Tower Load Distribution

Mon Feb 12 19:33:41 EST 1996

On 12-FEB-1996 01:18:25.5 bigdon said to the REFLECTOR

   > But that's only half of the story.
   > Go out and watch your stuff in a windstorm.  If it's anything like
   >around here, in a good storm you are gettting hammered by big gusts
   >every few seconds from directions which can change maybe 30-50 degrees
   >in direction with each gust.  And the timing between gusts can vary.
   >All of this can set up twisting, stretching,and bending dynamic
   >oscillations in the tower/mast/guy system. Don't forget slop in the
   >rotor.  All this sort of behavior is virtually impossible to model
   >particularly in the general form it would require to produce the kind
   >of computer program being discussed in this thread.
   > If all these perturbations manage to combine themselves
   > to locally overload something briefly, you can
   > kiss your big signal *goodbye*.
   > It can happen at wind speeds well below the peak wind speed for which
   >you thought you designed your system. My guess is these effects
   >account for many of the lost systems due to windstorms.
   > A classic case of this is the first Tacoma Narrows bridge which was
   >most certainly designed to withstand a bumper-to-bumper traffic jam of
   > fully-loaded 18-wheelers in both directions in combination with *static
   > loads* from some peak windstorm.  Yet it failed when lightly loaded
   >in a rather moderate garden-variety windstorm.  For those who have
   >never seen the video of what came to be known as "Galloping Gertie,"
   >the wind set up a swaying/twisting action in the main span that
   >increased in amplitude until the whole works deposited itself in Puget
   > Big Don

Big Don is correct.  What seems to be a simple problem, air flow around a "wire, mast, element, tubing" NOT!  I am away from my
reference books but I believe its called the Von Karman Vortex Effect.  What
happens is that a vortex is created and shed alternately from one side of
the wire and then the other.  This creates an unbalanced forces on the
cylindrical element ... first on one side and then the other.  If done
correctly mechanical resonance can be achieved....bad!

What appears simple is NOT!

73, Hank/K2UVG
jkahrs at

PS... I don't have one of those super sig blocks, but I am a retired Navy
Carrier Fighter Pilot with a MS in Aero I'm a professional radio
amateur and a professional golfer with a 25 handicap!

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