[TowerTalk] TOWER FORCES

[Tower2sell@aol.com]

Your basic unresolved question is how to resolve the the placement of the 
antenna above the guys.

The basic problem is that most people are not analyzing their towers 
correctly. The tower itself acts like a beam-colum supported at the base and 
guy points. For a one guyed system this becomes an easy problem - a simple 
supported beam with a cantilever. Now the wind forces act on the tower body 
and increase with height so this complicates the problem. Now to get real 
messy the wind forces also act on the guy wires. 

When multiple guy level are used, then the deflection of the guy wires and 
the tower body become a point of concern and a "balancing" act occurs between 
the stiffness of the guys and the tower. (Shear and Moment diagram stuff goes 
here.) 
So how does the placement of the antenna above the guys work? It is used in 
most commercial applications because they want to place large mounts and 
platforms at the top and this provides extra room for these mounts. Here is 
the trick- (don't try this at home and it should only be done by a 
professional) The extra bending moment at the top of the tower can be used to 
reduce the maximum bending moment in the tower. This is done by selecting the 
right guy wire size and spacing and the proper cantilever length for the 
antenna load. The other antenna loads below the top also play a role too. Let 
us not forget to include the P-delta effect of the compression load from the 
tower weight and the guy wire tensions. This is all done on a computer. The 
problem is worked in a few minutes that would take an engineer a week to do 
by hand.   (The output can  be massive)

The question of using 4 guys - DON'T

The extra guy only adds more wind area more down force. The major axis of a 
tower is when the wind blows into one wire. with 3 wires - 2 are 
nonfunctioning and with 4  wires -3 are none functioning. Besides guying 4 
wires to a 3 legged tower is not smart - you could overload one leg with the 
extra guy.

How does a guyed tower fail? Normally a whole section does not buckle in 
compression. Most of the towers will have a leg buckle form bending moment in 
the tower combined with compression. Another mode of failure is when the 
brace buckles in compression due to too much shear in the tower. Tension 
failures in legs are nearly impossible but could happen in the section above 
the guy wires and would be a bolt or a bolt bearing type failure.

Tower section properties should be listed in the tower details (catalog cut 
sheets) showing the section's I and S, the leg capacity and the brace 
capacity. The brace capacity is not the section's shear capacity.

The guyed tower is one of the MOST complicated engineering problems ever 
invented and there are not many computer programs that will work the problem 
properly and comply with the EIA-222 code.

I hope everyone is informed and confused. I had an English teacher that said 
that confusion is the beginning of knowledge. 

Let the knowledge begin.

Tower2sell@aol.com

--
FAQ on WWW:               http://www.contesting.com/towertalkfaq.html
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm