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Re: [TowerTalk] Screw Anchor Question

Subject: Re: [TowerTalk] Screw Anchor Question
Date: Tue, 17 Jan 2006 17:02:19 EST
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In a message dated 1/15/2006 4:35:22 P.M. Pacific Standard Time, writes:

>  My tower project is starting to come together, but I have a  concern that
came about while installing the screw anchors today.  Two  of the anchors are
in undisturbed soil, while the third is in soil that was  disturbed when the
house was built.  The soil was used to build up the  backyard of the house
about 8 months ago.  What prompted the following  question was that one
anchor went in CONSIDERABLY easier than the other  two.  The tower is a
planned 70 foot Rohn 25 installation with two  sets of guys with a tribander
and 2 element 40 on top. The anchor is on the  downwind side of the
predominate wind here.  

>  How  long before disturbed soil is considered undisturbed?  I am  concerned
that the anchor in question may pull out due to the soil it was  installed
in.  Are there any recommendations as to what I can do to  improve this
anchor and in turn make me sleep better at night after the  tower is up?
    Lots of folks have weighed in on this question so  I thought I'd throw in 
my 2 cents worth.
    I'm not a big fan of screw-in anchors,  particularly the ones that Rohn 
sells. They're pretty small and Rohn says they  are rated at "2500 pounds 
holding power" plus they're pretty small - 5/8" rod  with a 4" auger. With 
sets of 3/16" EHS guys on 100' of 25G for  instance, the proper tension of 400# 
times 3 sets of guys is 1200# with  NO WIND - cutting the capacity of the 
anchor by almost half. And the  other half is going to be used up pretty quick 
when a big wind comes up.  Obviously you can have less guys per screw-in and I 
strongly recommend it.  

And of course nothing is said about soil  holding capacity which the only way 
you're going to know for sure if it's  going to work in the first place. And 
you have to have a  geotechnical  survey (not cheap) in order to know more 
specifically about  your soil conditions. 
    Concrete anchors OTOH will go just about anywhere  - even sand if they're 
big enough. If you're going to put in industrial-type  screw-ins like AB 
Chance, etc., you will increase your reliability over the  funky Rohn versions. 
    And utility pole guying is WAY different than  tower guying - different 
vectors, forces, heights, etc. 
    Back to your original question - it takes YEARS  for soil to compact 
adequately back to where it was before it was excavated  and fluffed up so it's 
mystery why that screw-in went in so easily and it's  pretty obvious that 
it'll be a tower failure point waiting to happen. I'd  recommend a concrete 
anchor for that anchor at least. 

Cheers & GL with your installation,
Steve    K7LXC
Professional tower services for commercial and amateur
Cell: 206-890-4188

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