[TowerTalk] Strength of rotating towers

[Chris Burger]

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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