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Modeling a box section using Shell Elements

Modeling a box section using Shell Elements

Modeling a box section using Shell Elements

(OP)
Folks,
Recently I was experimenting modeling a box shape using shell elements in SAP and ETABS. I ran into the problem that the computed Moment of Inertia (MOI) was different from what the program was computing.

I modeled a pin-roller condition of a given span and backtracked I using the deflection equation. The actual I was 20% higher than the backtracked I.

I modeled the box using center-lines and thickness of the elements.

Has anyone else run into such a problem and if you did, what was your approach to modeling it right?

Try a 4' wide x 4' deep box with 2'x2' void (= 12" thick box).

RE: Modeling a box section using Shell Elements

Was this a concrete element or steel?  Is it possible you used a different E than the program?

RE: Modeling a box section using Shell Elements

A few things come to my mind. One, how long was your span? If you had a 4'x4' box with a short span, shear deformations could be controlling rather than moment deformations, which would not be accounted for in your equation. I would use a span/depth ratio of at least 10. How fine was your mesh? If you are using 4 node elements they may not be acting fully composite over their length. Especially with such a large cross section. I'd refine your mesh and see if the values converge. Also, how is your load applied? If you apply a point load at a single node in that type of model, you'll get large local deformations as well, making values around that area invalid. One more thing. Are you using the correct offsets for you elements? If you extrude the view does it create the box section your assuming? Remember that the thickness inputted extends out from both planes of the element.  

RE: Modeling a box section using Shell Elements

(OP)
E was correctly accounted for.
Span was 60'
Load was applied as a uniform force on the top slab.
Yes, extruded shape is almost the same as section assumed (except for missing chunks at centerline intersections.

RE: Modeling a box section using Shell Elements

What about your mesh? I would suggest at least two rows of elements per side to place a node at mid height each side.

RE: Modeling a box section using Shell Elements

(OP)
The mesh was very fine. It was a 6"x6" mesh. I would appreciate if someone could try modeling in their FEA program to verify results.

Span = 72'
Loads = 100 psf x 4' wide (400 PLF)

Width of top slab = 4'
Total Depth = 4' (modeled as 3' deep member)

RE: Modeling a box section using Shell Elements

Did you try simpler problems first?  If not, the you should.

Start with a cantilever plate bending about its weak axis with a point load at the tip.

Then do the same problem with the plate bending about its strong axis.

I suspect that the difference is either shear deformation or something slightly wrong with the boundary conditions in the shell model.  Try to use a cantilever so you eliminate the chance of screwing up your boundary condition at hte roller.

For what it's worth, I've done this kind of thing and after messing with it a while, the two models should be closer than 20%.  Maybe 2-3% different.

RE: Modeling a box section using Shell Elements

I just read your last post.  Your loading method might be part of the problem.  You applied the load as 4 psf across the beam.  This will make the top shells bend between the vertical walls and I don't know what this will do to the beam's overall stiffness.  This is an example of introducing too many variables at once.

You could also try a cantilever with an applied moment at the tip.  That eliminates shear.

Start simple and build up to the more complex problem.

RE: Modeling a box section using Shell Elements

Slickdeals,

You could model it as the sum of two square areas (one is negative):

  I = (4*4^3 - 2*2*3)/12 = 20 ft^4

Does that agree with your calculation?  You will not get the same result using centerlines.



 

Best regards,

BA

RE: Modeling a box section using Shell Elements

(OP)
@BA,
I know what the MOI should be. I want to model it right using shell elements so as to capture torsion in the box due to uneven loading etc.

As a result, I started the model but in making quick checks, I found discrepancies and hence, wanted to know how people typically model them.

Thanks

RE: Modeling a box section using Shell Elements

Not familar with thin shell structure in computer model, but know it behave differently from plate element. Have you checked the manul and make sure your elements meet the definition/limitation of thin shell element?  

RE: Modeling a box section using Shell Elements

slickdeals,

I have no idea what you just responded.  Perhaps you should abandon your computer program in favor of a clear understanding of the problem you are attempting to solve.  I don't believe you know what you are doing.

Best regards,

BA

RE: Modeling a box section using Shell Elements

kslee,

How could a 4'x4' section with a 2'x2' void be considered a thin shell structure?  It clearly is not.  The "shell" is 12 inches thick and spans four feet...hardly a thin shell.

Best regards,

BA

RE: Modeling a box section using Shell Elements

BA: That's my question too. Thanks for clarification though.

RE: Modeling a box section using Shell Elements

BARetired,

That is harsh, unnecessary and uninformed criticism not required in forums like this.

