Full height blocking along full length of diaphragm 1

[YoungGunner]

We have started doing projects in a new region and the contractors are pushing back on our details which require full-height blocking along the entire length of the diaphragm. They say the engineers in that area are allowing them to only put blocking and clips above the shear walls. The issue I have with this is that the diaphragm calcs assume the lateral forces are distributed across the entire length of the wall, and therefore would need blocking the full length of the wall to distribute it. If we have segmented walls, then the diaphragm may at times exceed it's own capacity if transferring only to the shear walls. Basically, those engineers are not using collectors. I understand with calcs I could probably justify otherwise, but we tend to be in Seismic Design Category D there and the numbers don't justify it.

The issue is further propagated by the engineers saying the H1 clips are enough to transfer the lateral loads where shear walls are not located, and I'm sure everyone on this board is well aware that Simpson specifically says that the clips aren't meant to be used for lateral transfer. I know BCSI says that technically trusses with 6" or less heels and spaced 24" or closer are assumed to transfer 50 plf lateral force, but this is incredibly low and the trusses in this area tend to have higher heels.

It also doesn't help that the IRC says explicitly that blocking is not required above openings in braced wall lines AND that the blocking is only required above the braced panels. To me that completely negates the point of a collector, but whatever.

To clarify the actual question:
1) Do you agree with those engineers and I guess the IRC that full-height blocking is only required at shear walls (the loads don't normally check out from a diaphragm capacity perspective FYI).
2) Do you have other ways of going about this? I love it when the engineers on this board unite together and agree, but unfortunately many in the industry aren't so noble to be part of this board and will continue to do thing their way, which forces us out of business.

 

[phamENG]

First - don't conflate braced walls and shear walls. While similar and intended to accomplish the same thing, they are NOT the same thing. If you use engineered shear walls, you're ripping the braced wall pages out of the code and throwing them away. Your entire lateral load path needs to be engineered. That's not to say you won't end up with something similar, and I usually try to arrange things so my shear wall designs mimic braced wall detailing as much as possible just to limit confusion about new-fangled details that "aren't the way my grand-pappy used to build houses!".

Simpson does NOT say they are not meant for lateral transfer. They have lateral load capacities listed right in the catalog. What they do say is that they are not a replacement for blocking. There's a difference. The blocking they are referring to is the blocking intended to prevent rollover at bearing. We're required to brace wood flexural members at bearing to resist torsion. So they're saying the H1 clip can't resist torsion in the end of the rafter/ceiling joist/truss, but it can transfer up to 510lbs along the wall so long as the member is independently braced.

So if you have an unblocked roof diaphragm, can satisfy torsional restraint in some other way, and your shear load from diaphragm to collector fits within the limits of the connector then you should be fine. The collector is often the top plate with additional nails at splices or straps over splices.

Be careful - Simpson does have a section in the catalog about combined loading, so you need to consider combined uplift and lateral load on the connector. They also have a blog post about it here.

Hope this helps.

 

[JLNJ]

Does the blocking over the shear walls have the capacity to transmit the shear loads? If the answer is yes then maybe the blocking at other locations isn't doing much other than mitigating joist seat rollover.

If the diaphragm needs more fastening than can be provided over the shear walls, then additional blocking and collectors seems in order.

I am not familiar with the IRC so I can't comment on its provisions.

 

[phamENG]

One other step in the load path that may be overlooked - you have to check your truss members for horizontal shear from the diaphragm to connector/plate connection. My guess is the 50plf has something to do with the ability of the truss plate to allow the load to pass from the top chord to the bottom chord. So if you do need blocking there, partial height (to get it above the joints in the truss) or V-notch blocking can be an option that allows you to maintain airflow at eave vents.

 

[YoungGunner]

phamENG, my post agreed with you on most accounts and I appreciate the insight of braced wall panels versus design shear walls (though the construction industry would blink if I ever tried to explain that to them) but I have to disagree with one comment. While Simpson's products do provide lateral capacities, this does not imply that the trusses or rafters themselves have the capacity to laterally transfer the load, even if prevented to rotate. For taller heel trusses, we would be trying to transfer lateral loads through bending of the thin chords and through the press plates. I don't think the truss manufacturers design their plates to laterally transfer anything. I'm sure all of this could be quantifiable to some degree, and surely there is SOME capacity there, but I doubt anyone has every tried to go through every facet of that calc, and therefore we just rely on full-height blocking which solves all problems. Except for the problem I'm dealing with, which is, the contractors AND engineers say we don't need it up there.

I do appreciate your insight and it confirms most of my thoughts. It's a hard industry to join.

 

[phamENG]

YoungGunner - if you read my second post, you'll see that I stated roughly the same thing about the shear transfer through the member and ways to resolve it.

 

[YoungGunner]

phamEng yep see that now. Thanks for the clarity.

I may be young in the industry, but I'm not willing to accept the standard as the standard without solid argument.

 

[phamENG]

No problem. Fighting inertia in this industry is exhausting and never ending. This biggest problem I encounter in the residential market is that the GCs like to sell you on how experienced and trustworthy they are - "I've been building the best houses around for 30 years!" - but then they hire the lowest bid for carpentry and don't check behind them. I had a conversation with a contractor client of mine the other day because they had covered the sheathing before the city inspected it. All the usual stuff - I've been doing this a long time, You know I don't cut corners, I only hire competent guys - you know they did it, etc. He wanted a letter to the city saying it was done properly. I spend 5 minutes on site and found that they hadn't even built half the lateral force resisting system on one of the levels - never mind the sheathing.

So there's entirely too much trust - the GC hires the trades, schedules them, and then gets mad at everyone else when things don't come together right when they should be coordinating and inspecting.

There's also a lot of turnover here - so that carpenter who used to do everything perfectly now has a new foreman while the experienced guy is out fishing, but nobody seems to account for it until after things go wrong.