10
years...


That is true but I would be willing to believe that 80+% will show
immediately.

wrote in message
oups.com...
All:

I'm trying to become better educated about the relative strengths of
different types of plywood, and right now I'm dealing with a 3/4" block
of seven-layer oak-veneer plywood.

I'm wondering what would you folks would consider to be the maximum
reasonable span that type of plywood should be expected to sustain for
a shelf holding a 27" tube TV? I'm presently planning a 36" span 22"
deep, secured to the sides of a cabinet via 1/4" dadoes in a solid oak
frame. The cabinet will be backed by 1/4" plywood, glued and screwed to
the shelf from the backside.

I've read some posts that say this type of plywood would be more than
adequate for even larger TV's, but a few others that say it will sag,
but none of the discussions I've seen have talked about how that sag
tendency varies with the number of layers in the plywood, so I have to
plead uncertainty.

Thanks for your kind help in advance.

David

Basically only the strata that are in one direction are actually
contributing to the bending strength. So in terms of solid wood consider it
as a sheet of wood half the thicness of the plywood . In practice support
stringers of solid wood are always a good the deeper the better....mjh

Leon wrote:

"Duane Bozarth" wrote in message
...


But that won't tell what the deflection will be down the road 10
years...

That is true but I would be willing to believe that 80+% will show
immediately.

Perhaps...lacking data but recalling experience I suspect how much would
be species and type of construction (of the ply) would play a part.
Obviously if it sags a half-inch under immediate load it ain't
enough...a smaller deflection that might be judged acceptable initially
could easily end up w/ quite a bit more over time I believe.

mike hide wrote:

wrote in message
oups.com...
All:

I'm trying to become better educated about the relative strengths of
different types of plywood, and right now I'm dealing with a 3/4" block
of seven-layer oak-veneer plywood.

I'm wondering what would you folks would consider to be the maximum
reasonable span that type of plywood should be expected to sustain for
a shelf holding a 27" tube TV? I'm presently planning a 36" span 22"
deep, secured to the sides of a cabinet via 1/4" dadoes in a solid oak
frame. The cabinet will be backed by 1/4" plywood, glued and screwed to
the shelf from the backside.

I've read some posts that say this type of plywood would be more than
adequate for even larger TV's, but a few others that say it will sag,
but none of the discussions I've seen have talked about how that sag
tendency varies with the number of layers in the plywood, so I have to
plead uncertainty.

Thanks for your kind help in advance.

David

Basically only the strata that are in one direction are actually
contributing to the bending strength. So in terms of solid wood consider it
as a sheet of wood half the thicness of the plywood . In practice support
stringers of solid wood are always a good the deeper the better....mjh

That's not actually quite right--the glued laminations add significantly
more resistance to bending owing to the glue than if they were not
present entirely (as your model suggests).

"mike hide" wrote in message
...
Basically only the strata that are in one direction are actually
contributing to the bending strength. So in terms of solid wood consider
it
as a sheet of wood half the thicness of the plywood .

Do you mean 4/7ths?
Plywood has a strong and a weak direction.
It is good to keep this in mind.

-j

In article WFaze.6536$Gv3.3851@trnddc02,
"HMFIC-1369" wrote:

Hmmm then you wait a year across the spectrum of atmospheric conditions....


Not to mention the re-runs of Everybody Loves Raymond.

I made a cheapo TV entertainment center, with pine ply. Only want to see
how it holds up thru the years. Also, it would serve me a better idea to
what I really want out of it when I make the next one, with RED OAK. I
have a Sharp 27". The plywood (that the TV is sitting on) is measured as
21"x37"

What I did was use biscuits on the sides (probably three each 2 sides).
The back was straight glued to the back. The front has a 1"x1" (pine).

The real reason I use 1"x1" in the front was to cover the front of the
plywood (those layer lines), but also see if it will support the TV as
well. You are asking the exact same thing on what I did. I made that
thing about 5 years ago and upon reading your remarks, I took a good look
at my thing now, it has NOT sag a bit!

Regarding how I did with the 1"x1" (it is actually 1-5/16", so it may be
a 1-1/2" x 1-1/2"?), I cut 1/2" off (top view), leaving 7/8" for the
front, and 1/2" off (back view), leaving 5/8" for the bottom support (but
really it's the "whole" "L" piece that supports the front). I hope you
understand my English.

The pine plywood was from Home Depot. The next red oak ply will be from
Alpine Plywood. But I might end up getting the red oak ply from HD if the
Alpine Ply is *way* overpriced. The ply will only serve as shelfing, the
rest (largely viewable) will be solid red oak (raised panels).

(Sorry to run on) I am also thinking about using 1/2" ply for the sides
and back only to help with the weight. But a WW professor at my UWM said
it will be same thing, either (with 1/2" or 3/4" ply) both will end up
heavy! He suggests to stick with 3/4", but I don't know yet.

My comment about you plan using dadoes, I would refrain from do that
because it could lead to breakage when moving it around? But thats me
being a "biscuit guy" (even I do use dadoes rarely).

