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Bending of Beams Questions

  1. Oct 14, 2009 #1
    Calculate the maximum B.M the symmetrical section can carry Plyweb with timber flanges (50x100 c.s.a) The maximum stress = 1.0N/mm^2

    The structure is essentially an I beam with dimensions as follows.

    The central beam has a width of 25 and height of 500.

    The four flanges which constitute the I shape have each a width of 50 and height of 100 and are flush with the top and bottom of the central beam.

    Since no units are given I have assumed all dimensions are in mm.

    B.M = W.L / 4 The Answer = 4.3kNm

    Total force acting upon the beam is the udl of 1.0N/mm2.
    Total width is 125mm therefore w = 125x1 = 125 N.

    W.L / 4 = 125^2 / 4 = 3906.25 Nm which is clearly wrong with respect to the answer given.

    Working backwards. 4300 N = W.L / 4 gives (4300 x 4) / L (where L = 125) gives a value of W = 137.6

    Any help would be appreciated as I am majorly confused over such a simple problem.
  2. jcsd
  3. Oct 18, 2009 #2


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    I don't believe you are interpreting the problem correctly. It appears to be asking that given the allowable bending stress of the material is 1 N/mm^2, calculate the moment of inertia of the cross section and determine the maximum bending moment that the section is capable of supporting, without exceding the allowable bending stress in the flanges. It is not asking for loading, nor does it give a span length or support conditions.
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