One more thing to think about and that is the type of guys used.
Although EHS is very strong, it is heavy and that leads to a deep catenary.
EHS may not stretch measurably unless badly over stressed and 600# tension
is 600# regardless of guy type, but deep catenaries have "give" and they can
resonate which is a very good reason to make sure they are properly
tightened and the proper size is used. Over size EHS guys will over stress
the tower when they are properly tensioned.
Another guy material is the Kevlar family. There are several brands
available. They are light and very strong. You can carry a couple hundred
foot coil on each shoulder. Try that with 1/4 or 5/16 EHS <:-)) These
materials have very little give, but they will stretch *slightly* more than
EHS, at least in the beginning. However the catenary in even 6600# test
Kevlar is so mall you have to sight down the line to even detect it and then
it is truly tiny.
This leads me to believe using any of the Kevlar family of guy lines would
lead to more effective use of torque arms than would using EHS steel. We'd
need an engineer to run the calcs on that to truly know.
BTW, both the Kevlar and EHS recommend tension at 10% of the rated strength.
Now, we've talked about both single point and the star, or 6 point guying
systems in reference to twisting and I don't think any one doubts that the
star system is far superior with torque arms when it comes to twisting. How
about lateral (site-to-side) resistance to motion between the two systems?
Nothing comes without a price.
For instance we know the cosin of 90 is 0 and a guy line perpendicular to
the line of force does basically nothing. Normal star, or 6 point guys are
spaced at 120 degrees which is a 60 degree angle to the line of lateral
force. Cosin 60 is just happens to be 0.5 or 50 percent. Figuring the guy
angle as 45 degrees to the vertical is 0.707 (familiar number) just what
kind of force is exerted on the guys? I realize this is a generalization
and a simplified one at that as the forces exerted are going to contain both
the twisting and lateral movement/forces. I'm going to stick with just the
lateral force for the example and hope I get the math right.
Lets say the wind is such that it exerts a 200# (very conservative) side
force directly away from the single point guy which is anchored at a 45
degree angle to the vertical. The guy exerts a side force of it's tension
multiplied by 0.707 or about 140#. However the force exerted on the guy by
200# of lateral force is the side force multiplied by 1.404 or, 280#
Now in the case of the star guying at 60 degrees and assuming the angle to
the vertical is still 45 degrees we still get close to 140 # of side force,
but it's at a 60 degree angle to the perpendicular and there are two of them
so we have 280#, but at 60 degrees we only have 50% of that force
perpendicular to the side of the tower, or 140#. Going back to wind and
200# wind load at 60 degrees to the guys we end up with 400# divided between
the two guys, or 200# each.
Now if I did my calculations correctly and didn't get lost along the way,
the star, or 6 point guy system reacts the same to lateral forces the same
as the perpendicular 3 point system.
The only down side the pressure applied to the tower by the guying system
which comes out to the 6 point putting twice the downward pull as the
standard 3 point. With the top guys at 600# tension as in my system we use
the sin of the angle of say 60 degrees for the top set or 0.866 (600#) =
519.6#. At the 60 foot level the angle is about 40 degrees or 0.6 and those
are tensioned at 400#. 0.6 X 400 = 240 # and the bottom tier are (guessing)
about 20 degrees or 0.3 and they too are tensioned at 400#. 0.3 X 400 = 120
#
That gives us (rounding) 520 + 240 + 120 = 880 X 3 (3 point guying) 2640# of
downward pressure on the base from the guying alone. The tower weighs 900#
and the antenna system is well over 300# for another 1200#, or 3,840# on the
base.
A 6 point would double the downward force from the guys to 5240# plus tower
and antenna weight.
As I said this is over simplified and I've probably made a mistake of two,
(never turned in a math paper without a mistake or two and I have a minor in
it) but it should be close.
Now for a very scary thought.
Guy tension is a function of the *guy* size and material. It is not a
function of the size tower. The size of the guy should be a function of the
size and height of the tower so we should be able to figure just how big we
can go with any particular tower.
Just think of a tower guyed with 1/4 inch EHS. Now think of a ham putting
up a 90 foot light weight steel tower and guying it with 1/4 inch EHS "for
strength". With those 1/4 inch guys properly tensioned he is going to have
_well_over_ ONE TON of weight on that little tower base.
I have dismantled 90 foot steel TV towers that used 1/4 inch steel guys.
The bottom three sections were "belled" into each other to the point it took
a jack to get them apart.
> Conservation of Momentum says that that torque energy has to go somewhere.
I agree that >the guys will resist it, and using torque bars/limiters
probably helps keep the tower from >twisting, initially, but those guys have
an elastic modulus and will stretch. Kurt, K7NV, a SE I
This varies greatly with the guy material and tension. The give in the
catenary is probaly far greater than any stretch in the EHS. Even Kevlar
has little stretch, but it has so little catenary that the stretch is
probably a much higher percent than in EHS steel.
>believe, published structural calculations on various tower/guy/antenna
configurations. His >website seems to be down right now (either via the
N1LO website or k7nv.com) but his
I assume those are www.N1LO.com and www.k7nv.com?
>conclusions were that in high winds with decent antenna loads, those towers
which have their >based fixed in concrete were most prone to failure; the
base is fixed, the torque energy just
I can see where this would, or could be a problem in heavily loaded towers
with large antennas, but I doubt the average ham normaly comes any where
near it being a problem. I see too many Aluminum and light weight steel
towers guyed with *untensioned* Radio Shack guy wire (the soft stuff) and a
tri-bander on top that have survived 70 mph winds with no apparent damage.
Quite often they don't even have a dirt base, but just set the bottom
section into the sand about a foot or so.
>twisted the tower apart. A pier pin base allows the tower to twist free
and is a better solution >except that, as stated, the guys will stretch, and
in high winds the tower will want to "lean"
Using the Kevlar materials I dought you'd even be able to detect any lean
with a 50 - 60 mph wind. OTOH I'd expect it to be substansial using EHS in
the conditions we normally use.
I've been up comercial towers guyed with one inch steel cables (looks like
wire rope) that had the pointed base on a pier pin. In 30 plus mph winds
the things didn't even vibrate. These towers did not use torque arms, but
the guy tension was shuch that if you hit one with a wrench it'd sing like a
loud tuning fork and it was a very high pitched note. Well above Middle C.
Were I going to use that approach I'd want to use a solid leg tower. OTOH
these commercial installations did not have a lot of twisting moment
comparied to the lateral force. I would expect most of the *big* Ham towers
OTOH to have a high twisting moment compared to the lateral force.
>over. The standard pier pin base is flat and all the compression forces
can be exerted onto >one leg and that leg will eventually buckle.
IMO you'd defiantly want the pointed base, but I don't think the towers most
of us use are adaptable to that kind of mount and guying when pushed near
their limits in height and wind load.
Roger Halstead (K8RI, EN73 & ARRL Life Member)
N833R, World's Oldest Debonair (S# CD-2)
www.rogerhalstead.com
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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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