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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