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Method of joints for bridge truss

  1. May 7, 2016 #1
    1. The problem statement, all variables and given/known data
    Calculate the forces for each member.

    2. Relevant equations

    Sum of forces in x=0
    Sum of forces in y=0
    Sumof moments about D=0


    3. The attempt at a solution
    Work is on attached image of problem. I'm having trouble solving for support reactions. At D there is

    a hinge. At A, C, and Fthere is a roller. I know that the hinge provides a horizontal and vertical reaction force where the rollers only provide a vertical reaction force. I know I can't sum the moments about a roller and I'm don't think I can sum the moments about the hinge (did this anyways as Im stuck). So how can I solve these support reactions?
     

    Attached Files:

  2. jcsd
  3. May 7, 2016 #2

    SteamKing

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    You can sum moments about any convenient point. Just because a roller doesn't support a moment does not prevent this.

    A structure in static equilibrium must have forces and moments sum to zero, regardless of reference
     
  4. May 7, 2016 #3
    That is the right thing to do right? I keep getting Fy and Cy to cancel out during my sum of the moments equation? So I have sum of moments about D to be 0=0?
     
    Last edited: May 7, 2016
  5. May 7, 2016 #4

    SteamKing

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    It's not clear what you are talking about here. From the diagram, it looks like the supports are located at D and F.

    The structure is symmetrical about the line BE. What do you think the reactions should be?
     
  6. May 7, 2016 #5
    There is a roller at A, C and F. There is a hinge at D.
     
  7. May 7, 2016 #6

    SteamKing

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    What's the purpose of the rollers at A and C? Your sketch is not clear on this.
     
  8. May 7, 2016 #7
    We are suppose to design a truss that can handle a load of 12.5 kip at the center of this bridge. There is a roller at the top left just under bridge, there is a roller at top right just under bridge, there is a roller on support underneath bottom right of bridge, and there is a hinge on support underneath bottom left of bridge.
     

    Attached Files:

  9. May 7, 2016 #8
    When I calculate support reactions for the truss I made with the 12.5 kip load in center I keep getting zero. That should not happen, unless I am mistaken.
     
  10. May 7, 2016 #9

    SteamKing

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    If the rollers at A and C can support reactions at the cliffs, and the roller at F and the hinge at D also have support reactions, then your structure is no longer a simple truss, but a statically indeterminate structure with four points of support but only two equations of statics to work with.
     
  11. May 7, 2016 #10
    So what you are saying is that there should not be rollers on top of the cliffs? That drawing is from my teacher as a guideline to get us started and such.
     
  12. May 7, 2016 #11

    SteamKing

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    Well, I don't know all the details of this project.

    I'm just saying that you can't analyze this structure as a simple truss when it isn't one. :frown:
     
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