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Forces on beam

  1. Jan 25, 2012 #1

    Ive attached my attempt at solutions please help me resolve question 1a

    thank you,

    exam questions.jpg

    exam answers1.jpg
  2. jcsd
  3. Jan 26, 2012 #2


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    The beam is statically indeterminate, so you need more than just the equilibrium equations to solve it. Using just the equilibrium equations won't cut it. Your shear and moment diagrams are conceptually incorrect; you have ignored the reaction force at B.
  4. Jan 27, 2012 #3
    How do you deduce that?

    so there should be a jump at point B in the sheer force diagram. What would the bending moment diagram look like?
  5. Jan 27, 2012 #4
    I agree absolutely.
    What way it requires to fix undetermination? Different approaches are possible. I can try to solve but it is not so fast.
    Try to find here in the meantime http://www.orlovsoft.com/mmsamples/mmpage01.html
  6. Jan 27, 2012 #5
    The first Case:
    http://img705.imageshack.us/img705/9721/tophysicsforum03.png [Broken]
    The second Case:
    http://img29.imageshack.us/img29/1944/tophysicsforum04.png [Broken]
    Both together:
    http://img824.imageshack.us/img824/9440/tophysicsforum05.png [Broken]
    I hope it helps. But if you need equations post here, I will try.
    Last edited by a moderator: May 5, 2017
  7. Jan 27, 2012 #6
    thank you SolidElast, but how did you get the numbers 18000,60000 for first case?
  8. Jan 28, 2012 #7
    For example, by Force Method. Be patient, it is simple but not is so obviously. You need only in physics process understanding. Your first task in general kind looks like this.
    http://img823.imageshack.us/img823/2784/tophysicsforum06.png [Broken]
    According to Hook Rule (liner deformations) we can apply superposition principle.
    So, detected task is presented by two separate tasks superposition.
    http://img513.imageshack.us/img513/6137/tophysicsforum07.png [Broken]
    http://img38.imageshack.us/img38/7364/tophysicsforum08.png [Broken]
    Now we can remember that total deflection in B point is zero:

    [itex]{\it wIScheme}_{{B}}+{\it wIIScheme}_{{B}}=0[/itex]
    From last equation we determine unknown [itex]R_{{B}}[/itex]
    Last edited by a moderator: May 5, 2017
  9. Jan 28, 2012 #8
    I don't understand, what is wISchemeB, wIISchemeB ?

    Is the pink line the bending moment of the beam?
  10. Jan 29, 2012 #9
    Deflections. The solution's main idea is fact that deflection of B point (for example B) is zero because there is fixing. The way is to combine equation of such fact.
  11. Feb 5, 2012 #10
    Although previous helpers are correct, there is another interpretation of the badly worded question part a i). If it had said "draw the approximate deflected shape and the approximate bending moment diagram", you could have done that without any physics. Maybe just an intuitive answer is possible, and worth 8 marks if you understand the relationship between types of support, loading, deflected shape and M diagram. Then, in part (ii) the points of contraflexure are given. This unlocks the indeterminacy and enables you to draw the moment and shear diagrams without indeterminate analysis. Symmetry also helps understanding in this question.
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