Wood Diaphragm to Wood Shearwall Anchorage 1

Mostly constructability I would bet. Most contractors like to sheath the walls while they're on the ground.

However I have done that detail numerous times.

True. The structural sheathing normally stops at the top of the double top plate.

I do not get the comment about substituting blocking for the framing anchors. The framing anchors are normally nailed to the double top plate and the vertical blocking with the diaphragm edge nailing penetrating the top of the blocking.

Mike McCann, PE, SE (WA)

The wall could be a partial depth shear wall, which is most often the case, and the rim joist is acting as a collector.

Ah there it is. 8ft tall typical walls so you have a panel joint at the top of the double top plate. My walls aren't typical in this project so this detail could work.

The additional blocking is a 2x on top of the double top plate (they reference figure 9.7).

Hey here is something from APA. It contains some connection details that may give you some insight or ideas.

Thanks for that link. I read through it. Seems like they don't like to transfer rim joist shear from the diaphragm directly to the sheathing, but would rather use the top plate. They'll use a rim joist to transfer shearwall shear from a wall above into the wall below though.

I don't dispute any of the constructability stuff. I agree 100%. There is, however, still a minor technical hiccup to consider.

With the detail as you've proposed it, the plywood will be forced to cantilever above the top plates to pick up the shear load. The cantilever distance would be the vertical height between the double top plates and the nails connecting the wall sheathing to the rim board. This cantilevering would result in in-plane bending in your wall sheathing.

Is it a big deal to have your wall sheathing cantilever up a foot or so to pick up the shear load? Surely not. However, we don't normally subject sheathing to in-plane bending when we're doing a "by the book", Malone style lateral design. Rather, we stick to the "shear panel" assumption and insist that all flexure be resisted by discrete chord members.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

I use the sheathing to connect for lateral and vertical loads. It's free, just lap it over the floorline.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

KootK,

Although I see your argument for in-plane bending I don't see that being a feasible failure mode. The rimjoist wouldn't allow for that kind of bending. I would still consider it shear panel action of sorts.

But I'm probably wrong.

Jayrod12 said:

The rimjoist wouldn't allow for that kind of bending. I would still consider it shear panel action of sorts.

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I think that you basically have to treat it as though the blocking weren't even there if the blocking won't be prevented from shifting laterally by some manner of connection to the top plates. Essentially, in this scenario, all the blocking does is provide a nailer to allow the roof and wall sheathing to be stitched together.

I believe that, in reality, the blocking would help some. You'd develops some kind of strut and tie system like you do with screws installed at an angle. The wall sheathing would be the tension bit and the blocking the compression strut. I think that it would take a considerable amount of lateral movement to get that working to any serious extent however.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

Alright you win this one yet again. However have you ever seen a rim joist or blocking without a few toe nails into the top plates? That should provide all the sliding resistance required.

Agreed. But then the toe nails would just be doing the job of the clips in Breyer's detail and we're back to square one with the detail.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

Touche,

Less toe nail required for sliding resistance as opposed to the uplift and sliding? (Grasping at straws now)

I never rely on toenails. I usually clip them off.

I would suggest, in lieu of the clips, to add flat 2x blocking above the double top plate and between the trusses or joists where the rim joist would be nailed to the flat blocking and the blocking to the top plate.

Just seems like a lot more labor though than just adding the metal clips.

Mike McCann, PE, SE (WA)

I guess I don't see any "cantilever" action here. The shear from the diaphragm is simply transferred into the rim board and the shear in the rim board is transferred into the vertical sheathing.

The double top plate and the rim board do not "support" the sheathing - rather they are simply elements used for connectivity and they just go along for the ride.

For the sheathing to "cantilever" that suggests that the double top plate is some kind of boundary condition for the sheathing, which it is not. There is no finger of God here pushing back on the double top plate resisting lateral movement such that the sheathing will bend like a cantilever in plane. The sheathing is what is securing the double top plate from moving laterally.

I would agree also that the so-called bending, if it even exists, is not really significant anyway.



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Actually m^2 brings up a good point with the 2x on flat. I generally have something there to screw drywall to. almost like a third top plate offset 1 1/2" in. this gets nailed down to the top plate. The rim joist nailed to this addition 2x.

But as KootK pointed out, it basically serves the same purpose as the clips so I would guess it would be contractor preference.

jayrod12 said:

Less toe nail required for sliding resistance as opposed to the uplift and sliding? (Grasping at straws now)

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You must be skinny. Skinny guys fight 'til they're burger. Love it.

Do you mean uplift due to wind suction? Certainly, if both forces are present then both must be accounted for.

JAE said:

I guess I don't see any "cantilever" action here.

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See below. I don't go isometric for just anyone you know. I think that the mechanics are more apparent if one considers the floor truss / top ribbon scenario where this often comes up. It removes the appeal of the strut and tie mechanism that I mentioned above.

JAE said:

There is no finger of God here pushing back on the double top plate resisting lateral movement such that the sheathing will bend like a cantilever in plane.

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The ubiquitous case is that of shear walls that do not run the full length of the roof diaphragm (mike20793`s point above). In that scenario the "hand of god" is just the restraint provided to the top plate by shear walls further down the line. As a particularly salient example, consider the sketch below as it might occur over top of a window opening.

JAE said:

I would agree also that the so-called bending, if it even exists, is not really significant anyway.

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No doubt. I really just tabled this as a possible explanation for why you never see details like this proposed in the industry literature. I also find that there is relatively little awareness of the "shear only diaphragm panel" concept that underpins virtually all diaphragm design in wood.

Were there to be a real problem, I suspect that it would be with the fasteners rather than the sheathing. The moment induces additional local nail shears not accounted for in the typical diaphragm design where we just pluck some handy unit shear numbers from the stock tables.

.




I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

Fun thread, and yes my situation is as you have shown. 24" attic floor trusses. Loads arent high though.

Kootk - I get what you are saying, but why wouldn't it act like a channel without flanges(the stud chords) instead of separate distinct panels? Just because the chords are gone? Its not like the chords affect shear transfer between panels. And let's assume that the panels are nailed to those 2x shorts you have at the end of the trusses in the similar way that the panels would be nailed to the studs below.

Jerehmy said:

Just because the chords are gone?

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Yes, precisely. Back when I was a wood truss maven, I used to try to get folks to throw a few nails into the truss end verticals for just this reason. Even that wasn't quite by the book, however, because there was no positive connection between the tension chord of the upper panel and the tension chord of the lower panel. That is, unless, you believe in transferring combined tension and shear through sheathing panels: Link.

Jerehmy said:

Its not like the chords affect shear transfer between panels.

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Without the chords, the flexural tension and compression forces that would have otherwise resided in them have to be transferred across the sheathing panel to panel joints that are normally designed to transfer only shear. That means:

1) [Shear + Tension] AND [Shear + Compression] interaction forces on nails usually design just for shear.
2) Compression needing to be transferred from panel to panel across joints that usually include a 3 mm gap.
3) Tension forces needing to be transferred from panel to panel across joints where the nails are very close to the edge of the sheathing and the edge of the supporting framing.

Again, this is just theoretical sport on my end. I don't really think that your detail is in any trouble.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.