Calculating Max Deflection of Simply Supported J Section Beam

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Discussion Overview

The discussion revolves around calculating the maximum deflection of a simply supported J section beam, focusing on the appropriate moment of inertia to use in the calculations. Participants explore the complexities involved in the deflection behavior of J beams, including the interplay between bending and torsional effects.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions which moment of inertia value (Iy or Imin) should be used for calculating maximum deflection, suggesting that Iy might be appropriate since deflection occurs along the y-axis.
  • Another participant emphasizes the need to use the moment of inertia corresponding to the perpendicular axis to the plane of deflection, noting that J beams have complexities due to coupled bending and torsional deflections.
  • It is mentioned that the bending deflection of J beams is coupled with torsional deflection, complicating the analysis unless under specific loading conditions.
  • One participant provides a series of assumptions regarding the J beam's characteristics, including the potential neglect of additional deformation in the absence of torsional forces.
  • Another participant counters that bending and torsion are inherently coupled in J beams, asserting that even without a torsional load, bending will induce twisting and vice versa.
  • A later reply suggests that the coupling of bending and torsion may depend on the relationship between the torsional moment of inertia and the deflection moment of inertia, proposing that if the torsional inertia is significantly larger, torsional effects might be negligible.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between bending and torsion in J beams, with some asserting that they are coupled while others suggest conditions under which torsional effects might be negligible. The discussion remains unresolved regarding the specific implications of these interactions on deflection calculations.

Contextual Notes

Participants note the complexity of analyzing J beams due to their asymmetric shape and the coupling of bending and torsion, which may not be straightforward. There are also references to specific theories and assumptions that may not be universally applicable.

phiska
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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!
 
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phiska said:
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!

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.
 
Clausius2 said:
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.

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.
 
Speed said:
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.

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 length of the section is small compared with the length 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.
 
Clausius2 said:
Here's my logics:
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.

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.
 
Speed said:
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.

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 I_{torsion} and I_{deflection}

If I_{t}/I_{d}>>1 then torsional effects would be negligible, don't they?. I am only asking.
 
Thanks everyone for your help!
 

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