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When you have a basement wall that needs to join up to a garage slab at the top of the basement wall, how are the footings for the garage slab placed, and how does the garage slab attach to the basement wall?

I've done a bit of research into foundations, and understand that footers need to be poured on virgin soil (undisturbed so it is compacted) in order to avoid major problems with settling later.

But if you excavate and pour the basement wall, then back fill behind it, the earth where you would want to put your garage slab footings would be the back-filled earth which you are not supposed to compact.

The IRC defers to ACI 332 for foundation and concrete walls, but I'm not finding anything particularly relevant to the problem. One website mentioned "Jump Footings" also called Discontinuous Footings, but it sounds like the maximum change in elevation can be five feet. It also sounds like this is used in cases where the garage has concrete walls, and the concrete garage walls are joined to a concrete house wall.

I've drawn a picture to illustrate the problem. It's a side view of the house footers with the poured walls, and the garage footers with the slab, and a gap is clearly visible in the problem area.

foundation question picture

  1. Is there a standard code-compliant way of dealing with this that doesn't require an engineer? (The county inspector says that if anything is listed in the IRC 2012, no stamp is required).

  2. If an engineer is required, what is the typical approach used?

UPDATE:

For greater clarity, the house does not exist yet, this is new construction. I'm designing the house myself, and want to work through as much as I can first and then just bring the problems to an Engineer all at once (if needed). If there is a standard solution in a code book, then no engineer is required in my area.

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Be careful. You are about to change the "structural dynamics" of the basement wall.

When concrete walls are poured, reinforcing steel is placed on the TENSION side of the wall. In your case, the concrete wall is designed as a retaining wall. That is to say, the concrete wall is NOT restrained at the top. Therefore, the reinforcing steel is placed in the outer one-third of the wall, (soil-side of the wall). (By the way, the floor joists sitting on the wall, parallel or perpendicular, do not count as restraint the top of the wall. The wood will shrink over time and the wall will be a retaining wall.)

If you restrain (connect) the top of the wall to the garage slab, you've changed the wall from a retaining wall to a beam. Now the tension side (and reinforcing steel) is on the inner (room-side) one-third of the wall.

If you connect the garage slab to the top of the wall, you'll have failure (cracks and possibly crumbling) depending on how "wet" your soil is...

Go see an architect or a STRUCTURAL engineer, not a CIVIL engineer.

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    This really doesn't answer the question. There are millions of homes with garage slabs and basements. Was every single one of them custom designed by an engineer? There must be an acceptable standard approach for this. – Mark Jul 8 '17 at 12:29
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    @Mark Yes, you are correct, "There are millions of homes with garage slabs and basements." However, their "structural dynamics" weren't changed by the homeowner. – Lee Sam Jul 8 '17 at 16:57
  • For greater clarity, the house does not exist yet, this is new construction. I've updated the question to reflect this. – Nick Jul 8 '17 at 18:16
  • @LeeSam - but neither was this one. – Mark Jul 8 '17 at 19:48
  • @Mark Oops...my mistake. I see you need the wall and connection into the garage slab designed. That's more than I can do in the "comment" format, so I think I'll add a second "answer". Sorry about that. – Lee Sam Jul 8 '17 at 22:02
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It's complicated, but I'll make a few assumptions:

1) Soil bearing is average: 1500 psf

2) Span (height) of wall: 10'

3) Moisture in soil against wall: none (I'm assuming you have sufficient perimeter drains at the base of the basement footings, so no moisture will lay against the wall.)

4) Loading: 2 story house with both the second floor and roof bearing on wall, plus the vehicle. (We call this "super imposed" Loading.)

5) Material: Concrete wall, (because it's easy for me to calculate. If it's masonry, let me know. The width of the wall will probably increase to 12" and the reinforcing steel will change to a module (spacing) of the masonry.)

So, looking at your drawing, you'll notice that the soil is pressing against the left side of the wall. This means the top of the wall wants to "pull away" from the garage slab, while the bottom of the wall is "pressing against" the basement slab. (Well, actually you don't have the bottom drawn the way we like. We like a 6" step down in the basement slab so that it is pressing against a 6" tall lip...and these calculations assume you make that change.

The top of the wall will need "hooks" in the rebar in order to fasten the wall to the garage slab.

I'd use: 8" thick concrete wall with #6 rebar at 12" oc EACH WAY with the VERTICAL rebar placed 3/4" clear from the face of the INTERIOR BASEMENT ROOM. This steel runs vertically and should lap the same size reinforcing steel out of the footing. Also, the vertical rebar should bend into the center of the garage slab. It should extend a minimum of 36" into the slab and have a minimum of 16" hook, (curved preferred) and thicken the garage slab to 8" thick where it laps the main garage slab...(see below.)

I'd also install the same reinforcing on the other side of the 8" concrete wall too. (That's due to reverse bending of the wall, because the end of the wall is fixed (rigid). if you hire an architect or structural engineer, they may eliminate this...

I'd make the garage slab 6" thick and reinforce it with #4 at 12" each way and extend it into the 36" zone by the wall.

Like I said, it's complicated. Good luck.

  • Oh, to answer your original question, no, there's nothing "standard" or "prescriptive" in the Code...especially for your situation: 10' high wall plus super imposed loading. You'll need some professional help to satisfy your Building Official. – Lee Sam Jul 8 '17 at 22:48
  • Could you elaborate on, "Well, actually you don't have the bottom drawn the way we like. We like a 6" step down in the basement slab so that it is pressing against a 6" tall lip"? Is there a site with a good picture of how the foundation parts are supposed to come together? – Nick Jul 17 '17 at 23:55

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