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Homework Help: Bending Moments

  1. Feb 20, 2006 #1
    Hi, I've got a little problem with a bending moment question.

    Here it is (really sorry for the bad diagram!)

    http://img474.imageshack.us/img474/7844/help15eu.jpg" [Broken]

    R1 and R2 are reactional forces, which using moments I have calcualted to be:


    Section A of the beam is a mild steel beam, section B can be presumed to have infinite strength (i.e. all of the bending is taken by section A).

    The value of 1000 x 9.81 is the weight of section A, do I take this into account in the bending moment diagrams?

    How do I go about drawing the diagram for this, here are two possibilities:

    http://img469.imageshack.us/img469/4135/help25cc.jpg" [Broken]

    In (b), I have said that section B can be treated as a wall, with a moment produced by the forces to the left of the wall.

    I need to calculate the maximum bending stress in section A.

    Any help would be amazing!

    Hope I've made any sense, if you need any more info, just ask.

    Thanks a lot, Oli.
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Feb 20, 2006 #2
    so in the first part. up until the length of B ends. it will be a flat line at 0 on the bending moment diagram. it pretty much doesnt exist for all our purposes... i did double check your reaction forces and those are fine.

    first you need to draw the sheer force diagram. i like option b since we are treating the section B like a wall. so we start with a 0 up until that point where B ends. then at 1.1m from the left.. youll have a force. since it is a wall. there will be a reaction sheer and moment force.. youll have to calculate those.. just pretend B doesnt exist and A is attached to a wall.. those are simple calculations.. then go through and map out the forces that go up/down like the reaction Y from the wall, 100*9.81, R2 and R1.. you SHOULD end up at 0 at the end of the beam. then just go through and calculate the area under the curve for each section of the Sheer diagram and thatll give you a moment diagram.. and the highest moment force you get throughout the beam. thats the force you use for the maximum bending stress calculations.

    if that doesnt make sense send me a PM and ill show you more in depth
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