[TowerTalk] Adding guys to self supporting towers

Roger (K8RI) K8ri-on-TowerTalk at rogerhalstead.com
Tue Aug 13 02:28:53 EDT 2013


On 8/12/2013 5:06 PM, Richard Karlquist wrote:
> The topic of adding guys to self supporting towers has been
> discussed a number of times on this reflector, but I am
> still not clear on exactly why it supposedly won't work.
> If I have figured crrectly, the worse case compressive load
> on a leg at the bottom of a trianguler tower
> is equal to the product of the horizontal windloading
> at the top times the factor h/[w*sqrt(3)] where h is
> the height and w is the width of a face.
> For example, a 100 ft windload at the top of a 50 foot
> tower with 18 inch face will induce 1925 lbs of
> compressive force.  If a the tower is guyed at the top
> with the guys at 45 degree angles, the compressive
> force is simply equal to the horizontal wind load, or 100 lbs.
> Much less than the unguyed 1925 lbs.

You forgot to add the guy tension.
This is a bit more simple than the analysis  that gave leg compression, but
  if i use the right trig function...it's a rough view of the forces.

The guyed tower will have the same downward load, PLUS the guy tension 
and the additional transformation from horizontal forces to vertical 
caused by the guy(s)
his is not a simple transformation, but assuming worst case where the 
wind lines up with one of the guys

If you have 1/4 inch Phyllistran at 440# tension (cos(45)*440 )*3 = 693# 
additional load on the base
The downard force from the horizontal wind is another 1637# for a total 
of 2330#

This is an increase of slightly over one ton and probably exceeds the 
capability of the lifting cable which does not have a great deal of 
excess strength.
Just the 693# from the guys which is less than EHS would be pushing your 
luck

Self supporting towers are much more complicated as they have leg 
compression that a pier pin mounted, guyed tower does not and must be 
stronger for a particular wind load


73

Roger (K8RI)

>
> It seems to me the guyed tower is much stronger and could
> handle a considerably larger wind load based on this
> simple analysis.
>
> The only way I can see that this wouldn't work is if a
> larger antenna resulted in torque loads that the tower
> couldn't handle.  For example, the HDBX series, well known
> for its poor torque strength, would be a poor candidate
> for guying.  OTOH, a light weight tower with a large
> face width might be able to take a lot of torque.  To
> facilitate this, you might want to build the tower with
> less taper than it typically has in the self supporting
> configuration, or maybe no taper.   All of this depends
> on wind area and boom length.
>
> Comments?
>




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