bracing top of foundation, residential

[planitarch]

Question about the allowable height of foundation wall on a typical wood frame residential project: does adding +1' wood knee wall above a standard 8'tall concrete wall (8" or 10" thick w/2-#5t&b) effectively cancel out the lateral bracing of the wall?

Some of my engineer colleagues won’t ‘sign off’ on that type of design, saying that without the floor joists tied to the sill plate/ anchor bolt assembly, the concrete wall loses its ability to resist lateral load of soil. They insist on retaining wall reinforcing. However, builders in the Chicago area have been getting by with this detail for YEARS without any special reinforcing or spread footings. Some architects (who I will not name) say it’s no big deal either, especially when you consider how foundation walls ‘step’ down around the back on most of these McMansions with walk-out basements. They rely on the weight of the structure alone to resist lateral load of soil, etc.

Tell a builder he has a retaining wall and he’ll look at you like you came from Mars….what to do? Counterforts?

 

[COEngineeer]

I agree to your colleagues. If you dont do any special reinforcing on the footings, that means you assume the bottom is pinned. If you have nothing pushing on the top of the basement wall (i.e. main floor framing), then your foundation wall wasnt designed properly. The floor diaphragm cannot take that much load. Thats why I like to design the foundation wall with fixed at the bottom and assume the floor diaphragm doesn't take any load. But you need additional reinforcing and bigger toe and heel to do this.

 

[CJJS]

thread256-166662

 

[HeberPE]

Just say NO! I've inspected quite a few concrete basement walls with stress cracks caused by this type of construction, and also basement walls for "daylight basements" that weren't adequately braced and then developed long diagonal cracks extending from the top corner diagonally down toward the bottom center of the wall span. If there are lots of jogs and perpendicular walls around the foundation perimeter, then maybe the adjacent walls will butress each other, but once the length of the wall starts to get long, you're just asking for trouble.

Builders may have built like this in Chicago in the past, but I bet those house plans were never prepared and stamped by an engineer either.

 

[COEngineeer]

HeberPE, the sad thing is.. there are a lot of engineers who do not design these basement walls like if it were a concrete tank/retaining wall.