"Top" or "Bottom" of Footing? 5

[TBellP47]

When placing foundation footings for frost heave protection, should the "TOP" or the "BOTTOM" of the footing (spread or strip) be located at the required minimum "frost depth"?

Also, is there a specific code (ACI, IBC, etc.) that requires one of these locations to be used with respect to the "footer" or "footing" when considering the required "frost depth"?

If no specific code covers this issue, is there a preferred, accepted industry standard or practice being used and, if so, which of the two locations would that be?

Thanks in Advance,
~TBell

 

[emmgjld]

My experience has been the local Building Official determines this.
My preferred usage is the Bottom of the footing.

 

[JAE]

Frost depth always has been and should be to the bottom of the footing.

You are trying to avoid a condition where frost occurs in the soil directly under a footing and in which case the soil expands (or rather the moisture freezes and expands within the soil matrix) and dislocates the footing.

Most specified frost depths from geotechnical engineers and other regional practices are usually a bit deeper than the true average frost depth so there is already a safety factor included in it.

For example, in my area, the average measured frost depth is 26 inches...but our "standard" depth for frost is 42 inches.

 

[CarlB]

I agree that "bottom of footing" is the standard in regards to frost depth.

A note on JAE's comment-

I don't agree the 42" footing depth versus an "average" frost depth of 26" is the correct picture of the safety factor. The frost depth value to consider is the design depth of frost, which is typically the 30-year maximum value. In Anchorage AK the design frost depth is about 10 feet, and the design footing depth for heated structures is also 42 inches. The "safety" is primarily the continuous building heat, along with the insulation on the foundation wall, which prevents frost below the footing.

 

[dik]

JAE's on the mark... but, I've encountered frost in Winnipeg parking lot area at 13'... and, in Lindsay, Ontario at 11', also in a cleared parking lot... just depends on temperatures, lack of snow and soil type. Frost protection in Winnipeg is typically 6' and in Lindsay it is 4'...

Dik

 

[msquared48]

dik:

That's not frost at that depth - that's permafrost. Totally different animal. That requires a totally different foundation solution. That requires providing insulating between the permafrost and the concrete foundation so that it does not melt. Have a real special design problem if the local yearly frost goes down to the top of the permafrost.

I believe what was being referred to here is a seasonal frost depth - one that goes away each year. In that case, the lowest level of the yearly frost layer should be above the bottom of the footing, and local frost depth.

Mike McCann
MMC Engineering

 

[JAE]

Below is a link to US "extreme" frost depths, from this site:

http://www.ngs.noaa.gov/PUBS_LIB/GeodeticBMs/#figure13

 

[JAE]

- forgot to mention it is in meters of depth.

 

[fattdad]

I use bottom of footing for frost depth for the perimeter footings. For interior footings, you can place them at a higher grade.

f-d

¡papá gordo ain’t no madre flaca!

 

[MiketheEngineer]

As stated:

Bottom of footing
Check w/ Local building officials

Here in St. Louis it is 30'' - 100 miles south it is 18"

Go figure

 

[dik]

msquared... nope, not permafrost... Dik

 

[concretemasonry]

I was on a project in minnesota where we placed a fill in unfrozen soil in July. It was for a construction haul road.

One year later in the next June, we had to make a cut through the road section for utilities. We found frost 10-15' deep that had to be removed to complete construction.

This was not permafrost, but was frost that was "driven" down by the reptitive loads and fact the road was clean and open all winter, so there was no insulation.

The same winter (after fill placement and before excavation), we ran into areas that had less than 1' of frost. These were protected by trees and a good snow cover and had no traffic or disturbance. - I remember when the official site thermometer said 52 below one morning that winter.

 

[CarlB]

Hoo boy, not the "frost driven down by traffic" theory again. (it's often heard from contractors). I'm still waiting for anyone to produce one study or model that explains how frost is driven down by traffic. The deeper frost under roads is entirely explained by the lack of snow cover and organics.

Sorry to digress...

And dik, yes that depth of frost is not unheard of in interior Alaska either, but is about the upper limit for "seasonal" frost areas.

 

[BigH]

There is one point that hasn't really yet been addressed - for locating the footing in relation to the depth of frost penetration, namely, [green]is the material on which the footing is to be placed "frost susceptible"?[/green] For freezing to affect the footing, the soil must be frost susceptible, you must have a source of water at such a depth that it can contribute to the formation of ice lensing and the soil is permeable enough so that water can 'travel' in the time frame of the winter months. In Terrace Bay Ontario, the frost depth is something like 9 to 10 ft (3 to 3.5m). I put my footings at 3 ft. Why? I was founding on very clean sand (non frost susceptible; the water table was down some 12 ft. So it didn't make any sense to take the footings down some 10 ft. Just a added thought . . .