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Sign convention of shear force and moment on beam

  1. Jul 8, 2016 #1
    1. The problem statement, all variables and given/known data
    I dont understand why the sign of moment of beam at left (clockwise) and moment at the right of beam (anticlockwise) are considered as positive...
    Why we cant assume the clockwise moment as positive and anticlockwise moment as negative?
    yS8bRDE.png
    2. Relevant equations


    3. The attempt at a solution
     
  2. jcsd
  3. Jul 8, 2016 #2

    SteamKing

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    It's just an arbitrary convention assumed by the author to keep calculations consistent.

    You can assume a different convention for shear forces and bending moments, as long as you maintain consistency.
     
  4. Jul 8, 2016 #3
    look at the diagram carefully, can i assume moment of beam at left (clockwise) as positive and moment at the right of beam (anticlockwise) are considered as neagtive..???
    in OP, the author assume moment of beam at left (clockwise) and moment at the right of beam (anticlockwise) are considered as positive..,which different orientation of moment has same sign.....
     
  5. Jul 10, 2016 #4
    This is similar to another post of yours, to which I replied: What definition of Moment are you using? There are two possible definitions, each of which can be derived from the other. The one I prefer is that the bending moment at a section is the algebraic sum of the moments on ONE SIDE of the section. You have to have a sign convention that should yield the same moment, whether you take it to the left of the section, or to the right.The significant drawings for sign are shown in the lowest line of the table you quote. Sagging moments are positive and are produced by a combination of a clockwise and an anticlockwise moment. Although in some situations it can help to ascribe a sign to a clockwise or anticlockwise moment (determining the overall equilibrium of a body - for example), when drawing bending moment diagrams, you need to consider whther the cause of the bending moment is having a sagging (+) or a hogging (-) effect. Nothing to do with CW or ACW .
     
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