At one time a 1610 ft (490 meter) TV tower at Caprock NM was the tallest
manmade structure on the planet. The option to put on de-icing
capability was not taken as it was decided (hoped) there would not be
enough icing to be a threat. When I was a Senior in High school that was
the only TV we could get. An ice storm in the winter of 1960-61 brought
the tower down. The ice storm took down 20 miles of overhead power
distribution poles leaving the town of Tatum NM a national disaster area
with truck mounted army generators brought in to preserve food in the
stores. Gas was pumped by hand from underground tanks into 5 gal cans
to fuel cars. School was closed until utility power restored (new poles
installed for 20 miles.)
Moral of the story... Cutting corners is not always economical
Patrick NJ5G
On 9/23/2017 7:12 PM, Wes Stewart wrote:
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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|