One way to distill the question of the joints (and applies to all
self-supporting towers)
If you just hand tighten the nut and bolt, (all 3) and then try to move
the metal tower legs up and down, how much does it move?
Goes to the question of oversized holes. Note structural joints are
allowed to have oversized holes, although washer requirements may be
different depending on the amount of oversize.
The tower design analysis provides the maximum vertical force (from
wind/moment), (which can easily be 7000-15000 lbs, depending on height
etc)...and if there's movement in the joint and you want to prevent it,
you can figure out the clamping force needed to provide the required
friction to resist it (for the mating surfaces present).
There's really no magic. On the one hand, we get all serious in our
analysis of wind moments for other reasons. That all translates into a
force on one leg. If we're serious about believing moment analysis, and
we don't want joint movement, shouldn't our analysis of bolted joints be
pretty simple? I mean, how else would you be deciding what's "good enough" ?
The wind-induced moment decreases as you go up the tower, so the lower
joints see more than than upper joints. And you don't want to
over-torque and strip the threads, especially as the diameter decreases.
And never-seize on the stainless steel bolts lubricates, so the torque
for a particular clamping force goes down by 20-30%.
While lots of variables affect the connection between torque and
clamping force, I think there isn't a common agreement that the goal is
a slip resistant joint, achieved through preloading of the bolts. (I
guess I'm commenting on the idea that seeing the lock washer clamped
down means anything good).
Now on the other hand, the goal might not be to prevent joint movement
in the worst case. If so, then there's lots of possible torque values
that are fine. But then you get the loosening question. A well-preloaded
bolt will have enough friction to resist loosening also.
I did a quick look at the numbers for those 3/4" stainless steel bolts
and was surprised they can handle a lot, assuming you don't gall the
threads when you torque them.
All these questions about torque etc, should really be about: what does
it take to make the joint not move under load, and what bolts are needed
to achieve that.
The lock washer question is a canard. Flat hardened washers would be
preferred in any structural joint that's sufficiently preloaded.
I think there's no answer to these bolt questions, because we all don't
agree on the goal for the joint. (but on the other hand, people seem to
agree on the torque/clamping force needed at the base of crankups:
what's the difference?)
-kevin
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