Wood Diaphragm to Wood Shearwall Anchorage
Wood Diaphragm to Wood Shearwall Anchorage
(OP)
Searched around, couldn't find anything that addresses my specific question.
For those that have Breyer 6th edition, Figure15.3b shows the connection situation I have a question about.
The diagram shows a framing anchor to connect the double wall top plate (chord) to the rim joist. It also says other connections such as toenails or blocking could be used in lieu of the framing anchor.
Why not just use the wall sheathing? I am racking my brain and can't figure out a reason why. Looked all over online and couldn't find anything. I feel like I'm overlooking something.
For those that have Breyer 6th edition, Figure15.3b shows the connection situation I have a question about.
The diagram shows a framing anchor to connect the double wall top plate (chord) to the rim joist. It also says other connections such as toenails or blocking could be used in lieu of the framing anchor.
Why not just use the wall sheathing? I am racking my brain and can't figure out a reason why. Looked all over online and couldn't find anything. I feel like I'm overlooking something.






RE: Wood Diaphragm to Wood Shearwall Anchorage
However I have done that detail numerous times.
RE: Wood Diaphragm to Wood Shearwall Anchorage
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)
RE: Wood Diaphragm to Wood Shearwall Anchorage
RE: Wood Diaphragm to Wood Shearwall Anchorage
The additional blocking is a 2x on top of the double top plate (they reference figure 9.7).
RE: Wood Diaphragm to Wood Shearwall Anchorage
RE: Wood Diaphragm to Wood Shearwall Anchorage
RE: Wood Diaphragm to Wood Shearwall Anchorage
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.
RE: Wood Diaphragm to Wood Shearwall Anchorage
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
RE: Wood Diaphragm to Wood Shearwall Anchorage
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.
RE: Wood Diaphragm to Wood Shearwall Anchorage
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.
RE: Wood Diaphragm to Wood Shearwall Anchorage
RE: Wood Diaphragm to Wood Shearwall Anchorage
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.
RE: Wood Diaphragm to Wood Shearwall Anchorage
Less toe nail required for sliding resistance as opposed to the uplift and sliding? (Grasping at straws now)
RE: Wood Diaphragm to Wood Shearwall Anchorage
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)
RE: Wood Diaphragm to Wood Shearwall Anchorage
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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RE: Wood Diaphragm to Wood Shearwall Anchorage
But as KootK pointed out, it basically serves the same purpose as the clips so I would guess it would be contractor preference.
RE: Wood Diaphragm to Wood Shearwall Anchorage
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.
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.
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.
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.
RE: Wood Diaphragm to Wood Shearwall Anchorage
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.
RE: Wood Diaphragm to Wood Shearwall Anchorage
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.
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.
RE: Wood Diaphragm to Wood Shearwall Anchorage
RE: Wood Diaphragm to Wood Shearwall Anchorage
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.
RE: Wood Diaphragm to Wood Shearwall Anchorage
2kip wind shear, ASD. so 4kip*ft. Let's say its continuous and 30ft long. So 4/30 = 133#. A single screw could do that and we'd be able to maintain the shearwall.
Yeah seems the loads are nothing to worry about. Even for much larger buildings, I can't see them getting too big.
I feel better about the situation now. I was worried I was overlooking something major.
RE: Wood Diaphragm to Wood Shearwall Anchorage
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.
RE: Wood Diaphragm to Wood Shearwall Anchorage
As for the cantilever - you have the end of the joists supporting the rim boards which would counteract any downward/upward force from the cantilever. So its really like a cantilever with a moment resisting support at the top. Free to translate but not rotate. I suppose there is still some flexural force though.
EIT
www.HowToEngineer.com
RE: Wood Diaphragm to Wood Shearwall Anchorage
The double top plate has no way to create a Sum V force as it is simply a loose double top plate attached to the sheathing itself.
I could draw a free body diagram of the full height sheathing (bottom of wall to top of rim board) and have a complete static condition without the "cantilever" action you are attempting to show. Simply sheathing in pure shear from top to bottom.
Again - for there to be a cantilever with a cantilever span of "e" per your diagram
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RE: Wood Diaphragm to Wood Shearwall Anchorage
The double top plate is a shear collector, from the roof or floor diaphragm to the shear elements below, particularly if it is needed to limit the h/d ratio to 3.5 or less, as in perforated shear walls.
Mike McCann, PE, SE (WA)
RE: Wood Diaphragm to Wood Shearwall Anchorage
They have cabinet file storage in this attic so the truss have double top and bottom chords. pretty decent reactions but 2" would probably work for Fcp.
RE: Wood Diaphragm to Wood Shearwall Anchorage
I was a little skeptical. I gave it a good read last night, however, and I'm on board. KootK approved!
Agreed. This bears some similarity to the strut and tie mechanism that I mentioned above. I would think that it would be less true for a floor truss ribbon and approach more true for something like full depth rim board.
Jeez... I was wearing braces the last time that I got three red exes on a single page assignment. My kids loved this and request more of the same.
I muddied the waters in my original sketch when I indicated that the collector force in the top plate would be SUM(v). In general, it will not be SUM(v). Rather, the collector force could take on just about any value, positive or negative, depending on the relative stiffness of the various vertical bracing elements up and down the line. See sketches A, B, and C below for an example of a case that would yield a shear flow diagram consistent with my original sketch.
The "finger of god" force is really a red herring when it comes to this debate however. The statics of the cantilevered sheathing above the top plate remain the same regardless of the value of the top plate collector force. See sketches D, E, and F below for the statics when there is no divinity at play. Panel two just cantilevers from panel one, without the benefit of discrete chords.
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.
RE: Wood Diaphragm to Wood Shearwall Anchorage
I'm not sure why then there is any concern over a small portion of that "cantilever" up at the top.
Your earlier statement "a minor technical hiccup to consider" concerning this cantilever isn't a hiccup at all - simply a feature of any typical shear wall, correct?
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RE: Wood Diaphragm to Wood Shearwall Anchorage
Mike McCann, PE, SE (WA)
RE: Wood Diaphragm to Wood Shearwall Anchorage
There is no real concern on my part. I tabled this as an explanation for why we don't see details like Jerehmy has proposed in the literature.
Not in my opinion. The unique feature here, in my mind, is that the upper portion of the shear wall doesn't have vertical "chords". That means that flexure has to, briefly, be resisted by bending stress in the sheathing. The design methodology that we generally use for wood diaphragms assumes that sheathing is only loaded in shear. This issue doesn't develop with typical detailing where blocking or rim board acts as a mini-diaphragm to transfer shear from the roof diaphragm to the shear wall top plates.
I don't feel ambitious enough to dig up the clause references but both the US and Canadian wood design codes contain language to the tune of "all wood diaphragms shall have discrete boundary elements on all sides". This is an expression of the "shear only panel" assumption built into our standard wood diaphragm design methods. Technically, Jerehmy's detail could be construed to be in violation of these clauses. I'm not, for a second, suggesting that anybody over at the AHJ is going to flag this 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.
RE: Wood Diaphragm to Wood Shearwall Anchorage
But overall this appears to me to be a minor issue. I was just responding such that I agree with manstrom way above.
The sheathing can be extended (assuming panel sizes work and no joints in the panels) and simply transfer the shear to answer the original poster's question.
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