I had a Cushcraft 80-meter vertical once that broke exactly in half and folded
over on itself. I had guyed it about 2/3 of the way up and apparently caused a
resonance that fractured it at one of the swaged joints. I had noticed some
vibration in the mast but ignored it, thinking that the guys would keep it up.
In retrospect, they probably exacerbated the problem. The uniform, non-tapered
diameter probably didn't help either.
The power line people use Stockbridge dampers to reduce wind-induced flutter in
their lines and guys. I saw lots them installed on the guys for what was then
the tallest man-made structure on the planet, the 2063' tall tower for KVLY TV.
Wind or vortex shedding is another technique used on some structures.
https://en.wikipedia.org/wiki/Vortex_shedding. ; (Note the mention of Heard Isl.)
Just today I again visited the site of a TopGolf driving range where I've been
watching them install a plethora of heroic tubular masts holding up a huge net.
I'm guessing they are at least six feet in diameter at the base and well over
100' tall. Since the last visit I notice they've installed a helical wrap up
the masts. When you start looking for this stuff it's everywhere. The old-time
radio antenna on my 1998 PU and the stubby antenna on the back of my newer car
both have helical wraps.
Wes N7WS
On 9/23/2017 6:24 AM, Bob Shohet, KQ2M wrote:
There is another potential factor that I have not seen anyone mention specifically, that of
“resonance”. The interplay of wind speed (constant or intermittent) and
direction, wind vortices around the tower guy and turnbuckle, turnbuckle friction, guy
weight, guy length and guy tension, also contribute to whether or not some form of resonance
may or may not be in play for a given guy and turnbuckle. This can introduce vibrations into
the turnbuckle which can facilitate additional movement (turning) and fatigue, just as
resonance has been known to destroy element tips, bridges, etc.
Movement from resonance can potentially explain “head-scratching” phenomena
related to turnbuckle movement and loosening of untethered turnbuckles, especially when only
one specific turnbuckle seems to be affected.
The bottom line is that as humans, we often unwittingly discount the very phenomena that can
cause a problem and potential catastrophic failure simply because we can not see physically
see the factors at work that are impacting a future weak point in an installation. Which is
all the more reason to take simple steps to protect against them when we can; “just in
case”. :-)
73
Bob KQ2M
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