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Re: Nashville, TN area foundation help

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OK... I am an old guy.

For smaller (30 to 40' span) PEMB's with just spread footings, hairpins are very efficient in transferring the lateral thrust from the anchor bolts into the slab and the slab WWR forms a tension tie. For longer spans you need to put in a tension tie in the form of a rod or rebar in a tie beam. If you have drilled piers, you are correct, they can take a lot of lateral thrust depending on the soil.

The best resource for PEMB foundations is Foundation Design and Construction Manual by Buettner, Fisher, and Manske (Computerized Structural Design, Inc.). It was written for Butler Manufacturing.

My own practice is to use a tension tie until you get beyond a 15.2 k service thrust load. Beyond that I use a tension tie. Again, if the soil requires them for gravity loads, I may use a drilled pier. The only potential problem is that you have to calculate the long term lateral movement of the top of the drilled pier. It is not a huge problem, but it has to be considered.

If you use a turned down edge of a slab in bearing, you generally have to discount the top 18" (no overburden to facilitate passive) which puts moment into the slab, and again, the passive pressure is allowed, but there is the potential for sideways settlement in some soil types. It is doable, but it adds some considerations.

The Foundation Design and Construction Manual also has some good information on using horizontal soil resistance.

Harold Sprague

From: Bill Polhemus <bill(--nospam--at)>
Reply-To: <seaint(--nospam--at)>
To: seaint(--nospam--at)
Subject: Re: Nashville, TN area foundation help
Date: Thu, 31 Mar 2005 11:40:44 -0600

Harold Sprague wrote:

In a former engineering life, I designed foundations for several PEMB manufacturers.

The first attempt was always to use the turned down slab with the hairpins to take the thrust out of the 3 hinged arch. If the thrust was too great, or you had a feature that precluded you from using the slab as a tension tie, you went to a tie beam and spread footings. In that case it did not much matter what the edge of the slab was like.

The US Army Corps of Engineers lists the frost depth at 22" for Nashville, but you should call the local building official if applicable.

Stepping right in it...

I never got the "hairpin" thing. It always seemed to me to be in the
category of "stuff the old guys did because they always did it that way."

Because My PEMB foundation designs are typically on drilled piers, with
edge beams typically on the order of 30" depth or so, I figure there's
plenty of "oomph" there to resist the thrust without worrying about

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