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Fixed end moment in beam

  1. Apr 27, 2017 #1
    1. The problem statement, all variables and given/known data
    I dont understand the fixed end moment AE and also fixed end moment EA ? shouldnt they be the same ?
    2. Relevant equations


    3. The attempt at a solution
    For the fixed moment AE / fixed moment EA , shouldnt it = -10(2) +5(2) = -10 , for fixed end moment , we always assume clockwise as positive and anticlockwise as negative , right ?
     

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  3. Apr 27, 2017 #2

    scottdave

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    If you use the right-hand rule, then clockwise will be negative (Thumb pointing away from you), while counter-clockwise will be positive (thumb toward you).
     
  4. Apr 27, 2017 #3
    So , in this example , the auhthor chose that the anticlockwise moment as positive ?
    So , the author's working of Fixed moment AE 10(2) -5(2) = 10 kNm , the 10kNm is anticlockwise moment ?


    then , why shouldn't the fixed moment EA = -10kNm ?
     
  5. Apr 27, 2017 #4
    In this thread
    https://www.physicsforums.com/threads/moments-sign-convention-in-beam.910959/page-2 , ( post#29)
    @PhanthomJay stated that we can choose either clockwise or anticlockwise as positive ?

    But , when we draw the bending moment diagram , we must always keep clockwise as positive ...So , that's why the -10kNm ( antoiclockwise moment ) has negative value in the bending moment diagram ?
     
  6. Apr 27, 2017 #5

    scottdave

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    So as long as your convention (right hand/ left hand) is consistent throughout, then you should be OK. The 5 kN pointing up from the left side of the pivot does create clockwise. The 10 kN/m pointing down on the left side creates anticlockwise. This creates a larger value moment so the anticlockwise wins.
     
  7. Apr 28, 2017 #6
    Bump
     
  8. May 2, 2017 #7

    PhanthomJay

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    There are lots of ways to go wrong with the minus sign. I urge you to use clockwise moments as plus when determining unknown reactions. And to assume the unknown moment reactions as clockwise .When you solve for it , and if it comes out minus, then it is actually anticlockwise.
    In your example, I would write the equation as 5(2) - (10)(2)(1) + MAE = 0. Solve MAE = +10, clockwise.
     
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