I'm wondering if we aren't using these terms interchangeably and they don't
necessarily mean the same to every one.
Tension: "To me" tension is the pull on the guy once it's installed which
usually works out to about 10% of the rated breaking strength of the cable.
Although some can call this arbitrary, it serves two purposes which are
related. Reducing the catenary to some acceptable level and preventing
oscillation of the guy lines. 10% on EHS leaves a decided catenary while
10% on Phyllistran leaves a catenary that is darn near impossible to see.
However due to the laws of physics you can not eliminate a catenary in a
line with a horizontal component.
The tension is pretty much a balancing act and part of a resonant system.
Think of the tower as a very low frequency tuning fork or pendulum. Deep
catenaries in cable that is not under enough tension will allow the tower to
move back and fourth with the mass of the cable in the deep catenary adding
to the energy. With heavy cable and a deep catenary I would expect under
some conditions the movement of the tower could be at the low frequency
resonant point of the guys. Not a good thing. Tightening the guys does two
things. It raises the natural resonant frequency of the guys and reduces the
amount the tower can move. A shallower catenary also does the same. However
we reach a point of diminishing returns as much past 10% takes a big change
in tension to make much of a change in the catenary.
I'd add that the tower and guys are not a simple resonant system, but have
multiple resonances and multiple modes which depend on guy length,
elasticity, catenary/tension, mass of the tower, mass of the guy, and
probably a lot of things I've forgotten.
If you want to check this out, find a guyed tower with 1/4 or 3/8" EHS guys
and with the owners permission, "whack" the guy with a soft hammer (rubber
or dead blow) 4 or 5 feet out from the anchor. You will see a "wave" travel
up the guy to the tower and be reflected. With really heavy guys under
thousands of pounds of tension, they will "sing" like a tuning fork and
you'll not be able to see the low frequency wave.
Pre-tension: as used in the Phyllistran instructions is putting a length of
line under tension and keeping it there *while* the Big Grip (TM) anchors
are installed. Rephrased, the anchors should be installed on the cable
while it is under the proper tension. I have a double cam like device that
grips the guy cable and hooks to a come along. In conjunction with a
tension gauge I can pull the cable tight, install the big grip in the proper
location and then release the tension. If I can get the anchor for the come
along in the right spot I can have the Big Grip in place to hook to the
turnbuckle. I can even install it while connected to the turnbuckle.
They used to recommend tightening the cables to 10% and then retightening in
a day or two. Now they just tighten to 15% and it'll be just a tad over 10%
in a day or two.
Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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