NBCC PART 9 Anchorage of building frame into foundation

[Said the Sky]

Hello all,

can anybody explain to me how 9.23.6.1 sentence 2c provides any anchorage into a 8" wide foundation wall for a home. We qualify as low wind and low seismic area, however I can't figure out how 2c could be subsituted in place of 2b (1/2" anchor bolts spaced at 2.4m O/C into concrete. Are the sill plates nailed into the side of the concrete through the 2x4 on edge?

I got a 3 storey home, which qualifies as part 9, so alot of the loads are quite small, but were at the cusp of where we could be considered part 4 (were just under the 600m2 outline area)

9.23.6. Anchorage
9.23.6.1. Anchorage of Building Frames
1) Except as required by Sentence 9.23.6.3.(1), building frames shall be anchored to
the foundation unless a structural analysis of wind and earthquake pressures shows
anchorage is not required.
2) Except as provided in Sentences (3) to (6), anchorage shall be provided by
a) embedding the ends of the first floor joists in concrete,
b) fastening the sill plate to the foundation with not less than 12.7 mm diam
anchor bolts spaced not more than 2.4 m o.c., or
c) embedding in concrete two 38 mm by 89 mm sill plates placed on edge and
separated by blocking spaced 1.2 m o.c.

 

[skeletron]

Is that the most recent version of NBCC 2015? See attached as that specific article 9.23.6.1(c) has been removed. This is from the download available on NRC website.

I would imagine that it would be a friction-based logic. I default to anchor bolts through sill plate (considerably different area of the country, though). But I also consider Part 9 design as not exclusively prescriptive, and the designer needs to consider loads, capacities, and also best practices. I think the route more aligned with prescriptive design would include calculating loads and capacities with the CWC Engineering Design Guide to Wood as you are able. There are silly clauses in Part 9 that probably don't follow best practices, but also may apply due to scale or economy of construction.

 

[Said the Sky]

Hi skeletron my clause is from NBCC 2019 version, I see this double sill plate used quite a bit for houses but definitely skeptical about how it works.

 

[skeletron]

The NBCC 2010, 2015, and 2020 editions don't have that clause. It appears that this is an exclusive hold-over in the Alberta Building Code 2019. This would make sense regarding your description of location (low seismic, low wind?).

If I had to throw a method and number at it:
-Wall plate is nailed to the side grain of embedded sill plate at the exterior side.
-Sheathing is carried down over the embedded sill plate and nailed to the face grain of embedded sill plate at the exterior side.
-Blocking between the sill plate ties the two together similar to ladder framing (ie. 2-toenails)
-Movement prevented in the direction of the wall by enclosing the blobs of concrete in the 48"x5" blocks of concrete between sill/blocking.
-Overturning prevented by giving bearing contact to the interior side plate at the blocking locations.
-Movement prevented by dead load + friction

So it's really only as effective as the blocking attachment = 2 toenails @ 48" o/c.
Relying on the dead load providing enough friction would only be as good as the distribution of dead load evenly around the frame. What happens when the wood shrinks? If you are relying on dead load contact, I would anticipate some issues.
I'd be surprised if this works with the numbers but maybe in low-wind and probably could squeak it out with q1/10 in the Farm Code in some areas.

I don't particularly like the detail and I'm surprised that there's no mechanical attachment (screw, bolt) connecting the two sill plates through the concrete stem at a minimum. At 3-storeys and closing in on 600m2 (6400ft2) I would be in a camp that tries to advocate for using a more standard bolted sill plate attachment. For the scale and economy of construction it's a better "risk mitigation" strategy that you can pencil out easily for a pretty low cost (in comparison to the square footage cost of the structure).

I'm not entirely convinced there is a difference in the loads between Part 9 and Part 4 especially for wind. Furthermore, Part 9 isn't a free-pass to use the listed solutions as-is. You still need to calculate out the loads (or use the prescriptive load tables in CWC Eng. Guide to Wood) and choose the best solution.

 

[Said the Sky]

Yes your right this is Alberta. I started doing some freelance work for a smaller residential structural firm that does only part 9 stuff and I am looking at alot of their details and it makes me think.,,twice. but I also see this detail for 2-3 storey houses.

I've check the dead load and the overturning is OK if the joists are orientated to land on the "shear walls". The shear walls are actually not shear walls, only exterior walls have OSB sheathing, the interior walls have 2 layers of 5/8"gypsum. So I guess in theory there is shear resistance. I just don't know whats helping against overturning.

I've attached the architectural detail of "what they have always done" and the sheathing doesn't connect to the sill plates. It stops top of rim board, but even if the sheathing extends to the sill plate, how is the sill plate anchored into the concrete.

as with all structural engineering projects, there is incredible push back and the "this is what we have done for past 100 years" "why are you re-inventing the wheel brother"

 

[skeletron]

It stops top of rim board, but even if the sheathing extends to the sill plate, how is the sill plate anchored into the concrete.

It appears that the ladder framing is doing the work. Relying completely on friction because sill+blocking surrounds a blob of concrete from the stem wall. Probably installed during forming. There's 3/8" OSB over the sill plate, so possibly that is nailed to the interior sill, although the degree of capacity is questionable.

