the increased stretch is just the first step. now figure the change in
angle of the deflection of the guy wire and calculate the force vector at
the end of that torque arm vs the force it would be on the tower leg
directly.
there are 3 parts i can see to this problem:
1. the difference in stretch of the guy wire for the same deflection of
the tower... this part is what you calculated.
2. the increase in the angle that the guy wire is deflected. this
increases the restoring force.
3. the increase in the arm length that the restoring force acts on.
just off the top of my head the biggest factor is probably #3. both 1 and
2 are likely sine functions of the deflection angle which changes only a
little bit for increased arm length. but the length of the arm that the
force acts on is directly proportional to the torque arm length.
> I just did some guy wire length calculations. I used a 100' Rohn 55 tower
> with a 12" torque arm at the top and a 80' guy radius. The guy wire
> length
> is 126- 6.28". I then rotated the tower 10 degrees and the guy wire
> length
> increased to 126' - 6.41" or 0.008% longer. If I connect the guy to the
> tower leg, I get an increase length of 0.006%. Since in real life the
> actual guy wire would not be longer this extra length would have to come
> from slack in the guy or stretch. I am not a PE, but it would seem that
> the
> is not enough different to provide much torque resistance with this
> system.
>
>
> ----- Original Message -----
> From: "Tom Rauch" <w8ji@contesting.com>
> To: <keith@dutson.net>; "TowerTalk" <towertalk@contesting.com>
> Sent: Wednesday, August 04, 2004 01:09
> Subject: Re: [TowerTalk] Rohn Torque Bar confusion
>
>
> But no one is saying they are equal Keith.
>
> I see a serious flaw in the idea a guying system does
> nothing to reduce twist. That idea only would be true if the
> guyline attached to the center axis of the tower.
>
> Consider a spoked wheel such as on a bicycle or even the
> drive wheel of a motorcycle or car with wire wheels. If the
> guyline does not resist twist, why do wire wheels resist
> huge amounts of torque without serious deflection?
>
> The reason is pretty simple. When the tower twists all three
> guylines have to reach a longer distance, tension increases
> a large amount. (You could view the tower as a wheel.) All
> three guylines, when the tower starts to wrap up, add force
> that opposes any twist. That's because the guylines are no
> longer in line with the center line of the tower. Each
> guyline adds an equal opposing force to any twist.
>
> Now that isn't exactly like a wire wheel with 50 spokes, but
> it still is a significant force when the twist tries to pull
> the guyline an extra inch or two!
>
> As a matter of fact, this is why tower with heavy insulators
> in guylines feel so "twisty". They have more sag to suck up
> out of the guyline, so the tower can twist and wobble more
> than a guyline with less sag.
>
> If a tower with a radius of one foot is extended to two feet
> in radius by the addition of a torque arm, the same increase
> in guyline end spacing would cause twice as much resistance
> to torque.
>
> The primary difference between a torque arm and a six wire
> star bracket is the star bracket already starts with the
> wires "deflected" from being in line with the center, so the
> initial movement is much less for a given increase in guy
> tension. The fact it is six lines doesn't help, because
> three are going slack while three are tightening.
>
> That's how it looks to me in my head-cad.
>
> I'm sure there is someone who can put numbers on this. The
> problem isn't that complex. I do absolutely know the idea
> guylines do nothing to resist twist is wrong, because I
> can't twist my tower that sits on a greased ball at the base
> insulator and it has NO torque arms of any type.
>
> 73 Tom
>
>
> _______________________________________________
>
> 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.
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
>
>
> _______________________________________________
>
> 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.
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
_______________________________________________
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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