>At 08:46 PM 4/16/03 -0400, K3BU@aol.com wrote:
>>That is nonsense!
>>If the guy wires can hold the load, the tower will be stressed less than if
>>self-supporting.
and At 10:09 PM 2003-04-16, N4AR wrote:
>Yuri, this is simply wrong. simple vector analysis will show you the vertical
>load guy wires apply to the tower as the lateral force increases. whether the
>tower can take it is a design decision made when the application -- i.e.,
>guyed or unguyed -- is determined.
Now, wait a minute, guys. I don't think Yuri is *always* wrong with that
statement. Let's look at some specific cases. If the guy wires are terminated
so far away from the tower that they're essentially horizontal, there is NO
added vertical load on the tower legs when the guy is attached or when the wind
blows. And if these guys are tensioned properly, they take the wind loading on
the antenna at the top of the tower, and the tower legs have to provide far
less compression strength or tension strength during a wind storm than if there
were no guy wires at all.
If, on the other hand, the guys were really steep, I might agree with Pete, but
at points in between you can't categorically say the tower is worse off. In
most *practical* situations, I believe the tower is *better* off for the reason
that Chuck, W1HIS, noted -- the angle of the guy wire is much more "favorable"
for vector loading calculations than the self-supporting tower legs are. That
is to say, the vertical forces created in the tower legs and guy wires for a
given wind speed and a given antenna loading depend heavily on the angle
involved in translating the horizontal wind force into a vertical tension or
compression force. When "reasonable" guy wires are added with proper
tensioning, the resulting force in the tower legs is substantially lower than
before the guys were added.
Real rough example:
10 square foot "antenna" at top of 80 foot tower, in steady 20 psf wind.
Horizontal force on antenna is 200 lbs. Overturning moment is 200 lbs times 80
feet, or 16,000 lb. feet. Assume self-supporting tower has 4-foot distance
from each leg to center-point of base, and wind is oriented directly in line
with one leg. The force in that leg is 16,000 lb. feet divided by 4 feet, or
4,000 lbs (compressive or tensile).
Now attach non-stretchy guy wires to the tower so they're at a 45-degree angle
with the horizontal. Tension the guys to 10% of their breaking strength, as
usually recommended. So we have an initial tension force in each guy of, say,
400 pounds. Then we get maybe 400 times 0.7 or less than 300 pounds of force
added to each tower leg just because we added guy wires. (If you're worried
about the dead weight of the guy wire material itself, use Phillystran. But
1000 feet of 3/16"EHS weighs only 73 pounds. Three guys, each 130 feet long,
weigh less than 35 pounds total. Double that for insulators and cable clamps
or guy grips. Not a big deal.)
Now let the wind blow horizontally. Since we tensioned the guy wires properly,
that 200 pound force on the antenna is taken up by the guy wire. If the guy
wire were horizontal, the added tension in the guy wire due to wind force would
be 200 pounds. If the guy wire were vertical, the added wind force in the guy
would be infinite. Secant function, perhaps? 1 divided by (1-cos)? Since the
guy is at 45 degrees, the added wind force in the guy is maybe 1.414 times 200,
or about 300 pounds. However, when we vector resolve the 300 pounds of
wind-induced guy tension into horizontal and vertical components, we get 200
and 200, respectively. Ain't 45-degree symmetry wonderful?
(I could be off by a factor of 2 here and there for radius vs. diameter, flat
plate vs. cylindrical elements, etc., secant vs. 1/(1-cos), etc., but we're in
the ballpark.)
Thus, during a 20-psf wind "storm", the total load on the tower leg from adding
guy wires is about 500 pounds -- a far cry from the 4000 we had before we added
guy wires.
That's why I don't think it's any contest FOR MOST REASONABLE ANGLES OF GUY
WIRES AND MOST HEIGHT TO BASE DIAMETERS OF SELF SUPPORTING TOWERS.
But I thought this thread started because someone was going to house bracket
standard Rohn 25 or 45 type sections designed to be guyed, and someone else
suggested adding guys to reduce vibration-induced noise in the wall of the
house. In that case, adding guys is NEVER worse --except, perhaps, in the
pathological case where you use broadcast industry guy wire material so big
that the 10% tensioning load alone is sufficient to compress Rohn 25 legs!
Bud, K2KIR
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