Walk Out Basement Foundation Detail

[GaStruct]

For a walk out basement in a climate where the frost line is only 12", how do you all normally detail the foundations? I prefer to have footings and walls on all four sides of the house, including the walk out side. This allows me to completely isolate the slab on grade which is the best way to prevent cracking.

However, most contractors in my area insist on only pouring footings and walls on three sides and pouring a turn-down slab on the walk out side. The contractors claim my way is more expensive, but, if you draw both to scale and you design the turn-down correctly (even just using the minimum as shown in the IRC), my way has almost the exact same concrete area. I also see a turn-down footing being more work as it creates a weird detail where the wall footings transition to turn-down footings. Additionally, it ties the slab to the already poured wall footings.

How does everyone else detail this? Attached are details for a turn-down (which I don't use for walk out basements) and a sub-wall (which I do use for walk out basements). Note that these are only sketches.

 

[dik]

I usually use high strength extruded polystyrene or something of that ilk...

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

-Dik

 

[TRAK.Structural]

How does a turndown at the walkout side tie the slab to the wall footings? It will be a separate pour so unless you are providing dowels there wont be a physical connection?

 

[GaStruct]

@Dik What do you mean? I'm inquiring about a turned-down slab vs sub walls.

@TRAK.Structural It is two pours but why wouldn't you have dowels connecting the two pours? Otherwise, there could be differential settlement. All the footings should be connected.

 

[TRAK.Structural]

Not saying you shouldn't provide dowels, just wasn't clear to me how they were tied together from your post.

Your contractor probably just has a process/sequence he likes to follow to minimize time and the number of pours. I don't see a big issue with either route. The turndown at the walkout side may even give you more sliding resistance if that is a concern.

 

[DTS419]

Be careful with connections. More is not always better. Excessive connections between concrete segments can lead to cracking because it restricts minor differential movements from thermal and settling that would otherwise be harmless.

 

[gte447f]

I think your preferred detail is probably pretty unusual in practice. I too prefer a floating slab that is isolated from wall footings in a lot of instances, but in this case, I don't really see any problem with the turndown slab approach, and I think it is very commonly used in this application. I don't agree that the turndown needs to be doweled into the wall footings to prevent differential settlement. By far the most common cause of settlement problems at the daylight side of walkout basements is construction on improperly placed fill. Dowels will not prevent this from happening.

 

[dik]

In areas with 4' to 6' of frost cover, the footing is stepped at the opening and frost protection is provided by the rigid insulation going away from the building and down the face of the foundation wall.

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

-Dik

 

[GaStruct]

@gte447f Thanks for your insight. I absolutely agree with you that the contractors aren't used to my preferred detail on the walk out side. However, they do my detail on the other three sides so they definitely know how to do it. Given all the advantages to isolating the slab I am inclined to keep showing it that way. Unless the contractor wants to take responsibility for it. But, they always want the engineer to sign off on their changes first though! It's always their idea but the engineer's liability. I'll have to disagree about not tying the footings together. I don't think I have ever seen that. All of the construction joint details between pours I have seen always show the rebar coming through the joint so the next pour is tied in. While maybe not critical in a residence, I believe it to still be best practice.

My only critique of isolating the slab is if excessive movement creates a lip at the transition between the slab and foundation wall. However, I don't often see interior slabs move that much after it has cured and dried. As long as the base was built correctly.

Side note, I don't like how turned-down slabs have all the load eccentrically on the footing. While the IRC prescriptively allow this for certain geometry, if you ever actually check the design, it always fails. If there's no brick ledge then the eccentricity is even worse. For a three-story house when all the walls are load bearing above, I just don't like it.