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

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

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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.
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Leon wrote:

But that won't tell what the deflection will be down the road 10 years...
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That is true but I would be willing to believe that 80+% will show immediately.
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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.
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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.
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Hmmm then you wait a year across the spectrum of atmospheric conditions....

work!
to
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You have a point but I wonder if the manufacturer will also consider heat radiation from a TV, Weight of a particular TV.
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Not to mention the re-runs of Everybody Loves Raymond.
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intrepid snipped-for-privacy@hotmail.com wrote:

I think that unless the shelf is somehow supported in the front you're going to have a problem.
I'm looking across the room at an entertainment center which we purchased to house the stereo and a 35" tube type television. It's 3/4" oak veneer plywood with solid oak face framing.
No sag in ~ 7 years BUT the width of the shelf is bisected by a vertical support which forms one side of a cabinet beneath the television. The other side (below the TV) is an open area with a shelf where the center channel speaker is stashed. The whole thing, including the "shelf" the television is sitting on is framed in 3/4 oak. There is no backing behind the TV.
The reason I fear for your success as described above is the apparent lack of support at the front of your 22" deep shelf. The weight of a CRT Television is concentrated to the front. I suspect that left to its own devices, your television may well wind up face down on the floor in front of that cabinet.
Also, minor point, but any reason you're only looking at 1/4" dado rather than 3/8"? I always thought the "rule" was the dado is one half the thickness in 3/4 or 4/4 stock.

Probably correct but by supporting it properly both front AND rear (with maybe a center piece) you should be okay no matter what.
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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
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mike hide 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).
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Duane Bozarth wrote:

...
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...
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Duane Bozarth wrote:

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.
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it
Do you mean 4/7ths? Plywood has a strong and a weak direction. It is good to keep this in mind.
-j
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why split hairs.... If you take my suggestions you are playing it safe.....mjh
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mike hide wrote:

But overdesign has a cost as well...
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