[TowerTalk] Guying a TX-472 bottom section

[Jim Lux]

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).