"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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