[TowerTalk] An Engineering Question

[R. Morris]

David Clemons wrote:
> 
> Hi,
> 
>         I have a question concerning guy wire anchor placement.  I have
> two towers (one is 83 feet of Rohn 25 with a Hygain 155BA and 105BA, the
> other is 99 feet of Rohn 45 with Cushcraft 204-CD and 402-CD.)  A recent
> edition of the ARRL handbook says that the anchor distance should be
> between 60% and 80% of the tower height.  I have two anchors on each leg,
> with the lower two sets of guys on the inside anchor and the highest guy
> alone on the outside anchor.
> 
>         Now the problem - the 13 acres of property behind me was bought
> last year and the new owners are putting up horse corrals for their
> boarding/training business.  We recently discovered that four of my guy
> anchors (both anchors on one leg of each tower) are actually on their
> property.  They have been nice enough to tell me it's ok, but two of them
> actually fall inside one of the corrals and I think it best to get them
> out of there.  I am not going to move the towers, so that means I have to
> put the anchors closer (the outer ones by 12 feet or so) and also
> probably shorten the towers somewhat.
> 
>         The underlying question really is "how much can I cheat on the
> angle of the guy wires?".  Now there are those among you who will
> correctly tell me that any cheating is dangerous, and I understand that
> concept.  However, I am also aware that there is a different amount of
> danger associated with the anchor at 59% of the tower height than at 39%
> or 29%.  Also, my use of two anchors takes some strain off the anchor for
> the top guys - thus making the issue of the strain on the wire and tower
> legs more likely to be the critical problem.  The guy wire is 1/4 EHS,
> which I believe is rated for 6600 pounds.  The wires are broken up with
> insulators, and some of the wires are fastened with deadends while other
> connections are made using 3 cable clamps on each side.  (If it mattered,
> I could replace all cable clamps with deadends.)
> 
>         Does anyone have any experience or idea of the reliability of an
> installation with the top anchor at 50% of the tower height?  How about
> as little as 40% (about 68% angle to the ground)?
> 
> 73, Dave Clemons K1VUT
> 
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Check the Rohn book on the fold-over pages. Can't get to them right 
now, but they use four anchor points at 50% or less...

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