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Tower Ratings - Stupid Question

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Subject: Tower Ratings - Stupid Question
From: (Chris R. Burger)
Date: Mon, 23 Dec 1996 08:46:28 +200
> > I hear people putting guy wires on self-support or crank-ups,
> Just had a local amateur here have his "guyed self suporting" tower com 
> down...  The tower began to retract into itself, and then bent...
It's been said before in this thread, but a lot of the postings seem 
to ignore the implications.

It's a fact of life that a guy line can only pull.  If you ignore its 
stiffness (a reasonable assumption considering the forces involved), 
the guy can only exert a force along its own direction.

Consider, then, what a guy does.  It pulls downwards, and outwards.  
The ratio between the two forces is determined by the angle of the 
guy line.  For example, if the guy is at 45 degrees to the ground, the 
"down" and "out" are equal, and are 71% of the tension in the guy.  
If the angle is 60 degrees (as might happen if you're pressed for 
space), the "down" is 87% of the tension, while the "out" is 50% of 
the tension.

For a 60 degree guy line with 25 kN breaking strain (that's around 
5600 lbs, for those of you with traditional values), the guy will 
exert a force of 22 kN (equivalent to just over 2 tonnes) down the 
tower before it breaks.  This situation would occur when the wind 
forces the tower away from the guy line.

Even for a 30 degree angle (for those with massive real estate), the 
force is still well over 1 tonne.  That is equivalent to having a 
1200 litre (315 US Gallons, 270 Imperial Gallons, or 0.2 cubic 
furlongs) water tank on the tower.

Ask yourself if you want those kinds of forces in your tower.  If the 
answer is no, don't do it.

Chris R. Burger

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