[TowerTalk] Stacked Yagis on Tower Legs

[Steve Maki]

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