[TowerTalk] Guyed Tower with Dyneema rope guys

[Jim Lux]

At 01:06 PM 1/7/2006, Richard (Rick) Karlquist (N6RK) wrote:
> > My company based in New Zealand  is looking at designing a guyed tower for
> > New Zealand conditions and using Dyneema  ropes, with are made from
> > "superstrong" polyethylene fibers. I'm interested in hearing from
> > anyone who
> > has used this material for guys. Also I'm particularly interested
>
>Antenna guying is what is known as "corner case" in terms of application:
>
>1.  Because of wind loading, you need high strength vs diameter.

I'm not sure that the wind load on the guy is a determining factor.  In any 
event, for thin wires, drag is a VERY nonlinear function of diameter, so 
it's a non trivial tradeoff.

>All the "miracle" ropes I've seen are only high strength per weight.
>I see that Dyneema is the same diameter as steel for the same strength.
>Thus the only important parameter defaults to cost per strength (see #3
>below)
>
>2.  UV exposure is worse than perhaps any other use.  I see that Dyneema
>loses 1/3 of its strength in 2 years.  Not clear what happens after that.
>
>3.  Cost is of the essence.  We already have Phillystran.  It seems to
>be perfectly acceptable in nearly all respects.  I can't imagine
>using any substitute unless there was a substantial cost saving.  Actually,
>there's nothing wrong with steel in antenna applications where you
>don't need nonconductive guys, and it's even cheaper.

The original poster was in NZ, so perhaps the cost factors are different 
(maybe aramid cables like Phillystran are not cost competitive in NZ?)





>4.  A rope that didn't stretch much might be worth spending extra bucks
>on.  I couldn't find anything definitive about that on the web, except
>weasel words about the fibers changing alignment with age.
This, to me, is the most critical aspect of guying with some alternate 
material.. the springyness.  Elasticity isn't necessarily bad, but does 
require looking at all the engineering.  You can't just slavishly copy a 
design intended for use with steel guys. I would think that one of the 
trickier aspects would be column buckling loads, since a stretchier guy 
allows more bend in the tower, which aggravates the column buckling problem 
(why you shouldn't lean too far when balancing on that empty beer can).