[TowerTalk] pipe anchors

[K8RI on TT]

On 10/29/2011 12:09 PM, Richard wrote:
> I live on a small city lot and am in the process of installing a 40 ft
> Rohn 25G tower in my back yard with a pier pin base. I'd really like to
> avoid using screw in anchors

Screw in anchors are notoriously unreliable.  Yes, I know there are 
those who have used them for years, but they are highly dependent on 
soil conditions and the quality of the anchor.  They are best avoided.

>in such close quarters and would prefer to
> use an elevated guy anchor like pipe set in cement. I originally thought
> that 11 ft would be great to clear my roof line but have been told that
> would require a pipe larger than what I can obtain around here. Revising
> my plans a 4 ft high pipe in the corners of the back yard would still be
> better than going to ground level. Can anyone give me an approximate
> idea as to what size pipe and what size base might be needed?

Elevated guys usually fall in the "not recommended" class as well. That 
said, I use them on a heavily loaded, 100' 45G but they are filled, 
large, braced, and set in 17,000# of concrete. I had to go that route to 
clear the house and driveway. Hence the overkill on the anchors.  Still 
after 10 years, the NW guy which is in the least stable soil (sand and 
clay mix) has moved about 8" to 10" out of plum, meaning that great big 
block of concrete is rotating in the ground.

>
> My tower is 40 ft, I would have at most 12 lbs of wind load on it, my

These are my thoughts and not a recommendations: <:-))

Fortunately a lightly loaded, 40' 25G does not take a lot of guying.
What you can get away with 95% of the time and what's recommended are 
two different things. Just remember that when you do not use the 
recommended guying you are playing the odds.  Actually all installations 
play the odds. It's just that with the recommended installation the odds 
are strongly in your favor for all but very unusual conditions. IE ice 
storms and abnormally high winds for your area.

Recently we had some unusually strong winds from the NE which is an 
unusual direction for us.  A friend found himself out hanging onto the 
NE guys at 2:30 AM in the rain after the tree they were anchored to, 
broke off. When there was a lull in the storm he moved his little 4WD 
SUV out in the yard and hooked the guys to the bumper hitch.

When I took down my old 90 footer (lightly loaded) I gave him the 
elevated guy anchors which were 4" pipe which like your location were 
set in heavy clay. However although his tower is only about 50 feet (I 
think) it is heavily loaded. He set one of those anchors in his sandy 
yard and it was immediately shifting.  We settled on welding reinforced 
"ears" made of 1/4" steel plate, to each side of the pipe giving it a 
wide foot print. 1/4" steel plate made it a lot heavier too.   Now the 
pipe (guy anchor) appears to be at least several feet wide and extends 
well down into the ground and is no longer moving.

The pipe is also going to be poured full of concrete to make it more 
rigid and to keep out water which around here would freeze and break the 
pipe.  With 4" pipe it wouldn't hurt to insert a 1/2" rerod down the 
center before pouring the concrete into it. That would also add more 
rigidity to the anchor. Remember that regular schedule 40 pipe is very 
easy to bend.  I prefer schedule 80, or structural steel tube but that 
does not come with galvanizing.  Use a "closed" eye bolt and install it 
"before" filling the pipe with concrete.

With elevated guys remember the attach point above ground acts like a 
lever so if you are running 400# tension 4' above the ground you have 
1,600 ft lb of leverage at ground level.  That is enough to bend smaller 
pipe or cause said pipe to shift position in the ground and it doesn't 
take a lot of wind to increase that force significantly.

"I would guess" that 4" pipe set at least 5 or 6' deep and extending 
high enough to give you a 4' elevated anchor, set in 400 or 500# of 
qukrete(TM) would suffice for your system as described. Please note, I 
said *guess*.

73

Roger (K8RI)

> soil is a pretty heavy clay with no sand and no rock at least down to 4
> ft., our wind zone is said to be 70 MPH. Any input would be greatly
> appreciated.


>
> 73
>
> Richard / N5YPJ
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