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?
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