RE: Frost footings - whats the big deal?

Hello Jim,

I would like to suggest a different way of looking at this.  Instead of "following the code", I believe that we should design structural objects according to structural engineering principles and then check to see that the design does not violate the code.  This may seem like just a play on words, but it is more than that.  The building code is not supposed to be a cookbook although one might get that impression from some of the seminars that are given about it.  In my opinion, that is why the code is getting to be so voluminous and so confusing, because many engineers expect it to tell them how to design instead of setting parameters for the minimum loads that must be resisted.  It is our job to determine the load path without reference to the "code" and then to design the elements for forces specified in it.

If this sounds different than what you generally hear it is because I work primarily on correcting existing problems with structures that were designed piecemeal with elements that "follow the code" but which do not add up, or connect, to form a complete load path without a weak link.  I have been doing this for over fifty years now and I am able to reflect on how this is increasingly occurring.

For your examples of the little things (that I agree are important), remember that unlike the main structure, putting collapsible foam under them may not prevent heave because they  would not have enough weight to crush that material.  Hopefully we do not need another code section to deal with that.

Richard Hess SE

-----Original Message-----
From: Jim Wilson [mailto:wilsonengineers@yahoo.com]
Sent: Tuesday, November 06, 2007 5:49 PM
To: seaint@seaint.org
Subject: Re: Frost footings - whats the big deal?

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@shaw.ca>
To: seaint@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

To: seaint@seaint.org

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