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Finding reaction and moment

  1. Dec 8, 2015 #1
    1. The problem statement, all variables and given/known data
    Lets say i have a structure like this , at A and B are fixed support ( it resists moment about , force in x and y direction) I wanna find the moment about the A , should I include the MA as well in my calculation or no ?


    2. Relevant equations


    3. The attempt at a solution
    moment about A = MB +MA + W(L/2) -RBy(L) = 0 ?
    or moment about A = MB + W(L/2) -RBy(L) = 0 ?
     

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  2. jcsd
  3. Dec 8, 2015 #2

    PhanthomJay

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    Suppose this beam was a simple cantilever fixed at A but free at the other end. Same applied loading. Would you include the moment at A when summing moments about A in your moment equilibrium equation?
     
  4. Dec 8, 2015 #3
    So, we don't have to consider MA when calculate the moment abour A?
     
  5. Dec 8, 2015 #4

    PhanthomJay

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    How did you come up with that conclusion?
     
  6. Dec 8, 2015 #5
    Based on your tone
     
  7. Dec 8, 2015 #6

    PhanthomJay

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    Oh, bad tone. Try the simple cantilever case, say a beam A-B of length L fixed at A and with a load P applied at the free end B. Now sum moments about A = 0. Does MA show up in that equation or not?
     
  8. Dec 8, 2015 #7
    no, how is it relate to the question i asked?
     
  9. Dec 8, 2015 #8

    PhanthomJay

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    In your problem, I cannot tell you whether or not to include MA in your equation when summing moments about A , because that is against forum rules . BTW, the problem is statically indeterminate. But anyway, as a hint, if you consider a simple cantilever of length L fixed at end A with the unknown moment M_A at that end and with a load P applied at the free end, then summing moments about A for equilibrium yields M_A - PL = 0. Thus M_A = PL as you would expect. The moment of any couple about any point is the couple itself. So back to your original problem about whether to include M_A in your equation...what say you now?
     
  10. Dec 8, 2015 #9
    yes , i should include MA when i wanna calculate the moment about A
     
  11. Dec 9, 2015 #10

    PhanthomJay

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    yes, correct.
     
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