6” Foundation Walls 1

[SinStrucEng]

I have a simple query. What is the risk or adverse conditions brought on by a 6” foundation wall detail?

Background:

Based on Ontario, Canada. My client builds single story bungalows, slab on grade, no basement. To date they’ve been pouring full depth walls (past frost line depth) and the footings and walls are wholly standard - 8” walls and footings that measure 24”x8”. No issues to date.

OBC may be allow me to build 6” walls rather than 8”. I will admit that I am yet to fully check all code clauses so this might change… BUT if nothing outwardly restricts me - is there a reason why I cannot or should not?

Thanks, SinStructEng

 

[skeletron]

Vibrator gets stuck in a 6" wall more often than not. Also less tolerance for anchor bolt placement.

 

[NorthCivil]

a lot of townhouses in BC are built on 6" foundation walls with only a single top bar for reinforcement (no grid of bars in the wall)

i've never designed it, just seen it.

i dont know how an engineer can justify it. but small residential construction is a cost sensitive, competitive race to the bottom. so i imagine once one company starts approving such a detail, they all are forced to.

havent heard of any issues, so long as there are no serious soil pressures.

it wouldnt qualify as the scariest foundation ive ever seen. such a construction method was typical in the 70s, and while in the guts of 50 year old houses, ive never seen an issue barring some serious lateral loads. Most that ive seen are in good condition.

 

[XR250]

We have tons of houses here built on 4" single wythe, brick foundations that are doing fine. Granted, they are only going 8" below grade max.

 

[dik]

The apartment building down the street (5 or 6 storey, I don't recall... 50 years back) uses 6" conc walls and 8" HC slab. Foundation walls are 10" likely.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[SinStrucEng]

To close the loop on this, thank you everyone that replied. We will be going with 6" walls on a trial basis for a few homes. Assuming no worries, we may adopt it where conditions allow. Our footing remain the same.

 

[Alex A.]

I don't see an issue with 6" foundation walls, as long as it is properly designed based on height of the wall and earth conditions. As some have said, many older homes have have been constructed with worse conditions (i.e. unreinforced concrete, brick, or rubble stone walls) and are still standing today. One shortcoming may be that you will not have enough room for a brick ledge if the home has stone/brick veneer, so these walls are more suited for stucco or siding construction.

J.T. Donald Consultants Limited
Markham, ON, Canada

http://www.jtdonald.com

 

[TLHS]

You can justify a 6" foundation wall in those conditions based on the Part 9 foundation wall tables, it looks like.

Also, it seems weird to me that the reinforcing requirements for insulated concrete form foundation walls are different than other concrete foundation walls, but that's a recent addition I guess so it's got more modern requirements. That's part 9 for you!

 

[Greenalleycat]

Our code has allowance for 'standard' or 'engineered' solutions.
Code 'standard' has minimum 165mm (6.5") though obviously your codes will be different.

Some thoughts from me
- 6" doesn't allow much tolerance for...anything really. May work on paper but could be challenging in practice (I've no idea what other detailing constraints are for your jobs in terms of anchor bolts, reinforcement, slab ties, steel to concrete connections, etc)

- We require 75mm (3") clearance either side of concrete poured directly against ground for durability reasons - if you only have a 6" thick wall then you cannot satisfy this constraint (if relevant)
This would be particularly relevant if using stirrups/ties - the bend radius of the stirrup will push it close to the foundation edge

- 6" is a thin strip so bearing capacity could become an issue

 

[TLHS]

Now I'm just stuck on the fact that Part 9 lets you do unreinforced concrete foundation walls at all sorts of heights. Even if it's historically worked for things like basements, it seems like a terrible idea in an area that might see seismic forces and displacements.

I guess the shear walls are basically working in line with the walls and there shouldn't be a huge amount of in plane bending as long as your surrounding soil is consistent and moves as a unit... but there's no ductility and where does your shear wall uplift go?

I'd be fine with the numbers not working out, but it feels like there's a load path issue.

 

[skeletron]

@TLHS: I think some/most seismically active places still have a declaration similar to this one. Yeah, there's some haywire stuff in Part 9. I don't view it as a step-by-step recipe, rather it is a bunch of possible ingredients that you can throw together and make soup. But you still gotta taste it (run the numbers) to see if it works.

I think 6" walls are :thumbs down: for many reasons already mentioned above.

 

[phamENG]

6" CMU walls are fairly common around here for shallow foundations (we have a 12" frost depth). Architects like it because a 2x6 wall will sit nicely on top of it and a floating slab won't have a joint 2" out from the wall. Makes floor finishes a lot easier.

I'm not worried about tolerances for these applications. If I have a 6" stud wall sitting on it, the anchor bolt has to be placed in the same 1.8333" band either way. And the face of block needs to align with the required face of wall above to the same tolerance anyway. Would I want to do it for a tall, reinforced wall? No. Because grout space and bar placement tolerances could get to be a pain eventually. But for our 3 and 4 course foundation walls...I have no issue with it at all.