On 10/9/2014 1:53 PM, Ralph Mowery wrote:
"rickman" wrote in message
I'm not sure what "sideways torque" is exactly, but an I beam is an I
beam. I don't think a ladder has any inherent weaknesses that would
preclude it from being used as a mast. Masts often have a rather smallish
diameter pipe at the top. The structure of a ladder is much more
substantial than that.

With adequate guy wires I expect a ladder would make a very good mast. The
only real issue is that of cost and convenience. I haven't priced tower
sections but I know long ladders are not cheap.

You will probably pay around $ 100 or more per 10 foot section of Rohn 25G,
one of the most common towers or about 30 to 40 dollars for a used section
in good condition.

If you look at a ladder, you will see the sides of it are made of sort of an
H section . The middle part is much larger than the side pieces. That
gives it the strength when it is laying against a wall as designed to be.

Actually you have this backwards. The middle part is called the web and
is there to support the two ends as well as add strength to bending in
one direction. The ends are called flanges and are where much of the
strength come from. They are separated from the center of the shape and
so have a large moment of bending. Even when the amount of material in
the flanges is less than the web, they add more strength because the
difference is more than offset by this larger distance. The ends also
resist bending in the other direction.


If the ladder is very long and a beam is mounted to the top of it, when the
wind blows it will tend to twist the ladder. The narrow sections of that H
form are not designed for much twisting. A 3 element beam can cause a lot
of twist in a 40 or more MPH wind .

Why would the wind tend to twist the ladder? Is your antenna highly
asymmetrical? I would be more concerned by the bending forces which can
be very high if the antenna is a considerable distance from the highest
guying point. If there are guy wires immediately below the antenna this
can be minimized.

There is only so much torsional force which will be applied. The ladder
is supplemented in resisting torsion by the guy wires if they are
properly done. There are the I beams of the side rails of the ladder,
but the ladder is also a larger I beam made up of the rungs and the side
rails. This adds extra strength.


A pipe will tend to resist twisting. Just look how small the drive shaft is
in most cars that have the rear wheel drive.

Yes, a pipe is an good shape to resist torsion, but this depends on the
thickness and diameter. The drive shaft of a car is *very* much larger
and thicker than any antenna mast likely to be used by a ham. The
weakness of a pipe is to bending. So a pipe must also must be guyed
extensively.

I had no idea tower sections were so expensive. A ladder is looking
better all the time.

--

Rick