Guy, that was a great discussion -- thanks.
When I think of "self supporting," my first thoughts go to crank-ups, since
those are the only self-supporting towers I've ever owned. Even the
crank-ups are very rigid, and the only movement is the slop between sections
which results in maybe 1/4" lateral play if you shift your body weight a lot
while fooling around up there. It never occurred to me to guy any of my
crank-ups, but of course they can also be cranked down if the winds become
extreme.
With 12,000 lbs of concrete at the base of my crank-up, tipping over is
something that is about as likely as the house blowing away. They'd
probably occur simultaneously during the next nuclear windstorm...
WB2WIK/6
"Success is the ability to go from failure to failure with no loss of
enthusiasm." -Winston Churchill
> -----Original Message-----
> From: Guy Olinger, K2AV [SMTP:olinger@bellsouth.net]
> Sent: Friday, April 18, 2003 10:06 AM
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] Guyed self-supporters (was "Concrete
> suggestions")
>
> 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.
>
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
> Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with
> any questions and ask for Sherman, W2FLA.
>
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