Thanks Hank
I didn't really understand this until I drew a top view of a tower in
AutoCAD. It was then easy to see that the torque arm offers little
resistance, since it's in line with the guy and that the six point system
offers considerably more torque resistance. I installed a six point torque
system on my 160' Rohn 65G tower and it made a noticeable improvement in
reducing the twisting of my tower. I wonder if twisting significantly
degrades tower survival in strong winds. Since rotating towers cannot have
torque systems, are they weaker than standard towers?
73,
John KK9A P40A
----- Original Message -----
From: <kr7x@comcast.net>
To: "Tower (K8RI)" <tower@rogerhalstead.com>; <TowerTalk@contesting.com>
Sent: Tuesday, August 03, 2004 18:12
Subject: Re: [TowerTalk] Rohn Torque Bar confusion
Roger etal:
You are partially correct and partially incorrect. You can't eliminate basic
vector statics from the argument of the usefullness of torque arms. I
specifically mean the style used for 55g and larger and made of channel.
If you can imagine the tower in section as a rigid triangle with the guy
attachments at the verticies (points) and the line of the guys extending
outward from the verticies in line with the center of gravity of the
triangle.( the intersection of the perpendicular bisectors of the opposite
sides). When the wind or other forces are a linear vector in the plane of
the triangle and through the center of gravity, the tower section will try
to displace laterally. This lateral displacement is resisted by the guys as
the vector summation of the forces in the guys. Equal and opposite total
reaction. The tower section is in equilibrium and will not translate in its
plane. Remeber guys only resist tension and cannot resist a lateral load to
their line of action.
When you translate the plane triangle section in rotation in its plane,
twist or torque load, initally the guys cannot resist the lateral
displacement as the guy's line of action is perpendicular to theaction
direction of the load. As the triangular section twists additionally then
there is created an angle between the line of force and the guy line of
action which can develop a vector resistance to the movement of the
triangular section. This value of resistance is a function of the sine of
the angle created. It wil start at 0 degrees at rest, sine of 0 degrees is
0.
This explains why the torsional resistance of the standard guying scheme is
so low and why if significant torsional forces are present in a tower they
twist.
In this case the farther away from the center the guy attachment point is
really doesn't help with the torsional strength the angle between the guy
and the line of force of the torque is what matters.
When you star guy or 6 point guy a tower with a torque arm mechanism then
you have guys that are better aligned to initally resist the twist from the
onset. Therefore this type of system is better able to resist torsional
loadings.
In a normal tower torsion is input into the tower by the antenna and the
rotator mounting plate as the rotator needs something to push against to
work. The top of the tower is one of the best places to put a 6 point guying
system if that is were the rotator is located.
Newton's laws and vector mechanics will always explain how these things
work.
Regards
Lonberg Design Group, Ltd.
Hank Lonberg, P.E.,S.E. / KR7X
President
> I've noted a bit of confustion on the effectiveness of torque bars.
>
> First, torque bars do work when rigidly installed.
> Second they work without resorting to 6 point guys (2 per arm)
> Third, they are more effective with the 6 point guy system than a 3 point.
>
> For those who say they don't work all you have to do is the math.
>
> When the arm moves it translates to linear pull on the guy. An 18 inch
arm
> is going to pull more on the guy than when the guy is anchored directly to
> the tower leg. With Kevlar guys this is substantial force as there is so
> little catenary and give in the guy line.
>
> The big question is; Do you need them?
> "My own opinion" is , It all depends. If the tower load is some long
beams
> I'd certainly use torque arms. If the antenna/wind load is minimal I'd
not
> bother.
>
> On my system which is most likely not typical
> http://www.rogerhalstead.com/ham_files/Tower29.htm and
> http://www.rogerhalstead.com/ham_files/Tower26.htm I do use the torque
arms
> at the top. I do not use them at the middle and bottom levels.
>
> The top guys are tensioned to a bit over 600# while the bottom two tiers
are
> about 450#
>
> I've been up there in winds near 30 mph and the top of that tower does not
> appear to move.
> However, looking up through the antennas at the clouds when the wind is
> blowing like that creates a tendency to leave fingerprints in the tower
> legs.
>
>
> Roger Halstead (K8RI, EN73 & ARRL Life Member)
> N833R, World's Oldest Debonair (S# CD-2)
> www.rogerhalstead.com
>
>
> _______________________________________________
>
> 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.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|