> Any help? Is there someone or some other reference that I can turn
> to? I have searched the internet and have not turned up any good
> leads. Is there any software available that will analyze a
> catenary shaped member? Are there any references, research papers
> or other documents that indicates at what point a solid element may
> begin to behave as a cable? Thanks!!
There's a whole chapter in Timoshenko's Strength of Materials volume
2 about beams with combined axial and lateral loads. The answer to
your question depends on the boundary conditions. If you assume rigid
spanwise support, axial loading starts to develop whenever the rod
has any deflection at all. That's because the length of the
deflection curve is longer than the original straight line between
the posts. Timoshenko shows how to calculate the corresponding axial
load and superpose the direct and axial stresses. You can compare the
stress from the direct load to that for transverse loading only to
see where one or the other dominates. There's always some
contribution from direct loading but you can assume the boundary
between the two behaviors as the point where the difference is less
than usual engineering accuracy--probably about 5%.
It's trickier when you have elastic supports like stanchions
supporting the rods because they also deflect under load which
reduces the axial load on the rod. but you can figure this out pretty
easily by hand. If you have multiple spans all the rods have full
axial support, except for the ones at each of the ends of the rail
where the stanchions can deflect inward unless they're pretty well
braced.
This effect is fairly easy to figure manually, but if your EOR is an
old shell-back who believes you can analyze the world by assuming
it's a cantilever beam, your work is cut out for you. Just make sure
you've designed your connections to account for the full direct
loading and the beam reactions both, and that you've taken steps to
assure that you've eliminated any local yielding that might make the
rod sag or the connections loosen. Anyone old enough to remember the
coated wire clothes lines and how they'd sag. You'd tighten the wires
up and put a little pre-tension so they were nice and straight, and
the next couple of laundry loads and a nice breeze would have them
sagging again because the wires stretched. Even after a detailed
explanation of all that, my mother was singularly unimpressed -- what
good was engineering college if she'd have to use clothes poles all
he life.
Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw@skypoint.com | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania
1864)
http://www.skypoint.com/members/chrisw/
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