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Shear force diagram and bending moment diagram

  1. Oct 15, 2016 #1
    1. The problem statement, all variables and given/known data
    theoretically, bending moment should ends at bending moment = 0 , am i right ?

    2. Relevant equations


    3. The attempt at a solution
    Here's my sketch of SFD and BMD ,my bending moment doesnt return to 0 , which part of my working is wrong ... to find bending moment (M) , i get M =Vdx , where V =shear force
    Vdx = area of the shear force diagram

    for the BMD , I let downwrads as positive ....
     

    Attached Files:

  2. jcsd
  3. Oct 16, 2016 #2
    or is there something wrong with the question ?
     
  4. Oct 16, 2016 #3

    PhanthomJay

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    You did well, the reason why the moment didn't quite close to 0 was because you rounded off the right end reaction to 102 when it should have been 101.67. Also, by convention, the moment diagram is mirrored, it should be above the axis, but otherwise looks good and your shear diagram is good. I know you are self studying and using a book with sometimes errors, both which makes it difficult to understand this stuff, but you are to be commended for working very hard to understand the principles, nice work,
     
  5. Oct 16, 2016 #4
    is it wrong to assign upwards is positive for SFD , and downwards as positive for BMD ? Or should i keep either (both upwards or downwards) as positive for BMD and SFD?
     
  6. Oct 16, 2016 #5

    PhanthomJay

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    your shear diagram follows convention. Note that the shear is getting less and less positive as you move from left to right. Accordingly, the slope of the moment diagram should be decreasingly positive, not decreasingly negative. Or another way to look at it is that the area under the Shear diagram between 2 points is the change on moment between those 2 points, which is positive, not negative.
     
  7. Oct 16, 2016 #6
    i mean is it a must to assign upper side of graph as positive for SFD and BMD ? or assign lower side of graph as positive for SFD and BMD ?
     
  8. Oct 16, 2016 #7

    PhanthomJay

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    It's not a must but it makes life easier ...those plus and minus signs are killers.
     
  9. Oct 19, 2016 #8
    can you take a look at my sketch of graph , i am not very sure about the shape of graph ....
     
  10. Oct 19, 2016 #9

    PhanthomJay

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    Other than what we talked about regarding the upside down moment graph and the non closure to zero due to rounding off, both the values and shape of both the shear and monent diagrams are excellent, very good work. Oh at x= 5 on your moment graph, that should be a nice smooth curve and not a discontinuity there, I think you may have penciled in the curve so it looks a bit uncertain there.
     
  11. Oct 19, 2016 #10
    ok , thanks . So , the shape of graph is correct ? at x = 5 , it's continuous , sorry , my sketch using pencil is messy.
     
  12. Oct 19, 2016 #11

    PhanthomJay

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    Yes shape is excellent. I just wish you had drawn the moment diagram right side up
     
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