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Homework Help: Particle statics - A bridge span

  1. Aug 18, 2006 #1
    Heres the problem.

    A bridge span is 5m high and 20m long. Find the verticle force and the horizontal thrust on each of the piers P in terms of weight suspended from the center of the span.

    I really hope you can understand the geometry of the problem without the drawing(there was a drawing in the book). Imagine the two piers as stumps that are 20m apart. From the upper corner of each stump, on the side facing the other pier, exits a rod in the direction of the other pier and up. The two rods meet at a height of 5m right between the two piers, so the cathete that forms an angle with the rod(hypothenuse) is 10 m long.

    I will name the weight F.

    Heres how I tried solving:

    I reasoned that the rod would transfer the force of the weight to the pier by means of a force which is parallel to the rod.

    I found that force by first finding the angle between the 10m length and the rob using tan w = 5 / 10.

    With the angle I find the componant parallel to the rod:

    sin w = F / F(parallel to rod) --> F(parallel to rod) = F / sin w

    I reason that the horizontal componant that Im searching for is the x componant of F(parallel to rod).

    I find it by cos w = F(horizontal) / F(parallel to rod)

    =H(horizontal) * sin w / F

    which becomes: F(horizontal) = (cos w / sin w) F

    = 2 F ; because cos / sin = 1 / tan, and tan w = 0.5

    I know for certain that my answer is wrong.
  2. jcsd
  3. Aug 22, 2006 #2

    Tom Mattson

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    I'm not seeing the geometry from the description, and I imagine that others aren't either. Could you perhaps scan the figure and attach it to your next post?
  4. Aug 22, 2006 #3


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    is this a triangle truss with base 20 m and height 5 m? :confused:
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