[TowerTalk] TOWER FORCES

[n4kg@juno.com]

The following is the response from K7NV to my 
inquiry about the basics of tower guying and
analysis of the resulting forces for the conditions
described.   de  Tom  N4KG

 > n4kg@juno.com wrote:
 > >
 > > Kurt,
 > >
 > > For the simplistic case where the antenna load is placed
 > > exactly at the guy wires (one or more levels), am I correct
 > > in assuming the horizontal load is carried by the guy wires
 > > and the increased guy tension is carried by the vertical
 > > compression of the tower?
 > >
 >
 > Yes, you have the basics. Much of how it is stated is a matter of
 > perspective.

Here's how I'm most comfortable viewing it.

The guys are the primary means of lateral support. Not like a
freestanding tower, where all loads are reacted at the base (but guyed
towers do have some amount of this behavior also).

So, the guys get loaded up in tension when horizontal loads are present.
Since that tension is acting along a path that is at an angle to the
tower (or ground, take your pick) it results in an additional
compression load on the tower.
I.E. the largest part of tower compression is a byproduct of the guy
tensions, which is a byproduct of the lateral loads applied to the
tower. 

That way we never lose sight of the big picture, who's responsible for
what.

 > > IF this is the case, then would it not be fairly simple to
 > > calculate the benefit of moving the guy anchors out based
 > > on the change in the angle of the guys with the ground at
 > > the anchors?
 > >
 > Also yes. Moving the anchors out will reduce guy loads and 
 > tower compression.
 >
 > The only other thing about the two above is the guy elongation.
 > In the first one, additional tower stress, due to bending, 
 > is caused  by guy elongation.
 >
 > In the second, additional gains in tower bending stress reductions
 > are to be made by improving guy angles, which result in lower guy
 > elongation.
 >
 > > I confess I don't have a clue about resolving the bending
 > > moments throughout the tower if the load is above the
 > > guys.  I assume the "rigidity" of the tower becomes a factor
 > > in those calculations but don't really know how to apply this
 > > throughout the tower.
 > >
 > > Is there a straightforward (albeit complex) way to resolve the
 > > forces in the tower when the load is above the guys?
 > >
 > > 73,  Tom  N4KG
 >
 > There are ways to do it, I actually wrote some code for my 
 > Commodore  64
 > to do it 10 years ago. I'll have to dig around and see if I can find
 > any of that stuff.
 > I gave up on it when I found this neat little FEA program. It takes
 > about 5 seconds for it to analyze a tower, all the time is in
 > setting it  up and processing the results.
 >
 > There are still varying bending moments in a tower with the loads
 > all applied at the guys. Mostly caused by the differences in guy
 > elongation due to the different guy loads, angles, and sizes.
 >
 > --
 > 73, Kurt, K7NV
 >
 > YagiStress - Software for Yagi Mechanical Design
 > Visit http://www.freeyellow.com/members3/yagistress/
 
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