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Posted by daestrom on February 25, 2009, 5:48 pm
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> Oh crap, I thought I had it until reading your reply!
>
> I thought (from previous examples), but expanding using your example:
>
> "For example, suppose there are three guys, each at 45 degrees from
> horizontal, and at compass bearings 0, 120, and 240 degrees from the
> tower. Suppose the wind is blowing from bearing 60 degrees, exerting
> 500 lbs force on the turbine. The additional downward force produced
> by the tensions in the two guys at 0 and 120 degrees will be
> 500 x sqrt(2), or about 707 lbs, making a total downward force on
> the tower base of 1707 lbs. The tensions in those two guys will each
> be 500 lbs. The third guy, at 240 degrees bearing, will be slack."
>
> That the additional vertical (downward) force on the mast from a 60
> degree windward breeze placing a 500lb horizontal load on the turbine
> would add 500lb to the base loading (through the guys and mast) and
> 707lbs of additional (45 degree) guy tension divided equally between
> guys at compass points 0 & 120?
>
> How can guys at compass points 0 & 120 have 707lbs each of tension
> without exerting an additional 707 x 2 vertical load on the base?
>
I'm not sure David's numbers are right.
First, looking down from above we have a force acting at 0 degrees and a
force acting at 120 degrees that have to add up to a net force of 500 lbf
acting at 60 degrees (opposite the direction the wind force is acting).
This forms a nice neat equalateral triangle with each leg equal to 500 lbf.
Now, for a guy wire that is 45 from the horizontal to generate a reaction
force of 500 lbf horizontally, it must have 500 lbf / sin(45) of tension 707
lbf. The third guy will be slack.
A tension of 707 lbf in the guy will create 500 lbf in the horizontal
direction from the tower and also 500 lbf downward. With two guys, the
total downward force is double that or 1000 lbf. Add to that the static
load mentioned and you have 2000 lbf total downward.
The issue is that the two guys are pulling 'against' each other horizontally
if you look at it from above, but pulling 'together' downward if you look at
it from the side.
daestrom
P.S. Hopefully I got it right this time, I've worked it out three times and
gotten three different answers :-/
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