>Did you ever calculate whether the antenna load was within the specs
>the tower manufacturer gave. Any tower can come down if overloaded, and
>from your description, you had quite a load on the tower. Also, you
>don't mention what the tower rating was.
Most towers are rated at a value of Sq ft in 50 or 70MPH winds. First the
recommended wind force load for many areas is usually 70 or 80MPH so the
50MPH actually rates the tower lower at the recommended wind force by quite
a margin. 18 or 30 sq feet at 50MPH is much less at 80MPH. It is
recommended if you are at or above this limit to lower the tower if high
winds are expected. The tower Mfg does not give wind limits for towers
partially cranked down, but Lou Tristao estimated that for each foot each
section is cranked down the wind force specs are increased about 3 sq feet
to a max of 12 extra sq ft. This only applies to his tower design and
perhaps to the later US Tower versions not Rohn or Tri-Ex. With these,
specific tests would need to be run to determine the extra wind force available.
This is for freestanding. Lou Tristao also designed his towers as guyed
crank ups for many years (CTL300 and 400 series, Mosley Towermaster
(CTL400)). This design extended to the Palmer Industy version, but
probably does not apply to US Tower unless guy rings are welded to the
outside tubes as he did with the Tristao/Palmer self-supporting crank ups.
To make this work you must insert metal pins to take the load off the wire
ropes. Otherwise you will keep the weight on the wire ropes and force
strong moments into the concrete base.
I have used the Tristao 71' self-supporting tower (15 sq ft @50MPH) cranked
down about 10" per section, and guyed according to the CTL-471 specs since
1969. The tower withstood Hurrican Camille in 1969 (77 MPH), two tornadoes
overhead which picked up the antennas and rotators and turned them 45
degrees (the Tristao/US Tower plate just sits in the top section using the
side weld bars as support. A bar makes the plate sit below the side bars
and uses the side bar to stop movement). The picking up of the rotator and
mast/antennas defeats this design and is its main weakness. But you cannot
crank the tower down if the plate is attached to the sides as with Rohn 45.
The tower also withstood The Hurricane last Sept (70 to 85MPH winds) in
Metro Atlanta. I lost all my wire antennas and the reflector on the KT34XA
turned to 45 degrees.
The guying and cranking down slightly allows the tower to hold nearly 24 Sq
feet at the rated 70MPH for my location. (34 sq ft at 50MPH per Lou).
>>It was such a pain to crank it up and down, that operators finally left it
>>up most of the time.
>
>I bought a Sears 1/2 inch drill motor to use on my Tri-Ex 52 foot
>crank up. I only takes a couple of minutes to crank it up or down.
This is a good idea as the big mechanisms sold by the tower Mfg add $$$ to
the cost ( up to $3,000). The first commercial attempt was Tow-Tec (a W2).
Perhaps the Sears drill motor is faster than the Tow-Tec (sold to The Hazer
people 10 years ago) which took 30 minutes for one ham's 64 foot alum crank
up to come down and nearly 50 minutes to crank up..
Perhaps Henry WA0GOZ could tell us how fast the sears drill motor works
and how he adapted it to the winch crank handle?
Towers are work, but fun!
73, Dave K4JRB
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