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.
>
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_______________________________________________
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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