infomet at embarqmail.com
Sat Jan 16 10:42:20 PST 2010
Well, we have certainly beaten this horse, although I doubt if we have
cleared anything up!
I want to make a couple of points.
I think the long guy, more horizontal, more catenary, WILL have a lower
horizontal stiffness than one more vertical. That will make the tower
deflection asymmetric for different wind directions, but not a big deal in
real cases, I expect. That's tricky though, because the steep guy point has
to move horizontally to develop horizontal reaction. If a steep guy were
very stretchy, large horizontal displacements would be required. An
extremely steep guy might break or overload the tower vertically before
developing the required reaction. So I'd say the anchor points are set out
far enough to take out the windload at the top of the tower while leaving
enough vertical capacity in reserve for the lower points. I'm sure there is
a crossover angle where the horizontal stiffness/tension is optimum.
Who said tensions should be based upon the strength of the wire...sounds
dumb! Suppose you had two identical wires with different rated strengths.
Would you tension them differently? I don't think so! Excess tension is
excess vertical load, which can really go up with many sets of guys. I
think the lowest tension that will produce the maximum allowable bending in
the design wind is where you want to be.
After all this, we have never even heard anything about the drag coefficient
of real towers. If we had that, calculating a reasonable estimate of tower
loads would be pretty easy! I'd love to put some in a wind tunnel and make
Now maybe someone will address the massive concrete bases recommended in the
catalogs? I maintain that if the tower is held vertical by the guys there
is near zero horizontal reaction at the base and all we need is enough base
AREA to keep the thing from sinking, plus enough horizontal restraint to
handle the windload on the tower between the ground and the first guys,
which isn't much. There is no overturning moment at all.
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