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Crushing of Plywood -- Really?

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.

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

RE: Crushing of Plywood -- Really?

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.

RE: Crushing of Plywood -- Really?

(OP)
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.

RE: Crushing of Plywood -- Really?

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

DaveAtkins

RE: Crushing of Plywood -- Really?

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.

RE: Crushing of Plywood -- Really?

(OP)
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.

RE: Crushing of Plywood -- Really?

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.

RE: Crushing of Plywood -- Really?

(OP)
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.

RE: Crushing of Plywood -- Really?

(OP)
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.

RE: Crushing of Plywood -- Really?

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.

RE: Crushing of Plywood -- Really?

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.

RE: Crushing of Plywood -- Really?

(OP)
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.

RE: Crushing of Plywood -- Really?

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

RE: Crushing of Plywood -- Really?

(OP)
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.

RE: Crushing of Plywood -- Really?

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

Maine EIT, Civil/Structural.

RE: Crushing of Plywood -- Really?

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.

RE: Crushing of Plywood -- Really?

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

RE: Crushing of Plywood -- Really?

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.

RE: Crushing of Plywood -- Really?

(OP)
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.

RE: Crushing of Plywood -- Really?

Kootk,

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

DaveAtkins

RE: Crushing of Plywood -- Really?

Lol. And a painful sigh.... But nice joke Dave. You and Mike should hang out. *sigh*

RE: Crushing of Plywood -- Really?

I'd be a little careful with video. Last thing you want is it to be used as proof against you that you were "aware" of deficient conditions because you could have seen it in one-second frame of a video across the way. Not sure if there is actual exposure to liability because of it, but that is how it has always been explained to me.

RE: Crushing of Plywood -- Really?

That sounds a little extreme to me, otherwise anyone could argue that you should have seen something because you had a 60th of second to see it in real life as well.

Maine EIT, Civil/Structural.

RE: Crushing of Plywood -- Really?

(OP)
@JD: thanks for tip. That's an excellent point. It's soooo hard not to resent what legal bullshit has reduced us to. I'm going to keep taking video but be discrete about it and store it on my personal Dropbox account.

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?

Attached is how the bearing allowable for wood structural panels are done.

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?

(OP)
I was waiting for you to make an appearance Woodman. Thanks for your contribution. Sooo... would you every let plywood crushing slide?

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?

It is 1.15.

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?

According to my PDS Supplements, plywood has an allowable face bearing varying between 105 and 360 psi, much less in general,than even a Hem-Fir sill plate.

Mike McCann, PE, SE (WA)


RE: Crushing of Plywood -- Really?

KootK:
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?

(OP)
Thanks Dhengr. I've had similar issues with brick on the outside of wood buildings and plumbing on the inside. Do framers balk at the sheathing blackouts? We've been debating that strategy.

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

RE: Crushing of Plywood -- Really?

Shim squeezing...finite....not issue.

RE: Crushing of Plywood -- Really?

The only time I have ever concerned about perp compression crushing (truss bearing aside) is during building shoring. You want the post shores to be tight to the underside of the structures equally everywhere, so that the load goes where you have intended it to, without redistribution due to excessive stiffness loss of the shoring assemblies (Kpost + Kblocking + Kstructure below. An average perp compression stress is around 400 psi and I would think that the plywood would be in the same ballpark, as I doubt the glue provides much if any resistance. As other have suggested, the displacement you are comfortable with is probably more important.

RE: Crushing of Plywood -- Really?

This was an interesting thread. I drove 6000 timber piles a few years ago and the plywood shims were indeed smoking when pulled out - that was one sign other than the noise the hammer made that it was time to change them. I remember looking at them but they were too hot to touch. I did not take pictures. The are confined which probably changes their response to being pounded. Again, thanks for the entertainment and education.

RE: Crushing of Plywood -- Really?

KootK
"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?

(OP)
Thanks Woodman. It's good to have at least one dissenting opinion -- particularly yours.

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

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