The shear walls are actually not shear walls, only exterior walls have OSB sheathing, the interior walls have 2 layers of 5/8"gypsum. So I guess in theory there is shear resistance. I just don't know whats helping against overturning.

Shearwalls with no hold downs. This requires the calculation of dead load to determine the right Jhd factor in the equation.
What is the LFRS of the house in your opinion if they are not actually shear walls?

as with all structural engineering projects, there is incredible push back and the "this is what we have done for past 100 years" "why are you re-inventing the wheel brother"

I get it. But you are also the engineer, so you get to draw your line. If they (house designer, contractor, etc.) are involving the engineer for an exclusively Part 9 structure, then they are relying on your engineering judgement. This is also starting to become a directive from some AHJ's that were riding fast-and-loose in the past 100 years. As stated above, prescriptive Part 9 solutions still require the use of the CWC's Eng. Guide for Wood Frame Construction. See Article 9.4.1.1.

Still want to rely on ladder framing? It's pretty easy to install an A34 or similar framing angle at the rim and attach a concrete screw. At least that would kind of meet the illustrated intent of CWC (sheathing over rim or framing angles), but I'm not sure how the numbers pencil out.

 

[Said the Sky]

I added a 3/4" anchor bolt at the end of all interior walls into concrete foundation. also added a bottom sill plate for which the rim board sits on, then added nailing angled nailing from the rimboard to the bottom sill plate, which is connected to the 3/4" dia anchor bolt. Going to see if this will cause a push back, if so I will go with your idea of A34 clip from the rim to the concrete, but whats the load transfer from the wall to the rimboard?

we have 3" nails at 6" O/C that nails the sheathing into the top of the rimboard, but that doesn't transfer the overturning forces to the rimboard as the nails are in tension, and these nails are usually just transferring the diaphragm (floor) forces into the rimboard. Only thing I can see working is add A35 clips from where the wall sheathing stops and clip it to the rimboard, then have another set of clips conntecting the rimboard to the exterior sill. And maybe two straps one at each end of the wall.

regarding the shearwalls I discussed this with the engineer working with this company I freelance for, and he said in the code specifically for low seismic and wind areas only exterior walls need to be sheathed, and interior walls do not, so if we check if there is no net uplift were covered for overturning. but the shear part I am not sure. He said he checks if gympsum shear is enough but I know that in O86 were not allowed to use gypsum more than a certain percentage of shear for each floor.

if it were me I would have 1/2" anchor bolts instead of the two ladders and one side sheathed OSB interior shear walls every 15m O/C.

i've attached the part 9 clause that says only exterior sheathed is allowed.

 

[skeletron]

but whats the load transfer from the wall to the rimboard?

Wall studs + sheathing --> bottom wall plate --> 16d nails plate to rimboard below

we have 3" nails at 6" O/C that nails the sheathing into the top of the rimboard, but that doesn't transfer the overturning forces to the rimboard as the nails are in tension, and these nails are usually just transferring the diaphragm (floor) forces into the rimboard. Only thing I can see working is add A35 clips from where the wall sheathing stops and clip it to the rimboard, then have another set of clips conntecting the rimboard to the exterior sill. And maybe two straps one at each end of the wall.

So, I'm confused now, because part of your post is saying "hey, this is the solution Sir Architect, Mr. Engineer, and Ms. Contractor are telling me works" and you're also saying (by not saying) "I've checked the forces for shear and overturning and it's all good because we're low seismic, low wind" and then part of your post also casts some shadow of doubt into if it actually works. This extends to the point that now you are convinced that you need framing clips (for shear) and straps (for overturning)...

he said in the code specifically for low seismic and wind areas only exterior walls need to be sheathed, and interior walls do not

Yes. I agree.

He said he checks if gympsum shear is enough but I know that in O86 were not allowed to use gypsum more than a certain percentage of shear for each floor.

I believe that is only for seismic. But also, if you're using gypsum for shear, you need to specify (and inspect) how the gypsum is attached.

if it were me I would have 1/2" anchor bolts instead of the two ladders and one side sheathed OSB interior shear walls every 15m O/C

Then propose that. Use your engineering tools (math) to show why it is a better solution. Interpret the Code with a level of scrutiny that advocates for best current practices to be used for the next 100 years. If your contracting engineer is telling you "this works, do it this way" then you need to do a little bit of discovery for your own due diligence. If you can't make it work then either you've discovered there is not a continuous load-path to the foundation or that the contracting engineer needs to help fill that gap or potentially do it themselves. This is why freelancing can put an engineer into a vulnerable position. You're trying to make something work, that you don't think works but someone else is telling you that it does without any logical explanation.

Not sure what the prevailing issue is here anymore. I realize your original question was "how does this work?" and my original answer attempted to point out a hodge-podge method to calculate the numbers for you to compare. I also was able to point out other reasoning to suggest why this particular detail has been removed from equivalent codes. However, I'm not going to detail a 3-storey >6000ft2 house on the bare minimum details.