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.
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.
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.
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.
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
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.
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).
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").
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.
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.
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