I Beam analysis - splitting into half and halving load yields different results to a full beam 9

[ILoveFEM]

Hey guys,

We are trying to analyse the bending of an horizontal I-beam that is attached to a ENCASTRE (fixed) end. Imagine it being bolted onto a wall, and then a point load of 20 kN is applied.

So we are now trying to split the beam in half, and model it on ABAQUS to help with processing time. We would then half the point load to 10kN. Here is a picture of the model on ABAQUS:

Now here is the full beam on ABAQUS:

We found that the Mises stress is different- maybe its caused by torsion or eccentricity of the beam? Do you guys have any idea why the results are different and how we can fix it on ABAQUS?

Thanks!

 

[TLHS]

This is a worrying way to approach this problem, because without careful work you will miss member stability issues. Lateral torsional buckling of cantilevers can be a significant problem, especially when they aren't braced at the tip. Without some sort of induced eccentricity and iterative analysis you aren't going to see a possible LTB failure mechanism.

Unless there's some specific reason why you need the complication of a detailed FEA, you're going to get more reliable results from traditional design methods. You can even see from the results that the analysis method is overkill. You have a stress gradient that simply scales linearly.

 

[rapt]

TLHS,

I was actually thinking of this on a long drive yesterday. Introducing artificial restraints to overcome an unnecessarily introduced eccentricity problem is very dangerous. Better not to try to take shortcuts! Even if it does not matter in this case it could be a bad habit to get into.

MacGrubber22,
Great to see you are psychic too! Can you read the bit I deleted as well! Hope you cannot read my mind or we could get into lots of trouble!

 

[Illbay]

Please tell me this is some sort of academic exercise. Otherwise, using ABAQUS to analyze a single beam? REALLY?

And what are you doing this for? Why would you split a real-world beam this way? If you're trying to analyze a Wide Flange, then that's what you should analyze.

I apologize that I didn't read the whole thread, if this has all been answered (TLR), but I'm pretty anal about analysis (who says engineers can't joke?) and this seems a bit much even to me.

"No one is completely useless. He can always serve as a bad example." --My Dad ca. 1975

 

[Tmoose]

I believe many, and maybe most, discussions and tutorials of half and even quarter models to take advantage of symmetry also mention the necessity of enforcing a symmetric boundary condition at the plane of symmetry.

http://simulatemore.mscsoftware.com/taking-advantage-of-symmetry-in-fea-msc-nastran/

Also when such modeling is inappropriate and should be avoided when result are potentially asymmetric, like buckling and basic modal analysis.

Maybe IloveFEM's instructor set a little trap with this assignment to create an enhanced learning situation.
Or, IloveFEM did not RTFI.