N4KG reply interspersed in text.
On Wed, 5 Aug 1998 09:16:08 +0008 "George T. Daughters"
<gdaught6@leland.Stanford.EDU> writes:
>
>
>hi all,
>
>do a thought experiment with me. imagine a 50 foot tower, guyed part
>way up, say at 25 feet. the guys are absolutely perfectly
>equally tensioned, and not stretchy. (remember, this is an
>imaginary tower) now say that there is a big antenna at the top.
>now let the wind blow. as others have already pointed out, there
>will be lateral forces at the base from the wind on the base sections
>(the bottom 25 feet) but there will also be much larger lateral
>forces (in the opposite direction to those arising from the base
>sections) due to the big antenna and the top sections of tower. it's
>like a lever with the fulcrum at the guy level of the tower.
>
>if anyone doubts this, i suggest the following REAL experiment:
>build a tower about 50 feet high, guyed at 25 feet, with a big
>antenna on top, and a free-floating, well-lubricated base...
>say a smooth steel plate on dry ice. build this installation more
>than 50 feet away from anything you care about. now, when the wind
>blows (even a gentle zephyr) DON'T stand anywhere near the thing.
>the lateral forces at the base will kick the base out (toward the
>wind!) and the whole works will come down!
>
>sorry... a long winded, but i hope understandable, demonstration that
>there ARE lateral forces on the base of a tower.
>
>73,
>
>George T. Daughters, K6GT
>.....................................................
QSL, BUT you must admit this is an extreme case.
Most people would put a second set of guys at the top,
which take most of the horizontal load of the antenna.
In your case, with a single set of guys half way up the
tower, let us assume an 8 sq ft antenna and 86.6 MPH
wind which exerts 30 lbs. / sq. ft. That places 240 lbs
of thrust against the top. Ignoring the tower load, an
equal thrust would be required at the base to hold it
steady. BUT, 240 lbs is not much of a load, not when
compared with maximum vertical load capacity of the
tower, which is around 15000 lbs for Rohn 25. With
a second set of guys placed at the top, you could
still put roller skates on the base plate and the tower
would remain standing, even if the guys stretched
unevenly (IMO :-).
Oh yes, if the guys stretch and the base is free to move,
it will do so to zero out the (small) lateral forces resulting
from the (small) tower movement due to guy wire stretch...
so I guess we are both correct. Bottom line: there may be
SOME lateral forces at the tower base but they will be
relatively SMALL compared to the vertical load, especially
if the tower is supported at two or more levels.
de N4KG
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