Simply Supported Beam Question

  1. 1. The problem statement, all variables and given/known data

    Question: Sketch the cross section of the gantry girder and indicate on it the position of the maximum bending stress. State if this stress is tensile or compressive for this particular example, explain how you decided

    2. Relevant equations

    There aren't any required

    Relevant information however: The Beam is Hogging and the Maximum Bending Stress is positive

    3. The attempt at a solution

    I've sketched the cross-section and previously indicated the maximum bending stress on the top to indicate the tensile factor.

    However this is wrong and instead is Compressive.

    What i struggle with is explaining why this is the case.

    ___________
    |..................|
    .......|....|
    .......|....|
    .......|....|
    .......|....|
    ____|....|____
    |..................|

    .......... [itex]\sigma[/itex]


    This is the cross section with the sigma sign beneath to indicate the max bending stress is compressive.

    Just to show visually what i've done.

    I just need help explaining why it would be compressive / how i decided. Just to clarify, i didn't decide this, i was told by my tutor but i have to figure out why and theres nothing out there on the internet that discusses this.
     
  2. jcsd
  3. PhanthomJay

    PhanthomJay 6,278
    Science Advisor
    Homework Helper
    Gold Member

    If a beam is 'hogging', it is deflecting upward, that is, it has upward curvature. This tends to expand (stretch) the topmost fibers and to compress (squeeze) the bottom fibers, as you can see by drawing a quick sketch. The top fibers would be in tension and the bottom fibers would be in compression. For a symmetric beam, these 2 stresses would be equal. If the beam is not symmetrical, which apparently is your case, the max stress occurs at the outmost fibers of the flange furthest from the neutral axis.
     
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