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Wood Diaphragm to Wood Shearwall Anchorage

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.

RE: Wood Diaphragm to Wood Shearwall Anchorage

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.

RE: Wood Diaphragm to Wood Shearwall Anchorage

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)


RE: Wood Diaphragm to Wood Shearwall Anchorage

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

RE: Wood Diaphragm to Wood Shearwall Anchorage

(OP)
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).

RE: Wood Diaphragm to Wood Shearwall Anchorage

(OP)
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.

RE: Wood Diaphragm to Wood Shearwall Anchorage

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.

RE: Wood Diaphragm to Wood Shearwall Anchorage

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

RE: Wood Diaphragm to Wood Shearwall Anchorage

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.

RE: Wood Diaphragm to Wood Shearwall Anchorage

Quote (Jayrod12)

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

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

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.

RE: Wood Diaphragm to Wood Shearwall Anchorage

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.

RE: Wood Diaphragm to Wood Shearwall Anchorage

Touche,

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 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)


RE: Wood Diaphragm to Wood Shearwall Anchorage

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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RE: Wood Diaphragm to Wood Shearwall Anchorage

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.

RE: Wood Diaphragm to Wood Shearwall Anchorage

Quote (jayrod12)

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

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.

Quote (JAE)

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

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.

Quote (JAE)

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 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.

Quote (JAE)

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

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

(OP)
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.

RE: Wood Diaphragm to Wood Shearwall Anchorage

Quote (Jerehmy)

Just because the chords are gone?

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.

Quote (Jerehmy)

Its not like the chords affect shear transfer between panels.

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

(OP)
Id say the compression wouldn't be much different. We'd have that 2x stud at the end.

RE: Wood Diaphragm to Wood Shearwall Anchorage

Another wrinkle with the floor truss example is that you're unlikely to get the trusses, studs, and sheathing joints to line up. That doesn't invalidate the load path but it definitely makes it messier.

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

(OP)
What if we just fastened those 2x's to the top plate, for arguments sake. I'm just curious what the tension would be.

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

At 133#, you could probably make due with just resisting dead load.

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 was about to post the combined uplift and shear detail. I think that is an OK detail, no? Kootk, you're not a believer?

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

Sorry KootK - I don't buy your iso drawing. You show a SumV value where there is none.

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 the double top plate must have something (finger of God) holding it from moving laterally and there isn't anything but the sheathing. But the sheathing cannot cause its own moment.

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RE: Wood Diaphragm to Wood Shearwall Anchorage

For this construction, the rim joist is actually a rim truss - similar to an end wall truss.

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

(OP)
They're 2x on the flat. That'd only leave 2" bearing on the 2x6 double top plate. That'll work for calcs but 4" bearing with cantilevered sheathing sounds better to me. Would you only have 2" bearing?

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

Quote (RFreund)

I was about to post the combined uplift and shear detail. I think that is an OK detail, no? Kootk, you're not a believer?

I was a little skeptical. I gave it a good read last night, however, and I'm on board. KootK approved!

Quote (Rfreund)

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.

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.

Quote (JAE)

Sorry KootK - I don't buy your iso drawing. You show a SumV value where there is none.

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

So the whole shear wall is a cantilever off the footing.
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

Perhaps wood sheawalls, but not CMU where reverse curvature can occur.

Mike McCann, PE, SE (WA)


RE: Wood Diaphragm to Wood Shearwall Anchorage

Quote (JAE)

I'm not sure why then there is any concern over a small portion of that "cantilever" up at the top.

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.

Quote (JAE)

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?

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

With all the adjoining joists/trusses, etc. the panels are typically nailed all around. I can see the issue at the edges where in tension there might be some discontinuity in the panel. Typically there are inter-story straps if this is a floor so the panel would be completely covered with perimeter elements on all sides. For roofs, maybe not. On the compression side there isn't much, if any, difference.

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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