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I-Beam deflection to find E

  1. May 14, 2010 #1
    I have a force vs displacement graph of an I-beam that is supported by 2 pin joints, and 2 equal forces on top causing it to bend.
    I am required to find the Elastic modulus of the beam.
    How do I do this?
    Am I using the equation: E/R = σ/y = M/I, or turning the displacement into strain?
    I also have no idea how to do this. Do I find the neutral axis and radius of curvature?
    Can someone please help me?!
    Thank you.
  2. jcsd
  3. May 14, 2010 #2
    Let's start with what you DO understand...please take a closer look at your graph and post your findings.
  4. May 15, 2010 #3
    Are you indicating a linear relationship?
    Force/displacement for all data points doesn't give a constant relationship. Am I meant to average this?
    Am I in the right direction??
  5. May 15, 2010 #4
    Are you sure it was 2?
  6. May 15, 2010 #5
    There were 2 supporting the bottom, and 2 applying force at the top.
  7. May 15, 2010 #6
    If you have two pin joints you have indeterminate horizontal reactions.
    Beams are normally supported with one end free to move ie on roller or sliding supports. Only one end is pinned.
  8. May 15, 2010 #7
    Well, our experiment didn't.
    How does this effect my calculations?
  9. May 15, 2010 #8
    How did you 'pin' an I beam?
  10. May 15, 2010 #9
    Can you draw a moment diagram for your beam using your measured loads?

    You need to relate the area under the moment diagram to EIy where y is the measured deflection.

    Have you heard about either the area-moment method or the double integration method?
  11. May 15, 2010 #10
    This image is what our situation looked like:
    http://www.physics.unc.edu/~aold/APPL150/MaterialsProject_files/image002.jpg [Broken]

    I have a moment diagram for the bending, which is also shown in the figure above.

    The area-moment method, I do not know, but I'm assuming the double integration method is for find the BMD?
    Last edited by a moderator: May 4, 2017
  12. May 15, 2010 #11
    Your beam is what is known as simply supported. It is not pinned.

    You do not appear to have measured the deflection , at the centre of the beam. The central deflection is given by

    [tex]EI\delta y = \frac{{Fa}}{{48}}\left( {3{L^2} - 4{a^2}} \right)[/tex]

    To calculate the deflection whre you are showing it, under one of the loads, requires one of the methods I have mentioned.
    The double integration method calculates the equation of the 'elastic curve' which is basically a geometric equation describing the variation of the vertical position of the neutral axis along the beam , by doubly integrating the differential equation representing moment diagram.

    You need to discuss these with your teacher if you need more than the central deflection.
  13. May 15, 2010 #12
    I make the expression for the deflection at the loads to be

    [tex]EI\delta y = \frac{{F{a^2}{{\left( {L - a} \right)}^2}}}{{3L}}[/tex]
  14. May 15, 2010 #13
    Sorry of this seems stupid of me, but, we took several data points, recording the load and the deflection due to that load.
    So, when the Force comes up in an equation, do I just take ANY load point and the corresponding deflection?
  15. May 16, 2010 #14
    Ok, so now I have calculated M, I, y, and σ, but I still can't find E without R. Am I correct?
    I'm not so good with integration to find R. Is there another way to find E?
  16. May 16, 2010 #15
    I cannot imagine any college would have its students carry out an experiment without studying the theoretical background.

    Did you understand the formula I presented in posts 12 and 13?
    If not you need to review your course material, perhaps you missed some part?

    Where does my formula mention R?
    Did you measure R?
    Perhaps that was because you are looking in the wrong place for your relationship and do not actually need it.

    Have you studied superposition in relation to beams?
  17. May 16, 2010 #16
    I don't know if this helps but if we take any beam and set up an x-y coordinate system, with y vertical and x starting at one end and running along the length of the beam.

    Then at any section along the beam ie any particular x value

    [tex]EI\frac{{{d^2}y}}{{d{x^2}}} = M[/tex]

    Integrate once

    [tex]EI\frac{{dy}}{{dx}} = \int {Mdx + {C_1}} [/tex]

    Integrate a second time

    [tex]EIy = \int {\int {Mdxdx} } + {C_1} + {C_2}[/tex]

    This is known as the double integration method.

    The second equation yields the shear force (diagram) along the beam
    The third equation yields the deflection along the beam

    Now it is obvious that [tex]\int {Mdx} [/tex] is the area under the moment diagram so we can do this graphically by working on the moment diagram and measuring the area under it.

    Engineers have also worked out standard loading conditions and superposition allows us to read these from tables and combine them.
  18. May 16, 2010 #17
    Ahhh, ok. I think I have it now.
    Thank you for all your help :)
  19. May 16, 2010 #18
    Hope your lab report goes well now and you can catch up fully on bending and beam theory.
  20. May 17, 2010 #19


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    Homework Helper

    If the beam were always within it's linear elastic range, each pair of force / defection points should theoretically produce the same value for E. In reality it won't for several reasons: measurement error, imperfect linear elasticity, etc. You should calculate E based on each point and see how close the values are. You might then average the values.
    Last edited: May 17, 2010
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