Small Wall on Slab vs Wall on Footer

[bb7100]

I am trying to understand the differences between a traditional footer, wall and slab pour vs the non-traditional thickened edge slab and wall. This is for a personal garage in my back yard. The garage is 24x24 and it is sunk into the ground 3 feet in most places except at the garage door.

(I put in rebar and drainage in the drawing just to show that it would exist, not to show the proper placement or type).

The benefits of the traditional method that I am aware of are as follows:
[ul]
[li]It makes it possible to install additional thermal barriers between the floor and the outside pours.[/li]
[li]It is possible to replace the floor independently (if it cracks etc).[/li]
[/ul]

The benefits of the non-traditional method that I am aware of are as follows:
[ul]
[li]Could complete in two pours instead of one.[/li]
[/ul]

I don't fully understand the load bearing implications of the 4' wall (with ~3' of gavel/earth back filled). It doesn't make much sense to me that the traditional would be much stronger than the non-traditional.

Can you help me understand why I shouldn't (or should) do the proposed wall on slab?

 

[cvg]

most garage slabs do not traditionally rest on the footing. a strip footing is constructed first allowing the framing to be completed. usually the framing is wood, it appears that yours is concrete retaining wall. Since you have a retaining wall, your footing needs to be constructed as part of the retaining wall and the slab may be poured at the end.

 

[RHTPE]

I would not be so quick to declare that the wall-on-thickened slab is not "traditional", as I have seen it detailed that way quite often. It really depends on the locale and its foundation requirements (frost depth, soil conditions and the like).

Either method can be designed to work for the application that you show.

Now, that said, the slab is most often placed after the building is closed in (particularly in residential work) for the convenience of the contractor. By that point in time, the base gravel will have been placed, and the slab would be shaded from the sun making placing & finishing less subject to the whims of Mother Nature.

Making the slab placement the very first operation subjects it to a number of detrimental conditions, i.e. weather, ground water, poor base material, difficulty in accomplishing accurate layout, etc.


Ralph
Structures Consulting
Northeast USA

 

[cvg]

I would not be so quick to declare that the wall-on-thickened slab is not "traditional"

I didn't say the wall-on-thickened slab is not "traditional", only that a slab on grade should generally not be sitting on top of footing.

in this case, the wall is retaining earth and must be designed integrally with a footing designed to resist sliding and overturning. A simple strip footing (as in case 2 above) will not work, the wall would tip over. the footing can connect to and form part of the garage floor if desired.

 

[bb7100]

cvg it was actually me that had called it non-traditional (because I haven't found many written references to that configuration).

The garage will be built with wooden framing. Since my back yard has an upward slope and I didn't want it to sit on top of the hill, I am excavating so that the rear of the garage sits into the hill. This is the reason for the small concrete stub-wall.

Since the stub wall surrounds the garage (except the garage door opening), I would think the perpendicular sections of the wall would provide sufficient support from the fill behind the wall, just like in a traditional foundation.

 

[bb7100]

http://theconstructor.org/structures/retaining-wall-design-principles-2/1333/

shows a few retaining wall designs for low heights that are similar in nature.

 

[cvg]

Since the stub wall surrounds the garage (except the garage door opening), I would think the perpendicular sections of the wall would provide sufficient support from the fill behind the wall, just like in a traditional foundation.

well, you need to do the calculations to prove that. and it really is not laterally supported all around, only at one end.