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Need help clarifying statics problem.

  1. Jul 30, 2012 #1
    http://postimage.org/image/8i5i2dtgr/

    http://postimage.org/image/8i5i2dtgr/


    1. The problem statement, all variables and given/known data
    What is the magnitude of the internal force in member BC?


    2. Relevant equations
    So I figured out the reaction force at A and D by taking the moments about point A to find the vertical force at D and then summing the x and y components to find the forces at A.


    3. The attempt at a solution

    So I solved for the reaction forces for A, and D already. Why can't I split the truss into three members? Why can't I take member AD and have a free body diagram for that member and solve for the Fx and Fy for Bx and By by summing the forces? Do I absolutely have to take the moment? Can I not do

    Ax-Bx=0
    Ay-By+Dy=0

    To find the components of B? This method doesnt seem to work.
    Why do I have to take the moment?
     
  2. jcsd
  3. Jul 30, 2012 #2

    PhanthomJay

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    This is not a pure truss since the horizontal and vertical members are subject to bending. When taking a FBD of AD, you must include the horizontal component of force at D, found by first taking a FBD of the vertical member.
     
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