Crushing of Plywood -- Really?
Crushing of Plywood -- Really?
(OP)
A colleague is looking at a wood building set out by an architect without much wood experience. As a result, spans are long and columns are very heavily loaded (for wood). In many instances, the governing mode of failure is bearing failure of plywood per Canada's wood design code. We're having to detail sheathing block-outs to achieve better bearing scenarios.
Our question is this: is plywood crushing really a failure mode worthy of consideration?
If we were talking about crushing of a beam, or even a sill plate, I could see cause for concern. The crushing will result in deformations that would be unacceptable. For 3/4" plywood however, I have a hard time envisioning anything that feels like a serious consequence.
Thoughts? I'm trying to be practical here. Anyone whose read my threads in the past will know that's a stretch for me.
Our question is this: is plywood crushing really a failure mode worthy of consideration?
If we were talking about crushing of a beam, or even a sill plate, I could see cause for concern. The crushing will result in deformations that would be unacceptable. For 3/4" plywood however, I have a hard time envisioning anything that feels like a serious consequence.
Thoughts? I'm trying to be practical here. Anyone whose read my threads in the past will know that's a stretch for me.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.






RE: Crushing of Plywood -- Really?
RE: Crushing of Plywood -- Really?
-10"x10" wood column.
-2x ill plate.
-3/4" plywood.
-10" wide wood beam.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Crushing of Plywood -- Really?
DaveAtkins
RE: Crushing of Plywood -- Really?
I've never paid a whole lot of attention to crushing failure of stuff like that, I liken it to shims for steel. My thought is how much can they really crush before there is no air left to crush out? likely that amount is within an acceptable range.
RE: Crushing of Plywood -- Really?
Back to the issue dujour: what bad things come of plywood crushing? Will a 3/4" thing get crushed down to 1/4"? Will a fragment of oriented strand burst though the carpet and blind a child? Will the multifamily unit tip over like those Kyoto buildings post-EQ? Or will a jury of my peers simply find my judgement to be lacking?
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Crushing of Plywood -- Really?
Advantech may be a different story.
I rarely ever consider it, however, and just check the plates.
RE: Crushing of Plywood -- Really?
1) Plywood bearing = 4.28 MPa taking into account load spreading factor etc.
2) #1/#2 SPF sill plate bearing = 4.88 MPa.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Crushing of Plywood -- Really?
Once upon a tech diploma, I was a residential wood truss designer. It was a common occurrence for major girders to fail the sill plates in crushing. We'd alert the EOR and suggest steel bearing plates or running the posts through. Pretty sure it never happened... ever. We conveniently designated non-truss-to-truss connections to be beyond our scope.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Crushing of Plywood -- Really?
Maine EIT, Civil/Structural.
RE: Crushing of Plywood -- Really?
"Compression perpendicular to the grain is generally not considered to be a matter of life safety. Instead, it relates to the amount of deformation that is acceptable in a structure. Currently published values of bearing perpendicular to the grain are average values which are based on a deflation limit of 0.04 inches when tested in accordance with ASTM D 143. This deformation limit has been found to provide adequate service in typical wood-frame construction."
Maine EIT, Civil/Structural.
RE: Crushing of Plywood -- Really?
For context, my colleague adds "One thing to add, no sill plate under the glulam columns (referring to the assembly you listed). Not that it matters, because plywood crushing would govern over it. Generally it’s glulam columns sitting on LVL beams/LSL blocking – which have about twice the crushing capacity of plywood."
Regardless, we seem to be coming to a consensus opinion here.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Crushing of Plywood -- Really?
Bob
RE: Crushing of Plywood -- Really?
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Crushing of Plywood -- Really?
Maine EIT, Civil/Structural.
RE: Crushing of Plywood -- Really?
In my experience, I ignore the plywood crushing. If you read the footnotes, the crushing limit is something like 5% deformation. I can deal with a 5% of 3/4" "failure".
For heavily loaded columns, cut out the sill plate. A 5.5" square PSL post will overstress and crush a bottom plate. They even sometimes build it this way.
RE: Crushing of Plywood -- Really?
Bob
RE: Crushing of Plywood -- Really?
Maine EIT, Civil/Structural.
RE: Crushing of Plywood -- Really?
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Crushing of Plywood -- Really?
Tell your friend the plywood is probably OK, but it might start smoldering
DaveAtkins
RE: Crushing of Plywood -- Really?
RE: Crushing of Plywood -- Really?
RE: Crushing of Plywood -- Really?
Maine EIT, Civil/Structural.
RE: Crushing of Plywood -- Really?
Realtime Google earth would be a nice solution to this. I'll make a separate account under KootKAccounting74FU.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Crushing of Plywood -- Really?
As you can see in it, the allowable is much lower than the average. So as is not be good engineering to use the average value. I can see why some people may try to say it is okay.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Crushing of Plywood -- Really?
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Crushing of Plywood -- Really?
TehMightyEngineer - when it comes to the number of jobsite photos, I'd up the exponent in your equation to 1.15. At least that's been my experience.
KootK - I agree, although with my older cameras I'd look like a news reporter or similar walking onto the site. Contractors get very nervous when that happens.
It's been my experience that there are never too many images recorded, and the one you really want (after you return to the office) is the one you DIDN'T take.
Ralph
Structures Consulting
Northeast USA
RE: Crushing of Plywood -- Really?
Maine EIT, Civil/Structural.
RE: Crushing of Plywood -- Really?
TehMightyEngineer - Oops, me bad. I thought I had seen 1.10 as I was typing my comment and didn't look back before posting.
Ralph
Structures Consulting
Northeast USA
RE: Crushing of Plywood -- Really?
Mike McCann, PE, SE (WA)
RE: Crushing of Plywood -- Really?
On std. stick framing (typ. stud wall framing) the crushing of good sub-flooring (plywd., etc.) is generally not a significant issue, and platform framing is such an efficient system of framing. The story settlement will generally be fairly consistent and uniform. But, the settlement, crushing and shrinkage does add up over several floors, so it can’t be ignored as relates to holddown systems and ext. wall sheathing buckling, or more rigid mechanical systems, etc. Fc⊥ and shrinkage should always be considered. With 10"x10" sq. columns and 10" timber beams (gluelam bms?), I would not interpose sub-flg. or std. framing lumber under the columns. Cut the sub-flg. and your “-2x ill plate” out around the column. [If it’s sick, cut it out] I would put the post right on top of the wood beam, likely with a stl. bearing pl. to distribute the post load to the beam to account for the beam’s Fc⊥. Another thing you want to pay attention to with large columns (large concentrated loadings) is relative differential settlement, crushing, shrinkage, btwn. these high point loading locations and the remaining std. stick framing. This same thinking can apply where wood/timber framing bears party on wood columns, but also on steel (no shrinkage or shortening) and masonry which might actually grow a little in height. It is fairly common to see the middle bearing wall being a low point, when the lower level beam bears on wooden posts, and the joists span both ways to masonry bearing walls.
RE: Crushing of Plywood -- Really?
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Crushing of Plywood -- Really?
RE: Crushing of Plywood -- Really?
RE: Crushing of Plywood -- Really?
RE: Crushing of Plywood -- Really?
"Sooo... would you ever let plywood crushing slide?"
The problem with wood/plywood/OSB crushing is that the wood never recovers from it. You can overstress the bending of wood for a short time 10 min, 7 days, a month. Which is why there is a load duration factor for this of 1.6, 1.25, 1.15.
No I would not let it slide. I would be willing to design it with every possible perk allowed by engineering.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Crushing of Plywood -- Really?
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.