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Engineering Mechanics Question

  1. Mar 2, 2016 #1
    1. The problem statement, all variables and given/known data

    The right triangles of the plane truss are all the same. What are the forces in members BH and HG?

    Diagram for question is below, or a link:
    http://puu.sh/nsw1k/aad8a8b134.png [Broken]

    2. Relevant equations

    n/a

    3. The attempt at a solution

    Reaction force at A + reaction force at E are equal, therefore both = 15kN.

    It was after this point I became stuck.

    I calculated the forces in the x axis:
    ABcos60 + AHcos30 = 0
    Then in the y axis:
    ABcos30 + AHcos30 = 15kN

    Not sure if the above is correct, and not sure how to continue with the question. Any help would be great!!!
     

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    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Mar 2, 2016 #2

    haruspex

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    You can get more equations by considering the balance of forces at each node. But that could generate a lot of equations and a lot of unknowns.
    Using the symmetry, there are 7 unknowns.
    It will help if you can find an easy one to start with. I suggest G. What do you notice there?
     
  4. Mar 3, 2016 #3
    Perhaps that GC is equal to 12kN? Not entirely sure if that's right but if so I could work from there.
     
  5. Mar 3, 2016 #4

    haruspex

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    No, that isn't right.
    What forces act at G? What are their directions?
     
  6. Mar 3, 2016 #5
    I wouldn't think there are any forces in the x axis since the total force in the x axis is zero.

    Does the force at GC act upwards? Equal to the total downward force, so 30kN?
     
  7. Mar 3, 2016 #6

    haruspex

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    In analysing forces in such interconnected systems, you need to focus on one node at a time, just thinking about what that node experiences directly. The only forces node G feels are from the three members that meet there, it doesn't 'know' anything about the externally applied loads. (Only nodes B, C, D feel those.)
    The forces from the three members meeting at G must balance both vertically and horizontally. That allows you to write two equations involving the forces in those members. What equations do you get?
     
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