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[TowerTalk] earth anchors & screw-in anchors

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Subject: [TowerTalk] earth anchors & screw-in anchors
From: (W0UN--John Brosnahan)
Date: Wed Feb 26 15:24:55 2003
>There's an old engineering exercise where you have a steel piano wire and 
>a big rope in parallel supporting a weight. The piano wire is very stiff, 
>so takes all the load, and fails.  Another exercise is the "welded gusset 
>on the bracket" that makes it weaker, because it stiffens it, and actually 
>increases the stress.


You raise some interesting points, but the analogy above does not apply because
the guys are not in parallel.  They are separated by a long section for 
flexible tower.  So the tower does the equalizing.  Same with the three guys
at any given level. The tower moves around enough to equalize the tension in
the three guys (under static load--winds are different of course).

>The Rohn approach is a good one, sort of following the "put all your eggs 
>in one basket, and then watch that basket very carefully" 
>philosophy.  Design the one anchor with a huge margin and then hang 
>everything off of it.  Of course, if you "cheat" and put an unreliable or 
>underspecified anchor in, then you've defeated the purpose.

Actually Rohn does not put all of their eggs in one basket for many of
their larger towers and uses separate guy anchor points.  Rohn certainly
has an "equalizer plate" that must serve some purpose but I have never
been totally convinced that it is necessary.  One example--if you have
an odd number of guy levels then the middle guy is always in the middle
of the equalizer plate and receives no benefit from the equalizing.

I really don't think it is all that complicated (but then I am not a PE
but rather a physicist with a lot of tower experience).  I think the
flexibility of the tower itself is greater than any equalizing that might
be done in (at least) the smaller Rohn towers (25/45/55).

I have done a lot of towers up to the 200 ft level from the tropics to
the arctic region and have seen no functional difference whether I
use one guy point or multiple guy points in each direction.   The
multiple guy points do have the advantage of no single-point-of-

The only weak link I have seen is the corrosion of the guy rod
in the soil over long periods of time.  I strongly suggest following
Rohn's suggestion about covering the guy anchor that is not in
the concrete but still under the earth is a tar/paper wrap.

Other than these thoughts I don't have anything much more to
add and I am always open to better ideas as long as there
is some scientific justification.  Just hate to see things made
more complex than they really are.  I guess Occam's Razor
is my guiding philosophy.

73--John   W0UN 

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