HVAC penetrations through plywood shearwall bottom plate

[skimboard20]

Hi all,

I'm working on a project where a contractor needs to drill holes in a few plywood shearwall bottom plates in order to run HVAC ducts. The bottom plate is 2x6, and the required penetration is about 5", so we can assume the bottom plate is essentially gone for about 12" of shearwall length (less than one stud bay). The bottom plate is nailed to joist blocking, with anther shearwall stacked directly below.

How big of an issue is this? My initial thought is that we are losing the bottom plate nailing along the length of the penetration. But I can figure out a way to get a few more nails into the bottom plate either side of the penetration. Are there any other considerations I need to address?

Thanks for the help!

 

[KootK]

In principle, I think that you're okay here. We typically assume that the wall shear delivery is uniform-ish by the time that it gets to the bottom plate. To the extent that is the case, the bottom plate is not technically a collector and, therefore, does not need to be axially contiguous.

That said, I'm sure that the real shear distribution at the bottom of the wall is, in reality, not perfectly uniform. As such, it would be nice if there were an axially contiguous plate at the bottom of the wall. It would also be nice if I had a Porsche rather than a rusty 2012 GMC Terrain. One must pick their battles, in life and engineering. This bottom plate thing would not be mine.

 

[SE2607]

Qualitatively, I agree with KootK. Quantitively, if you want to put numbers behind it, you can easily do so (in my feeble mind, anyway). You do not state the length of the shear wall or the total shear load. If the total shear wall length is 10 feet and the shear load is 2, 000 lbs, then the load on the sole plate nailing is 2,000/10 = 200 PLF. If your sole plate nailing is 16d @ 6", then the load on the nail is 200(6)/12 = 100 lbs./nail. If you have one 12" penetration, then the load on the sole plate nailing is 2,000(10-1) = 222 PLF and the load on the sole plate nailing is 222(6)/12 = 111 lbs./nail. If you sole plate nail has more capacity than 111 lbs., you are ok. If not, you can change the sole plate nailing along the entire shear wall to, say, something like 4". I would use the same approach for small openings in the shear wall such as electrical outlets. If you go through this exercise, you will find KootK is correct 99.6% of the time and you can sleep easy at night.

 

[milkshakelake]

Something to think about...If the bottom plate is spliced with the rim joist, the rim joist can carry the entire chord (T-C for the diaphragm) and shear wall transfer load over the hole. You just need some detailing to make sure the end of the rim joist butt joint doesn't coincide with that hole.

 

[dold]

I would use the same approach for small openings in the shear wall such as electrical outlets.

My unluckiest coworker ran into a plan-checker in Anchorage, AK one time.... FTAO straps on every penetration in the shearwall, including all electrical outlets, because "stress concentrations". What a nightmare.

 

[skimboard20]

Hi all,

Appreciate the responses. SE2607, thanks for the quick quantitative breakdown. Main take away is that it's a safe assumption that shear transfer is near uniform once you get to the bottom plate. Make sure sill plate nailing accommodates the loss of transfer due to the penetration.