foundation wall tied to S.O.G.

[paullaup]

In our office, we usually like to tie the top of foundation walls to the slab on grade. We usually cast the slab over the foundation wall and add rebar dowels.
Some Architects don’t like this detail because it conflicts with the energy code. They sometimes ask us if they can float the slab. i.e. no contact between the slab and foundation wall.
I want to use this detail, but all I can offer the architect is that it provides lateral bracing for the top of the foundation wall where the seismic loads are at its greatest.
Although, I believe this to be enough reason to do this detail, are there any other reasons (code directives, etc) why the slab should be tied to the foundation wall??
Any contributions to this is greatly appreciated
Thanks

 

[jheidt2543]

I live in the northern midwest US and on this I would have to agree with the architects (god forgive me!). The detail is a poor one IMHO for the following reasons:

1. For engergy conservation reasons: the slab will conduct cold in the winter and heat in the summer.

2. For frost heave reasons: If the subgrade along the perimeter foundation under the floor slab freezes, the slab will heave and crack. Yes, perimeter insulation normally prevents this, but in some really cold years there can be a fair amount of movement.

3. For settlement reasons: should the foundation wall settle, or move at all, the slab is moved with it and then cracks. If the wall doesn't move, but the subgrade under the slab settles, the slab cracks. Either way the slab cracks. So, let the slab "float".

4. Pouring of the floor slab: If the slab is to be poured over the foundation wall, then it has to be poured before the walls go up, which prevents the enclosure of the building. It also means that the floor slab is exposed to all the construction operations while the walls go up and the roof go on. In the interest of getting a good job on the floor slab, we ALWAYS pour the floor after the roof is on the building (well, nearly always). Then the wind, rain and snow are not damaging the floor finish.

For these reasons, my prefered detail is to separate the foundation wall and the floor slab.

 

[paullaup]

jheidt2543,
Thanks for your response. All those reasons make sense, but if I floated the slab, what would brace the top of the wall? Maybe it's something abvious, but I'm not seeing it? In some cases the dead weight alone would probably be enough, but in a one story building with only roof load, there is little dead load.
... any thoughts?

 

[dik]

The passive soil pressure on each side of the wall? Have to be a little more careful in loading dock areas, of if the wall has fill on one side only... it's in the cost of the detail.

I've also used foam as a thermal break with reinforcing steel 'z' bars cast into a split... and the one leg of the 'z' is straightened into the slab...

 

[NigelB]

Paul,
If you float the slab instead of pouring it over the foundation wall, then you should dowel the slab to the wall to control vertical movement. This is particularly necessary at a doorway to protect against a trip hazard due to heave or settlement of the slab.
In the Dallas Texas area it is standard practice to pour the slab over the foundation wall, and to put an expansion joint between the foundation and the exterior pavement.
Nigel

 

[jheidt2543]

Paullaup,

I agree with dik, that the backfill on both side of the foundation wall provides enough support to recieve "normal" building wall loads. A short 3' or 4' foundation wall 8" thick is pretty stiff even if it is unreinforced.

Now, if the foundation wall is a basement wall, that is different. But, then you have first floor framing to provide lateral support to the wall or much more rebar.

I guess it also depends on how tall the foundation wall is and what kind of floor loading you have. But, I would still refer to my first post and leave the floor "float".

 

[SperlingPE]

Assuming that the wall is not a basement wall, float the slab on top of the wall. To provide a thermal break and pour stop, provide one course of cmu block (tied to the wall) and rigid insulation between the block and slab.