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Truss and Suspension bridge.

  1. Mar 29, 2007 #1
    How will I calculate the forces involved in a truss and suspension bridge, my teacher did not explain this fully and I'm confused as hell.
     
  2. jcsd
  3. Mar 29, 2007 #2

    radou

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    Depends on the geometry and the loads applied to your bridge. If would be great if you posted a pic.

    Basically, all you have to do is apply the equations of equilibrium to your system. I can't be any more specific with the given data.
     
  4. Mar 30, 2007 #3
    Erm, let's just say a 100 metre bridge with trusses with an angle of 60 degress. The bridge weights 20 000 N.
     
  5. Mar 31, 2007 #4

    Pyrrhus

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    Well, a picture and your attempt will be a good start.
     
  6. Mar 31, 2007 #5
    I don't have anything because I don't know how to calculate the forces.
     
  7. Mar 31, 2007 #6

    radou

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    Do you know how to calculate the support reactions? Your bridge must have some supports. Did your teacher explain this? It's the first step.
     
  8. Mar 31, 2007 #7
    I don't know how to do it.

    Like I said, a bridge like so.
    [​IMG] How would I get those calculations of the trusses?
     
  9. Mar 31, 2007 #8

    radou

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    Well, after calculating the support reactions, you can calculate the force in every truss element by analyzing every node separately. Start with the node(s) where you have only two (out of three) unknown forces.
     
  10. Mar 31, 2007 #9
    How did it come up with those calculations though?
     
  11. Mar 31, 2007 #10

    radou

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    Which calculations? Do you mean, the calculation of the reaction forces at the supports?
     
  12. Mar 31, 2007 #11
    Yes, the numbers on the members in the diagram.
     
  13. Mar 31, 2007 #12

    radou

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    I assume the numbers represent the forces in the members, and the colors represent compression/tension. As I said, you can calculate every force by looking at every node separately. Imagine you "cut out" the leftmost node. You have two unknown and forces and one familiar force. Set up two equations of equilibrium, one for the "y-direction", and one for the "x-direction". You can calculate the forces in the members from these equations.
     
  14. Mar 31, 2007 #13
    But, that looks like an equilateral triangle, why is there different calculations?
     
  15. Mar 31, 2007 #14
    Also, I the program used a matrix to solve the forces, what would the matrix calculation formula be?
     
  16. Mar 31, 2007 #15

    radou

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    What matrix are you talking about?

    The calculations seem wrong. For example, I got ~28.87 for the leftmost blue truss element.
     
  17. Mar 31, 2007 #16

    AlephZero

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    The triangles are not equilateral in the drawing.

    The answers are correct (to 2sf) for the shape as it is drawn. I get 30.05 for the force in the leftmost blue truss.
     
  18. Mar 31, 2007 #17

    radou

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    Of course, for some reason, I totally ignored the mesh. :uhh:

    Alpha[X]^2, sorry for causing eventual confusion.
     
  19. Apr 6, 2007 #18

    dfx

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    Read up on "pin-jointed analysis of trusses". The "matrix" is probably just Cramer's method to solve simultaneous equations for the member forces.
     
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