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Three-Force Member

  1. Oct 30, 2008 #1
    1. The problem statement, all variables and given/known data


    2. Relevant equations

    3. The attempt at a solution
    The prof told us to treat the three-force member as an equilibrium of particle question. So here is the FBD and the triangle I set up for the particle equilibrium:

    I don't know where to go from there.. I mean I know it's a right angle triangle, where I know the hypotenuse, but that's really it. Help? :)
  2. jcsd
  3. Oct 31, 2008 #2


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    Your first FBD is good. It is usually best to break up the known (3kN) and unknown (A and B) forces into their x and y components. Then just use the 3 equilibrium equations (sum of x forces =0, sum of y forces = 0, and sum of moments about any point = 0) which will give you the required 3 equatiions to solve for the 3 unknowns A, B, and alpha.
  4. Oct 31, 2008 #3
    Thanks Jay. I know how to do it with the equilibrium equations actually. But I'm having problems doing it using the three-force member theory...
  5. Oct 31, 2008 #4


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    That's a pain theory to use, but if it is required by the problem, the steps are shown here at
    http://darkwing.uoregon.edu/~struct/courseware/461/461_lectures/461_lecture15/461_lecture15.html [Broken]
    I've never used the graphical method.
    Last edited by a moderator: May 3, 2017
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