[TowerTalk] Weak Link in Guy System?

[Pete Smith]

At 04:18 PM 11/18/00 -0000, Barry Kutner wrote:
>Overengineering isn't always safe. For example, suppose you use 
>guy wire rated at 20,000 lbs. on a Rohn 25. Follow the rules and 
>tighten to 10%. The downward forces from the increased guy 
>tensions could cause the tower to buckle in between guys. (the 
>numbers are for illustrative purposes only, not calculated to buckle 
>the tower).

I think it's a little more complicated than that -- if I understand the
analysis properly, the reason for pre-tensioning is to take most of the
slack out of the guys so that any additional force resulting from the wind
causes them to operate mainly in "stretch."  The 10 percent is presumably a
rule of thumb that relates the weight of the guy cable to its breaking
strength and applies enough pre-tension to put it in the right operating
mode.  But what about Phillystran or fiberglass, both of which are much
lighter for a given strength?  I put 6700 lb Phillystran on my top guy
station, and with 400 lb tension the Philly is visibly straighter than the
4000 lb EHS with the same tension.  I see no reason in that case to crank
the Philly up to 670 lb.

On K7NV's web page there is some interesting discussion of guyed tower
analysis he has done using finite element methods.  From memory, in one
example he replaced (analytically) the 3/16 EHS on one guy point with 1/4"
EHS and significantly reduced the stress on one area of the tower, because
the heavier guy stretched less for a given tension, so the tower deflected
less.




73, Pete Smith N4ZR

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