The old Rohn catalogs only gave load ratings based on the
full rated height of each tower type (200 ft for R25, 300 ft for R45).
No deratings were called out for lower heights. All guying
illustrations showed guys equally spaced along the full height
of the tower (i.e. top guy attached to the top of the tower).
The new Rohn catalog (mine is dated 1994) shows increasing
loads for lower heights. This is a welcome clarification but the
examples show the top guys for R25 placed 5 ft down from the
top of the tower and the top guys for R45 placed 9 ft down from
the top of the tower.
The ratings are appropriate only if ALL of the antenna load is
placed along the open tower above the top guy. If you extend
a mast above the top of the tower, the bending moment applied
to the tower can become excessive.
To minimize the bending moment applied to the tower, the
largest antenna should be placed as close to the guys as
possible. With the guys 9 ft down from the top, the only way
minimize the bending moment and rotate the antenna is to
use an expensive ring rotor or side mount just above the
guy wires. If the large antenna is placed one foot above
the top of the tower the bending moment will be 10 (ft) X
the antenna windload vs 1 or 2 (ft) times the windload if
the guys are placed at the top of the tower.
The bending moment for an antenna at the top of a 12 ft mast
will be (12 + 9) X the antenna windload vs. (9 + 1 or 2) X the
antenna windload if the guys are placed at the top of the
tower.
Example: Consider two antennas with 8 sq ft of windload
placed 1 ft above the top of the tower and 12 ft above the tower
top with a wind force of 30 psi (87 mph). The windload forces
will be 240 lbs at each antenna.
With the top guy 9 ft below the top of the tower, the bending
moment is (9+1) X 240 + (9 + 12) X 240 = 7440 ft lbs.
With the top guy 1 ft below the top of the tower, the bending
moment is (1+1) X 240 + (1 + 12) X 240 = 3600 ft lbs.
The bending moment contribution of the mast and tower
was not included in these calculations.
Notice that the guying configuration shown in the new Rohn
catalog results in more than twice the bending moment
applied to the tower for these moderate sized antennas
(similar to TH6 for example). Full size 3L40's or 2L80's
will be much worse.
When using a rotating mast to support more than one
antenna above the top of a tower the top guys should
be placed as close to the top of the tower as possible.
This is the ONLY way to safely realize the full capacity
of a guyed tower with multiple antennas. The amateur
community needs to encourage Rohn to provide load
ratings for their guyed R25 and R45 towers with the
guys placed at the top plate or first step down from
the top.
You may ask, "Is the bending moment at the top of
a guyed tower a real consideration?"
I'm glad you asked. The answer is a resounding YES!
One of locals had a Force 12 6L20 (8.3 sq ft) mounted
above a Force 12 2L40 / 2L80 (13 sq. ft.) on 130 ft of
Rohn 45 with the top guy 9 ft below the top of the tower.
The rotor was approximately 5 ft down from the top of the
tower.
A storm ripped the top plate off the top section causing
the mast and antennas to plummet to the ground. He
suspects a cold weld may have contributed to the
accident but the above calculations show that the load
born by the tower is considerably higher when the guys
are placed below the top of the tower.
de Tom N4KG
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