[TowerTalk] Loose guys on towers

[Roger (K8RI)]

Al Williams wrote:
> 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 at msn.com>
> To: "towertalk" <towertalk at 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,
Ahh..isn't that 3/16" for a 25G?  I'd have to hunt up my book but IIRC 
25G uses 3/16 while 45G uses 1/4"

73

Roger (K8RI)
>  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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