[TowerTalk] earth anchors & screw-in anchors

[Jim Lux]

At 01:00 PM 2/26/2003 -0500, Pete Smith wrote:
>At 11:57 AM 2/26/03 -0500, RICHARD BOYD wrote:
>>W3LPL uses a separate anchor for each individual guy wire.  Just another 
>>"data point."
>
>This is another point that makes tremendous good sense.  Rohn seems to 
>lean heavily, in their catalogue, toward bringing all of the guys on a 
>given side to a common anchor, and then using an equalizer plate to 
>transfer all the loads to a common anchor rod.  Of course, if that rod 
>were to fail, for whatever reason, the tower would be free to fall over 
>like a tree.
>
>It seems to me that it would make good sense to attach at least the top 
>set of guys from a tower to a separate anchor a bit further out.  Doing so 
>would tether the tower in the event of a failure of either guy anchor on a 
>side, and might give you a chance to retrieve things.  It would also give 
>you the option of spreading the top guys a bit farther than the others, to 
>ease stacking.

But, when you use multiple anchor points, you run the risk of not sharing 
the load among the guys as intended. The relative tension in the guys will 
change with their relative length, which changes with temperature.  If you 
have separate attachment points, you'd need to design each guy for more 
(conceivably, all) load, or design some sort of load equalization into the 
system (i.e. maybe the tower flexes a bit).

The same kinds of issues need to be dealt with if the guys have different 
"spring constants", either because they are different lengths, or because 
they are different materials.

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.

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.

Just goes to show that nothing is as simple as it seems at first glance...


Jim Lux, P.E., W6RMK