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Homework Help: Shear force and bending moment

  1. Sep 14, 2018 #1
    1. The problem statement, all variables and given/known data
    I was asked to find the shear force diagram and bending moment diagram for this load combination...But , i have problem of getting the BMD now . i am not sure which part is wrong , can anyone point out ?

    2. Relevant equations


    3. The attempt at a solution
    MA = 5(2)(1/2)(2/3 x 2) + [ (10x4x4) - (10x4x0.5x(2+(2/3)(4)) ] +2(10)
    , thus MA = 73.33kNm.
    For , VA , i gt (5 x 2/2) + (10x4/2) + 2 = 27kN
    I have sketched the SFD as attached , but i have problem of finding the area below the SFD to get moment , how to do this ?
     

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  2. jcsd
  3. Sep 14, 2018 #2
    I doubt that my MA = 5(2)(1/2)(2/3 x 2) + [ (10x4x4) - (10x4x0.5x(2+(2/3)(4)) ] +2(10)
    , thus MA = 73.33kNm. is correct or not ?
    Can someone help to check ?
     
  4. Sep 14, 2018 #3
    I get

    $$ M_a = 5*2 + \frac{5 \cdot 2}{2} \cdot \frac{2}{3}\cdot 2 + \frac{10 \cdot 4}{2} \cdot (2+ \frac{1}{3} \cdot 4) = 83.33$$

    and

    $$V_a = 27$$
     
    Last edited: Sep 14, 2018
  5. Sep 15, 2018 at 12:31 AM #4
    why you ignore the moment caused by the 10kN force ?
     
  6. Sep 15, 2018 at 9:40 AM #5
    There seems to be something wrong with the original figure. If you have a region of 5 kN/m it should be a rectangular region, not a triangle. Same for the 10 kN/m. Or is the 5 kN/m at the peak of the triangle, and the load distribution is non- uniform?
     
  7. Sep 15, 2018 at 3:55 PM #6
    If you mean the distributed 10 kN/m force, then I do not. It is the third value I am adding.

    The total force of that triangular distribution is 10*4/2 , and the equivalent force will act at the centroid of the distribution, which is 1/3 * 4 from the left side of the triangle. Finally add 2m to find distance to the fixed support.

    From my experience, the 5 kN/m force represents the peak of the triangle.
     
  8. Sep 15, 2018 at 5:47 PM #7
    OK. Then I'll give the problem a shot so we can compare.
     
  9. Sep 15, 2018 at 6:26 PM #8
    @dss975599 and @Chestermiller

    I found a mistake in my calculation. The first term should be 10 * 2 , not 5*2 as in post #3. This will give a Ma= 93.3 kN*m.
    I verified this using an online calculator.

    https://skyciv.com/free-beam-calculator/
     
  10. Sep 15, 2018 at 9:57 PM #9
    Thanks for your answer , i have obtained the SFD as in the online calculator , but i have no idea to get the area under the SFD to get the BMd. Do you know how to get the area under the SFD so that i can plot BMD based on SFD ?
     
  11. Sep 16, 2018 at 1:35 AM #10
    @Chestermiller @CivilSigma here's my working ... At x = 6m , i get M = -61kNm , but not -8kNm as provided by the online calculator , which part of my working is wrong ? for x = 0 and x = 2 , i gt the M value same as the online calculator
     

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  12. Sep 16, 2018 at 1:41 AM #11
    here's my trying ... Can anyone point out which part of my working is wrong ? I have been looking at this for the whole day
     

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  13. Sep 16, 2018 at 8:15 AM #12
    I can't make out what you have written in the photo. But, first of all, for the direction of the moment you have drawn in your original figure, I get +93.33 kNm (in agreement with @CivilSigma) rather than your -93.33. For the shear force, I get the following:

    ##V=-1.25x^2+27## for (0<x<2)
    ##V=1.25x^2-15x+47## for (2<x<6)
    I integrate to get the moment variation $$M=93.33-\int_0^x{V(x')dx'}$$
    This gives me:
    ##M=93.33-27x+\frac{1.25x^3}{3}## for (0<x<2)
    ##M=110-47x+7.5x^2-\frac{1.25x^3}{3}## for (2<x<6)

    This gives values for M of 42.67 kNm at x = 2 and 8 kNm at x = 6
     
  14. Sep 16, 2018 at 8:22 AM #13
    ##V=1.25x^2-15x+47## for (2<x<6)## ,may i know how do you get this ?
     
  15. Sep 16, 2018 at 9:46 AM #14
    and how do you get ##M=110-47x+7.5x^2-\frac{1.25x^3}{3}## for (2<x<6) ? By integrating ##V=1.25x^2-15x+47## for (2<x<6) , i gt ##M=0.42x^3-7.5x^2+47x## for (2<x<6).....
     
  16. Sep 16, 2018 at 11:30 AM #15
    For the distributed loading, I got

    ##w=2.5x## for (0<x<2)
    ##w=2.5(6-x)=15-2.5x## for (2<x<6)

    For the shear force, I integrated $$\frac{dV}{dx}=-w(x)$$subject to the initial condition V(0)=27

    For the moment, I integrated $$\frac{dM}{dx}=-V (x)$$ subject to the initial condition M(0)=93.33
     
  17. Sep 16, 2018 at 10:18 PM #16
    how do you get 2.5(6-x) ? I dont understand
     
  18. Sep 17, 2018 at 6:41 AM #17
    What is the equation for the straight line passing through the points (2,10) and (6,0)?
     
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