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Homework Help: Shear Diagram - Beam with 3 supports

  1. Mar 14, 2014 #1
    1. The problem statement, all variables and given/known data
    Draw the shear diagram for the compound supported beam.

    2. Relevant equations

    Statics... so sum of moments at any point or forces in any direction are 0

    3. The attempt at a solution

    I summed the forces in the y direction, as well as the moments at A and moments at B, but using these 3 equations causes the variables to cancel out when solving for the reactions. I get 0 = 0...
    I guess I cannot use two moment equations, but then how am I to solve this? I'm in first year statics.

    My equations are...
    [tex]0 = \sum F_y = A_y + B_y + D_y -3*6 - 5[/tex]
    [tex]0 = \sum M_A = B_y*3 + D_y*9 -3*6*3 - 5*7.5[/tex]
    [tex]0 = \sum M_B = -A_y*3 + D_y*6 - 5*4.5[/tex]
    Counter-clockwise moment is positive
    Last edited: Mar 14, 2014
  2. jcsd
  3. Mar 14, 2014 #2


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    You have a pinned joint at point C. Try splitting the single beam into two parts: A-B-C and C-D. Write separate equations of static equilibrium for each piece of the beam. What's the bending moment at point C?
  4. Mar 14, 2014 #3
    Sorry, is the bending moment at C zero?
    I tried getting isolating for [tex]C_y[/tex] but I keep getting [tex]C_y = 0[/tex]
    Is the upward force at pin C the same as the "shear force" at C?
  5. Mar 14, 2014 #4


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    Yes. The pin cannot develop a moment.

    If you write equilibrium equations for beam C-D, you'll see that Cy cannot be zero.
  6. Nov 17, 2015 #5
    Hi Teeth, I had a query.

    By taking the moments about Point A and B, You have formulated the equations 2 and 3.
    Is this way of generating equations right ?

    Looking forward to a reply from you soon.

    Aman Ratan
  7. Nov 17, 2015 #6


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    Teeth hasn't been around PF since posting this question a year and a half ago, so I hope you are not looking for a reply any time soon.

    PF generally frowns on resurrecting old posts like this.

    It is OK to link to an old post if you have a particular question about it, but you should start your own thread in that case.
  8. Nov 17, 2015 #7
    Thanks SteamKing
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