Well your engineering chops are way above mine, so take this with a few
grains of salt.
You've correctly identified the two parts of the equation.
Part one: deciding how well a clamp can grab onto a small tower leg. IMO
that by itself is enough to indicate spanning a face with our hardware.
After all, many hams move to 3" masts to avoid slippage.
That gets us to part two, which is my own, maybe unfounded fear about
welds (where small rod Z-bracing is welded to thin wall tubing)
eventually tearing away from the leg from constant swinging forces over
the years if concentrated on one leg. My uneducated view of the
situation is that tall skinny guyed towers are like wet noodles, and
twisting force on one leg changes the vectors on the components from
simple compression and tension to more of a lever action on the leg/Z
brace connection.
Maybe tower manufacturers have envisioned this method of mounting large
yagis, but somehow I doubt it.
-Steve K8LX
On 06/20/20 16:23 PM, Grant Saviers wrote:
I admit my statics expertise is a bit ancient but I don't understand the
reasoning for the CW of attaching swing arms to multiple legs for
"structural reasons". So educate me if my analysis is missing something.
It seems to me that any moment (torque) applied to a tower by an offset
wind load acts thru the tower section neutral axis (center), no matter
how the mechanical attachment is designed. The lateral force on the
tower at the attachment is the same no matter the attachment method.
Hanging on one leg is the same as attached to all three when we consider
the tower as a relatively stiff structure. That is true for most all PE
analysis which use the tower section structural properties only, not the
stress in the detailed bracing.
Further, any offset load across two legs is up on one and down on the
other. A lever is created with a pivot on the near leg. Good news is
the lever force multiplication isn't much for usual offset distance vs
tower face width. I agree there are fastening and ease of fabrication
advantages for using more than one leg (eg crossed angles) and perhaps
that is what is meant by "structural advantages". Potentially the wind
stability of the load is improved if a multi-leg mount carries thru to
the antenna. I think common for microwave antennas.
The exception might be when the moment exceeds the strength of the
bracing in twist of a leg but I think that will take a mighty big moment
considering the torsional yield strength of a leg tube/bar and the
number of welded braces resisting it. Then most of the time we are
trying to have a wind balanced yagi mounted so that moment is minimized.
Another constraint is the maximum permitted vertical load per leg,
but for most Rohn towers, production ham antennas, and heights that spec
is hard to exceed.
What does matter is the total amount of offset loads (moments). The sum
of the moments is being resisted by the guys and the tower base. That
could be significant with all antennas at same azimuth, boom aligned
with wind. Parking antennas on opposite sides creates canceling moments.
A wire rope or Phillystran guy attached to a leg has very little
capability to resist these moments. So if using multiple offset antennas
on rings or swing arms, double guys with torque arm stabilizers are a
good++ idea for the guys to take twist loads, so to not exceed the tower
twist strength towards the base.
Thinking this thru was more than academic with multiple offset loads on
my big tower. If I've got it wrong please update my engineering.
Grant KZ1W
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