I like the slack guy idea but you must be aware that when the wind does blow
and the guy(s) take tension that a compressive force is contributed by the
guy to the tower. As always the greater the angle where the guy meets the
tower the less compressive force is generated by wind. As mentioned a lot
lately a 45 degree angle on the guy puts a downward force on the tower of
one unit of force for every 1.414 units of force on the guy. Note that for
every pound of lateral force on the tower due to wind there will be a pound
of force downward on the tower with 45 degree guys. I'm not arguing for or
against guying any tower designed to be free standing, just pointing out
that even a "slack guy" system generated compression in a wind.
Patrick AF5CK
-----Original Message-----
From: K7LXC@aol.com
Sent: Tuesday, August 13, 2013 9:01 AM
To: towertalk@contesting.com ; richard@karlquist.com
Subject: Re: [TowerTalk] Adding guys to self supporting towers
The topic of adding guys to self supporting towers has been
discussed a number of times on this reflector, but I am
still not clear on exactly why it supposedly won't work.
If I have figured crrectly, the worse case compressive load
on a leg at the bottom of a trianguler tower
is equal to the product of the horizontal windloading
at the top times the factor h/[w*sqrt(3)] where h is
the height and w is the width of a face.
It seems to me the guyed tower is much stronger and could
handle a considerably larger wind load based on this
simple analysis.
Comments?
First, I'm not an engineer so these comments are semi-educated ones.
Second, the capacity of a tower is limited by the leg strength so you don't
want to add unnecessary downward compressive force on the legs; e.g. guying
a free standing tower.
BUT IMO if the guys have little or no tension on them, then they don't
add appreciably to the compression on the legs but some of the wind forces
are transferred from the legs to the guy wires which is the purpose of the
guys. Guys in this case are more tethers than guys; that is, they limit
the forces on the legs without adding much to the total leg load. Since a
self supporting tower doesn't require guy wires, adding them in the above
scenario is more like a belt-and-suspenders approach with some added tower
safety margin.
A possible problem is that with a bunch of slack in the guy wires they
will tend to wind-slam when the wind gusts. That's why you want a minimum
of initial tension.
Cheers,
Steve K7LXC
TOWER TECH -
Professional tower services for amateurs
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