At 12:51 PM 8/4/2005, you wrote:
>This question has been bugging me for a while. I've been trying to figure out
>why tower guys are preloaded. I haven't worked out why that is an advantage
>over simply tightening them enough to take up the slack. I run the math on
>wind loading and such and the preload always seems to increase the stress on
>the tower. There must be something I am overlooking.
>
>Gary
>
>N0GW
Gary--
Preloading IS just taking out the SLACK. Taking out the slack in a precise
manner. As soon as you pull the guy wire off the ground there is
tension in it.
The proper slack removal, balanced with the downward force on the tower
is the preload. Yes, 10% is an arbitrary number, but is the right answer for
Rohn's engineering when using EHS cable. Even with 10% tension you will
be surprised at the sag -- easy to see when sighting along the guy wire.
Using Phillystran, the preload was (and may still be, not sure with their newer
materials) 15%. This was to compensate for the somewhat greater stretchiness
of Philly. Under load the Philly would stretch some at first and tend to relax
to the point where the load was finally about 10%.
Guys with a lot of insulators will sag more than guys with none at the 10%
preload, but that is still the right answer to keep the tower solidly standing
without overloading the legs. And still having plenty of "reserve" strength
when the wind starts blowing.
--John W0UN
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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