] [Thread] [Date] [Author]

[Mark Allen Jones <jonema(--nospam--at)wwc.edu>]

Hello,
     I will apologize in advance for the cross posting.  The basic issue of this is both very 
general and obtuse (at least no one on this campus has seen anything close to a solution.)
     Now to the problem.  Basically, I need an equation for a 3D catenary/parabola.  I am 
analyzing a suspension bridge.  Normally, the cables are all planar.  In this case the top 
supports are offset from the roadway a considerable distance making the main cable curve in 
both the plan(top) and profile (side) views.  Basically, take the cable as it hangs between the 
two supports and rotate it about the supports to meet the roadway at the mid point.  The 
roadway load is uniform and the vertical cables are evenly spaced.  The line between the 
support points is parallel to the roadway, just offset. (let's not make this any more difficult 
than we have to.)
     I have talked with a couple of engineering and math professors.  So far, this is what 
we've discussed.  Unloaded a string suspended by two points forms a catenary.  With a uniform 
load the shape turns into a parabola.  The major conclusion is that it will form a parabola in 
the plane of the two supports and the midpoint.  The chief objection is that the weight of the 
main cable is significant (the cable is 4.5 feet in diameter) therefor you have loads in the 
plane and a load out of the plane.

TIA,
Mark Jones, EIT, aquapontifex
jonema(--nospam--at)wwc.edu
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