Whats with this "rule of thumb?" rohn catalog specifying tension should be
10% of the breaking strength?
The electrical anaylsis is that a filter capacitor for a 12 volt circuit
should have a voltage rating of 120 volts!?
k7puc
----- Original Message -----
From: "Its from Onion" <aredandgold@msn.com>
To: "towertalk" <towertalk@contesting.com>
Sent: Saturday, January 16, 2010 12:07 PM
Subject: [TowerTalk] Loose guys on towers
"Adding more guys wont give you what you need because with extra guys there
is extra loading on the tower"
Rohn specifies that guys should be tensioned to 10% of the breaking strength
of the guy size that is recommended for a particular tower. One rule of
thumb is 8% if the guy is out at 100% of tower height, 10% if at 80% of
tower height (standard Rohn drawings) and up to 15% if the anchor point is
at 65% of tower height. You lose a lot of wind load in this last type of
installation.
For Rohn 45, 3/16 EHS is recommended, having a breaking strength of 4,000
lb. Therefore, 400 lb tension is appropriate for Rohn 45 tower. The primary
failure mode for Rohn 45 is in compression of the legs, so it is important
not to over tension the guys, resulting in greater compression of the tower
legs. 1/4 inch EHS is 6650 and tension should be 665 pounds for where 1/4 is
specified by Rohn for the upper guys.
If you leave guys 'loose' that can cause all types of issues from
'galloping' to 'snap-through'.
Galloping info;
http://www.tower-technologies.com/Vibration.htm<http://www.tower-technologies.com/Vibration.htm>
Tower failure;
https://www.nprlabs.org/apre/PREC2008_VirtualNotebook/2-02_TowerCatastrophes/2-02-01_SillimanPresentation.pdf<https://www.nprlabs.org/apre/PREC2008_VirtualNotebook/2-02_TowerCatastrophes/2-02-01_SillimanPresentation.pdf>
Snap-through typically occurs when the guys are allowed to become loose, and
a high wind is blowing on the tower. Even in normal conditions, the upper
guys work at a narrower angle to the upper tower than the lower guys,
reducing their effectiveness at restraining sideways bending. If the guys
are allowed to become loose, the working angle is further decreased. If the
loads are high enough, or the guys are too loose, the angle between the guy
and the tower will reduce to zero, and the guy can no longer restrain the
sideways motion. The result is that the upper tower “snaps through” and
falls over.
Lee
KE4VYN
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