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[TowerTalk] stress in guy wires

Subject: [TowerTalk] stress in guy wires
Date: Tue, 16 Aug 2011 15:43:11 -0400 (EDT)
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Fellow Hams,
I believe I have one way of measuring the stress in the guy wires. I would  
like to have some comments whether I'm right or not.
Consider the guy wire as a string in a guitar. The vibration in a guitar  
string follows a relatively simple formula that you can check at Wikipedia. 
I made some thinking (which may be dangerous) and calculations and found  
that the stress in a guy wire can be measured by observing the speed a wave 
runs  up and down the wire. If you hit the wire (with e.g. your hand) you 
will notice  that a wave is going up that wire and returns after a short time. 
By measure  this time and know how heavy the wire is, you can calculate how 
much force the  guy wire is tensed.
The stress (in Lb) = wire weight (in Lb/foot) multiplied by (the length of  
the wire) squared divided by (the return time of the pulse wave) squared 
and the  whole thing divided by eight(8). The formula looks like this in a 
spread sheet :  "=1/8*A1/A2*(A3/A4)^2" there A1/A2 is the wire weight per 
length unit, A3 is the  guy wire length and A4 is the time it takes for the 
to return after you  "hit" the wire. The formula is off by ~1/2 % as you 
should divide by 8.03806  instead of 8 to be more precise. Be aware that a 1% 
error in the time  measurement results in ~2% error in the result.
I measured ~.75 s on my 100 feet 5/16" steel guy wires (56.6 lb/250 feet)  
which indicates 500 lb. Reasonable?
The time might be too short to measure for short guy wires or for  Phillys 
wire. I leave the method to measure the time for these applications  to 
somebody else to figure out.
Hans - N2JFS

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