[TowerTalk] Fwd: Less Than Optium Guys

[Hans Hammarquist]

 I want to remember from my calculations, when I did the plans for my tower, that the worse scenario windloading on a 120 deg guy system that when the wind arrived at 15 degree off the wire hit by the wind. The reason for this is that the wire is loaded not only by the wind but also from one of the wires on the opposite side as the tower will send a force to that wire due to the side vector from the wire directly loaded by the wind. I was a little surprised by that result myself but if you want to be picky.
Another thing I found was that the 120 deg system is a lot more sensitive to misalignment than the 90 deg. I therefore chose to use 4 guywires instead of 3. I always recommend 4 guywires when asked.
73 de Hans - N2JFS
 
 
-----Original Message-----
From: Lux, Jim <jim at luxfamily.com>
To: towertalk at contesting.com
Sent: Sun, Dec 11, 2022 12:27 pm
Subject: Re: [TowerTalk] Less Than Optium Guys

On 12/11/22 9:18 AM, Grant Saviers wrote:
> A thought experiment is to consider what happens as the guys are 
> separated by even more than 120 degrees.  As 180 degrees is approached 
> the tension needed in each guy to resist wind splitting the angle goes 
> to infinity.  I think we would all agree only two guys opposing won't 
> resist the wind between them and the tower falls down.  Conversely, a 
> pair of guys at the same angle each only need one half the tension.  
> So at some angle between them the tension in each exceeds all the wind 
> on one guy.
>
> So then how to do the calculation.  To simplify the compound angle 
> force vector calculations, I considered the tension to oppose a 1000# 
> force on the antenna, if the guys were at 90 degrees and horizontal 
> with the antenna.  Then calculated the increased tension needed if the 
> angle to the tower was 45 degrees.
>
> So from the thought experiment, we know the tensions at 120 degrees 
> are higher than at 90 degrees. Maybe somebody would calculate the 
> exact angle when it exceeds the wind directly on one guy.


Considering 4 guys 90 degrees apart. Wind aligned with one of the guys. 
if there's *no displacement* then all the wind load is on the upwind 
guy, plus its installation tension. The side guys see their installation 
tension.  the down wind guy sees installation tension less the wind load.

However, the tower isn't perfectly rigid nor is it rigidly attached to 
the ground, so it bends slightly downwind - now the side guys see some 
increased load.    So the problem isn't quite a simple as it seems.  
It's kind of an interesting problem, and solving it analytically (as 
opposed to with some sort of systematic FEM approach, where you step the 
wind direction around 360 degrees) would be "an exercise for the 
ambitious student" even if you picked a model that is simple (single 
beam in compression, hinged at bottom - or, fixed-free beam with simple 
bending and no torsion)

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