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Homework Help: Bending Moments

  1. Dec 10, 2013 #1
    1. The problem statement, all variables and given/known data

    Hollow circular beam with outside diameter 350mm, and wall thickness of 60mm

    E=60 Gpa

    beam length 2m

    Force applied to beam at midpoint of beam (1m from each end)


    Force applie from the Top down, Ra , and rb from bottom up

    2. Relevant equations

    Used o(bending)=My/I

    3. The attempt at a solution

    Got I correct as 599.29x10^-6(m^4)

    I think the bending moment is 100knm, but put in 50knm(wrong)

    I was wondering if y=0.175(half of outside diameter in metres)

    i got 14.60MPA(10^9) by doing, 50,000[bending moment]*0.175[y]/599.29*10^-6

    When replacing 50,000 with 100,000 i get 29.201MPA, and for the microstrain i get:


    rearrange for strain

    strain = 29.201*10^6/65*10^9 = 44.92495067*10^ -5 or 44.92 microstrain

    Attached Files:

    Last edited: Dec 10, 2013
  2. jcsd
  3. Dec 10, 2013 #2


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    y is measured in meters, not meter^2.

    Bending moment is measured in N-m

    You need to brush up on units and how to calculate bending moments.
  4. Dec 10, 2013 #3
    that was a mistake, an editing error

    as later shown, y=0.175mm, ( which is the half of outside diameter )

    could you eloborate on what i need to brush up on, where i can attain this information?
  5. Dec 10, 2013 #4


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    Add to the list above, need to work on metric system. Half of 350 mm is 175 mm, not 0.175 mm (which is a teeny-tiny measurement)

    Working out bending moments starts with drawing a free-body diagram of the beam and figuring out the loads.

    Once the FBD is in static equilibrium, you can construct the shear force and bending moment diagrams for the beam.

    By inspection of the beam, it's easy to see that Rl = Rr = 100 kN. You should be able to figure out the moment from this information.
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