The finish carpenter and myself are in conflict about the strength of plywood. We need to cantilever 5/8 inch CDX plywood over the end of a pony wall (34″ tall) for our counter top. My understanding is that the grain of the plywood should run perpendicular to the wall direction for maximum strength. The finish carpenter says it doesn’t matter as there are 4 plies to the plywood and therefore there is no strength bias in either direction. I think the inner plies are of a “higher grade” ie. less knots and therefore have more strength than the outer plies. What do you all think?
Foxtrotkilo
Replies
5/8" CDX for a countertop? The C stands for the grade of the face veneer, the D is for the backside veneer and X is exterior glue. The inner plys are likely less than D with voids. Use a different material for this job.
Beat it to fit / Paint it to match
Sorry, my mistake. The plywood is for subtop to be under granite. Foxtrotkilo
Perpendicular is strongest in this application...
How much cantilever...
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming
WOW!!! What a Ride!
8 inches. The plywood is a subtop for granite. FK
FK...
Why worry about the strenght... 8" needs some sortta support...
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming
WOW!!! What a Ride!
We've been using 3/8" x 5" steel plates, spaced every couple of feet. Screwed w/ 3" #12s, & some PL Premium. Very lo profile, way strong. Don't worry, we can fix that later!
Right<G>, like this: Don't worry, we can fix that later!
I can't see the steel plates in that picture ;)
jt8
Ha, I told you they're low profile<G>...here's the view from Australia :-) Don't worry, we can fix that later!
But you should have used 3/4 AC Fir in a 7 or 9 ply...
now rip it out and do it right..
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming
WOW!!! What a Ride!
If it's regular plywood with all plies about equal in thickness then it's a good bit stronger with the grain running perpendicular to the supporting edge, that is, the grain is sticking out in the direction of the cantilever.
If it's veneer plywood with only very thin plies on the surface, then it's stronger with the next underlying plies running perpendicular to the supporting edge.
Fox, it's significantly stronger. Don't ever take structural advice from a trimmer...most don't even own a framing square!
blue
PS No attacks please...I'm only jiving....I'll accept good natured jabs at my trimming ability however.
Warning! Be cautious when taking any framing advice from me. Although I have a lifetime of framing experience, all of it is considered bottom of the barrel by Gabe. I am not to be counted amongst the worst of the worst. If you want real framing information...don't listen to me..just ask Gabe!
It has less to do with the number of plies in either direction and a lot to do with where those plies are within the plywood. Think of it like an I-beam.
Plies on the interior of the I-beam have less mechanical advantage. Mostly they only have to handle shear forces associated with keeping the top and bottom surfaces connected so they act as a unit.
Those closest to the surface, top and bottom, carry the weight. Grain running the direction of the span handles this stress much more effectively.
Thanks for the excellent explanation. I knew the outer plies determined the strength but now I can print your explanation out and mysteriously leave it where the finish carpenter can read it.
Thanks again, Foxtrotkilo- this forum is great!!
I have an opinion on the original question, but no facts to back them up so I'll stay quiet on that however,
You said:
>> Think of it like an I-beam.
Plies on the interior of the I-beam have less mechanical advantage. Mostly they only have to handle shear forces associated with keeping the top and bottom surfaces connected so they act as a unit. <<
Not sure if I can go along with that analogy. I thought an I beam got it's rigidity from the web, and the top and bottom flange were just there to stabilize the web. Here is an idea: think of a flitch beam... where does it get it's strength?
And just to clarify - IMO, this stuff has nothing to do with plywood... Matt
> I thought an I beam got it's rigidity from the web, and the top and bottom flange were just there to stabilize the web.
No, the web is just there to hold the top and bottom flanges in place. The top is in compression and the bottom is in tension. It's the extreme top and bottom that contribute the most to section modulus. Note that the flanges are generally a bit heftier than the web.
-- J.S.
When a beam is loaded in bending, the forces near the middle of the span are tension in the bottom surface and compression in the upper surface. The web carries essentially no load except to hold the upper and lower flanges in position. That's why webs in steel beams can have lots of holes in them near the middle of the span. Usually holes are not made because they cost more in labor to cut than they save in material, but if weight were a factor, such as in an airplane, holes are made in the web.
Near the ends of the beam, or in a really short beam, there is nearly no bending force, but rather vertical shear. Sheer forces are carried by the web and the flanges do almost nothing. In fact, in commercial steel construction you'll notice that the flanges of steel beams are not even attached to columns. Only the webs are.
A flitch beam is made from a flat plate only for convenience. It could be done with less steel if it were made in the shape of an I-beam, but it would be an awkward, impractical shape.
OK - most of what you and JS said makes sense.
Riddle me this though... How come with I joists, dimentional lumber, and floor trusses, if you want a stiffer floor, you go to a taller member ie: a taller web? BTW - can you tell me the weight of a piece of steel that 1/2" x 9" x 14'? This is not just a quiz - it's for a real application. I had a little steel book around here somewhere, but havn't seen it in several years. Matt
Thats heavy stuff! I worked a steel supply shop for three years right out of high school. Most of our "fish plates" (flitch plates?) were 1/4 x 9's x 17' for garage headers. Occasionally we'd have one of those 1/2 special order deals....not fun!
I don't know the weigths per foot any more...try US steel or weights and measures .com.
Aww..just wait..someone in here will know it, they always do!
blueWarning! Be cautious when taking any framing advice from me. Although I have a lifetime of framing experience, all of it is considered bottom of the barrel by Gabe. I am not to be counted amongst the worst of the worst. If you want real framing information...don't listen to me..just ask Gabe!
I found a steel weight calculator:
http://www.mesteel.com/cgi-bin/w3-msql/goto.htm?url=/info/carbon/convert/metalcalculator/metalcalculator1.htm
Matt
Edited 11/17/2004 8:06 am ET by DIRISHINME
214 pounds. A taller member is stiffer. You could increase the stiffness with bigger flanges but it's more efficient to make it taller. But you do lose headroom so sometimes it's a tradeoff.
The further apart the flanges are from one another, the stiffer the beam.
Steel has a density of 490 pounds per cubic foot. Ben did the math for you.
> Riddle me this though... How come with I joists, dimentional lumber, and floor trusses, if you want a stiffer floor, you go to a taller member ie: a taller web?
Because section modulus is proportional to the square of the height.
-- J.S.
My response to the original Q. This might have been mentioned. 8" isn't all that much for the granite to stick out. Assuming a 2x4 wall, 1 1/2 overhang on the other side, 8 inches is very little. What does the granite guy say? I'd be incined to use other supports. How is the ply planned to be finished? A job with granite tops certainly needs a nice finished product under the tops.
My advice is to skip the ply and use a decorative angled support.
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