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Homework Help: 2 Force Members and Trusses!

  1. Feb 27, 2010 #1
    1. The problem statement, all variables and given/known data

    See Figure

    2. Relevant equations

    [tex] \sum F_{x} = 0 [/tex]
    [tex] \sum F_{y} = 0 [/tex]
    [tex] \sum M = 0 [/tex]

    3. The attempt at a solution

    See Figure

    I'm fairly certain the force I drew at point D isn't at a 90 degree angle, but if its not I can't find the perpendicular distance for the horizontal/vertical distances at the point D.

    This has been giving me a really big headache and alot of stress lately so some help would be greatly appreciated!

    Attached Files:

  2. jcsd
  3. Apr 2, 2010 #2
    * Use geometery to find lenght of members
    * Then using method of joints, forces in the members can be easily found.
    Last edited: Apr 2, 2010
  4. Apr 2, 2010 #3
    I see you are using squared paper, but not drawing everything to scale. To do so would help you check the use of 3-4-5 triangles. Interestingly, these problems do not really depend on distances but on angles. If the units were km, the answer would be the same. This emphasises that the ratios of distances are the key to full understanding.
  5. Apr 4, 2010 #4
    Hey mate,

    I am an applied mechanics tutor so I should be able to see you finish this problem under guidance. You are told to find the axial forces running through these members.

    Step 1. Pick a set a global x-y coordinates to work with. This may be anywhere on the structure but for convenience I recommend doing so on the node A.

    Step 2. Have yourself an internal force convention. This should be somewhere in your notes. i.e. tension positive, compression negative etc.

    Step 3. Use the equilibrium equations you stated to find the reactions.

    We now run into a problem here. You have two fixed supports. This structure is indeterminate by one degree. If indeterminate structures is what you are here for please reply back and confirm this so I may continue.

    I have assumed you are new to solving statically determinate structures. Correct me if I am wrong.
  6. Apr 4, 2010 #5
    I understood FH to be a vertical link, so that F behaves like a roller support. That makes it stat det.
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