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Homework Help: Maximum stress in beams with different cross sections

  1. Nov 10, 2012 #1
    1. The problem statement, all variables and given/known data
    Here is the problem:
    I have to calculate the maximum stress for A, B and C.

    2. Relevant equations
    I used:

    σ = y*F*L / 4*I (but I don't think this is right)

    σ = maximum stress
    y = Perpendicular distance from to neutral axis
    F = load
    L = length of beam
    I = moment of Inertia

    3. The attempt at a solution
    σ = y*F*L / 4*I

    I got the following:
    A = 1.241
    B = 5.562
    C = 4.085
  2. jcsd
  3. Nov 10, 2012 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Your problem statement is not clear. Is stress bending stress? Is it shear stress? Is it combined stress?

    In any event, your formula for stress is suspect. First, you should draw a free body diagram of your beam with the applied loads. Construct the shear and bending moment curves from that.
  4. Nov 10, 2012 #3
    Yea you need to be a bit more specific with the problem statement. Although I assume your problem is for maximum bending stress. In that case your equation is suspect still. Your maximum bending stress will occur where the bending moment is maximum and the selected point is the maximum distance from the beam's neutral axis. So to even consider your problem you'll need to derive the bending moment function first and find the point at which it is maximum, then compute the neutral axis location on each cross section and find the maximum distance from each.
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