Personally I'm a fan of this. It provides a confident tie between walls. It's likely one of the reasons wood buildings tend to outperform the design calculations. You know, one of those unaccounted for alternate load paths.

I suppose so, but requiring something that has no basis seems like it could get us in trouble. I personally believe any extra strength comes from the extra materials we don't account for. Drywall, exterior finishes, construction bracing, and non-load bearing framing.
What would this single ply 2x overlap provide?
From a gravity load stand point, the end post of either wall would have to fail for the top plate to transfer any load to the adjacent wall. If the end post fails the single ply top plate will fail right after it.
From an uplift stand point, the overlapping plate could be used to transfer the load across the corner however most engineers require uplift restraint each side of the corner or properly fasten the corner posts to get a force transfer along the entire height of the wall.
From a lateral load stand point, assuming there are 2 nails in the overlap, there would be a 2-300 lb lateral capacity of this connection. Not very much...I guess it is good to have though. I would then question how a lateral load would get to this joint. I assume the adjacent wall... which should be designed to be supported by a very large roof or floor diaphragm.

I'm not opposed to it, just curious why.

1) Floor Diaphragn chords
2) Shearwall drag chord
3) Load transfer when vertical elements don't stack: 24" oc vs 16" oc or just the shoulder stud of an opening or a beam at floor opening. Without the double plate, all joists/rafters/beams must have a stud underneath (+/- 1").
4) out of plane wind load transfer from stud to floor. More critical in taller walls as sometimes metal angles are required with longer nailing especially at the roof as opposed to floor which will have a rim for purchase.

Most wood design texts address this in detail. I am not a fan of the single plate either. Green building concepts be damned as they aren't think about the structure as a 3d concept rather only the vertical component. Code overlap is 24" with (2) 0.128x3" nail at 24" or "splice plate" (see below). It is likely the lap has (4) nails. If you have ever done a wood drag strut you know how valuable the double top plate is and specifying longer laps and more nailing.

Also don't forget if you don't double plate then the wall height for pre-cut studs will be off. You cause all kinds of issues and extra costs and waste for the framer and finishers if you don't follow conventions.


IRC 2012:
"R602.3.2 Top plate.
Wood stud walls shall be capped with a double top plate installed to provide overlapping at corners and intersections with bearing partitions. End joints in top plates shall be offset at least 24 inches (610 mm). Joints in plates need not occur over studs. Plates shall be not less than 2-inches (51 mm) nominal thickness and have a width at least equal to the width of the studs.

Exception: A single top plate may be installed in stud walls, provided the plate is adequately tied at joints, corners and intersecting walls by a minimum 3-inch by 6-inch by a 0.036-inch-thick (76 mm by 152 mm by 0.914 mm) galvanized steel plate that is nailed to each wall or segment of wall by six 8d nails on each side, provided the rafters or joists are centered over the studs with a tolerance of no more than 1 inch (25 mm). The top plate may be omitted over lintels that are adequately tied to adjacent wall sections with steel plates or equivalent as previously described. "

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MAP

focuseng,
I agree with all of those functions for a double top plate and agree that a single ply top plate can be problematic. My question isn't regarding the top plate, or even lap splices along the length (for drag loads, etc) but rather to the code requirement that the top plate be "overlapping at corners and intersections with bearing partitions. So, they are asking that all bearing walls have overlapping top plates with any intersecting bearing wall top plate. In essence, a butt joint wouldn't be allowed.

Ah! Now I see.
In that case I have no answer other than it is a nice easy way to set walls. It certainly makes perpendicular partitions stronger tied. My experience would say that I couldn't find a skiled carpenter who would want to do it any other way. But that shouldn't necessarily be a code issue

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MAP

Laterally, there is the diaphragm completeness argument. I don't buy it, personally, as it's really all about the fasteners and they may be spaced away from the corners anyhow. And it's just too pedantic.

Like Jayrod, I like it for general integrity purposes. With the lap, the plates can serve as crappy girts at the top of the walls, helping to stabilize them both in the final condition during construction. Which is not to say that they should be relied upon explicitly to perform those functions.

The requirement would be a pain for panelized walls I would think.

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.

If we are just looking at the extreme outer corners of the diaphragm, wouldn't the force be 0 at these points?

In my opinion the lap is a good idea at all exterior corners/jogs because it is the termination point for the top plates so why not lap the plates, but any intersecting interior wall should just butt joint the exterior wall top plate.

Quote (RESENG2)

If we are just looking at the extreme outer corners of the diaphragm, wouldn't the force be 0 at these points?

Nope. Conventional procedures assume uniform shear along the edge of the diaphragm.

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.

Boundary edge as a collector, yes it would be uniformly loaded along the edge but it would also be resolved along the edge.
Boundary edge as a chord, the force would be zero at the outer corners.

Respectfully disagree on both counts.

Quote (OP)

Boundary edge as a collector, yes it would be uniformly loaded along the edge but it would also be resolved along the edge.

Only if your structural diaphragm is considered to be of a depth less than the full physical depth of the diaphram and you're doing a bunch of fancy stuff with transfer diaphragms inboard of the diaphragm edge. This all hearkens back to our "shear panel" assumptions as they pertain to wood diaphragm design. In all cases, you need a boundary element to run the full length of whatever you're calling "the diaphragm". And that's a code requirement as well as a theoretical necessity. The overwhelming majority of wood diaphragms are structural diaphragms right out to the corners.

Quote (OP)

Boundary edge as a chord, the force would be zero at the outer corners.

The chord force will be zero at the edge but the diaphragm unit shear along the chord will actually be at it's peak value.

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 am not suggesting that a boundary element is not required at the diaphragm edges.

On point #2, I was referring to the force in the top plate/(chord), which you agree is zero at the corners.
My point is there is no force in the top plate that needs to be resolved at the corners.

I guess more to the point of the OP: the requirement is not just for boundary walls which may or may not be utilized in the diaphragm design but explicitly if they are "bearing partitions". So an interior bearing wall is REQUIRED to interrupt the double top plate of an exterior bearing wall when it intersects. Does not need to be an outside corner. It seems to me this is more a stabilizing joint to brace the wall to another wall. But an arbitrary one based on conventional construction practice. The necessity of double top plates and lap joints therein is not the question but rather the mandated link between two intersecting walls.

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MAP

Quote (OP)

I am not suggesting that a boundary element is not required at the diaphragm edges.

Yeah, I get it. You're suggesting that a boundary element is not required to extend to the corners. I'm suggesting that's rarely the case and may have something to do with the code provision that you're inquiring about.

Quote (OP)

My point is there is no force in the top plate that needs to be resolved at the corners.

Yes, and the fact that you're focusing on that point suggest that you may not have fully digested mine (original post and sketch). You don't need a chord at the corner because there's a big force in the chord. You need a chord at the corner because there's a large unit shear in the diaphragm at the corner and that cannot develop in the absence of the chord extending right to the corner.

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.

My gut instinct says: primarily this requirement is to provide "dummy proof" bracing during construction, secondarily, it provides an alternate load path for lateral forces while in-service.

Quote (FoxSE14)

primarily this requirement is to provide "dummy proof" bracing during construction

Ditto. Kind of like tie joist detailing in open webbed steel joist construction.

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.