Tall Crawlspace Foundation Walls

[TRAK.Structural]

Ok this question is for the residential experts out there. See the photo, I see this type of system frequently where I live where homes have sloping grades on the property. Essentially a CMU foundation wall that extends anywhere from 1 to 4 feet above crawl space grade and then a wood framed wall on top of this until it reaches the first floor framing. Wood knee walls range in height but I've seen 6 to 8 feet tall on the high end. My questions relate to the residential code and how to interpret this condition:

1. Does this scenario fit the prescriptive provisions and therefore NOT need an explicit engineered design? (Assuming there is less than 4ft of retained soil on the outside of the CMU wall)
2. Does soil constitute "permanent lateral support" for the base of the wall, or is that specifically reserved for slabs in contact with the wall?
3. Sometimes I review other consultants drawings and I've never seen a detail showing this condition; I've even noticed this for homes that are already built with this exact condition. Has anyone ever designed and detailed this way?

 

[phamENG]

I don't live in an area where this can happen, so I don't have person experience with it. Here's what the IRC has to say:


What are you quoting for number 2?

 

[SWComposites]

if you are in a seismic zone, those cripple walls are notorious for falling over in an earthquake, and thus need to resist shear loads from the building above.

 

[ResidentialStructGuy]

Usually any thing higher than 14" I check to resist seismic loading.

 

[TRAK.Structural]

What are you quoting for number 2?

See below for number 2. I know this is specifically for more than 4ft of retained soil, but still unclear as to what exactly qualifies as lateral support for the base of these CMU walls. Sliding friction and passive pressure on a footing provide resistance, but is that the intent of this part of the prescriptive code? It matters for considering out of plane wind load on these walls.

if you are in a seismic zone, those cripple walls are notorious for falling over in an earthquake, and thus need to resist shear loads from the building above.

Typically they are framed just like any other exterior wall with sheathing and anchors into the top course of the CMU so in plane shear loading isn't much of a concern. I'm more curious about out of plane lateral loads. The wood framing is probably fine b/c of my previous sentence, but with a prescriptive CMU wall there isn't likely any grout or rebar in the CMU except at the top course. So any out of plane lateral reaction at the base of the wood walls must be resisted by a cantilever CMU wall without grout/rebar.

Usually any thing higher than 14" I check to resist seismic loading.

In plane or out of plane?

I would think out of plane seismic (in my area) is minimal due to very low self weight of the wall itself and the weight associated with floors above will be distributed to perpendicular walls by the diaphragm.

Are you checking the CMU as a cantilevered wall with an out of plane lateral reaction at the top for wind/seismic on the wall?

 

[XR250]

They do this around here to save money (obviously). I can never ge the numbers to work - especially with 4 ft, of backfill and un-reinforced 8" CMU with some wind load. Honestly, I have never seen an issue with one in practice.

 

[phamENG]

but still unclear as to what exactly qualifies as lateral support for the base of these CMU walls.

Ah - got it. Anything that stops it from moving - friction, passive pressure, etc. Friction is not a great one to use, but typical mortared bed joint at the footing with the footing embedded to frost depth is typically sufficient.