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Simple Truss Problem

  1. Nov 27, 2013 #1
    1. The problem statement, all variables and given/known data

    The truss is suspended by two pin joints at A and B. Each of the four segments is 3 m wide and 4 m high. Find the reaction force at A


    2. Relevant equations


    3. The attempt at a solution

    sum of forces y = -ay + by -f=0

    Can someone tell me what the moment at B is?

    Mb = -4*w*F + 0*Ay - Ax*h = 0

    i dont know if that is right. Ay and ax confuse me
  2. jcsd
  3. Nov 27, 2013 #2
    Hmm I got that Ax = -3F but I can't figure out what Ay is
  4. Nov 27, 2013 #3
    Any thoughts?
  5. Nov 27, 2013 #4
    Hmm, a friend told me Ay = F but.. I don't understand why?
  6. Nov 27, 2013 #5
    Once you know Ax, and, then, Bx, consider the moments of the forces about the point where F is applied. That will give you an equation linking Ay, Bx, and By.
  7. Nov 27, 2013 #6
    Thank you

    but I don't know what Ay and By are, so that wouldn't help me would it?
  8. Nov 27, 2013 #7
    You have an equation that relates them.

    You have posted another equation with them in #1.

    Two equations, two unknowns, problem solved.
  9. Nov 28, 2013 #8


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    Are you sure that both joints are pinned hinges and that there is not a roller support at B that would make it incapable of supporting vertical load? If the support at B was a roller support , then Ay = F. But if both A and B supports can support vertical loads, then the problem is statically indeterminate because you have 4 unknowns Ax Bx Ay and By, and only 3 equilibrium equations. It only affects the forces in AB, in either case. You may assume Ay = By = F/2, if both supports are full hinges capable of taking loads in the x and y directions.
  10. Nov 28, 2013 #9
    Yes indeed. Using my "method", I end up having two identical equations, as was to be expected if I had thought straight. Not sure what I was thinking.
  11. Nov 28, 2013 #10
    "Ay = By = F/2"

    but Ay is -3F and By = 3F (which is the correct answer)

    Also there are no rollers in this problem only pins.
  12. Nov 29, 2013 #11


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    That is not the correct answer for Ay and By. That is the correct answer for Ax and Bx (you correctly solved for Ax in your 2nd post). Note: It is assumed that x is the horizontal axis and y is the vertical axis.

    Did you state the problem exactly as written?
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