Tom wrote:
>[...]
>Example:
>A 100lbs. of horizontal force restricted by a 45 degree guy arrangement
>would result in horiz. & vert. force components of 70.7lbs each. The same
>loading with a 22.5 degree guy arrangement would result in a horizontal
>component of 38.3lbs and a vertical component of 92.4lbs. The vert. comp
>keeps going up as you approach the tower, but you'll never exceed the 100
>lbs. of force put in the horizontal axis, agreed??
No. You have not balanced the horizontal force, so your system is not
equilibrium. Force = mass times acceleration. There will be an
acceleration of the tower due to the net 29.3 lb force (your 100 lb
horizontal force minus the 70.7 lb guy force). The tower will move (flex)
until the horizontal force in the guy wire is 100 lbs. Since the guy wire
is at an angle, the tension in the guy wire will need to increase, and in
the first example, since it is at 45 degrees, there will wind up being a 100
lb horizontal force in the guy and a 100 lb vertical force, for a net
tension along the guy of 141 lbs. In the second example, the guy tension
will increase until there is 100 lbs horizontal and 241 lbs vertical, for a
net tension of 261 lbs.
Consider the following real world experience. You wish to hinge up 30 feet
of Rohn 25 (it's on a hinged base). The tower sits flat on the ground, you
(and a bunch of buddies) pull a rope attached to the top of the tower from a
point about 4 feet above the base. How hard do you need to pull? Too hard,
you can't lift it up.
(Vertical force on tower due to gravity is about 120 lbs. Why doesn't
pulling on the rope with 120 lbs of force do the job?)
That having failed, you pick up the top of the tower and set it on top of a
step ladder, so the top is now 8 feet off the ground (putting the tower at
an angle of about 15 degrees). You pull the rope from the same place and
it's a lot easier, right? (Have fun with the vectors - this one is
complicated).
As the tower goes up (and gets closer to 90 degrees from the ground), it
gets even easier, right? Since the weight of the tower never changes, why
does it get easier? Because of the angle you are pulling on the rope versus
the angle of the applied force (gravity) you are trying to overcome.
For what it's worth, I've successfully hinged up 30 feet of Rohn 25 by
myself without the use of block and tackle. Of course, the tower been
lifted up to about a 20 degree angle with a step ladder and I was standing
on a roof top that was 15 feet above the base. It's all in the angles.
Try looking at the problem from a different angle. :-)
-- Bill N7VM (ex N1BR, AI6E)
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