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Determining static equlibrium reactions

  1. Jun 25, 2008 #1
    I'm having trouble getting my head around how to determine the direction of the forces of reaction and of moment/torque in static equilibrium systems.
    For example how do I determine the reactions for the system attachment and why are they as they are.

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  2. jcsd
  3. Jun 26, 2008 #2
    Come on? Cant anyone help me??? This ought to be simple, but surprisingly no one knows.
  4. Jun 27, 2008 #3


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    Well, not that simple. Try isolating BCD and take FBD's of members BC and CD. Note that there can be no moments at joints B, C, and D. Solve for the reactions at B and D, then analyze AB as a cantilever with a distributed load and concentrated load at the far end. Assume small deflections. Note that your units for F are stated incorrectly, it should be in kN, not kN-m.
  5. Jun 27, 2008 #4
    I am not familiar with everything the exact English terminology so there may be small discrepancies in what I'm trying to say.
    Anyway, didn't notice the F units thing...
    Well I do know that the directions do no matter for the internal joints, all that is important is that they are opposite for each joint so as to keep the equilibrium.
    The problems I'm trying to solve are usually only about finding the magnitude, they don't involve vector quantities. So thats why I need to determine the direction(i.e. sign in the equilibrium equation) of the forces on the end points. Which for this case are A and D.
    But then again - the solution has some negative numbers which don't really make sense to me.
    Anyway, thanks.
  6. Jun 27, 2008 #5


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    I don't know what numbers you are getting because you haven't shown your work, but if you get negative numbers it might just mean that you assumed the wrong direction for the reactions at A and D, or, if the solution was given to you with a negative value, it might just mean that the direction of the force at the end point reaction is down. Perhaps you can visualize that the couple produced by the equal but opposite forces, F, tend to attempt to rotate frame BCD counter clockwise, resulting in a downward reaction at D and an upward reaction at A. Also note that at A, there must be a reactive moment acting at that end point.
    Last edited: Jun 27, 2008
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