<< This brings me to the following: I wonder if there
is engineering/scientific basis for the 10% tension rule? What if 3/16
EHS or 4000 lb Phillystran were tensioned to 600 lb? Is this unsafe?
Tnx/Barry >>
Hi Barry,
I'll take a turn on this one.
Guy cables that are not perfectly vertical act like extension springs in
two ways:
Mode 1) They change length relatively easily without significant
elastic stretching as the droop in them is pulled tight, resulting in a
very low spring rate until all the slack is pulled out, as you approach
the proverbial "straight line between two points."
Mode 2) Once they are tight, they can still change length mostly by
stretching elastically, although only with much larger changes in
tension (much larger spring rate).
So....before a guy wire can really do its thing, which is to keep the
tower legs from moving, (ideally), it must pull tight for upwind guys or
already be tight for downwind guys. Upwind guys will increase their
tension, and downwind guys will release their tension to balance the
forces (of wind, let's say) that are trying to move the tower. But
beacuse of the elasticity effect, the tower *must move* first to reach a
new equilibrium. It flexes.
How do you decide when you have pulled all the slack out? Thanks to
gravity, it is very difficult to get the guy wire to be a perfectly
straight line unless it is vertical. There will always be a "catenary"
curve in it that includes excess slack, even when the cable is pulled
well beyond 10% of its breaking strength.
Well, at some point, you have to pull it so tight that the tension
starts to make the guy wire stretch elastically (going from mode 1 to
mode 2 above.) And the cable still isn't perfectly straight.
I believe that the 10% of breaking strength rule has been worked out to
where, for the weight of a typical cable, practically all the slack has
been pulled out, putting the cable into mode 2 as described above. If
you preload your wire with much more tension, you are simply reducing
its ability to absorb additional load from wind before you reach its
breaking strength.
However, (don't we always run into these), if you reduce your guy anchor
spacing from the base below the 80% of tower height, then an increase of
guy preload to 15% of breaking strength (600 lb for 3/16 EHS) helps
compensate and control tower flex without cutting too far into your
reserve cable strength.
Another factor: the larger the diameter of the guy, for the same
material, the higher will be its spring rate, and the better it can
resist a change in length (and movement of your tower) for the same
loading force. Since it is heavier, it requires more preload tension to
pull out the slack. This the 10% rule keeps up with things.
Using a thicker guy gives you more control over the flexing of your
tower since it has a much higher spring rate, and much larger forces are
required to make it change length.
If you play a stringed instrument, you can see this effect when you
change, say, from extra-light gauge strings to medium gauge. It's a lot
tougher on your fingers to fret them!
Barry, I hope this helped you and the group.
--...MARK_N1LO...--
______________________________________________________
Get Your Private, Free Email at http://www.hotmail.com
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|