frost footing vs. monolithic slab

[thejack]

when do you draw the line when deciding frost footing with cmu wall for slab support, vs. monolithic slab.

reason i ask is that I will have to bring in 4-5' of fill to get slab up to grade. then a retaining wall, etc(not in that order specifially). OR should I do the frost footing and cmu instead?

is there a code to follow in regards to this?

best,
dc

 

[ToadJones]

Going the fill route will probably cost more.
I ran into this on a few houses years ago.
General contractor wanted a slab house. The site was such that it took about 6 courses of block to get strip footing below frost. Then, he had to purchase and truck in tons and tons of fill.
Going with a full basement at 11 or 12 courses + concrete basement floor would have been significantly cheaper.

Check into the fill prices closely. I imagine you'll have to properly compact that fill as well.

Other route is a crawl space.

 

[thejack]

oh sorry, this is for a garage slab. forgot to mention(24x16)....

 

[concretemasonry]

If it is an attached garage, then use the same foundation system as the home. That is relatively obvious because they are connected.

If it is an detached or independent, lightly loaded structure, use a stem wall system with a floating slab for the garage floor.

Here (Minnesota) the best long term preference is a stem wall system (top about 6-8" above the floor) for the vertical loads and to provided the desired curb inside the garage to make cleaning easier and allow the floating floor to be sloped for drainage. If the slope is substantial, a block wall system can be more economical because of the ability to use with stepped fotings.

Since it is just a floating slab, the thickness and soil compaction is up to the engineer that is familiar with the the site and soils. Excess excavation materials can be used to fill the exterior at minimal cost and provide protection an increase the depth beyond the mythical frost level.

Dick


Engineer and international traveler interested in construction techniques, problems and proper design.

 

[thejack]

it is detached, and i am also in MN. (weeps)


i guess that was my main question "the thickness and soil compaction is up to the engineer that is familiar with the the site and soils." ... there is no engineer involved as it is a residential garage. it is not economically viable to hire one either. sooooo.... to add a bunch of fill and compact in lifts and top with a monolithic slab, or stem wall.......( u prob already answered this, but is there a rule of thumb as to when you automatically consider the stem wall vs. the mono floater?) or will the mono slab function just as well structurally as long as the sub grade is prepared properly( compacted to max density).

dc

 

[ToadJones]

Sorry, I may have missed the boat on the OP.

By mono slab, are you referring to a FPSF...Frost Protected Shallow Foundation?
If so, there are many details found on the web for FPSF.

 

[thejack]

sounds like it means the same. by monolithic, i mean footing and slab all in one...

typically i pour a 10"x10" footing tapered up to 4" in the middle. with 2 #4 bars continuous around footing, wire meshed, class 5 sub base....