[TowerTalk] Rohn FK25 failure point?

[k6ll@juno.com]

On Thu, 27 Jul 2000 21:32:02 -0600 Sylvan Katz <jskatz@sk.sympatico.ca>
writes:
> 
> Rohn specifies that "Safe load at the Apex" for a 58' fold over  
> FK25
> tower is 100 lb. Where is this tower likely to fail under a higher 
> apex
> load? How much  under rated is the 100 lb spec likely to be?

I can see exactly what you're thinking, but I wouldn't push this limit
very far. I've got a 48 footer that is just slightly exceeds the spec
limit. When that tower is folded over at ninety degrees, the winch
cable is tight as a drum, the two back guys are tight as a drum, and the
two front guys are completely slack. Even if you try to minimize the
dynamic
loads by cranking the winch slowly, there will be a certain amount of
bobbing up and down, which increases the forces. It's worrisome.

Possible failure points are buckling of the boom, failure of the
cheesy little guying tabs, hinge failure, moment failure of the base
section due to the narrow guying pattern, and who knows what else.

Older Rohn catalogs used to list an additional spec "safe moment
at the hinge," which was the product of "safe load at apex" times
"hinge to apex distance." I think that is still a valid way to
figure loading if the various weights, such as rotor, mast, and
antennas, are not placed exactly at the apex.

Safe moment at the hinge for the fk25-58 would
be 100# x 35.5' = 3550 ft-lbs.

example:
40# rotor 33' above the hinge = 1320 ft-lb
60# antenna 36' above the hinge = 2160 ft-lb
20# mast centered 35' above the hinge = 700 ft-lb
total 1320+2160+700 = 4180 ft-lbs, which exceeds the limit of 3550.

Dave Hachadorian, K6LL
Big Bear, CA
K6LL@juno.com

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