Lookout Foundation Wall - Lateral Bracing

[MidwestSE]

Hi Everyone,

Here's a question for residential projects. Cantilevered concrete stem walls are popular in my area for lookout basements.

The governing residential code says you can use propped cantilever stem walls for wall runs of up to 35' before a perpendicular wall is required to brace the end of the propped cantilever wall run.

The question is this: would you think a 2' jog in the wall as shown in the picture would be justified as a wall long enough to be considered a perpendicular wall sufficient to brace for the end of a 35' run? I have a project where there are a couple such jogs in a 70'+ long run.

Thanks!

 

[KootK]

would you think a 2' jog in the wall as shown in the picture would be justified as a wall long enough to be considered a perpendicular wall sufficient to brace for the end of a 35' run?

It's hard to imagine that the 2' wall, acting as a cantilevered shear wall, could possibly resist the overturning demand imposed by the 35' span unless the 2' wall is, itself, laterally braced at the top. You could run some numbers on that to find out easily enough though. You'd be able to utilize some of the 35' wall run dead load to resist the overturning which will be helpful.

 

[BAretired]

It helps. So does the enlarged footing under the column, but it depends on the height of fill retained by the wall. The width of footing appears to be only about 2'-0", not much for a retaining wall.

BA

 

[MidwestSE]

Thanks for the input!

 

[KootK]

Is the long run of wall really not braced laterally by a diaphragm at the top? I as, because when I've encountered the business with the returns in the past, it's usually been in reference to temporary construction conditions before the diaphragms are installed.

 

[MidwestSE]

Correct - it is not braced by a diaphragm at the top. It's a stem wall that sticks up +/-4' above the basement slab on grade and has a half-height wood wall on top of it. Its called a "lookout basement" wall and it's used a ton in my area. It's code approved and used all the time on smaller production-style homes. For smaller homes, the rules are very strict on how long of a wall you can use. A lot of contractors try to use them on bigger custom homes that are well outside the accepted use limitations, and try to put those little jogs in the wall and say "that's bracing the next 35' of wall"

 

[8-of-Trades]

Could you tie into the frost slab ratwalls for your lateral resistance? They look long enough to count for your perpendicular wall if they are tied in and matching height.

"You don't know what you don't know, until you don't know it.

 

[MidwestSE]

8-of-Trades - in my area the frost walls and the slab are, 99% of the time, never connected for single family residential projects. I could try, but I'd doubt it would get built properly!

 

[KootK]

I don't love it but, on occasion, I've treated things like this a bit like sheet pile walls. The SOG restraining lateral movement inwards and the footing plowing into the soil providing lateral restraint outwards to form a moment resisting couple. It's dicer for suction loads on the wall of course but, then, there's usually less demand that way.

This kind of thing probably ought to be a truly cantilevered retraining wall. That said, I do realize how tough of a sell that is in the residential marketplace.

 

[XR250]

4 ft. of backfill wont take much more than a standard footing to act as a cantilevered wall (32" +/-) - assuming the wall and footing are properly reinforced. It is an easy sell in my neck of the woods and is really common.