[TowerTalk] Guyed tower stuff

[Kurt Andress]

Michael Lamb wrote:

Referring to K6NA's comments on Rohn and Triex tower construction, and my previous comments about Rohn tower twisting under compressive load....

> Does this mean that having my 190 foot 55G towers rotate inside rotating guy rings is going to reduce the chance for the towers twisting into a pile of rubble in a big wind storm?????  Or, does it place more concentrated angular moment at the bottom where the rotator resists any turning?
>
> 73/Mike, N7ML

Hi Mike,
Don't lay awake at night worrying about this one! You wisely chose 55G, which is a really strong section. Here's my 10 minute explanation on your tower.

Your tower is going to behave just like a non-rotating tower, buried in concrete, with one exception. That exception is that the rotating guy rings prevent the tower from getting any anti-torque support from the conventional torque bars.

This has been a point of contention from John, W0UN, regarding how effective the torque bars can be. John is right (IMHO), that the torque bars can't and don't do anything until the tower rotates to some point where the guy is able to exert a resisting force on it. I believe that the torque
bars are effective, and empirical experience from others supports this.
The op hits the antenna rotator switch, or the wind blows at the right angle. The tower starts to wind up. At 1 Deg of rotation not much happens. At 2,3,4,5,6...degrees of deflection the guy has more beneficial influence. John mentioned that his experience at K0RF indicated something on the
order of 45 deg of tower rotation at the top, for a non torque bar installation, with a full sized 40 meter antenna.
I'll throw in a note here that the "W" braced Triex tower will also wind up. Since all of the Triex bracing is diagonal, it will simply wind up less. Glenn, said as much in his post.
So, my opinion is that any torque bars are going to result in less tower rotation than no torque bars. Even if the absolue difference is 45 Deg to 30 Deg. This will result in lower torsional loads at the tower base. That's usually where the rubber meets the road, or the steel meets (or hits)
the dirt.

Back to Mikes question. The rotating tower will wind up freely, generating the max torque at the base, or rotator. If the offset rotator drive system is weaker than the tower (for torque loads), then something will fail in the drive and save the tower (my choice, easier to fix). If the
rotator and drive are stronger than the tower section, then Mike gets to call the crane company to help him fix it, or pick up the pieces.
It may be that, the loads on this tower never approach failure of either the drive or tower components. Since, guessing is popular, I'd guess that 70-80 Mph winds are not a problem with this installation. 90-120 Mph would make me really want to think about it!
If I knew what was on the tower, I' know more about how much to worry! Stacked 30 SqFt 80's result in more sleepless nights than stacked 6 SqFt tribanders.

Oh, BTW, forgot to mention the difference between tower torque caused by antenna (or tower) rotation and tower compression. They are two completely different sources for tower torque.
Antenna/tower rotation torque loads are developed by the acceleration of the rotating device and the antenna mass.
The tower torque caused by the vertical components of the guy reactions are defined by the wind speed and associated antenna & tower loads. They are only caused by the tower bracing configuration.

My info says 55G doesn't come down very often, unless the trees that Mike doesn't have, take out the guys, or overload them to to simply pull the tower over and fail the sections in lateral bending, as in K8DX's case.
73, Kurt
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