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Calculating wind turbine tower loads - Page 6

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Posted by vaughn on February 25, 2009, 8:42 pm
 


The professionals use dtrain guages to do the measuring & turnbuckles and/or
sliding preforms to do the adjusting.


Is this a trick question?  ;-)

Yes, on reflection I think all of the rigging weight gets transfered to the
base.  Any wizards out there agree or disagree?

Vaughn



Posted by Curbie on February 25, 2009, 9:05 pm
 
On Wed, 25 Feb 2009 20:42:09 GMT, "vaughn"


Sorry about the dumb question, but there seems to a lot of
counter-intuitive math results in involved in this subject.

Thanks for the help.
Curbie

Posted by daestrom on February 25, 2009, 10:06 pm
 

Yes, all the weight of the rigging (except for the tie down blocks
themselves) is supported by the tower.  Just think about the guy-to-tie-down
connection.  It isn't 'stiff' and can't 'push' up on the guy wire at all.
So the vertical component of all that rigging has to be supported by the
tower base alone.

As you rightly pointed out, the guy is typically under a preload tension
even with no wind, so the net force vector the block is exerting on the guy
is in a straight line into the ground along the line of the guy.

Now, "How much preload?"  That's a tricky question and I'm not sure what I'm
about to say is the whole story, so caveat OP.  The idea of a preload is to
tension the cables such that the tower won't have any 'slack' and move back
and forth.  If it did, then the motion and sudden 'jerk' when a cable comes
tight will be a much larger shock load and could snap the cable(s).

But you can't just 'snug them all done' on a calm day and call it good.
When the wind starts to blow and one cable (or a pair) are under more
tension, they will stretch a bit.  If the downwind guy was just 'snug'
before the wind started to blow, now with the stretch in the upwind cable,
the downwind cable will become slack.  A sudden change or any vibration
could cause the tower to bounce sideways and shock load this now-slack guy.

So I think you'll find that professional guys are tensioned up to something
a little more than what they'll see in wind loading.  That means when the
wind blows the 'upwind' cable has almost double the preload tension in it,
and the 'downwind' cable is not quite slack.  Sort of like pre-tensioning
the rebar in pre-stressed concrete.

But I could be wrong on this, so be sure to double check this.

daestrom

P.S.  So obviously if the wind load is expected to be 100 lbf on a 45 degree
guy, the 'calm day' preload on all four guys would be a bit more than 141
lbf (say, 150 lbf).  And on that windy a day the upwind cable has
150+141)1 lbf tension (and so does its anchor block!!!).

P.P.S And four guys, each preloaded to 150 lbf would mean an additional base
load of 150lbf*sin(45)*4B4lbf on a calm day.  Under the design wind it
would be (291 lbf + 150 lbf + 150 lbf + 9lbf)*sin(45)B4 lbf


Posted by Jim Wilkins on February 24, 2009, 2:52 am
 
The physics of this is called 'Statics".
http://darkwing.uoregon.edu/~struct/courseware/461/461_lectures/461_lecture=
8/461_lecture8.html

High school algebra and geometry are enough to understand the basics,
the trouble is that most explanations assume you will go further and
need calculus so they use it early on.


Posted by z on February 25, 2009, 5:54 pm
 david.williams@bayman.org (David Williams) wrote in


*laugh*

Thats just something I'd figure out just after putting the blades on...


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