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Simple truss question I can't seem to get right

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  1. Jul 2, 2016 #1
    1. The problem statement, all variables and given/known data
    p6v4LnW.png

    bRYWUNP.jpg

    ignore all the other writing!

    3. The attempt at a solution
    I can't tell where I'm making a mistake. Probably a massive oversight somewhere but I don't know where!

    First find the reaction forces at the pin and roller by treating everything as a rigid body. (I am now aware this isn't necessary since I can start at joint D since one of the equations has 1 unknown, but this method is still bugging me)

    sum F_x = 0
    R_Dx + R_Ax - 5000sin(15) = 0 (equation 1)

    sum F_y = 0
    -R_Ay - 5000cos(15) = 0
    so R_Ay = -4829 N

    sum Moments about point A = 0
    (2*R_Dx) - (5*5000sin(15)) = 0
    so R_Dx = 3235 N

    so plugging R_Dx into equation 1 gives
    R_Ax = -1941 N


    starting at joint D

    sum F_x = 0

    R_Dx + BD = 0
    so BD = -R_Dx = -3235 N

    but the answer says it is 12.89 kN compression!
    I can't continue because everything else will be wrong.

    What am I screwing up on?
     
  2. jcsd
  3. Jul 2, 2016 #2

    SteamKing

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    It appears you have accounted for the moments about A due to the horizontal component of the load at C.

    However, additional moment at A will be created due to the vertical component of the load at C, since this component is offset horizontally from the line A-D.
     
  4. Jul 2, 2016 #3
    Dang, can't believe I missed that.

    Thanks for the reply!
     
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