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Finding the tension in a cable of a bridge

  1. Aug 8, 2008 #1
    1. Compute the cable tension at the midpoint, the quarterpoint and support for a 1600 foot span with a sag at the midpoint of 100 feet, when the cable is loaded with 10 kips/foot spread uniformly along a horizontal line.

    Midpoint Cable Tension=?
    a)3200 pounds
    b) 16000 pounds
    c) 32,000 kips
    d) 63,000 kips

    2. Relevant equations

    3. The attempt at a solution I have no idea what to do. the only things that come to mind are the vertical and horizontal forces. Also setting the torque to 0. No real idea I repeat.
  2. jcsd
  3. Aug 9, 2008 #2


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    Science Advisor
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    Hello, arared, and wecome to PF!
    Cables suspended between 2 points, under their own weight, take the shape of the catenary, which often can be approximated by a parabolic curve. Cables that are uniformly loaded by external distributed vertical forces, which appears to be your case, do take on a parabolic shape. The horizontal tension is easily calculated by a simple formula, and the other tensions can be calculated from that value. Are you given any data other than what is given, or are you being asked to derive the formula? I could give you the formula for the midpoint tension, but that would give you the answer, so I have to keep it a secret for now, until you clarify the assignment.
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