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Bending of J Beams

  1. Mar 25, 2005 #1
    If i have a simply supported J Section beam, when i'm calculating the max deflection, what value of I do i use???

    I have already had to calculate Ix, Iy and Ixy.

    I would imagine that as deflection occurs along the y axis that i would use Iy.

    Or do i use Imin? I have already calculated this too.

    Any help would be gratefully appreciated as i'm completely stuck!!!
  2. jcsd
  3. Mar 26, 2005 #2


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    I don't know what axis reference are you using, but in this problem you have to employ the moment of inertia correspondent to the perpendicular axis to the plane of deflection. Think of it, each beam section is going to turn infinitesimally around this axis when deflecting.
  4. Mar 27, 2005 #3
    It is not so straightforward for J-beams. The bending deflection of J-beams is coupled with the torsional deflection, in all but the most special loading (via shear centre) case. You will need to pick up a structural mechanics book, I'm afraid, because it is not easy to explain how to do the sums. It should be under "asymmetric beam theory" or some variation thereof.
  5. Mar 28, 2005 #4


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    Here's my logics:

    1) I am assuming a "J" beam is a beam which has a section with a shape of a "J". Right?. Anyway he doesn't say nothing about the thickness of the J.

    2)If the characteristic lenght of the section is small compared with the lenght of the beam, it doesn't matter how the section is deformed, although we know that after the movement the "J" doesn't remain being a "J".

    3) In absence of torsional forces and under the above assumptions it can be neglected any additional deformation but that the correspondent to the neutral beam line.

    I know there is an special theory for beams or bars with small section thickness, but anyway don't believe torsional effects are present when you are applying a vertical load. It breaks the symmetry.

    If some other member wants to disagree with me, feel free. I don't remember this stuff very well.
  6. Mar 28, 2005 #5
    No, the whole point is that bending and torsion are coupled here. Even if you do not apply a torsional load to the beam, because of the coupling if you bend it, it will twist. Likewise if you twist it, it will bend.

    Only in very special cases are bending and torsion not coupled. Often engineers use these special cases (such as I-beams, square tube) because they are easy to analyse.
  7. Mar 30, 2005 #6


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    Maybe you're right. I promise you structure eng. is not my best. Your words sound fine. Anyway, I think the coupling will depend somehow in the relation between both inertia moments [tex] I_{torsion}[/tex] and [tex] I_{deflection}[/tex]

    If [tex] I_{t}/I_{d}>>1[/tex] then torsional effects would be negligible, don't they?. I am only asking.
  8. Apr 6, 2005 #7
    Thanks everyone for your help!
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