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#1
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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 |
#2
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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. |
#3
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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 |
#4
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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! 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 |
#6
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"HMFIC-1369" wrote in message news:fiaze.6523$Gv3.3103@trnddc02... If your looking for education.... contact the plywoods manufacturer.... They are the only ones who can supply you with specifications.. no guess work! Actually you can cut the piece of plywood to the size that you are going to use, place the TV that you are going to use on top of the plywood elevated on the ends, and measure the deflection. |
#7
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Leon wrote:
"HMFIC-1369" wrote in message news:fiaze.6523$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... |
#8
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Hmmm then you wait a year across the spectrum of atmospheric conditions....
"Leon" wrote in message . .. "HMFIC-1369" wrote in message news:fiaze.6523$Gv3.3103@trnddc02... If your looking for education.... contact the plywoods manufacturer.... They are the only ones who can supply you with specifications.. no guess work! Actually you can cut the piece of plywood to the size that you are going to use, place the TV that you are going to use on top of the plywood elevated on the ends, and measure the deflection. |
#9
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"HMFIC-1369" wrote in message news:WFaze.6536$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. |
#10
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"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. |
#11
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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 |
#12
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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. |
#13
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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). |
#14
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"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 |
#15
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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. |
#16
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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. |
#17
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"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 |
#18
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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... |
#19
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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 |
#20
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"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. |
#21
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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. |
#22
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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... |
#23
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"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. |
#24
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"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. |
#25
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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... |
#26
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"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 |
#27
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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. |
#28
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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 |
#29
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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 |
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