Foundation of a concrete shearwall in a low-rise concrete structure with a bearing wall system?

[33mmrrdd]

What is the common foundation of a concrete shearwall in a low-rise concrete structure with a bearing wall system? Since most of the lateral load will be resisted by the shearwall, I've noticed that a shallow foundation (wall footing) is not enough, thus requiring a deep foundation. Am I right?

 

[Lomarandil]

Shallow foundations can resist lateral load through friction, particularly when the vertical load of the bearing wall is reliably present. I would be surprised to see that deep foundations are required for a low-rise structure unless your soils are particularly poor for vertical load capacity.

 

[33mmrrdd]

Sorry, I forgot to mention I was referring to the overturning moment that will be carried by the shearwall. Since most of the lasteral load was carried by the wall, I noticed that the overturning moment produced by the lateral load is significantly larger than the resisting moment, so the wall is unstable.

 

[jayrod12]

I would say the foundation type is too much a function of local soil capacity and typical construction techniques. Where I am, it's not uncommon to put a single storey building on deep foundations because our regular 40 foot thick clay is pretty volatile as far as volumetric changes goes.

 

[brTCP]

A continuous footing is the most common solution in this case.

 

[lexpatrie]

I'm not convinced the foundation is typically checked for this by a lot of engineers.

https://www.woodworks.org/resources/five-story-wood-frame-structure-over-podium-slab/

(That's not really fair, because that's a woodworks document meant to walk people through the design of the wood elements, but if you're not paying attention and following their approach, you're not going to check the foundation for overturning or sliding.

The continuous foundation is pretty typical, as brTCP indicates, and if the building is long enough there's a fairly large Sx involved in resisting that moment, so just because an engineer doesn't explicitly check it doesn't mean it will be a problem, but it can't be precluded out of hand, and it's not great that it's omitted. It would likely be more problematic if a) structures more routinely saw 50 year wind loads, b) "unestablished" load paths, and c) inertia, as in the wind is a three second gust outside hurricane zones, larger than planned dead loads, etc.

Source:

https://bayarearetrofit.com/shear-wall-overturning/

(with scribbles by me)

Stress distribution on the foundation would change if there is a larger moment as the soil can't do a tension load. This sort of load would increase the downward pressure on the footing (P/A + M/S at the extreme far end) that might affect the reinforcing in the strip footing beneath the wall.

ETA - I'd be more concerned with the transverse moment in the strip footing beneath the wall, as the cantilever past the wall itself is fairly small, as depicted. Usually longitudinal reinforcing here is sized based on either minimum reinforcement, 4/3 of that required by analysis (whatever that may be, for example presuming dead snow and live loads only and sizing based on a presumed 5' depression the continuous footing has to span across), etc.