Mark:
What I believe I know about the pier/pad style of foundation is that it
exploits the mechanical properties of the covering soil. (Short tutorial
for those who care: a pier and pad foundation consists of two parts: a
thick slab of reinforced concrete at the bottom of the excavation and a
pier, or slimmer vertical column of reinforced concrete, poured on top of
the slab or pad. Kind of like an inverted letter T in the hole.)
The overturning moment of the tower is resisted by the soil that
surrounds the pad or slab. That is, when the wind blows against the tower
and the tower tries to topple over, the wide footprint of the pad pushes up
against the overlying dirt. If the pad is of the proper size, i.e., it has
been designed with the soil's mechnical properties in mind, there will be
enough dirt to keep the pad, and hence the tower, from overturning. (You
are correct in your interpretation of Rohn's drawings.)
The mat foundation is nothing more than a blob of concrete whose entire
mass is needed to resist the overturning moment of the tower and wind load.
(Again you are correct.) One could construct such a mat of concrete
entirely on top of the ground; the soil provides minimal overturning
resistance. In general that means you'll probably need more concrete with
the mat design. The pier/pad design uses the soil (it's free!) to provide
additional resistance to overturning.
The one drawback to the pier/pad design is that you have make sure you
properly backfill the hole. Remember, the dirt lying above the pad provides
overturning resistance. It must be compacted to nearly its original density
and strength. You don't just fill the hole around the concrete with dirt
and slap at it with the back of the shovel. You must dump a half a foot or
so of backfill into the hole around the concrete, then punch it with the end
of a two-by-four or similar tamping tool until it's compacted. Then dump in
another six inches of dirt, punch at it some more, etc., etc., etc.,
etc.,.....
You might also be able to have a local PE design a third alternative:
a simple concrete column. He'd take the soil's properties into
consideration, as usual, to determine how big it would have to be. I'm sure
you'll be safe with a PE's design even if it isn't from Rohn's catalog.
Good luck and let us know which way you decide to go.
73 de
Gene Smar AD3F
----- Original Message -----
From: <EL34GUY@aol.com>
To: <towertalk@contesting.com>
Sent: Friday, February 04, 2005 11:10 PM
Subject: [TowerTalk] SSV foundation
> Hello Everyone,
>
> Its been over 60 degrees here today and Im starting to think about putting
> up an 80ft Rohn SSV tower I have. According to the rohn documentation, I
have a
> choice to either go with a mat foundation that's one big chunk of
concrete
> or with a pier pad configuration. The pier/pad looks like it depends more
on
> earth support than the sheer weight of the concrete in the mat
configuration.
> The pier/pad requires 3 holes vs 1 for the mat. Does anyone know which
> foundation method yields the best support for the tower? I havent seen
anywhere
> where theres a sq ft rating for either method. Im in a fairly windy area
on a
> hilltop and would like the best strength possible. My soil is also very
dense
> and has a lot of clay in it so it packs very well. BTW the sections Im
putting
> up are 7,6,5,4. Thanks for any feedback you might have. 73
>
> Mark
> W0NCL
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
> _______________________________________________
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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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