[TowerTalk] Guyed self-supporters (was "Concrete suggestions")

[Guy Olinger, K2AV]

Seems to be an awful lot of air time trying to kill fleas on the elephant.

An earlier post is correct that the tower flexibility necessary to engage guy wires is quite missing on self supporting towers.

I have an 80 foot Trylon. Even if I swing my 250 pounds back and forth at the top, I can't get it to sway even a 1/4 inch. It is significantly RIGID.

I have been at the top of guyed towers, and my own self-supporter in high winds, and the movement in the wind at 100' on a guyed tower will take your breath away. I never have felt that at the top of the Trylon. It simply does not move like a guyed tower.

It is true that there is a huge overturning moment at the base of a self-supporter, a compression and lifting in a heavy wind. But the tower and base are engineered to handle that moment routinely. Personally I have never heard of a case of a properly installed self supporter being OVERTURNED by a high wind, hurricane or otherwise. 

Therefore adding guys to help keep a self-supporter from being OVERTURNED may feel nice, but it's guarding a bank with no money inside.

I have heard of **ONE** case of the top third of a Trylon being folded over by two inches of radial ice followed by 50 mph winds. That's turning a 30 sq ft rated tower into a 500 sq ft sail. But that's FOLDED OVER, UP the tower, not overturned.

Given the rigidity of the tower, and the fact that a guyed tower HAS TO MOVE to create the counter force opposite the wind, here is the question...

***Would a self supporting tower's movement in the wind reach a failure point before the guy could provide enough counter force to prevent it?***

Guying a self supporter may make you feel better, but if the self supporter isn't designed for the load forget it. You can STILL lose the upper section with guys on it. Or stated another way...

That two-inch radial ice plus 50 mph would have ruined that Trylon, even with guys on it, anyway. It just wouldn't have folded over the same way. If it was guyed at the top, it would have folded in opposite beneath the rotator instead. Once the bend damage establishes someplace, even just a little, the rest is history.

I have a suspicion that adding guys at the top may actually weaken a self-supporting tower with a mast and top load by providing a fulcrum at the top of the tower that is otherwise not in the equation.

This is because the tower is deliberately designed to flex more at the top and flex gradually less as you go down. This has the effect of SPREADING the flex moment along the entire height of the tower. 

The safety of the tower depends on all the moment being evenly spread. 

If the total moment in a wind is 5000 pounds compression toward the east, that must be spread out over the tower, AND is a constant sum that is maintained as long as the wind is steady. 

Now add top guys. Suppose that the windload on the antenna is 1000 pounds (4000 on the rest of the tower).

You now have added a 1000 pound force to the west down at the level of the rotator. The wind is pushing the antenna at top of mast to the east. The force is transmitted through the fulcrum at the tower top guy point. It now appears down at the rotator point pushing opposite direction toward the west. This means that there must be the effect of 6000 pounds to the east elsewhere, extra compression on the downwind side, to maintain the overall sum of 5000 pounds to the east. 

The tower was not designed to have a fulcrum point at the top. It was designed to be proportionately flexible at that point.

This stuff is NOT simple. It is NOT intuitive. It is deep doodoo PE stuff. 

Gets back to the prime directive. DO WHAT THE MANUFACTURER SAYS. Unless guys are in the manufacturer's contruction details, then DON'T.

73, Guy.