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When does a “stem wall” that is part of your foundation become a “concrete foundation wall”?

Joe.B

Joe.B

SAWHORSE
Staff member
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Dec 4, 2020
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Location
Myrtletown Ca
Assuming a prescriptive residential code design (no RDP stamp) and assuming seismic design category D or higher.

I have always looked at Section R402.2 and Table R402.2 and determined that the minimum compressive strength for a foundation is 2500psi.

The following section was brought to my attention:
R404.1.3.3.1 Compressive Strength
The minimum specified compressive strength of concrete, f 'c, shall comply with Section R402.2 and shall be not less than 2,500 psi (17.2 MPa) at 28 days in buildings assigned to Seismic Design Category A, B or C and 3,000 psi (20.5 MPa) in buildings assigned to Seismic Design Category D0, D1 or D2.

I am interpreting this as when prescriptively designing “Foundation and Retaining Walls” per Section R404 then the minimum compression strength needs to be 3000psi vs. 2500psi. The problem is that code doesn’t tell us when a foundation becomes a foundation wall. My gut intuition is that anything up to 24” above grade would just be a “stem wall” that is part of the foundation, and not a “foundation wall”. I would start calling it a “foundation wall” above that height.

Code definitions do not help. Code does say: “Where terms are not defined through the methods authorized by this section, such terms shall have ordinarily accepted meanings such as the context implies.” The dictionary definitions do not help me either, to broad. Google searches bring up lots for “stem wall” or “stem wall foundations” which fit my interpretation.

A google search for “foundation wall” produced these:
“A wall below the first floor extending below the adjacent ground level and serving as a structural support for a wall, pier, column or other structural element.”
“Foundation walls have a key role in buildings. They are structural elements, but they also isolate the building interior from environmental factors.”
“Foundation walls accomplish two main functions: structural support and environmental control.”
“Foundation walls are the walls that are typically seen in the basement of houses. The bottom wall is on a footing with a slab on grade attached to it.”

Do any of you have any insight you could share? Or a direction to point me in?

Thanks.
 
For example, in the image I posted (prescriptive in seismic design D) would that "stem wall foundation" be required to be 3000psi or is 2500 acceptable.
 
Yeah, the issue I'm having is that all of the references in R402 and elsewhere in the code reference 2500psi. Buried in R404 Foundation and Retaining Walls is this requirement to go up to 3000psi. If the need for the "foundation" (footing and stem wall) to be 3000 then why doesn't that show up in 402?

And why this makes a difference? That brings up another question:

If the Residential Code tells you you need 3000psi instead of 2500, does that trigger the need for special inspections? Residential code doesn't have any mention of special inspections. I would argue that if it's a purely prescriptive residential code design then it would not require special inspections, even if 3000psi concrete is required.

Thoughts on that?
 
Do your batch plants split hairs that close with concrete mix? Everything around here is around 4000 psi standard.

I think any vertical plane above the footing is a foundation wall. I do not have a source to cite for that opinion.

Break reports would usually be a no unless you really think they are using weak concrete. In which case, they can be required. It's pretty much your discretion as the BO.

Other thing you could do is get batch plant tickets. They won't be precise but they might make you feel better.
 
Yeah, around here we see mixes ranging from a "slurry mix" all the way up the scale. I think the highest I've seen is 6500psi. Every truck is required to have a batch ticket. Based on what I understand of the following code section, concrete over 2500psi requires special inspections, per building code. But not by Residential Code if you do a purely prescriptive design, that's my point.

Section 1705 Required Special Inspections and Tests​

1705.3 Concrete Construction​


Special inspections and tests of concrete construction shall be performed in accordance with this section and Table 1705.3.
Exceptions: Special inspections and tests shall not be required for: [OSHPD 1R, 2 & 5] Exceptions 1 through 4 are not permitted by OSHPD.
  1. Isolated spread concrete footings of buildings three stories or less above grade plane that are fully supported on earth or rock.
  2. Continuous concrete footings supporting walls of buildings three stories or less above grade plane that are fully supported on earth or rock where:
    1. The footings support walls of light-frame construction.
    2. The footings are designed in accordance with Table 1809.7.
    3. The structural design of the footing is based on a specified compressive strength, f 'c, not more than 2,500 pounds per square inch (psi) (17.2 MPa), regardless of the compressive strength specified in the approved construction documents or used in the footing construction.
  3. Nonstructural concrete slabs supported directly on the ground, including prestressed slabs on grade, where the effective prestress in the concrete is less than 150 psi (1.03 MPa).
  4. Concrete foundation walls constructed in accordance with Table 1807.1.6.2.
  5. Concrete patios, driveways and sidewalks, on grade.
 
