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!

 

[Mustaine3]

- added symmetry constraint?
- useful element type?
- how is the load applied?
- what is that stress concentration on the full beam?
- is U1 equal between the models?

 

[rb1957]

how do you account for the symmetry of the full beam, compared with the asymmetry of the 1/2 model ?

another day in paradise, or is paradise one day closer ?

 

[corus]

Restrain the cut plane in the x direction for symmetry.

 

[Denial]

Elaborating on Corus's comment.[ ] You should restrain all nodes on the symmetry plane against X-translation, YY-rotation, and ZZ-rotation.

 

[corus]

Denial, As far as I can make out it's a solid model so there are no rotational dof. You'd be correct if it was a shell model.

 

[GregLocock]

It seems to me to be great pity that we have produced a generation of engineers who do not understand the concept of 'shear centre'.

Cheers

Greg Locock


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[rb1957]

@corus ... so constraining the translational freedom should be sufficient, in this case.

@greg ... it would appear so ...

another day in paradise, or is paradise one day closer ?

 

[Denial]

RB1957.[ ] If your comment to Corus was a question, the answer is "yes".

 

[gravityandinertia]

@GregLocock:

Before I write what I want to say, I recognize that you have contributed much on these boards, so I don't want to say that this is aimed at you, it's just that your comment in this thread is inline with a trend I've seen lately from experienced Engineers that has been bugging me personally. This more of a self-rant really:

Why does 1 young Engineer not knowing what a "shear center" is, mean that an entire generation doesn't know? That's a BIG extrapolation. On average people only retain 70% of what they are taught and lose knowledge they don't use from there. In this case, this particular Engineer may have had bad professors, a role that didn't require him to use the knowledge for a while or is expanding his knowledge in a field he doesn't come from.

Every generation has good Engineers and bad. I don't understand the need to create a war between different generations, that doesn't do much for the profession as a whole. Every Engineer will go through the whole cycle from inexperienced to experienced eventually and I'm sure everyone has forgot topics from their studies that have to relearn. If anything, the way to raise the profession is to make each generation better than the last through mentoring where the topics of importance are taught by someone who has applied them many times.

@ILoveFEM

That being said as Corus said, the solution is to apply a fixture in the x-direction which creates a symmetry condition. Since symmetry is essentially having equal and opposite geometry and loading, the forces that pull that center line out of plane, will be resisted and the reactions on that fixture should be equal to what you would get if you sectioned it and used statics to calculate the internal forces at the center. Using FEA tools does go hand in hand with understanding how to calculate things by hand. In this case, I'm assuming you are learning the tool, since this is a simple problem. So make sure you check that you are in the ballpark of normal stress in the z-direction by doing a hand calculation on the bending stresses.

As analyses get more complex, I always start simple. Make sure the geometry is correct. Run a simple load case (Point load/Uniformly Distributed). Check it against a hand-calc. This proves out the material properties and the softwares settings. From there I add more detail to the geometry of the model and detail to the loading if necessary. Anytime I'm unsure, I double check everything (model, materials, settings). The easiest way to make sure everything is working is to start simple and build more detail to your model.

 

[JAE]

it's just that your comment in this thread is inline with a trend I've seen lately from experienced Engineers that has been bugging me personally

It seems to me that both you and GregLocock have seen trends and have both commented on those trends.



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[rb1957]

All generalisations are false ...

In my experience, many kids coming out of school don't have the basic knowledge I expect. I have also had to stop one candidate solving stiffness equations (to solve a redundant structure) in an interview ... clearly this guy knew his analysis (just needed time to develop the experience to apply it). The cause, I believe, is that universities are giving students too much material (a mile wide, and an inch deep); and this is why you now need a Masters degree to get the depth in a field (that we got when we were taught ... a furlong wide and a yard deep). There are also social issues and ageist issues from both sides of the argument (young "entitled" upstarts, mouldy oldie fogies).

another day in paradise, or is paradise one day closer ?

