Does the IBC/IRC confirm the requirement for diaphragm continuity through detailing? 1

[StrEng007]

Why does the 2021 IRC not address diaphragm continuity (Diaphragm boundary to blocking to top plate) through detailing? It is inferred in the definition of diaphragm (see below), but not shown in any figures or prescriptive outline.

We constantly run into the issue of contractors not wanting to provide full height blocking above the top plate because "that's not the way we do it" or "that doesn't provide adequate ventilation". The SBCA has issued technical papers addressing bird blocking to help maintain load path while providing ventilation. I get it, people don't want to do the extra work.

It's unfortunate that a prudent engineer becomes the less popular engineer because the firm down the street is willing to ignore Code requirements. However, the issue I'm seeing is that our Codes don't provide much effort in showing that it's a requirement to begin with.

When the IBC/IRC provides the following figure, this only validates the contractors argument that diaphragms don't need to be attached to full height perimeter blocking. However, these details are typically provided to either confirm the braced wall panel attachment, or the method to keep a truss vertically plumb. THESE DETAILS DO NOT PERTAIN to diaphragm continuity.

I understand load path, and understand the Code's requirements, but why has this not been addressed yet. Or maybe it is and I'm just looking in the wrong place.
And I know some good engineers who argue that lateral loads travel through the truss chord & connection to the top plate. Or that partial height blocking of the truss is "good enough for low lateral loads". Or that "portal frames" are established between the truss chord and roof diaphragm. Where I practice, we have to rely on the full perimeter/boundary of the roof diaphragm for load transfer.

 

[lexpatrie]

The IRC doesn't really "do" diaphragm in a purely rigorous sense, there is a requirement for blocking above the braced wall panels (as you show), the presumption is this is "good enough" fir a nin engineered system.

In the building code, where engineers are involved, it's more under "complete load oath" and "standard of care" than anywhere formalized and you can substantiate that in the Haug disciplinary actions in Florida where there was no detailing of the diaphragm boundary and they fined him.

It may be codified elsewhere, say, in ASCE 7, or SDPWS, but as I provide them as a matter of course the regulatory background on it isn't all that interesting to me. Even in a peer review the above should carry weight (or at least absolve you as you're not the design engineer, they can ignore you at their discretion or peril).

Some engineers will show the blocking full depth at alternating spaces (so I have heard, I don't have any drawings that actually show that).

The prescriptive approach in the IRC becomes increasingly risky in higher seismic, higher wind zones, and for higher end eave height. The rafter or truss has 3 toenails (at most 2 active per direction) or hurricane clips which have some strength, even if they aren't rated for that load direction, not that "some value not established by testing" is going to save you in front of the Board, if there's a failure and somebody notices what you did. (Contractor either followed your design or installed what the code called for..)

There are some technical articles (perhaps SBC magazine) but they aren't "at hand" right now.

 

[lexpatrie]

Blocking for diaphragm load transfer, (newly created).

 

[TheDW]

I share your frustration in this area. R602.10.8.2 goes as far as saying that heel heights of less than 9.25" in SDC A,B,C specifically do not require blocking.

I've also seen a lot of these rationalized using the hold down hardware to resist overturning but I am not a fan. If a shear wall is lightly loaded with say 200plf, thats 400# at the top of the truss (24" spacing) with a 1.5" width, so the uplift on the hold down with a 9" heel is 2400#. And you would need it in both directions. These sections make the conversation that much less fun.

 

[StrEng007]

lexpatrie, thanks for creating that. Here are two more contributions. Looks like the first one accompanies the document you provided from SBCA

https://sbcindustry.com/system/files/sbca/research-report/node/1166/srrheelblocking.pdf

https://seblog.strongtie.com/2012/1...member-between-a-trussrafter-is-not-optional/

I've also seen a lot of these rationalized using the hold down hardware to resist overturning but I am not a fan.

I agree, this is not the way to go. Even Simpson Strong-Tie doesn't stand behind that.

 

[lexpatrie]

DW, keep in mind the prescriptive code isn't something we are obligated to abide by if its not required under the prescriptive code and if you believe the blocking needs to be there for a structural reason, show it in your drawing as EOR.

Cross grain bending does appear to be necessary for load transfer if the blocking is omitted....

 

[TheDW]

For sure. I work a lot in SDC D so the IRC backs us up on that front fortunately.

But, the way I see it going in SDC A,B,C, is that a contractor ignores the blocking in our drawings, and when we push for it they could be savvy enough to be in the owners ear citing a specific code section that says I'm wrong, making me appear difficult and unreasonable, and at that point it doesn't matter that I'm right. These sections just make it more difficult to come out of these arguments looking good.

 

[lexpatrie]

Updating it now StrEng007.

That first one looks identical, unless the date is somehow different?