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Re: Frost footings - whats the big deal?

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In the code, there are HDD limits on using thermally protected footings. I think it's around 4000-4500, but don't quote me.  I'm well south of you, and we're in 5500 HDD territory, so it's not really an option anyone takes.  I will say that - for deck footings - it has become popular with some builders to rent a bobcat with a 2' auger that is used to plant trees.  They drill holes for the footings and just fill 'em with concrete. They have about a 6' reach, though around here our frost depth is only 24". 

I once had a similar discussion with an architect on a PA project concerning an exterior slab adjacent to an assisted living building.  The stock detail in the office was a turned down slab edge to frost depth - i.e. scrape off 8" of soil, dig a 3' trench around the perimeter, put 4" stone in the center and then place 4" of concrete plus the trenches.  I asked what would happen when the soil 8" below the slab dropped below 32 degrees.  We decided for the cost increase and the client's purpose, we would dig a 3' hole and fill the center with 57 stone so there could be no moisture present to form an ice lens.
Jordan


Jim Wilson wrote:
I did find it and that is very informative.  The trick is to know when following the code may not be the best answer.  Justifying deviations from the code is of course essential to sound engineering judgment.  The foam option is one such legitimate alternate in some cases.  It may be a reasonable way to retrofit an existing structure, even if it doesn't completely meet code requirements.
 
Its the little things, however, like front stoops and exterior slabs doweled to permanent foundations, precast exterior basement doors bolted to foundations, etc. that are very often done without frost footings.  Yet they are considered acceptable (or ignored out of ignorance) by inspectors and other code enforcement individuals.  I would submit that damage caused by frost heave under these structures can be much worse than frost action acting on a simple deck footing.  It would be nice to find some practical "commentary" on how to apply the frost footing requirements in gray areas.  And perhaps also on other ways to use foam insulation to mitigate frost heave (when not necessarily a frost protected shallow foundation).
 
Jim

 
----- Original Message ----
From: Daryl Richardson <h.d.richardson(--nospam--at)shaw.ca>
To: seaint(--nospam--at)seaint.org
Sent: Monday, November 5, 2007 7:32:16 PM
Subject: Re: Frost footings - whats the big deal?

Jim,
 
        I responded to this in detail on 10/17/2005.  If you can't find it in the archives I can re-send it to you.
 
        One thing you can do to prevent your problem is to provide ground insulation.  2" of Styrofoam buried a foot or so below the surface (to protect the Styrofoam will provide about the same protection as four or five feet of soil.
 
Regards,
 
H. Daryl Richardson
----- Original Message -----
From: Jim Wilson
Sent: Monday, November 05, 2007 8:35 AM
Subject: Frost footings - whats the big deal?

Frost footing issues are a real hot-button topic with local inspectors.  But in some cases I think they are going overboard in their interpretation and critical review.  Are there any legitimate references or theories on how or when frost depth requirements can perhaps be relaxed?
 
For example,
-Are frost footings appropriate when adding a small addition to an existing structure not on frost footings?  If a house has worked for 100 years with 18" deep footings, won't adding 48" deep footings next to them cause potential heave differentials?
-If an existing garage slab opening is not on frost footings because the rest of the foundation has 4' of dirt piled around it, and the slab has no cracks after 40 years, isn't it acceptable to leave it alone without underpinning this small area?
-If deck footings go down 24" instead of 48", is that acceptable, even if it is not ideal?
 
These are similar to situations I have seen where inspectors are drawing a line in the sand.  The costs they are subjecting homeowners to to make repairs seems to exceed the risk of some possibly slight foundation movement.
 
Local frost depth in northeast PA is 42" or 48".  Ground is often very sandy and rocky with positive drainage. 
 
Jim Wilson, PE
Stroudsburg, PA
 

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