The discussion centers on the sign convention for shear force and bending moment in structural analysis. The author defines upward shear force as positive on the left-hand side (LHS) and downward shear force as positive on the right-hand side (RHS). Participants question the inconsistency in sign conventions for moments, noting that clockwise moments are treated as positive on the LHS while anticlockwise moments are positive on the RHS. The consensus emphasizes that the direction of rotation, rather than the direction of the applied force, determines the sign of the moment.
PREREQUISITESCivil engineers, structural analysts, and students studying mechanics of materials will benefit from this discussion, particularly those focusing on beam behavior under various loading conditions.
chetzread said:
move on to the lower part of note, i found that the author take the clockwise moment at LHS as positive while at RHS , the author take anticlockwise moment as positive,why is it so?Clever Penguin said:If I had a beam, and applied a force acting upwards on the left, it would rotate clockwise.
If I was to instead apply a force acting downwards, but on the right, the beam would still rotate clockwise.
Sum of the clockwise moments = sum of the anti-clockwise moments. It is not about whether the force is acting up or down but whether the result of that force would cause it to rotate clockwise or anti-clockwise.
So, the moment clockwise considered as positive in this case?Clever Penguin said:If I had a beam, and applied a force acting upwards on the left, it would rotate clockwise.
If I was to instead apply a force acting downwards, but on the right, the beam would still rotate clockwise.
Sum of the clockwise moments = sum of the anti-clockwise moments. It is not about whether the force is acting up or down but whether the result of that force would cause it to rotate clockwise or anti-clockwise.