Modify an existing turn down slab.

[johnpaul23]

Hello,
So I have a project that is turning an existing garage (stick framed about 20'x20') into a two story ADU with a rooftop deck. So essentially 3 stories. The current garage is on a turn down slab 12" thick x 12" width. So not only with I need to increase the width, but also make the footing 24" to meet building code. Anybody know of simple solution? Dig up around current slab and epoxy rebar into a new 12 x 24 beside it?

 

[jerseyshore]

I'd think a typical 2 or 3 stage underpinning procedure would suffice for something like this since I'm assuming you are keeping the same footprint and have to drop the footing down for frost 3 ft +/-.

 

[XR250]

@JS - where are you getting 3 ft from?
In these situations, I typically spec underpinning with helical piers or spot footings every 5 or 6 ft.
I try to avoid enlarging footings with epoxied rebar as I feel like there is a good chance it won't be installed properly
I suppose you could get a geo tech out there to evaluate the soil. It may be fine as is. Also, you could try to arrange your framing plan to introduce new loading in discrete areas to minimize the underpinning.

 

[DoubleStud]

I agree with XR250. Helical piers would be your best bet. They have a bracket that they can install on the face of the concrete. Also we want to make sure we are not relying too much on the foundation to span from pier to pier, perhaps you use beefy rim board around the perimeter of second floor. This way you can guarantee the distributed loads will be shared by studs directly on top of the helical pier.

 

[johnpaul23]

Awesome. Thank you all for your replies. I will look into the underpinning and helical piers.

 

[milkshakelake]

Agree that if you use helical piles, make sure to make wood or steel posts going up from them directly instead of having the turn down slab taking moment. 12"x12" turn down slab is nothing, and won't be able to take any kind of loading over its span unless the load goes directly into a support, whether it's underpinning or helical piles. It's okay to use the turn down slab for bearing, but not much more than that.

If you go with the underpinning option, make sure the underpinning pit is deep enough to epoxy bars upward into the turn down slab.

 

[XR250]

Agree that if you use helical piles, make sure to make wood or steel posts going up from them directly instead of having the turn down slab taking moment. 12"x12" turn down slab is nothing, and won't be able to take any kind of loading over its span unless the load goes directly into a support

Isn't the flexural capacity of a 12x12 beam around 10 ft-kips ultimate? That seems like more than nothing over a 5 or 6 ft. span.

 

[DoubleStud]

XR250, I prefer to ignore it since it is existing and I have no idea the condition and the reinforcement.

 

[XR250]

I suppose that is fair, however, how do you justify foundation underpinning to remediate settlement of an existing foundation?

 

[milkshakelake]

I guess it isn't nothing, but I've seen small foundations like that with unknown concrete and reinforcement. It might have high water content, bad soil, or other quality control issues. They would tend to crack or settle, destabilizing the whole building. Some contractors wouldn't want to touch anything near a foundation like that, preferring to take down a structure and rebuild it (in the context of doing an excavation next to it).

With spread footing underpinning, it wouldn't necessarily solve settlement issues, but you're bringing the loading to discrete points that are well supported.

 

[jerseyshore]

@XR250, I just used 3 ft as an example for frost depth which is common in this area, obviously it varies depending on what state you're in.

OP, do you need to enlarge the foundation because the town is requiring it or because bearing capacity is exceeded? I looked at a house last week with a 12" wide footing and that's typically good enough for a regular 2 story house.

I do helical pile underpinnings nearly every week, but if the soil and bearing capacity are acceptable, with the price of helicals these days, if you can find a reasonable mason it's usually cheaper to pour concrete.

I'd compare the cost and schedule. A good helical installer could finish that job in 1 day if access is easy. That may be worth it even if the price is a little higher.

 

[johnpaul23]

@jerseyshore if we use the min. code of 1500 psf bearing, the proposed loading would be about 2000 psf on the 12x12 footing. So we'd need to size up to at least 18" wide.