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[TowerTalk] earth anchors & screw-in anchors

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Subject: [TowerTalk] earth anchors & screw-in anchors
From: (Pete Smith)
Date: Wed Feb 26 15:40:07 2003
At 11:00 AM 2/26/03 -0800, Jim wrote:
>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).

I'm NOT an engineer, but if Jim's point is that the dynamic loads on a 
tower are equally shared among the guys at all levels when they are 
attached to a single equalizer plate and anchor, then I'm 
unconvinced.  K7NV has done some finite element analysis on guyed Rohn 45 
that seems to demonstrate that deflection of the tower, under load, is very 
unevenly distributed among the different levels of the structure.  Seems to 
me that this must reflect different tension on the guys at different 
levels.  I wish Kurt were around to comment, but I suspect he's off in NZ 
tending to composite spars at the America's Cup (at least I hope so, for 
your pleasure, Kurt...).

Jim's point about different guy materials rang a bell with me.  I know that 
my Phillystran top guys stretch about twice as much as EHS.  Fiberglass is 
somewhere in between, and the first 21 feet of my middle guy set are that 
material.  Only the bottom guy set is all EHS.  Does the Rohn equalizer 
plate really have the ability to manage all this variation and apply it 
neatly to the single guy anchor?  Or might it be better, in my situation, 
to anchor the top guys separately, set their static tension per spec, and 
rely on the huge margin of the entire system to ensure that if the wind 
ever blows hard enough to approach the breaking point, I'm going to be 
worrying about a lot of things other than my tower.

>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.

This may be what they are doing, but it still worries me.  Sure, you can 
inspect the anchor rod down to the point where it enters the concrete block 
anchor, but if you've parged it with an asphalt sealant, it can be hard to 
see what may be going on under the coating.  Even then, there's also the 
possibility of failure through electrolysis inside the block.  It may be a 
choice among non-optimum solutions, but I think there's a legitimate case 
to be made for the scheme I proposed.

73, Pete N4ZR
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