Basement Wall Trust Resisted by Slab on Grade

[kevin555582]

Hello All,

I have been reading a lot of great threads on here about how people design (and make assumptions about) basement walls and how to resist loads, especially at the footings. One aspect I have not seen discussed (maybe it exists but I can't find it) and am curious about is how everyone views the interaction/support from the basement floor slab on grade.

What I have in mind is the following:
1. Typical basement wall of any height.
2. Heavily loaded laterally but lightly loaded axially.
3. Wall spans from the footing up to some diaphragm.
4. Supported by a spread footing.
5. Wall is adequately attached to the footing.
6. Typical residential 4" thick slab on grade just above the footing separated from the wall by a 1/2" expansion joint.

From what I have read and learned from other engineers it seems like once there is a slab-on-grade supporting trust at the base of the wall the load path is complete. But what are the limits of this? Would you only check the bearing strength of the concrete at the slab edge? When would you do a further analysis of the floor slab? Would you ever consider the slab to buckle upward away from the soil?

I don't have a specific design in mind to share but have been doing taller and taller basements, one was on a lake so there was a little bit of a hill being retained by the wall (with less soil on the resisting side of the house). Sometimes I get soil reports with good 4,000 psf soils which shrinks footing widths (which reduces sliding resistance). All of this has just made me curious how other engineers view the limits of the slab on grade to resist thrust at the base of a basement wall.

 

[driftLimiter]

Hey Kevin,

There are a few things to consider for slab resisting shear on grade. First is the simplest and most useful. Friction of the slab on grade Is the first load path I consider. If there isn't enough capacity there you could tie the slab to some footings or below grade wall edges and use additional passive there. This concept is pretty much like a Deadman anchor. Another way to get strength is to tie the slab in to shearwalls and use it as a reinforced diaphragm. ACI has provisions for diaphragm slabs on grades, it can cause issues with joints and typical detailing so I think this is only used as a last resort.

TLR
You should consider the capacity of the slab to resist sliding shear at the base of retaining walls.

 

[le99]

4. Supported by a spread footing.

Spread footing or strip footing? For a 4" thick slab thickness, how do you connect the wall to resist the out-of-plane lateral force?

 

[SteelPE]

Sounds like you are doing mostly single family residential designs. In your instance I would just follow the criteria outlined in the IRC and then move on. That is, as long as your project falls within the parameters of the IRC. If it doesn't, then I would engineer a solution for the wall that I am comfortable with.

I currently don't have the IRC memorized, but I don't think your lake house foundation wall with the hill above would comply with the tables of the IRC. So an engineered solution would be necessary... which may require the wall to have a bigger footing (with a heal and a toe) in order the gain proper sliding resistance.

I believe there was an old thread in here from years ago. The thread talked about using slabs on grade (SOG) for sliding resistance. I seem to remember the consensus being that if the SOG was to take this lateral thrust then it would need to be designed and reinforced properly (which it never is in residential applications).

The system you are designing has been a thorn in my side for a while. I don't think I could ever "prove" the IRC tables using my standard calculating techniques. This probably has to do with my initial assumptions being somewhat conservative.... which is why when designing these systems I would just default to the IRC.

 

[KootK]

My thoughts:

1) In my experience, engineers nearly always use the SOG to resist lateral thrust at the bottom of foundation walls. When I've looked at the numbers, it's often somewhere between difficult and impossible to get things to work based on friction and/or passive soil resistance at the footing unless the footing is earth formed into a dense bearing medium or pinned to that medium somehow.

2) In my opinion, the most competent and commonly relied upon mechanism of SOG shear transfer is not to use friction between the SOG and the soil but, rather, to push across the building and into the basement wall on the other side if conditions allow for that. One of the limits on this is where you might have a vertical step in the SOG or a trench drain etc that would interrupt this load path.

3) I once looked at upwards buckling of an SOG. That's pretty tough to make happen. You'd almost need to be working with an unreinforced SOG with some significant non-levelness in there.

4) For all that I've said above, I suspect that the reality is that these things work out because:

a) Design soil loads rarely come to pass and;

b) You probably do get some decent friction and passive resistance at the footing.

Good drainage behind your wall is probably your best friend in this.

 

[XR250]

Koot's #4 is my vote. I have been in hundreds of basements and can never remember an instance when there was not a gap between the slab edge and the foundation wall or expansion joint and foundation wall. I believe the IRC allows the unbalanced fill to begin at the slab for design. I typically design from top of footing because of this.