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Homework Help: Shear/moment on a beam

  1. Nov 30, 2011 #1
    1.http://en.wikipedia.org/wiki/Shear_and_moment_diagram on this page is a shear moment diagram. I can't work out how to resolve the reaction forces. please help.

    2. M=D*F

    3. I tried to use the moment equation at the points where there are reaction forces. Ie the two pin joints and the wall support, but this gave me the wrong numbers. I think it might be beacuse i've made a mistake with the 50 k/ft turning force. please show me how to work out the forces. thank you.
  2. jcsd
  3. Dec 1, 2011 #2


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    Hi, tmer, welcome to these forums!

    In that Wiki example, the beam is statically indeterminate, so you cannot solve for the reaction forces using statics alone, you must use other methods involving deflections, moment distribution, or other means, which are tedious and a bit difficult. But in this example, the reactions have been calculated for you, and when you sum moments about any point, using the loadings and reactions given, assumed correct, then that sum must equal zero, being sure to watch plus/minus signs (cw vs. ccw moments).

    The moment of a force about a point is, as you note, force times perpendicular distance. The moment of a couple (the 50 ft-kip moment you have referred to, which is a couple, not a force) about any point is just the couple itself (it is always 50 ft-k no matter which point you choose to sum moments about). When drawing the shear diagram, the couple is 'ignored' when calculating the shear value at that point of application, because it does not change the shear force; when drawing the moment diagram, the couple adds to the moment at that point of application.

    If you are new to shear and moment diagrams, Wiki in general is not a good place to start learning, as it generally starts with a complex example and proceeds to the more complex.
  4. Dec 2, 2011 #3
    Thank you for your reply. I think It is odd that they put a statically indeterminate example there, but I see what you mean. Thanks.
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