This is the story of the near failure of three 200 foot AB-105 towers at
W3LPL 24 hours before the 1994 ARRL DX Phone Contest. For about 12 hours
during the thursday before the contest, the temperature was 31 degrees and
a light mist fell continuousy, freezing on all of the antennas, towers,
wires, guy cables, and ropes, followed by a strong 40 mph wind. When I
arrived at home around 6:00 pm it was very apparent that three of the 200
ft towers were leaning over by several feet, and many of the elements on
the Yagis had flipped from horizontal to vertical. The ice had started to
fall as the temperature went above 32 degrees and the partially melted ice
laying on the ground was one inch thick! Only one antenna was destroyed,
a 56 foot boom 20M beam that had accumulated a tremendous amount of ice.
The boom drooped so much that the guys supporting it dropped under the
then bow shaped boom and collapsed it! Several insulators supporting wire
antennas snapped, reducing the stress on the towers somewhat.
The near collapse of the towers was caused by failure to consider the
possibility of massive ice load in selecting the height of the top set
of tower guys and in the installation of thousands of feet of wire
antennas and ropes hanging off of the towers without considering the
tremendous ice load they could apply to the towers.
The fourth 200 foot heavy duty AB-105 tower was completely unaffected by
the massive load because of several factors:
- The considerable additional strength of the heavy duty AB-105 tower
- The top guys were only 10 feet from the top of the tower, the three
light duty AB-105s had their top guys 16-18 feet from the top.
- The ropes supporting the many wire antennas hung from this tower
passed thru the pulleys then AWAY from the wire antennas, NOT back in the
same direction as the wire antennas (and ICE) load.
Lessons learned:
- If ice is even a remote possibility, design your antennas and towers
for it! I never thought one inch radial ice was even a remote possibility
here... Don't forget ice loads from wire antennas attached to the towers!
- Ice does not necessarily drop off antenna elements symetrically! When
it drops off one side first, the element is likely to flip to vertical
from the tremendous weight of ice on the other side! The elements must be
fastened to the boom attachment strongly enough to successfully
resist this very large torque. Vertical elements could collapse the boom.
- Guy placement should consider ice loads! Mine were much too far below
the top of the tower when everything was coated in one inch of ice!
- Ropes should attach to a pulley on the tower, then pull away from the
antenna they support, not back towards the load.
- Ropes should attach to the tower near a guy point so the guys take the
ice load to the maximum extent possible.
The towers did not fall down, we managed to repair the most critical
antennas so that our score in the 1994 ARRL DX Phone Contest was only
minimally affected, and the guying was totally redesigned and redone.
Six weeks later a 120 MPH microburst struck, destroying most of the
antennas and twisting the top of two of the towers, but thats another
story... We managed to get enough antennas repaired by late May so that
K3MM (then KF3P) could enter the WPX CW Contest with nearly full antenna
capabilities.
73!
Frank
W3LPL
donovanf@sgate.com
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