Frost Depth Requirements for Unheated Slab on Grade 1

[JKJohn]

A brief description of the project:
A pre engineered steel frame building. The building is used to assemble heavy power generators. The assembly will take place in the middle of the building on a moving station. The station will move in and out of the building on steel tracks, which extend 20-ft away from the building, so it could be picked up by a gantry crane and placed on trucks. The tracks need 18-in deep foundation and be poured monolithically with 8” reinforced slab. There is no foundation on the perimeter. The column foundations are shallow foundation and extend below the frost depth. The frost depth is 30-inch. And the building is open and not heated. Does the track foundation inside the building need to be to the frost depth? PS I am assuming the track foundation outside the building needs to meet the frost depth requirement.

 

[MiketheEngineer]

Yes

 

[RVSWA]

Mike's right.

 

[BigH]

Any reason to pour the slab monolithically with the track foundations? Assuming that the soil is competent, you might want to think about using insulation to protect the track foundations and slab. Another point to consider is whether the soil you have is frost susceptible. If it is clean sand and the water table is, say, 10 ft down, there is little need to carry footings to frost depth - you need frost susceptible soils, water and cold temps to cause frost heave (via expansion of pore water - not that much) or formation of ice lenses. Such would not occur in clean sands and gravels. However, if you have silty soil, then frost protection - by going below the depth of frost or equivalent in insulation will be necessary. Follow cold weather concreting protocols if placing concrete in the winter. One of my earlier jobs was warehouse floors that heaved some 2 inches - but heaved during construction - once construction was over (and winter) they settled back down and caused no more problems.

 

[JKJohn]

Monolithically for two reasons: no need for forms and making sure there is no difference in elevations.
I guess the slab as well needs to meet the frost depth. 30-inch slab is a lot.
I found publication at HUD for FPSF:

http://www.huduser.org/publications/destech/desguide.html

Do you recommend using this guide?

 

[oldestguy]

Hold on guys:

How fussy is the cart and contents for exactly being level?

If you reinforce things so that differential frost action won't result in "faults" between slab sections (and it will crack sometime), can you live with some uneven floor and track elevations?

Take a look at the cost of doing all that "frost depth stuff" and you may find you can live with a moving slab, dependent on the seasons.

How's come you know the frost depth is 30 inches? If you have clean sand, it may go much deeper maybe, but won't heave. If you have clay, it probably will heave some, but frozen depth may be only 12 inches. What is the local history with slabs on grade?

I'd step back before doing all that work and think about it.

Closed cell insulation does wonders if you really are fussy.

 

[JKJohn]

Oldestguy,

I think you hit it right.

If is very important that the cart and content are exactly leveled to the point that they have hydraulics to level it up. That being said, there are some limitation to the hydraulic movement, and they don’t want the slab/foundation to heave and don’t want uneven floors.

I looked at the cost of all “frost depth stuff” and it is very expensive, and that is why I turned it to you guys to help me find a less expensive way of doing it.

As for the frost depth, it was determine by a geotechnical report. And the soil under is silty sand.

Anymore information about the closed cell insulation?

 

[CarlB]

If you can tolerate no frost heave movement to the slab, you could remove soil to 30" below the slab & replace it with non-frost susceptible (NFS) soil. This is standard pavement design methodology in subarctic regions. When NFS soil freezes it will not wick moisture nor promote ice lense growth. A soil with less than 5% fines (passing the # 200 sieve) is generally NFS. Consider closed-cell insulation to reduce the frost depth & therefore the depth of NFS required.

A possible insulated section:
About 6" NFS at bottom as there weill be very small amount of frost penetration through the insulation, plus a smooth subgrade is needed to place the insulation. Then 2" board insulation. Then 12" NFS over this. Six insulations would probably be enough (thermally) but is difficult to compact just a 6" lift over insulation.