R109.1.5.2 Special inspections. For special inspections, see California Building Code, Chapter 17.
 
bill1952 said:
To your thread title question, I believe a stem wall is always a foundation wall and has to meet those requirements.
Click to expand...
I'm kind of leaning that way too, as long as it doesn't then trigger a special inspection. If the reference ICE pointed out means this would trigger a special inspection then I'm leaning towards not calling it a foundation wall unless it crosses some undefined threshold. I say undefined because code doesn't define it for me. If I'm left to make the call then I would say it would become a foundation wall if the grade on either side is substantially different, or if it's more than 18" above grade, or if it separates conditioned/unconditioned space.
 
All footings for walls below grade are part of the foundation no matter the height of the wall, if there is unbalanced backfill or constructed of concrete masonry or wood. Whatever you call the wall it is part of the foundation that braced wall panels, alternate bracing and other above ground connections are relying on for them to work properly in a seismic or high wind event.

As far as a special inspector for 3,000 psi concrete one is not required under the IRC.
Do not mix the codes.
 
I just checked ACI and they provide this definition for a "wall".

Wall - Member, usually vertical, used to enclose or separate spaces.

I think that fits well with my interpretation.
 
mtlogcabin said:
All footings for walls below grade are part of the foundation no matter the height of the wall, if there is unbalanced backfill or constructed of concrete masonry or wood. Whatever you call the wall it is part of the foundation that braced wall panels, alternate bracing and other above ground connections are relying on for them to work properly in a seismic or high wind event.

As far as a special inspector for 3,000 psi concrete one is not required under the IRC.
Do not mix the codes.
Click to expand...
I agree on both points. I feel just fine telling them they need 3000psi minimum if that doesn't trigger a special inspection.

Most residential construction I see is designed with a perimeter foundation for the main house, and slab on grade for the garage. Houses are generally one or two story, no basements. We don't have frost depth issues here so footings are typically not excessively deep. In order to keep the minimum clearance for the crawl space we will see an 18" concrete stem wall above grade. Of course there are exceptions, but generally I don't see anything called out over 2500 psi for this type of foundation. Someone else is saying that these stem walls fall under 404 and need to be 3000psi, and have special inspections. It seems excessive to me. I'm okay with bumping up to 3000psi, that's a minor issue. Special inspections is where I think I would draw the line.
 
Joe.B said:
If the reference ICE pointed out means this would trigger a special inspection
Click to expand...
R109.1 Types of inspections. For on-site construction, from time to time the building official, upon notification from the permit holder or his agent, shall make or cause to be made any necessary inspections.....

From there the code lists various generic inspections such as foundations and framing.

Then there is the other inspections:
R109.1.5 Other inspections. In addition to inspections in Sections R109.1.1 through R109.1.4.2, the building official shall have the authority to make or require any other inspections to ascertain compliance with this code and other laws enforced by the building official.

Included are inspections for elements that are not generic and likely not found with every construction. These are required inspections.

R109.1.5.1 Fire-resistance-rated construction inspection. Where fire-resistance-rated construction is required between dwelling units or due to location onproperty, the building official shall require an inspection....

R109.1.5.2 Special inspections. For special inspections, see California Building Code, Chapter 17.

R109.1.5.3 Weather-exposed balcony and walking surface waterproofing. Where balconies or other elevated walking surfaces are exposed to water from direct or blowing rain, snow, or irrigation, and the structural framing is protected by an impervious moisture barrier, all elements of the impervious moisture barrier system shall not be concealed until inspected and approved.

The way that I interpret this bit of code is that Chapter 17 of the CBC has been invoked by R109.1.5 Other Inspections. It is incumbent upon the permittee to be aware of a requirement for special inspections of any element present that is listed in Chapter 17 of the CBC as requiring special inspection. Note that the CBC does not give a blessing to concrete based on a batch ticket. If you want credit for concrete it shall be documented by special inspection and a laboratory tested specimen.

So there is no if here. Well there is the if that if it exists it will trigger the requirement.
 
Last edited:
ICE said:
R109.1 Types of inspections. For on-site construction, from time to time the building official, upon notification from the permit holder or his agent, shall make or cause to be made any necessary inspections.....

From there the code lists various generic inspections such as foundations and framing.

Then there is the other inspections:
R109.1.5 Other inspections. In addition to inspections in Sections R109.1.1 through R109.1.4.2, the building official shall have the authority to make or require any other inspections to ascertain compliance with this code and other laws enforced by the building official.

Included are inspections for elements that are not generic and likely not found with every construction. These are required inspections.

R109.1.5.1 Fire-resistance-rated construction inspection. Where fire-resistance-rated construction is required between dwelling units or due to location onproperty, the building official shall require an inspection....

R109.1.5.2 Special inspections. For special inspections, see California Building Code, Chapter 17.

R109.1.5.3 Weather-exposed balcony and walking surface waterproofing. Where balconies or other elevated walking surfaces are exposed to water from direct or blowing rain, snow, or irrigation, and the structural framing is protected by an impervious moisture barrier, all elements of the impervious moisture barrier system shall not be concealed until inspected and approved.

