As others point out the sustained typical pretension of 600# per guy
tends to pull over the post over time. For 100' tower and three guy
levels a ballpark horizontal force is about 1000# at the top of the 7'
post. Then consider a storm that may raise the force by 5x or so.
A big factor in a proper design is the soil and how it resists these
forces. The "post" choices are a back guy to a deadman chunk of
concrete, a large concrete pad to resist the overturning moment and
sliding forces, or a deep enough hole with enough concrete weight and
surface area to resist the moment. Depending on the soil properties one
(or two combined) will likely be best for cost and ease of
construction. So there is no simple "formula" and that is why many
homebrew designs are leaning over.
This is a design where professional help is really needed. The strength
of the post is the easy part.
For my new 140' R65 tower the PE designed 7' post was in the ground 8'
with several yards of concrete. To avoid the mower conflict, I decided
to mulch around a standard Rohn anchor buried base as a simpler, cheaper
and easier to implement design. (also PE approved).
Grant KZ1W
On 7/17/2015 6:12 AM, Marsh Stewart wrote:
Fred,
Depending on your soil, under recommended guy tension your guy post might
start to "lean in" after a while.
I had 60' of guyed Rohn 45G in service for about 23 years. The guy posts
where 8" ID 1/4" wall steel pipe. Each guy post was 10' long - 6 feet up and
4' in the ground in 2' X 2' X 4' of concrete. Each guy post was also filled
with concrete. After some time the guy posts began to lean a bit in at the
top. The guy posts did not bend. If I were going to do it all again, I'd
make the "face" of the guy post bases to the tower much wider than 2'.
73,
Marsh, KA5M
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Fred
Keen
Sent: Thursday, July 16, 2015 11:37 PM
To: john@kk9a.com; TowerTalk@contesting.com
Subject: Re: [TowerTalk] guy posts math
Well John my cab tractor is about 7'4". I'm in the process of putting up a
70' rohn 45g and have set 8 x 21 beams (8" deep x 21#s per foot) 12' long,
4' down into 3' x 3' x 4' concrete bases to allow me 8' of clearance.
(The bases of the beams have three 3/4" x 30" long rebar welded on both
sides of the sunken beam inside a rebar cage.)
My engineering reference scoffed at using pipe as the wide flange beam is so
much stronger.
Fred KC5YN
Sent from Yahoo Mail on Android
From:"john@kk9a.com" <john@kk9a.com>
Date:Thu, Jul 16, 2015 at 7:07 PM
Subject:Re: [TowerTalk] guy posts math
Why does he want them this high? To do it correctly the post and concrete
need to be professionally engineered. According to mechanical engineer
K5IU, elevated anchors require many yards of concrete per anchor and a lot
of steel. They are expensive! I used 4' high anchors made from I-Beams at
my P40A QTH when I needed them to clear the house roof. I use Rohn buried
anchors at my home station and the guys wires are only low close the anchor.
I do not see this as an issue. I can easily cut the lawn around the anchors
and cables with my John Deere zero turn 72" mower and the anchors are not in
an area where anyone normally walks.
John KK9A
To: "towertalk@contesting.com" <towertalk@contesting.com>
Subject: [TowerTalk] guy posts math
From: Lee George AK4QA <ak4qa@msn.com>
Date: Thu, 16 Jul 2015 15:34:01 +0000
Does anyone have a the formula for guy posts?
I have a friend that wants tall guy posts (7 feet) for a 100 foot tower so
He can walk under them. I need to show him the stress that is involved in
that as opposed to 2 feet out of the ground. I've always used the wooden
pole rule of thumb; for every foot up you need 3 feet down.
Also, if you have the calculation for the back guy (i.e. earth screws) well
my friends, that would be gravy on my biscuit!
All the best and
73,Lee AK4QA
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