Date: Mon, 12 Aug 2013 14:06:29 -0700
From: Richard Karlquist <richard@karlquist.com>
To: towertalk@contesting.com
Subject: [TowerTalk] Adding guys to self supporting towers
The topic of adding guys to self supporting towers has been
discussed a number of times on this reflector, but I am
still not clear on exactly why it supposedly won't work.
If I have figured correctly, the worse case compressive load
on a leg at the bottom of a triangular tower
is equal to the product of the horizontal wind loading
at the top times the factor h/[w*sqrt(3)] where h is
the height and w is the width of a face.
For example, a 100 ft wind load at the top of a 50 foot
tower with 18 inch face will induce 1925 lbs of
compressive force. If a the tower is guyed at the top
with the guys at 45 degree angles, the compressive
force is simply equal to the horizontal wind load, or 100 lbs.
Much less than the un guyed 1925 lbs.
It seems to me the guyed tower is much stronger and could
handle a considerably larger wind load based on this
simple analysis.
The only way I can see that this wouldn't work is if a
larger antenna resulted in torque loads that the tower
couldn't handle. For example, the HDBX series, well known
for its poor torque strength, would be a poor candidate
for guying. OTOH, a light weight tower with a large
face width might be able to take a lot of torque. To
facilitate this, you might want to build the tower with
less taper than it typically has in the self supporting
configuration, or maybe no taper. All of this depends
on wind area and boom length.
Comments?
--
Rick Karlquist
N6RK
## Who told you you cant guy a free standing tower ?
Its done all the time, including HDBX towers, Delhi,
and also Trylon towers. But it has to be done right. What
you end up with is the strongest tower in town.
## worse case is when the wind blows between guys. Then
the downward force on the tower is DOUBLE what it is, vs
wind blowing exactly inline with any one guy wire. On a Trylon
T-500-72 or a T400-80, the weak spot is at the junction of the
5th and 6th sections..from the ground, IE: 40 ft level. That is done
intentionally, so when in the FREE standing mode, the tower will
fold over at the 40 ft level..vs folding over full length..at the base.
## The UST HDX-689 is the same way, the 3rd section up from the bottom
has the weakest legs, and is where the tower will fold over..not the base.
You can see that on their eng specs.
## OK, getting back to guying a free standing tower, its usually done to
provide that extra bit of support in heavy winds. For yagi install and
removal,
two guys are removed...and yagi raised-lowered on the opposite side
of the remaining guy wire. Once yagis are back up, the 2 guys are put back
on. You have to be careful if a mast is sticking way out the top of the tower.
The top two sections need to be beefed up a bit, usually with extra 2 x 2 angle
steel on all 3 sides. On any trylon, that’s at the top of tower, and again
down
4 ft from top......and again at top of next section down...and again 4 ft below
that.
Done correctly, it will also eliminate TQ issues. Rotor goes 8 ft down from top
of tower..where there is 2 x 2 bracing. More 2 x 2 bracing 4 ft below the
rotor.
## IE: TQ gets transferred into the tower at the rotor plate – tower leg
interface.
I have guyed free standing towers, both with 3 guys per level..every 120 degs
....and also 4 guys per level....every 90 degs. I had 3 yagis at the top of a
free standing
tower back in the 70s... turned by a PP rotor..and had the entire mess in a
hurricane,
not even an issue..... but it was guyed, with 4 guys per level, and 3 levels
of guying,
12 guys in total.
## The usual deal these days is just 3 guys on a Trylon T-400-80, or a
T-500-72. When you have one of those 220 lb optibeam 40-10m yagis
on top of a trylon, then more weight with a mast and rotor, etc, the tower
does not inspire confidence. It needs just a bit more support.
## You can also guy the super titan towers....the ones with the .25 inch thick
legs, and .3125 inch thick legs, and also .375 inch thick legs at the base.
## The stress on the cross bracing is actually reduced when guys are added to
a
free standing tower, not an issue.
later.... Jim VE7RF
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