 

[gravityandinertia]

@JAE

I agree with you. I have made a comment in a similar "generalisation" fashion, but I did specifically mention "trend" meaning general direction/attitude, meaning I see more experienced Engineers expressing that attitude than not, but I certainly never said that ALL do, which is what I was writing about.

rb1957, said it all really, "All generalisations are false..."

 

[JAE]

Agree....yet there is also that which we call hyperbole.



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[ukbridge]

I just wanted to say I 100% agree with what gravityandinertia said.

 

[rapt]

GregLocock,

Seeing I gave you a star I suppose I need to defend it. Yes the generalisation that we have a generation of engineers who do not understand shear centres is probably a little extreme. For a start, my son has been graduated as a structural for 3 years and he has an amazing understanding of statics even while at university, so it is at least a generation - 1 . But he was an exception rather than the rule from what I can tell.

Most older engineers I know have the same problem with many younger engineers. Many of them tend to use computers to circumvent their limitations in knowledge and understanding of statics and areas of design and design codes rather than using their understanding of statics etc to justify their computer analytical results.

Our rule tended to be that if you did not understand the area of analysis/design and have experience in applying and analysing the results, then you researched and practiced and taught yourself. Only then did you use someone else's analysis/design tools to help you become more productive. And you used your understanding and knowledge in that area to analyse the results from the software to make sure it satisfied your understanding of the theory.

Many younger engineers have never developed that understanding and will blindly use software and accept its results. Yes, it is probably a problem with the system and possibly the lack of understanding of some older engineering bosses of the limitations of using software and instant productivity, in that they have to produce designs to pay for their existence. We had to produce also! But it still happens a lot. And it is frustrating!

In this case, did they really need the complexity of the model shown to determine the stresses in a Cantilever I beam with a point load at the end. Or was it a test to see if the simplification worked in a trial for a more complex problem? We are not told. If they simply wanted the stresses in a cantilever I beam with a point load, they could have saved a lot of time by simply using their calculator.

Whatever, someone doing this level of computer analysis should have a very good understanding of statics before they start. And someone with a very good understanding of statics should understand the concept of shear centre and would realise before they start that changing it to a Channel section rather than and I Beam would have significant effects!

 

[MacGruber22]

Whether wholly truthful or not, negative generalizations about young engineers don't help a person solve their immediate question. Just provide the guidance needed, and take the soapboxing over to Pat's Pub.

"It is imperative Cunth doesn't get his hands on those codes."

 

[GregLocock]

Waawaawaa. So, usefully our ill educated friend could have tried moving the centroid of the point of application of the load outboard until the beam stopped twisting. One might hope that at that point the stress patterns would match, other than locally to the load (St Venant's principle), and he would have learned something about shear centres in a fairly practical manner.



Cheers

Greg Locock


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[rapt]

MacGruber22,

You obviously do not understand how dangerous we "old" rambling engineers think this problem is! There has been discussion as to whether blindly answering questions actually helps the questioner (other than giving a quick fix for his current question) into becoming a better engineer. Or whether the answering process should try to lead the questioner on the path to finding the solution for himself thus teaching him to develop his own thinking and solutions (like engineers are supposed to do). An even older engineer than me sent the following reply to me during a discussion on Australian Earthquake Code development discussions over the weekend

"Unfortunately engineers these days unless you tell them what to do they won't do it as they will become bush lawyers in interpreting the standards to suit their needs."

So now we have to develop cook books with a recipe to solve every situation rather than design codes (for a good example, look at ACI318)!

He was already given the reason for the problem and the solution.

To me rb1957's answer is the correct one as it makes him think through the reason for the problem and the solution himself rather than handing it to him! The next best is GregLoccoks 1st for the same reason (other than his generalisation).

But these would not provide an instant answer and solution! They make the questioner think it out.

 

[MacGruber22]

rapt - I absolutely agree with guiding people to the solution to encourage the thought process - the approach used above, not so much. I can almost hear the irritation in the keystrokes.

"It is imperative Cunth doesn't get his hands on those codes."