[TowerTalk] Adding guys to self supporting towers

[Grant Saviers]

I can't follow your analysis, so here are some numbers for an HDX589 
from my PE analysis:

EIA-222-F 85 mph 3 second gust 71 mph fastest mile no ice

leg compression 41538#  base connection moment 80330 ft-lbs  lift cable 
force 6332 lbs.  Big numbers compared to your calcs.

A guy at 45 deg will add 1lb of downforce for every 1.414 lbs of 
tension.  So, just the pretension of 1/4 EHS would add about 1500 # with 
3 guys.

My casual review of the PE analysis convinced me that the engineering of 
self supporting towers has thin safety margins, around 1.3 to 1.6.  Like 
many structures, the weight vs load vs strength is a complex set of 
tradeoffs.   An optimized design would have all safety factors for all 
components the same, minimizing weight and cost.  The tradeoff for a 
tower maker is making multiple towers from the same components, thus any 
particular model isn't optimized.  Would a guy on the fixed section 
help?  On the 589, it appears that the third section has the least 
safety factor, it's likely every tower model is different.

That said, I would bet on the hoist cables as the failure point if guys 
were added.  The guy pretension alone adds 24% load to the hoist cable. 
There are many posted anecdotes of cable failures, whether they are poor 
maintenance or something else, and seem to outnumber leg failures by a 
significant ratio.  Others have noted the as designed cable load is 
higher than what is usually accepted practice, certainly way above "man 
rated".

Grant KZ1W


On 8/12/2013 2:06 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.
>
> 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?
>