[TowerTalk] Rohn Fold over summary / final consideration

[Sylvan Katz]

SUMMARY

Thank you for your responses. They were very helpful indeed. In summary
the consensus appears to be that the most likely place for a Rohn FK25
to fail is near the hinge when the top section is winched close to
horizontal. Also it seem that calculating the moment is better than
simply using the apex load. In other words I should use  3550 ft-lb
moment as the  safe spec (35.5' x 100 lb) instead of the "100 lbs at
apex load measure".

FINAL CONSIDERATIONS:
The information I have received from TT has been extremely valuable. I
now have a 58' FK25 erected in my yard. It is guyed at the hinge and the
apex with 3/16" EHS at 36.5'.  I am preparing to install the antenna,
rotor and mast. I want to pass my calculations for the two
configurations I am considering by the group for a final inspection
before I proceed. I would rather be safe than sorry!

The distances to the hinge in a normal FK25 according to the Rohn spec
sheet are

base to hinge -  22.5'
hinge to apex  - 35.5'

My tower is a used NASA weather collection tower. For reasons I do not
know it has a slightly different configuration. A local manufacturer
constructed the hinged base plate. The measurements are:

base to hinge - 25'  (3' base section  + 2 x 10' section  + 2' bottom
hinge section)
hinge to apex - 33'.  (2 x 10' sections + 8' top section  + 5' top hinge
section)

My target configuration is an optimized 2 el - 3 band Gem quad (with
separate feed lines for each band) at 64' rotated by a T2X (just because
I have one). Here are the two configurations I am considering:

Configuration A.
2 el Quad at 64' (4.5 sq ft)                       22    lb.  x 41'  =
902   ft-lbs
16' carbon steel mast (2.7lb/ft)              43.2 lbs x 33'   = 1426
ft-lbs
(center of mass at apex)
T2X (28 lbs) 8' below apex                     28   lbs x 25'   =
700   ft-lbs
-----------------------------------------------------------------------------------------

TOTAL
3028 ft-lbs
% safety factor (3350-3028)/3550 = 9%

Configuration B.
2 el Quad at 64'                                        22   lbs  x
41'   =   902  ft-lbs
11' carbon steel mast (2.7lb/ft)              30  lbs  x 36.5' = 1095
ft-lbs
(center of mass 2.5' above the apex)
T2X (28 lbs) 2' below apex                    28  lbs x 31'     =  868
ft-lbs
-----------------------------------------------------------------------------------------

TOTAL
2865 ft-lbs
% safety factor (3350-2865)/3550 = 18%

It is obvious from the above that calculations that configuration B has
twice the safety margin of configuration A. However, the above
calculations do not include the weight of the coax and misc. hardware.
If the safe moment of 3550 ft-lbs is accurate then using configuration A
I have enough of a margin to install 15 lbs of coax and misc. stuff at
an average height of 2 ft above the apex. Configuration B gives me
enough margin to install 20 lbs of coax and misc stuff at an average
height of 2 ft above the apex.

All of the above calculations assume that the Rohn spec includes the
weight of the guys, the thrust bearing and rotator shelf as part of the
tower and not part of the apex load.

Observation, corrections and comments would be very much appreciated.

tnx ... sylvan

PS  - I tried to find a supplier for a  moly-chrom mast but I could not
find any available is Western Canada.

--
Sylvan Katz
Saskatoon, Sask
VE5ZX & G0TZX



--
FAQ on WWW:               http://www.contesting.com/FAQ/towertalk
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com