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Re: frost heave

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MessageThere are actually TWO reasons why you drop the edge of a slab lower.
The first is frost heave and the second is interior heat loss.

Frost heave: The way frost heave action was explained to me, the heaving
forces are so high you won't be able to design to resist them.  Even if you
could, the ice lens could still develop and would push the soil under the
footing down instead of pushing the footing up.  When the ice melted, you'd
have a void and mushy soil under your footing.

Heat Loss: The perimeter footing typically has a layer of insulation
adjacent to it (at least they typically do around here), which helps prevent
heat loss through the slab, into the soil, and out of the building.  If the
footing is not there, the perimeter of the slab will be cold during the
winter.  This will increase HVAC loads, and may not meet local energy
conservation codes.

I've never designed a post-tensioned slab, but I have specified them on
apartment complexes.  The foundation company used (based out of Texas)
designed the slab using our loads and the geotech report.  The perimeter had
a 10" wide trench footing down below frost depth (36").  The footing was
nominally reinforced and poured monolithically with the slab.

It probably isn't economically feasible with only a 24" frost depth, but if
you absolutely do not want to dig a trench you can look into a "Frost
Protected Shallow Footing".  You extend a layer of insulation out
(horizontally) from the edge of the building.  Heat from the interior flows
into the soil under the insulation and keeps the ground from freezing.  The
insulation keeps the heat from escaping into the atmosphere.  Here is a link
to a report:  http://www.cs.arizona.edu/people/jcropper/desguide.pdf

Good luck,

----
Jason Kilgore
Leigh & O'Kane, L.L.C.
jkilgore(--nospam--at)leok.com
816-444-3144
816-444-9655 (FAX)
----- Original Message ----- 
From: Pat Clark
To: seaint(--nospam--at)seaint.org
Sent: Monday, October 13, 2003 3:55 PM
Subject: frost heave


I am looking for any references or input on designing a p.t. slab in an area
that typically requires frost protection down to 24".  I am wondering if the
slab could be designed to resist the heaving forces from the possible frost
lenses.  And if so, what sort of criteria could we design the slab for to
approximate those loads?

I am designing the structure, and consulting with a p.t. engineer out of Ca,
so they do not have any idea of what those loads would need to be.

Any recommendations would be most welcome.



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