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.
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