New construction attached garage slab - to bear or not bear on frost walls and full height foundation wall

[BakingFool]

The diagram shows a section view of a draft of my new home. Cars will enter the two car garage towards you, over a 4' frost wall. There are two sections of 4' frost wall that extend on each end of the full height foundation wall on the left, and there are 4' frost walls at the rear of the garage and on the right.

There is a roughly 2" wide step in each wall to create a curb that is the same width as the 2x6 stud wall above. The way this diagram was drafted, it is ambiguous whether the garage slab bears on the walls or not. Google doesn't give me a definitive answer, and it may be a case by case basis.

This is in zone 5A, frost line 44", soil has a lot of clay. There will be fill needed - 6" to 1-1/2' above existing grade under the slab.

My question is, should the slab be poured on top of the 2" wide step in each of the walls, or so it does not bear on them? Also, at the entry I assume the slab is poured over the cutouts for the doors/vehicles, so it has to bear at least on that wall, correct?

Thank you in advance,
Don

 

[BakingFool]

Here is a diagram of a proposed modification of the foundation as first drafted in the post above - which incidentally was done by a draftsperson, not an architect or engineer. This modification changes the common section of the foundation between the house and garage to 10" thick to accomodate the joists being dropped down. It was when I went to modify the above diagram that I noticed the slab appears to bear on the walls.

Note in the diagram attached below that the step the joists sit on has been carried out on the frost wall extensions. This was proposed so the frost walls do not sit proud 2" of the framing above on these two sections of wall.

If the weight of the slab on the 14 foot long 10" thick wall section is undesireable, I am wondering if all the frost walls can be stepped on the inside of the garage (including the two shown stepped on the opposite side in the diagram), slide the two short sections towards the garage 2" so that the sides facing the house align with the soil side of the 10" wall, and the slab bears only on the five frost wall segments.

 

[bill1952]

In a similar unheated garage I built 3 years ago I zone 6 - 48" to frost line - foundation was continuous all around. At garage door top of stem wall is finished and extends into garage about 4". It was actually a separate pour.Apron butts to it at same elevation with an expansion strip. Floor in garage "floats" inside stem walls all around. It's 1/2" higher than top of stem wall under door, with a galvanized bar at that edge. The 1/2" "step" was a detail I saw in a fine homebuilding many years ago and appeared to me for keeping rain from blowing in far.

So I'm for floating unheated garage slab on a heavy vapor barrier on a well compacted base, and if I do it again I'd do same but skip remesh and use fiber in the mix, and no cuts. I believe the unrestrained edges will allow it to shrink evenly and not crack. I'd look at the expense of the metal fibers.