So, to answer your question, YES, it will support it fine!

In fact, I am planning to make the new one this summer (which is why I
was asking about raised panels issues :P ), I am looking to do 36"x24",
to play it "safe" in case I get a 32" TV. I don't want a larger tube TV.
I was also thinking about "what if" I get a HDTV (or whatever is out
there today), it will have to have something different (they are so wide,
like 48").

Chuck

I'm wondering what would you folks would consider to be the maximum
reasonable span that type of plywood should be expected to sustain for
a shelf holding a 27" tube TV? I'm presently planning a 36" span 22"
deep, secured to the sides of a cabinet via 1/4" dadoes in a solid oak
frame. The cabinet will be backed by 1/4" plywood, glued and screwed to
the shelf from the backside.

"J" wrote in message
...
"mike hide" wrote in message
...
Basically only the strata that are in one direction are actually
contributing to the bending strength. So in terms of solid wood consider
it
as a sheet of wood half the thickness of the plywood .

Do you mean 4/7ths?
Plywood has a strong and a weak direction.
It is good to keep this in mind.

-j

why split hairs.... If you take my suggestions you are playing it
safe.....mjh

mike hide wrote:

"J" wrote in message
...
"mike hide" wrote in message
...
Basically only the strata that are in one direction are actually
contributing to the bending strength. So in terms of solid wood consider
it
as a sheet of wood half the thickness of the plywood .

Do you mean 4/7ths?
Plywood has a strong and a weak direction.
It is good to keep this in mind.

-j

why split hairs.... If you take my suggestions you are playing it
safe.....mjh

But overdesign has a cost as well...

You can try the "sagulator" - a calculator for determining wood
deflection. For your dimensions (36x22), red oak sagged .04". They
didn't have oak plywood. Maybe someone can tell us how oak plywood
compares to solid oak strength-wise.

Here's the URL for the calculator:
http://www.woodbin.com/calcs/sagulator.htm

....bob

"mike hide" wrote in message
...

"J" wrote in message
...
"mike hide" wrote in message
...
Basically only the strata that are in one direction are actually
contributing to the bending strength. So in terms of solid wood
consider
it
as a sheet of wood half the thickness of the plywood .

Do you mean 4/7ths?
Plywood has a strong and a weak direction.
It is good to keep this in mind.

-j

why split hairs.... If you take my suggestions you are playing it
safe.....mjh

Perhaps a lot more safe than you imagine.
Bending strength is proportional to the square of the thickness.
Deflection is proportional to the cube of the thickness.

Why not just consider it to be the same as "regular" wood at least in the
strong direction? That would be a lot more accurate.

BobN wrote:

You can try the "sagulator" - a calculator for determining wood
deflection. For your dimensions (36x22), red oak sagged .04". They
didn't have oak plywood. Maybe someone can tell us how oak plywood
compares to solid oak strength-wise.

Well, here's all you would ever want to know... :)

http://www.fpl.fs.fed.us/documnts/fplrn/fplrn059a.pdf

It shows that basically there's only about a 10-20% difference for same
species as compared to ply--somewhat less than I'd have thought. I
skimmed the (somewhat lengthy) report and if I got the gist right, it
concluded for there testing the difference was within the statistical
uncertainty. (Longitudinal to face sheet comparison)

I also found to my surprise that my impression that the laminating had a
significant effect on the transverse bending moment was also mostly in
error--there's an effect, but it's not nearly as large as I had
expected. I suppose my perceptions were flawed owing to the fact that a
4x8 sheet is only half the width in the transverse direction so the
impression apparent strength in that direction is amplified
artificially.

Duane Bozarth wrote:

mike hide wrote:
....
Basically only the strata that are in one direction are actually
contributing to the bending strength. So in terms of solid wood consider it
as a sheet of wood half the thicness of the plywood . In practice support
stringers of solid wood are always a good the deeper the better....mjh

That's not actually quite right--the glued laminations add significantly
more resistance to bending owing to the glue than if they were not
present entirely (as your model suggests).

Actually, I found that

http://www.fpl.fs.fed.us/documnts/fplrn/fplrn059a.pdf

shows you're a lot more nearly right than I thought, Mike...

Their test data shows neglecting the transverse plies gets pretty close
to the right answer than I thought--although it's a long report and I
haven't had time to read it thoroughly, there's one apparent
inconsistency I don't quite see how follows because it also seems to say
is essentially same as solid lumber of same dimension but seemed to
refer to nominal dimension unless I somehow missed the nomenclature...

"Duane Bozarth" wrote in message
...
Leon wrote:

Perhaps...lacking data but recalling experience I suspect how much would
be species and type of construction (of the ply) would play a part.
Obviously if it sags a half-inch under immediate load it ain't
enough...a smaller deflection that might be judged acceptable initially
could easily end up w/ quite a bit more over time I believe.

IMHO a far better method would be to double up on the plywood and reinforce
with 3/4" solid wood around the perimeter. About 10 years ago I built an
Oak veneer plywood cabinet double thickness at 1.5" thick total with 3/4"
trim around the perimeter. Setting on top is an old Mitsubishi 35" tube
type TV. Weight approximately 200#'s. No sag at all.

