Shear force diagram and bending moment diagram

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Homework Statement


theoretically, bending moment should ends at bending moment = 0 , am i right ?

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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 ....
 

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or is there something wrong with the question ?
 
  • #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,
 
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  • #4
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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,
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?
 
  • #5
PhanthomJay
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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?
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.
 
  • #6
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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 ;same convention w
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 ?
 
  • #7
PhanthomJay
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It's not a must but it makes life easier ...those plus and minus signs are killers.
 
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  • #8
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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,
can you take a look at my sketch of graph , i am not very sure about the shape of graph ....
 
  • #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.
 
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  • #10
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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.
ok , thanks . So , the shape of graph is correct ? at x = 5 , it's continuous , sorry , my sketch using pencil is messy.
 
  • #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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