On 10/7/12 9:07 PM, SPWoo wrote:
Hi Steve,
I need to guy the tower because I've exceeded the tower's design
limit by a factor of 3.7.
That's a pretty healthy oopsie...
If my two yagis were both mounted one foot
above the thrust bearing I'll be below the limit. However having one
yagi at one foot above the bearing and another one seven feet above
the bearing generate enough bending moment that exceeds the tower's
limit in the retracted position (3,700lb-ft vs. 1,000lb ft. at 122mph
wind speed).
What you really need to know is whether the "limiting load" is going to
be helped by guying. For instance, if the limit is bending on something
other than the bottom section (not unusual in a tapered tower, which
most crankups are), then guying the bottom doesn't necessarily help.
I didn't do my homework before I put everything in.
Hindsight is always 20/20. Getting a new tower is out of the
question. So I have two choices, do without my 40m yagi or guy the
tower. I read the UST FAQ document and it does say it's ok to guy
the bottom section of the tower. There are no details on how to do
it. I didn't go with a traditional guyed tower because I don't have
enough space for one. However I do have space to guy the bottom
section of my retractable tower. I get extreme winds here (>100mph
once or twice a year, >70mph ten days a year). UST did the calcs.
for me at 122mph and they said the tower could handle 15.6 sq. ft.
when retracted.
Do you have the calculations in hand? Can you tell where the limiting
load was? (is it bending of the bottom section?)?
However my Skyhawk yagi is rated at 8.5sq. ft. which
is too optimistic. The true number is more like 12.2 sq. ft.
projected. 8.5 sq. ft. is most likely an 'effective' windload for
flat members. The TH-11 is about the same size as my Skyhawk and
it's rated at 12.5 sq. ft. My F12 D240 is rated at 5.8 sq. ft.
projected. I'm unsure if the 5.8 figure is optimistic or not but
that's what Mark at F12 told me. So in a nut shell I don't want to
lose my 40m yagi and that's why I'm looking into guying the tower.
I think it would be worthwhile to hire an engineer to look over the
calculations. You're in that zone where it might work, it might not, or
you might be in a situation where you could reasonably gamble because
the cost of (not too likely) failure is small (no death and destruction).
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