Concrete Wall with NO footing

[cal91]

I have an old concrete building, 6" concrete walls X 25'-0" high. They run about 1'-9" into the ground, but have no footing.

There are 18" X 18" Pilasters every 20'-0", supported by a 3' X 3' footing.

Contractor is adding a second floor inside the building, supported by new walls just inside the 6" concrete exterior walls. He wants to know if we can use these walls for shear so we don't have to make the new walls shear walls.

The minimal footing scares me. Just the weight of the wall tributary to a 3X3 footing is 37.5 kips. Assuming the footing supports the wall thats 4200 psi. Or 3750 psi assuming the wall bears all of it's own weight directly to the soil.

There's not even a geotechnical report, so I can't justify using more than 1500 psf...

Not sure I even feel comfortable doing anything to this building even if I'm staying away from the walls. Any thoughts or suggestions?

 

[KootK]

1) trite answer is that it's fine if the numbers work and there's not an AHJ prohibition against this.

2) if it's me, I'd be looking at all of the wall shear as getting delivered to the discrete footings. Who knows if the wall bottoms are even in contact with the soil any longer or what that's worth in terms of shear capacity.

3) expect #2 to have some important implications for wall detailing.

4) what kind of soil do we expect here? If the walls are socketed into stone, that might be a bit of a different conversation.

 

[Deker]

Is it possible that this is a tilt-up wall supported only at the panel ends and interior pilasters by piles under a small pile cap?

 

[emmgjld]

No project location or even regional soils info. Very difficult to say anything specific.
This is similar to what was used for expansive soils, A voided wall (sometimes the wall is in contact with the subgrade soils) and isolated pad system. The 6 inch wide wall is smaller than I would care for, in most cases.
Again, No project location or even regional soils info.

 

[BridgeSmith]

Deker's query about piles seems like something worth investigating, assuming you don't know what is or isn't under the footings. If there's a pile or drilled shaft supporting those footings, then that changes everything, possibly even the need for the new wall to support the second floor.

 

[NorthCivil]

I see you have a nice big hole dug there.

Why not throw down a bit of crush, drill and epoxy a bunch of 15M rebar into the existing pilaster & walls, form a nice big cage, and fill her up with 30MPA?

form a nice big foot for that existing pilaster.

Sorry for the crude explanation - though these things tend to be a bit crude, don't they?