The way that I interpret this bit of code is that Chapter 17 of the CBC has been invoked by R109.1.5 Other Inspections. It is incumbent upon the permittee to be aware of a requirement for special inspections of any element present that is listed in Chapter 17 of the CBC as requiring special inspection. Note that the CBC does not give a blessing to concrete based on a batch ticket. If you want credit for concrete it shall be documented by special inspection and a laboratory tested specimen.

So there is no if here. Well there is the if that if it exists it will trigger the requirement.
Click to expand...
This is exactly why this is troubling to me. There are definitely some if's here.

If the stem wall portion of a foundation is simply part of the foundation subject to R402 then it's only required to be 2500psi.

If the stem wall is in fact a foundation wall, then it is also subject to R404 and would have to be 3000psi because I am in Seismic Design Category D/E.

If the Building Official determines that R109 implies that a special inspection is required, then a 3rd party agency will be required to take samples and provide reports.

If we "suddenly" start imposing this people are going to have a cow.
 
ICE said:
see California Building Code, Chapter 17.
Click to expand...

1705.3 Concrete Construction​

Diagram
Special inspections and tests of concrete construction shall be performed in accordance with this section and Table 1705.3.
Exception: Special inspections and tests shall not be required for:
  1. Isolated spread concrete footings of buildings three stories or less above grade plane that are fully supported on earth or rock.
  2. Continuous concrete footings supporting walls of buildings three stories or less above grade plane that are fully supported on earth or rock where:
    1. The footings support walls of light-frame construction.
    2. The footings are designed in accordance with Table 1809.7.
    3. The structural design of the footing is based on a specified compressive strength, f 'c, not more than 2,500 pounds per square inch (psi) (17.2 MPa), regardless of the compressive strength specified in the approved construction documents or used in the footing construction.
So, if the footing is designed at 2,500 pounds per sq inch and the concrete that is used is 4,000 PSI no special inspector is required.
 
Joe.B said:
If we "suddenly" start imposing this people are going to have a cow.
Click to expand...
Steak dinners all around. When you question if a stem wall is a foundation wall, ask yourself if there is a difference beyond semantics. If the wall is supporting a building it seems to me that it functions just like a foundation wall. I find it impossible to fit a stem wall into the construction that would not.

So the status quo is to accept batch tickets for the record and unless someone says "foundation wall" it's not. Why upset the apple cart? It is imperfect but so much of what gets done is. Has there been a problem? An arbitrary dimension for the height of a stem wall that converts it to a foundation wall is not better that allowing what has been the norm for a long time.

I don't know but maybe you guys do it differently there but the picture that I posted is how it is here ...monolithic...the foundation and any assumed wall is all part of the same pour.....wall and all. Given that the footing is 3000, so is the wall.
 
mtlogcabin said:

1705.3 Concrete Construction​

Diagram
Special inspections and tests of concrete construction shall be performed in accordance with this section and Table 1705.3.
Exception: Special inspections and tests shall not be required for:
  1. Isolated spread concrete footings of buildings three stories or less above grade plane that are fully supported on earth or rock.
  2. Continuous concrete footings supporting walls of buildings three stories or less above grade plane that are fully supported on earth or rock where:
    1. The footings support walls of light-frame construction.
    2. The footings are designed in accordance with Table 1809.7.
    3. The structural design of the footing is based on a specified compressive strength, f 'c, not more than 2,500 pounds per square inch (psi) (17.2 MPa), regardless of the compressive strength specified in the approved construction documents or used in the footing construction.
So, if the footing is designed at 2,500 pounds per sq inch and the concrete that is used is 4,000 PSI no special inspector is required.
Click to expand...
What about the stem wall? Usually both are done in one pour around here.
 
ICE said:
Steak dinners all around. When you question if a stem wall is a foundation wall, ask yourself if there is a difference beyond semantics. If the wall is supporting a building it seems to me that it functions just like a foundation wall. I find it impossible to fit a stem wall into the construction that would not.

So the status quo is to accept batch tickets for the record and unless someone says "foundation wall" it's not. Why upset the apple cart? It is imperfect but so much of what gets done is. Has there been a problem? An arbitrary dimension for the height of a stem wall that converts it to a foundation wall is not better that allowing what has been the norm for a long time.

I don't know but maybe you guys do it differently there but the picture that I posted is how it is here ...monolithic...the foundation and any assumed wall is all part of the same pour.....wall and all. Given that the footing is 3000, so is the wall.
Click to expand...
The detail I posted in post #3 was just an example, not specific. That picture does represent what I usually see, but as a monolithic pour. I've never seen what you show in post #7.
 
mtlogcabin said:
So, if the footing is designed at 2,500 pounds per sq inch and the concrete that is used is 4,000 PSI no special inspector is required.
Click to expand...
The parameters of the OP included 3000 psi concrete. The statement that if 2500 psi is required but 4000 psi is used, special inspection is not required is absolutely true and was never in question.
 
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