[TowerTalk] Stacked Yagis on Tower Legs

[Grant Saviers]

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