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.
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
> 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.
> TowerTalk mailing list
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.
TowerTalk mailing list