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[TowerTalk] "Self-serving" Failure Mode

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Subject: [TowerTalk] "Self-serving" Failure Mode
From: (Roger L. Elowitz)
Date: Fri, 04 Apr 1997 20:52:03 -0500
Hi Gang,

The response on the debris field thread has been terrific and tremendously
enlightening. So many interesting points of view!

Not long ago I had a question regarding the determination of guy line
tension and angles and stuff... and it was carefully explained to me on the
reflector about the vector forces operating on the tower structure and the
effect of guy line angles on those forces.  (Many, many great posts on this
subject.)  Now, with the "debris field" subject being brought up... I
instantly had the mental image of a tower being somewhat like avery blunt
arrow... pointing into the ground and the multiple guy lines being tensioned
not unlike  bow strings.

Of course, under "normal" conditions the "arrow" goes nowhere- just gets
pressed towards it's base!  

Then, Bill, VE5FN  seems to have expressed it all.  "It all depends" on just
what fails first. If the "arrow" (tower) buckles, the guy lines (bow
strings) will force the tower to its knees in a small debris field.
However, if a guy anchor gives way such as a bow string failure.... all
manner of possibilities exist as falling structures can impact lower guy
lines tearing them loose and setting up fascinating chains of events... none
of which are remotely predictable especially when compounded by variables
such as ice load and wind speed and direction.

I'm sure all of us would like a nice neat rule we can point to so that we
can substantiate our positions before some zoning authority (or higher
authority... like the xyl).  But I guess the time has to come when we
recognize these things aren't as simple as we'd like to believe them to be.
I believe the dangerous operating  word here is "self-serving."  

My (please don't laugh) 3-section push-up mast with three antennas on it
.... fell over like a fallen tree when one set of guy lines let loose. (The
wood-screweye pulled out of the garage roof.) The mast buckled at the first
level which was bolted to the house and the upper two sections fell onto the
house roof with the Ham-M rotor just putting a golf-ball sized hole in the
house roof and all the antennas went dangling over the edge of the roof.
(Snapped the boom of the 23-el atv beam clean in half although it hit nothing.)

I guess I'm kinda glad it didn't fold up like a spaghetti on the garage roof
or it would have done far more damage.  Of course I learned the lesson about
not using the wood screw eyes again.  Then again, this "tree-felling"
failure mode has to be accounted for especially when there are multi-Kv
overhead powerlines nearby.  Wishing for the "spaghetti mode" just won't cut
it.  But it is a nice thought nevertheless. 

Thanks all.  I've really learned something from this thread and will
continue to learn as I stay tuned.  "An open mind is an easy thing to fill."

Roger, K2JAS


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