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[TowerTalk] earth anchors & screw-in anchors

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Subject: [TowerTalk] earth anchors & screw-in anchors
From: (Jim Lux)
Date: Wed Feb 26 16:12:49 2003
At 02:24 PM 2/26/2003 -0600, you wrote:

>>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 
>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).

Is the tower really able to equalize it... it's constrained by the other 
guys.  I think the question is not quite so simple.  I started to do a 
quick analysis on this, but got sidetracked.  As you point out, winds are 
another factor.

>>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.

For more than 2 levels of guys, you're right, but maybe that's "good 
enough" for a N+1 redundancy calculation.  I haven't seen Rohn's analysis...

>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).

Maybe, maybe not.. without seeing the analysis you can't really tell.  My 
thing really isn't structures, but I've dealt with them enough to know that 
the loads and stress doesn't always work the way you intuitively think it 
does, particularly for over-determined systems, which a guyed tower is. 
Maybe, that's what makes a good structural engineer: they intutitively get 
it right.

As soon as you start assuming that the structural elements aren't rigid 
(i.e. you assume the tower will flex to adjust), a rigorous analysis gets 
pretty complex, pretty fast.  You can either stay with the simplifying 
assumption of rigid bodies, and put in a design margin, or you can do a lot 
more analysis, which frankly, will probably depend on a lot of assumptions 
of dubious certainty about soil and material properties, and workmanship. 
Kind of like modeling antennas with NEC.

>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-

But, is one big, really good, guy point better or worse than multiple not 
so good guys?  Hard to tell. It's probably that either works, and one is 
more economic in some situations, and not in others.  Certainly one could 
design either so that you'd be reasonably certain it would work.  This is a 
standard question in reliability analysis.. is adding redundancy actually 
increasing the probability of failure.

>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.

There's lots of justification for concern if you're pushing the limits of 
the materials (soil in particular).. On the other hand, if you're not 
pushing the limits, then simple is good...


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