[TowerTalk] Preload and Tension

[Steve Maki]

Pre-tension is so named because it's the tension in
a guy with no wind present (pre-wind, I guess). I like
"initial tension" better.

Obviously, the exact amount of pre-tension is less important
on a lightly loaded tower than a heavily loaded one. We get called 
occasionally to retension guys on towers where they are trying
to push the loading a bit without doing an expensive beef-up. We
might relax some bays to 7% and tighten others to 15% on the same
tower. That seems to be the most common range.

73,
Steve K8LX

K8RI on Tower talk wrote:
> 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
> 
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