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.

 

[CELinOttawa]

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

 

[jdgengineer]

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.

 

[TehMightyEngineer]

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.

 

[KootK]

@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.

 

[woodman88]

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.

 

[KootK]

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.

 

[RHTPE]

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

 

[TehMightyEngineer]

It is 1.15.

Maine EIT, Civil/Structural.

 

[RHTPE]

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

 

[msquared48]

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)

 

[dhengr]

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. F_c⊥ 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 F_c⊥. 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.

 

[KootK]

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.

 

[Ron]

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

 

[MacGruber22]

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 (K_post + K_blocking + K_{structure 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.

 

[IFRs]

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.

 

[woodman88]

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.

 

[KootK]

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.