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Fixed end beams-Direction of moment

  1. Jan 1, 2013 #1
    Hi,I am trying to find out how to decide the direction of moment (clockwise or anticlockwise) for a fixed end beam (for example).
    As far as I know the direction is towards the side of the beam which is compressed.

    The link below shows some examples with directions of moments.
    Can someone explain to me why the directions are like this?

    http://www.colincaprani.com/files/notes/SAIV/SAIV - Exam Handout.pdf
    Last edited: Jan 1, 2013
  2. jcsd
  3. Jan 1, 2013 #2


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    :eek::eek::eek: What a horribly confusing set of diagrams! :eek::eek::eek:

    He doesn't say whether the M's are meant to be the reactions, or the moments on the end of the beam in a free body diagram.

    And the arrows seems to be showing the direction of the actual (positive) moments, not a consistent sign convention.

    There seem to be too many "unknowns" to "explain" exactly what the author meant, but those statements should point you in the direction of figuring out something that YOU understand (which is more important in than trying to second guess the author, IMO)
  4. Jan 1, 2013 #3
    Any such table should be accompanied by a clear set of sign conventions.

    Presumabably Dr Caprani is using the same ones taught on your course?

    It rather looks as though Dr C is using the mathematicians and programmers reverse convention of anticlockwise moments positive and clockwise moments negative at any point.

    This is different from the usual engineering convention that would refer to a short length of beam with two moments (one at each end) with the convention that if the pair generates hogging or sagging moments in the section they are positive or negative (or the other way round).

    This alternative convention leads to MA and MB having the same sign in the first three entries in your table, unlike the mathematicians convention.

    Note, however, you reported difficulty with some entries but did not say which ones?
    Last edited: Jan 1, 2013
  5. Jan 1, 2013 #4
    He shouldn't have put a negative sign on those moments on the right-hand-side. The moments shown on the diagrams are the reactions, and are positive as drawn.
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