Great question, Chris! Wish I had the answer. In the past
when I have posed questions related to torsional limits of
tower sections, they seem to have fallen on deaf ears. Most
of the familar design equation that are prevalent in the ham
literature seem to address overturning moment. There seems
to be a dearth of information on torsional load limits.
You might want to talk to Kurt Andress, K7NV. His
mechanical design software "Yagistress" may have the
capabilities you need to address this problem. Intuitively,
I was thinking that mounting the base of the rotator on a
lossy torsional spring mount might be helpful. This would
transfer the rotational energy of the unconstrained rotating
sections into the fixed tower. In the static case, I don't think
this buys you anything because the rotating section would
experience the same amount of static torque once the lossy
rotator mount "wound up" tight. The lossy rotator
mount would, however, lessen the shock loads from transient
loads like stopping, starting, and wind gusts (imagine a car
hitting a springy wall instead of a brick wall). Of course if
you shock mount the rotator, you have to make sure the
system is damped enough that it doesn't oscillate, else your
tower might turn into a fascimile of the infamous Tacoma
Narrows Bridge.
Of course, I am not a structural engineer, so I may have this
all wrong. I would strongly urge you to talk to guys like Kurt,
or perhaps Dick Weber, K5IU to get a really credible answer.
BTW, sounds like a nice setup. Look forward to hearing you
on the air.
73 de Mike, W4EF......................
----- Original Message -----
From: "Chris Burger" <ChrisB@prism.co.za>
To: <towertalk@contesting.com>
Sent: Monday, October 29, 2001 9:13 PM
Subject: [TowerTalk] Strength of rotating towers
>
> My rotating tower is now up and running, awaiting only a suitably heavy
> rotator. The bottom 17 m is stationary, with something like 29 m rotating
> above it. Right now, the top beam (20M4) is at 42 m. For those with 19th
> century values, that's about 56, 95 and 138' respectively.
>
> The stationary tower is guyed, as is the rotating tower at two levels.
I'm
> using commercial tower sections, similar to Rohn 55 but somewhat heavier.
> I've adhered to the manufacturers' engineering specifications, except that
> the guy anchor points are slightly further out from the tower than
> specified.
>
> However, I've been wondering about the strength of rotating towers.
> Clearly, overturning and shifting loads are handled as well by rotating
> rings or bearings as they are by direct attachment of guys. However, if
> corkscrewing due to excessive torque is a possible failure mechanism of
such
> towers, using rotating rings or bearings will substantially reduce the
> strength of a tower. In other words, the torque exerted by the top beams
> relative to the rotator base, during start/stop operations or asymmetric
> wind loading, could cause the tower to twist and collapse inside the guy
> rings. Normally, if the guys are attached directly to the tower or even
> through torque arms, this torque will be arrested or distributed over a
> shorter piece of tower. Presumably, the shorter section will suffer less
> tortional strain in absolute terms (i.e. not angular but linear), and will
> therefore not reach critical misalignment with resultant failures.
>
> Has anyone investigated this issue? If so, what are the findings?
> Obviously, the ratios and tradeoffs will be slightly different for my
tower
> than for Rohn 55, but any findings on Rohn or other towers will be very
> useful.
>
> I'll summarise any insights that come my way.
>
> Chris R. Burger
> ZS6EZ
>
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>
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List Sponsored by AN Wireless: AN Wireless handles Rohn tower systems,
Trylon Titan towers, coax, hardline and more. Also check out our self
supporting towers up to 100 feet for under $1500!! http://www.anwireless.com
-----
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