Barry Kutner wrote:
Hi Barry,
Comments interspersed....
> Sorry if this is a dupe, but I never saw it reflected back to me...
>
> Many thanks to Kurt, K7NV, for his recent tower load analyses.
> Verrrry interesting...
Thanks for considering it!
> In his examples of guyed Rohn 45 with the base in concrete, the
> weakest point was the lower sections, related to movement of those
> sections.
>
> His analyses assume the usual standard of the guys being adjusted
> to 10% of their ultimate strength, in these cases 400 lb for 3/16
> EHS or 4000 lb. Phillystran. When larger guys were used, tensioned
> at 600 lbs., the safety factor was improved, not surprisingly.
The increased safety factor is not caused by the increased pretension. It
comes from reduced elongation from the larger size cable.
>
> This brings me to the following: I wonder if there is
> engineering/scientific basis for the 10% tension rule?
Yes. My understanding of the pretension is that it is required to get the
spiral construction of the steel cable to tighten up enough to have the
cable exhibit linear elongation behavior.
At very low loads, 0% - <10% of the breaking strength, the load vs
elongation is non-linear. The plot of this relationship is a curve,
making it very difficult to predict how it will behave.
The object of the pretension is to get the cable all firmed up so it will
act in a linear predictable fashion.
My guess is that this has been determined in the test lab on many pieces
of cable. Trying to do it with a calculator would probaly wear out any
good battery, and still be questionable.
The Phillystran cables require a similar but different approach
(different material, different physical construction). The cables are
tensioned to 15% of breaking. After approx 1 hour they relax and need to
again be tensioned to 15% of breaking. After that they will relax to the
10% of breaking figure, and are reported to remain stable.
> What if 3/16
> EHS or 4000 lb Phillystran were tensioned to 600 lb? Is this unsafe?
Not necessarily unsafe, but not as safe as the specified pretension.
Depends on the safety factors of the original design.
By increasing the pretension you are increasing the compression in the
tower, which contributes to the combined tower stresses. If tower stress
is the limiting factor, it's not a good move.
And, you are adding the additional load to the cables before the wind
blows. If cable strength is the limiting factor, it's not a good move
either.
--
73, Kurt
K7NV "That's K7 "Nevada" (ex - NI6W)
YagiStress - The Ultimate Software for Yagi Mechanical Design
Visit http://www.freeyellow.com/members3/yagistress/
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