Frost Protected Shallow Foundations ASCE 31

[bookowski]

No experience with doing a frost protected shallow foundation and I'm wondering if I'm missing something here, first glance appears too good to be true.

In my region (nyc) the code requires 48in. for frost protection:
1805.3.1 Frost protection. Except where otherwise protected from frost, shallow foundations, pile caps, and other permanent supports of buildings and structures shall be protected from frost by one or more of the following methods:
1.Extending a minimum of 4 feet (1219 mm) below the lowest adjacent permanent exposed grade;
2.Constructing in accordance with ASCE-32; or
3.Erecting on solid rock.

ASCE-32 seems pretty straightforward, based on my air freezing index (afi) they allow a footing depth of only 12" with exterior vertical insulation and no horizontal extension.

Going from 48" to 12" simply by adding vertical insulation of R=4.5 doesn't seem right to me. The architect is really into it and it solves some other issues with excavation but I'm skeptical. Am I missing something here?

 

[oldestguy]

I didn't read any text but /Figure 1 is goofy. Most of the other figures seem to make sense. Even their arrows as to the direction of cold seem OK.

this one is goofy.

 

[bookowski]

Yeah, the concept makes sense - I was just surprised how extreme the savings is. Excavating 12" and pouring 1ft of wall instead of 48" excavation and 4ft wall by adding an inch or two of insulation? Didn't seem right to me.

 

[KootK]

I haven't dug into the literature but, around here, it's usually 24" down and 48" out, recommendations coming from geotechnical engineers. Granted, this seems to be boiler plate and we don't benefit from the coastal climate moderation that you do. I do find your result surprising. It's got a "if this is so great, why isn't it being done all the time?" feel to it.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[bookowski]

No out reqt for low afi - does seem too good to be true though , 48" to 12

 

[BSVBD]

I am in Wisconsin.

This is a typical grade beam detail for us:

Based on IBC, my supervisor, an architect, determines the required insulation.

Both my supervisor and i hate grade beam foundations for PEMB due to the inevitable eccentric footings and the complicated rebar design within the footing to resist uplift.

We understand our contractor-clients' perspective in that it certainly is less expensive, therefore, we deal with that accordingly.

What we dislike even more is when the client wants a grade beam addition to an existing frost-wall structure.

Fortunately, i don't have to deal with that liability.

 

[CarlB]

I assume the 48" foundation depth is necessary to be below the maximum frost depth, so that depth would likely apply to heated or unheated buildings.

The 12" foundation depth for a FPSF in NYC is for only a heated building, defined as one which has a minimum monthly average inside temperature of 63 F or greater.