Crushing of Plywood -- Really? 1

[KootK]

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.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[CBSE]

Can you provide any sort of drawing to show just what you are talking about? I'm not sure I follow what is crushing the plywood.

 

[KootK]

Just imagine this assembly, from top to bottom:

-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.

 

[DaveAtkins]

Won't the sill plate crush before the plywood? The load can spread laterally through the sill plate.

DaveAtkins

 

[jayrod12]

My thoughts too. The sill plate doesn't stand a chance in comparison to the plywood.

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.

 

[KootK]

Like I said, it's a friends project. A friend who doesn't have an Eng-Tips account. She tells me that the other stuff works and I'm taking that at face value. She's using engineered lumber for the sill plates. Apparently its crushing capacity is actually better than the plywood's. Plywood's capacity is surprisingly low.

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.

 

[XR250]

I am pretty sure the compressive strength of the standard plywood is less than that of a spruce or fir sill plate.
Advantech may be a different story.
I rarely ever consider it, however, and just check the plates.

 

[KootK]

Yeah, I've never worried about it before either. Unconfirmed values provided by my colleague:

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.

 

[KootK]

Related anecdote:

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.

 

[TehMightyEngineer]

I can find the reference for you if you like but if I recall correctly the bearing "failure" is something like a 1/16" crush of the wood fibers. I've honestly considered it more of a serviceability failure than anything else unless the member being crushed is also taking flexure or some other stress which crushed fibers could reduce it's resistance to.

Maine EIT, Civil/Structural.

 

[TehMightyEngineer]

Ahh found the reference in Breyer's book for wood design per the NDS:

"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.

 

[KootK]

Nice. Thanks for digging that up for me TME.

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.

 

[BUGGAR]

We use plywood cushions in pile hammers all the time. Sometimes built up in layers to several inches thick. The purpose is, of course, to cushion the immediate hammer impact. At the end of a drive, the pile cushion frequently smolders and burns from the energy input. The plywood is flattened to a percent of its original thickness (I never measured how much) but it stills holds together while it is tossed from the hammer. It is somewhat confined laterally within the pile hammer anvil.

Bob

 

[KootK]

Now that's some solid anecdotal evidence. Thanks Buggar.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[TehMightyEngineer]

I'd love to see a picture of the plywood following driving if you have one BUGGAR.

Maine EIT, Civil/Structural.

 

[manstrom]

Late to the party. Technically plywood / osb has a lower Fc perp than sill plates. A Southern Pine sill plate has a Fc perp of 565 psi. This often controls post and stud design. Plywood has something like 350 psi for Fc perp. Designing around the plywood limits stud and post capacity greatly.

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.

 

[BUGGAR]

I wish I had photo'd the smoldering plywood "pills" but they were just pieces if scrap to be discarded. All eyes were on the piles to observe splitting at the top (even Gerwick got involved at one time). This was the "new" Dumbarton bridge. The pile cushions went into the bay and were later retrieved and trashed. Who'd a thunk that what was then scrap is now a subject of interest.

Bob

 

[TehMightyEngineer]

I'm convinced that the useful number of photographs taken on a job-site will always follow the formula: required pictures = n^1.15 (where n is the number of pictures you actually took)

Maine EIT, Civil/Structural.

 

[KootK]

Nice. As of late, I've been taking 20-30 seconds of high resolution video the minute that I step on site. Same thing if I'm looking at a specific issue. It costs nothing and I find that the video will often capture something that I wish I'd taken a picture of. I'll be a happy camper when Google earth goes real time.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[DaveAtkins]

Kootk,

Tell your friend the plywood is probably OK, but it might start smoldering

DaveAtkins