[TowerTalk] Guywires

[jimlux]

Kevin Normoyle wrote:
> Is this analysis wrong?
> http://wiki.contesting.com/index.php/Guyed_tower_study
> 
> If so, where?
> 
> It's a 45g analysis. I would think that means 25g sees more deflection 
> at the top.
> 
> I didn't understand people talking about 1" deflections.
> 
> What are the typical worse case deflections 100' 25g towers see in bad 
> conditions? (at the top say, given whatever guy conditions you like?)
> 
> I guess the analysis above doesn't address dynamic oscillations or 
> torsion issues.
> 
> Assuming you don't have enough damping, it would seem flexing is good 
> (so stress isn't transmitted to the base?)
>

Some flexing is good. What the typical analysis does is look at the 
forces on the vertical tubes and diagonal braces (the braces are longer 
and skinnier, so even though they're not loaded as heavily in absolute 
terms, they may be closer to their buckling point).  For the most part, 
these are thin columns loaded in compression, so buckling is one of the 
concerns.

When you start looking at multiple guy points, as well as nonuniform 
wind loads, the calculation gets complex.

There's a fair amount of margin in the manufacturer's recommendations to 
cover variations in installation, variations in material properties and 
manufacturing (weld size, for instance), as well as aging and usage 
effects (that ding in the side of the tube has a big effect on buckle 
resistance, as anyone who has stood on an aluminum beer can knows).

That margin is why anecdote isn't a good guide; that is, "I put up 130 
feet unguyed, and sure it waves a bit in the wind, but I just hold on to 
  a leather belt wrapped around a cross brace with my teeth to hang on 
while I beat the rotor into submission" stories are totally believeable, 
but not a good basis for design.

And, hey, what's acceptable for a ham with 40 acres (if it didn't fall 
down last winter, it wasn't big enough) isn't necessarily the case for a 
public safety agency putting up a VHF repeater antenna at HQ.

>