-------- Original Message --------
Subject: Re: [TowerTalk] Tower Base Round or Square?
Date: Wed, 26 Oct 2011 14:02:17 -0700
From: Jim Lux <jimlux@earthlink.net>
To: W0MU Mike Fatchett <w0mu@w0mu.com>
On 10/26/11 11:26 AM, W0MU Mike Fatchett wrote:
> Not an expert either. I think this has come up before. As long as you
> have the same amount of concrete or more at the describe depth you
> should be ok.
>
> Mike W0MU
>
> J6/W0MU November 21 - December 1 2011 CQ WW DX CW
> W0MU-1 CC Cluster w0mu.net
>
>
> On 10/26/2011 11:28 AM, Richard Thorne wrote:
>> I'm finally getting some dirt work done in the next day or two at my new
>> place which will include holes/concrete for my 45g tower.
>>
>> I plan on using a pier pin install, so does it matter if the base is
>> square or round? The rohn book calls for a 2' 6" square hole 4' deep
>> (I'm designing the tower based on 90mhp winds with a height of 120').
>> It would be easier to use a 36" auger to dig the hole. Since I'm going
>> with a pier pin I don't see that there would be any twisting moments on
>> the concrete base, it would only be there to hold the tower up. But I'm
>> not an engineer, hence the question.
>>
Here in southern California, round piers/caissons are fairly common for
retrofit installation things like street lighting, signals, and signage.
You can knock out a section of sidewalk, drill the hole, lower the
rebar cage, fill it up, replace the sidewalk, and you're done.
As you note, it's pretty easy, if you have access to the auger
equipment. (heck of a lot easier to drill a 3 foot diameter hole 4 feet
deep than to dig that square hole by hand)
Consult a local Engineer (or your planning department) to find out what
sort of depth vs diameter is appropriate or typical in your local soil
conditions.
The torsional loading isn't really an issue. The friction between the
surface of a round pier and the surrounding soil is pretty large, and
when you pour the concrete, it's inherently up against undisturbed soil.
They have traffic signals cantelievered out over 4 lanes of traffic
with howling winds blowing on them. I can't imagine that the torsional
load of even the largest amateur antenna is anywhere near a bunch of
stoplights and road signs on a 12" diameter tube.
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