Frost Depth for Interior Grade Beams in Unheated Spaces 1

[SoleSteel]

We are having a discussion in the office about frost depth requirements for certain interior grade beams. Previously on wood garages and breezeways for multifamily units, we've been specifying frost depth requirements for the interior grade beams in these areas as they are unheated. We are getting some pushback from contractors on the depth of these beams. I've heard justifications like water would not typically penetrate these locations since there are exterior grade beams sealing the perimeter of the building and that some breezeways are only open on one side so snow would likely not pile up in these locations.

I would like to either reaffirm our position on the issue or come up with a better solution. How have you guys/gals treated this issue on your interior beams in non-heated spaces?

 

[MotorCity]

I agree that for a large building, surface runoff probably would not find its way to the interior building foundations.

But you still have to deal with the possibility of groundwater (depending on where the water table is). Not saying it is or is not an issue.....place your bets.

From the contractor's perspective, why should he care? He is getting to build whatever is on the drawings.

A reasonable compromise might be to use shallower grade beams with adequate drainage piping.

 

[oldestguy]

I'd opt for some undercutting, replaced with clean sand or clean gravel, allowing for shallow depth foundation. 18 inches might be suitable, depending on water table and soil type. Experience with outside slab heaving may influence the decisions.

 

[SoleSteel]

By the way, we typically deal with 36" frost depth here in Colorado, depending on the jurisdiction and the location and the application is interior grade beams for PT slab on grade.

 

[JAE]

I've had numerous geotechnical engineers tell me that, over time, the moisture content in soils below a building will gradually rise.

If the building is unheated, and is perhaps vented or arranged that the interior temperature can get as low as the outside air, then as a prudent structural guy that I am, I would think about my client and/or owner and insist on frost depth footings unless some other means of prohibiting frost heave (i.e. the granular over-excavation idea above).

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[SoleSteel]

@MotorCity, I feel that groundwater freezing would be more of an issue than surface runoff and that would depend on, as you said, the water table elevation. The issue is that a particular client is giving us feedback from the contractors and I think it's important that we have a firm and sensible position on their feedback. We don't want to be/look unreasonable. The drainage piping idea is interesting. Are you thinking piping on the outside of the building to drain surface runoff away from the structure or embedding a pipe in the shallower grade beam to drain groundwater?

@oldestguy, I just left a comment that our application is PT slab on grade where they will typically excavate the grade beams and fill them with concrete before adding gravel and vapor barrier on the soil in between the beams. I assume your idea of undercutting is for the entire layout of the floor or was it only for the interior beams?

 

[JAE]

Your floor would have as much, or more, exposure to heaving than the footings I'd think.

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[SoleSteel]

@JAE: I think it is a matter of explaining it to the client that it's in their best interest. The granular over-ex idea is interesting, but that would have to be underneath the slab right? After the grade beams are poured, then the grids between our beams usually have a 5" layer of gravel and vapor barrier placed above them before a 5" slab pour above that. If ground level is 8" below top of slab, then that would mean our gravel base starts at 2" below ground level. Wouldn't the over-ex granular fill have to start much deeper than 2"? Seems to me like that might be a costly option (an option nonetheless). Please correct me if I misunderstand.

 

[JAE]

Yes, you'd have to get the granular base down deep enough to avoid any heaving.

In freezer warehouse projects that we've done (and seen done by others), for example, the top slab is supported over about 8" of rigid insulation. This is then supported over about 10" of granular material with a heating system in the middle of the 10" depth. This is either an electric system or a glycol system.

Without the heating system, you would then have to look at the potential temperature of the inside (vs. the outside) and then adjust the depth of footings or granular accordingly.

So if the exterior frost depth footings are at 36" below grade, your interior footings may be at 30", or 24" or something proportionate to the low temperature.

Alternatively, there could be only the insulation that might help further reduce the depth (look up info of frost-protected footings).

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[Mjkkb2]

I always locate them as if they were perimeter elements (down below the frost line). No pushback (yet).

 

[structSU10]

I want to make sure I understand your terminology - is the grade beam designed to span over the soil, not relying on it for support, but relying on the exterior wall / grade beams? Basically, is this a structural slab on grade? If so, could the interior beams / slab be casted over void forms to preclude frost heave issues, and thus be shallower?

 

[oldestguy]

The floor slab and the grade beam are different with respect to risk against movement. Yes, if you don't want your floor to move and neither want the grade beam to move, you do a deep undercut for the building. However, I'd put more effort into keeping the grade beam where built than the floor slab. Thus limiting the special resistance to heaving probably limits that work to the grade beam and let the floor move as it will.