"BobN" wrote in message
oups.com...
You can try the "sagulator" - a calculator for determining wood
deflection. For your dimensions (36x22), red oak sagged .04". They
didn't have oak plywood. Maybe someone can tell us how oak plywood
compares to solid oak strength-wise.

Here's the URL for the calculator:
http://www.woodbin.com/calcs/sagulator.htm

...bob

IIRC that is for solid wood, not plywood.

Leon wrote:

"BobN" wrote in message
oups.com...
You can try the "sagulator" - a calculator for determining wood
deflection. For your dimensions (36x22), red oak sagged .04". They
didn't have oak plywood. Maybe someone can tell us how oak plywood
compares to solid oak strength-wise.

Here's the URL for the calculator:
http://www.woodbin.com/calcs/sagulator.htm

...bob

IIRC that is for solid wood, not plywood.

Correct, but follow the link I posted--their testing indicated on the
order of a 20% or so discrepancy from solid material on a quick
perusal...

"Duane Bozarth" wrote in message
...
Leon wrote:

"BobN" wrote in message
oups.com...
You can try the "sagulator" - a calculator for determining wood
deflection. For your dimensions (36x22), red oak sagged .04". They
didn't have oak plywood. Maybe someone can tell us how oak plywood
compares to solid oak strength-wise.

Here's the URL for the calculator:
http://www.woodbin.com/calcs/sagulator.htm

...bob

IIRC that is for solid wood, not plywood.

Correct, but follow the link I posted--their testing indicated on the
order of a 20% or so discrepancy from solid material on a quick
perusal...

Duane, from an engineering point of view if you take a xsection of a piece
of plywood and assume the center ply to be the neutral axis [ the axis about
which the plywood bends ] and then consider the plywood sheet as a simple
beam then each of the plies running the same direction as the center ply
will contribute to the bending strength.per the "engineers simple beam
bending theory"
The cross plies will contribute little to the bending strength directly, but
indirectley they separate by varing degrees the contributing plies from
the neutral axis there by acting as webs and allowing the contributing plies
to act as caps . So in my view even though the separation is small some
additional bending strengthis should be added.......mjh

mike hide wrote:


Basically only the strata that are in one direction are actually
contributing to the bending strength. So in terms of solid wood consider it
as a sheet of wood half the thicness of the plywood . In practice support
stringers of solid wood are always a good the deeper the better....mjh


Duane Bozarth wrote:
That's not actually quite right--the glued laminations add significantly
more resistance to bending owing to the glue than if they were not
present entirely (as your model suggests).


moreso than that. even if the cross plies were made of air they would
increase the stiffness of the panel- for the same reason a torsion box
or hollow core door is empty inside.

All:

I just wanted to take a moment to thank ALL of you who provided such
good information and opinions on my project. Your advice was very
helpful.

I modified my cabinet design to include a 3/8" dado for the TV shelf
rather than the 1/4" I had originally intended. I've also added a
vertical cross-section of plywood beneath the rear edge of the shelf.

I further added a vertical shelf support that subdivides the formerly
open large area into two equally sized compartments. The vertical
support should transfer most of the sag to the cabinet floor, and I'll
also add an additional member under the shelf to transfer that load
directly to the floor.

The cabinet went together better than I expected, and I'm down to a few
niggling oops-shouldn't-have-done-that-so-I-hafta-fix-it items, then I
can trim it out and prepare for finish. Inevtiably, I suppose, it's
tending to twist a bit at the top, but I suspect that once it's in
place that won't be as prevalent a problem (I hope). Suggestions for
that welcome, too :)

Thanks again, all.

David


wrote:
All:

I'm trying to become better educated about the relative strengths of
different types of plywood, and right now I'm dealing with a 3/4" block
of seven-layer oak-veneer plywood.

I'm wondering what would you folks would consider to be the maximum
reasonable span that type of plywood should be expected to sustain for
a shelf holding a 27" tube TV? I'm presently planning a 36" span 22"
deep, secured to the sides of a cabinet via 1/4" dadoes in a solid oak
frame. The cabinet will be backed by 1/4" plywood, glued and screwed to
the shelf from the backside.

I've read some posts that say this type of plywood would be more than
adequate for even larger TV's, but a few others that say it will sag,
but none of the discussions I've seen have talked about how that sag
tendency varies with the number of layers in the plywood, so I have to
plead uncertainty.

Thanks for your kind help in advance.

David

Plywood was born to bow and twist. The secret is to screw,
nail or glue it down before it gets started.


wrote:


The cabinet went together better than I expected, and I'm down to a few
niggling oops-shouldn't-have-done-that-so-I-hafta-fix-it items, then I
can trim it out and prepare for finish. Inevtiably, I suppose, it's
tending to twist a bit at the top, but I suspect that once it's in
place that won't be as prevalent a problem (I hope). Suggestions for
that welcome, too :)