[TowerTalk] Preload and Tension

[K8RI on Tower talk]

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
www.rogerhalstead.com