Frost Line for Grade Beam with Piles

[PT999]

If piles are driven, with a concrete grade beam poured over the pile cap, does the bottom of the grade beam have to be poured below the frost line, or having the piles driven below the frost line complies with placing foundations below the frost line.

 

[JAE]

We always put the grade beam below frost. Otherwise the frost will heave the gradebeam - and ...even if you somehow design for the heave load (how you calculate that I don't know) you will still have the deflection in the beam and the subsequent damage to elements supported by the beam.

 

[JAE]

I might add that for frost...or for expansive clays, if you correctly design a void space below the beam, then you don't have to drop it below the frost level.

This would include a temporary void form and loose fill retainers on the sides of the beam to keep the void open beneath.

 

[jike]

I agree with JAE. This is our practice also.

I have, however, seen a few drawings by other firms that did not have the bottom of the grade beam below frost. They required a chisel point (frost point) on the bottom of the grade beam. I don't know how well this works if at all. I do not recommend it.

Perhaps someone else can comment.

 

[ghghghgh]

I agree with the others for perimeter grade beams.
How about interior grade beams? I've heard opinions that the building heat (unheated structures exluded) will prevent heave, and therefore interior grade beams need not be below frost, but I have not seen this in any code.

 

[Slowzuki]

A frost protected design using eps or xps insulation may help you if you don't want go below the frost line.

If you are in a normal location, which I assume is the case, even without a frost protected design the interior portions will not freeze.

 

[vmirat]

Here's a very interesting article about frost heave from our friends in Canada...

http://irc.nrc-cnrc.gc.ca/cbd/cbd026e.html

 

[JAE]

jike - funny you should mention the chisel point on grade beams. I had a project in San Antonio some years ago where a very large mansion was built on a stiffened slab on grade - very common in that region. The idea is to create a sort of waffle slab system on the ground - with the ribs spaced about 8 to 16 feet o.c.

The bottoms of all the grade beams were angled such that one vertical side was deeper than the other and the bottom of each grade beam was then sloped at about a 45 degree angle as you looked at the cross section.

I guess the idea was to allow the heaving clay to slide past the beam...but then you wonder, what the heck is holding the building up? Aren't you also knifing down into the soil with your gravity loads?

Our project was a complete excavation UNDER the whole slab and an installation of drilled piers and supplemental steel beam framing to completely support the house. The knifing (chisel point) didn't work obviously.

 

[haynewp]

The piles are only under part of the grade beams. The rest of the grade beam (above frost line level) will be subject to heaving if not designed for (for example with a void form).

http://www.mcgonagall-online.org.uk/poems/pgdisaster.htm