Tom:
When I refer to a star or six point guy arrangement I mean 2 guys from each guy
point on the tower. With 3 anchor points on the ground you get a 3 pointed star
in plan.
I am rethinking my earlier statement as Chris stated a system with the the guy
alignment through the center of rotation has no torsional resistance. However
if you do load it such that it trys to twist the guy line of action moves in
relation to the center of rotation and therefore creates an eccentricty, a
small distance but the guy force is fairly large. This is what the guy will
contribute to resisting the torsion. Say 1" horizontal displacement at the guy
point attachment and 100 foot guy length with 600 lbs inital tension. Assuming
no additional tension do to guy elongation the rotational resistance from the
guy force in line is 600 in-lbs. The horizontal component is about 6 in-lbs. As
you can see I overstated the horizontal. If there are 3 guys then the total
resisting force is (600+6)*3=1818 in-lbs or 151.5 ft-lbs. Not an insignificant
number.
The increase is not really linear as it is a sine/cosine function. I retract my
earlier argument re the lateral component.
Again a full application of vector mechanics wins again, you just have to make
sure encompass the whole problem not a portion. I thank Chris for providing the
spark and apologize to Roger if I was overbearing in my response. Tom I thank
you for you practical wisdom, as usual.
Regards
Lonberg Design Group, Ltd.
Hank Lonberg, P.E.,S.E. / KR7X
President
> 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.>>
>
> ....but eventually the tower twists enough that guy tenison
> tries to pull it back. I KNOW this happens because when
> moron's improperly install or guy a tower and the anchors
> are not aligned with the point of the tower, the tower gets
> all shaky and twisty no matter how tight the guylines are.
>
> This is because the tension of the guylines is trying to
> twist the tower, rather than hold it in position.
>
> As the guyline is moved out on torque arm the effective
> radius of the tower increases, and rotational movement
> decreases enough that it is VERY easy to feel.
>
> It isn't quite true to say guylines don't help keep the
> tower from turning. Anyone who has climber a tall tower
> with a point base that allows rotation, like an insulated
> tower, knows the guylines greatly contribute to rotational
> stability. When those needless do-nothing torque arms are
> added, the tower becomes much more stable.
>
> The only question I have is if the increase in resistance to
> twist translates into higher wind survival ratings, or if
> the tower can "twist" so much without contributing to
> failure the rating does not change. Of copurse even if the
> survival remains the same, I still prefer to NOT have my
> tower twist more than necessary.
>
> > 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.
>
> I don't agree. While the angle remains the same, the lever
> distance from center is increased. If you double that
> distance, it is a two-fold increase in resistance to twist
> for the same increase in guyline tension caused by the
> twist.
>
> > 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.
>
> That's certainly true, and you don't need a six point guy.
> You can use a three guy anchors and split each guyline into
> two ends some distance from the tower. If you keep the angle
> small, there is almost no difference at all between six guy
> points and three.
>
> 73 Tom
>
>
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