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Bending moment Diagram for fixed-fixed beam

  1. Mar 9, 2015 #1
    1. The problem statement, all variables and given/known data

    There is a beam, say length L, with a force F UP on the left built-in support and a CLOCKWISE moment. The right side has a force, F, DOWN and a COUNTERCLOCKWISE moment

    i.e F up, cw moment -------------------------------------------------------------F down, ccw moment

    I am required to draw a bending moment diagram for this. The only way I've ever been shown is the method of cuts/sections which I hope you are familiar with this. If I work from the right hand end, the equation I get is different from working from the left hand end.

    2. Relevant equations

    3. The attempt at a solution
    As stated above, I'm not sure if the method of sections works here although I was never made aware that it had occasions where it was useless. How can I get a BM diagram here?

  2. jcsd
  3. Mar 9, 2015 #2


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    Try breaking this problem up into two separate problems: the first one with only forces, and the second one with only moments.

    For the first problem, what would the shear force diagram look like? What would the BM diagram look like?

    For the second problem, what would the BM diagram look like?

    Combine the two BM diagrams et voila.
  4. Mar 10, 2015 #3

    Hi - thanks for that. I understand this. Just one Question; how did you know to do this c.f. other methods e.g. section method? W.r.t the sectioning method - If I work from right to left (call right 0, and left L) then I get the correct diagram. Working from left to right gives me the wrong answer!
  5. Mar 10, 2015 #4


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    I can't comment on what you did without seeing your work.

    What I recommended was just splitting one problem into two (simpler) ones, and then using superposition to combine the results.

    You have studied superposition, haven't you?
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