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Slab on grade bracing basement wall.

Slab on grade bracing basement wall.

Slab on grade bracing basement wall.

(OP)
In my area it is not uncommon for below grade garages to have CIP basement walls with the bottom of the walls braced simply by the slab on grade. Typically with an isolation material between the wall and slab. When the garages are 3 or 4 stories deep the reaction at the bottom of the wall is pretty large. While this appears to work from experience, I cannot provide a calculation to show it works. Passive pressure of the embedded wall footing is not enough. The compression stress on the SOG is small, but what is the load path after it get's in the slab? Not enough friction, sloped slabs to drains, slab folds do not guarantee a load path to the opposite wall etc.

What ways have you used to show a complete load path?

RE: Slab on grade bracing basement wall.

I don't rely on slabs on grade for the bracing of basement walls. My approach is to include ground beams between opposing faces of the retaining walls that act as struts to brace them. The continous footings beneath the walls work as beams in a horizontal plane. I'm in a seismic area so it's really a basic requirement for every large project we design around here and it ties all the foundations together.
The owner can then do whatever he wants with that slab on grade (sump pits, holes for infrastructures, whatever).

RE: Slab on grade bracing basement wall.

I'm in a non-seismic place and all basement walls are assumed to rely on SOG for restraint. There's friction and passive but, as you pointed out, those often do not work by the numbers. And nobody's really checking the numbers anyhow. One time, I attempted to look at the SOG as a structural compression member. Like you said, compressive stress isn't a problem. As it turns out, buckling isn't a problem either. Run the math on what it would take to buckle a slab on grade upwards, against it's self weight, and you'll come up with an impressively big number.

I'll pay a little more attention to things if there are significant elevation changes in the SOG near the wall or if I expect that future SOG removal really is something that needs consideration. As with all things related to retaining walls, I think that our best friend is that reality that the design loads never come to pass -- or even close. If they did, all those shear reinforced, 10" walls three stories below grade would be bowing in over top of the hoods of the cars parked in front of them. There's a reason designers never check basement wall deflections...

Quote (OP)

but what is the load path after it get's in the slab?

Across the space and into the wall/soil on the other side, like avscorreia's tie beams. Hence my concern for the SOG elevation changes.

The real paranoid folks among us sometimes worry that the slab on grade might shear off the bases of our columns. Wouldn't that be a kick in the nuts.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Slab on grade bracing basement wall.

What are you bracing the top of the wall with? Another slab on grade in the direction opposite of the basement or a wood/steel diaphragm? I've never done anything 3-4 stories deep but I've done quite a few that were braced by a slab on grade or a steel deck diaphragm at the top. I prefer not to use wood unless the load paths are clear. I'm also lucky/unlucky to work in an area with highly expansive soils so my slab on grades are riddled with deep heavily reinforced grade beams so I will occasionally design the basement walls as cantilevered if necessary.

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