With 'black-box' computing probably rife in some areas of engineering these days (only due to the rediculous timeframe engineers agree to design projects now), it is better that someone asks the question than not.

Obviously slickdeals is in the process of understanding the problem as he has posted this question.

RE: Modeling a box section using Shell Elements

Slenderbeam,

It is neither harsh, unnecessary nor uninformed.  I have no problem with anyone posing any question to gain a better understanding of an engineering problem.  I do have a problem with people responding to an honest attempt to answer their concerns with a flippant, silly commentary.

 

Best regards,

BA

RE: Modeling a box section using Shell Elements

I'm having trouble finding any "flippant, silly" commentary in these posts. The guy asked for help in finding a solution.  

RE: Modeling a box section using Shell Elements

OCI,

I offered "the guy" help in finding a solution, namely to calculate the moment of inertia using the properties of two squares, the 4'x4' square and the 2'x2' void instead of using "shell elements", my reason being that taking the properties of the section on the basis of centerline "shell elements" will provide an incorrect answer to the problem.

I'm not sure why we are belaboring this issue.  If you have something constructive to say, I will be happy to respond.  Otherwise, I suggest we simply forget it.   

Best regards,

BA

RE: Modeling a box section using Shell Elements

(OP)
@BA:

Quote:

my reason being that taking the properties of the section on the basis of centerline "shell elements" will provide an incorrect answer to the problem.

The sole purpose of posting this question was precisely a way to find out the right way to account for what you mention above.

My answer was not flippant. What I meant to say was that I know the answer to the problem, meaning the moment of inertia of a box shaped section.

I have uneven loading on the webs of the box, which will lead to some torsion in the box. I want to be able to capture that torsion. In my efforts to do that, I started by modeling a box with a simple span and check the MOI as a first step.

I really appreciate everyone trying to help here, but I do not understand what I said that ticked BA off.

Quote:

don't believe you know what you are doing
.In order for you to say this, you should have a complete idea of everything about the problem. I would rather you suggest ways to approach a problem rather than offer a flippant comment like that. I think your comment was far more flippant than anything I ever said. I did not take anything personally because I don't have a huge ego to satisfy.  

RE: Modeling a box section using Shell Elements

slickdeals:

Simply change the "shell" element to "plate" element. You shall be able to accomplish what you have in mind. Since the aspect ratio of your members does not seem to fit the conventional defination of shell element, the computer program may have defult value for certain type of element that over-ride your imput. (While thickness holds the key for shell element, I don't think I has much of meanings as it has in beam element)

RE: Modeling a box section using Shell Elements

kslee1000, I do not believe that will work.  In SAP2000, the difference between a shell and a plate is in the degrees of freedom that are in the element stiffness matrices.  Plates only have out-of-plane bending.  Shells have those plus membrane stresses.  This is the usual difference between plates and shells.  He clearly needs membrane stresses in the elements for the model to work correctly.

That being said, he might not have a "thin" shell.  SAP gives two basic options for shells: thin shells (ignore shear deformation) and thick shells.  He might need to model them as thick shells.  

This is just one of the things he needs to weed-out as the problem.  He actually needs to start with a much simpler problem or two or five to isolate the key parts of the behavior.

RE: Modeling a box section using Shell Elements

slickdeals,

I was out of line in my earlier post.  I apologize.

Best regards,

BA

RE: Modeling a box section using Shell Elements

(OP)
I was not expecting an apology, but apologies accepted and no hard feelings. I apologize also in case you thought my comment was flippant.

I am isolating the problem and how to simplify it like 271828 says. I am sure the bridge guys do this on an everyday basis and maybe someone can point me in the right direction. I have never designed one personally. This is a one off thing for me.

RE: Modeling a box section using Shell Elements

271828:

Thanks for your informative response. As a person out of computer field for a long while but some remote memories, I agree fully with your points. The bottom line is the model does not seem to fit shell structure as defined by theory (for which the programs were based upon), therefore, the results could not be fully relied upon, unless a verification was made by hand cal, which the OP has done and exposed the deficiency (a good move for him). Now is time to review the defination & limitation imposed by the programs, and modify the model accordingly, or change approaches all together.

RE: Modeling a box section using Shell Elements

There is an error in shell elements modelling sections if you model the flanges and web sections as a complete section. For isntance, at the corners of the box you'll effectively have material overlapping each other. Try modelling each part of the section separately so that you'd have a gap between the flange and the web at the corner. Then tie the two nodes together, at each corner. See if that works better. Of course it shouldn't have been modelled with shells as the thickness of each shell is relatively large compared to the overall section size and in such a case you may miss out on any non-linear variaiton in stress through the thickess or get incorrect stresses at the corners. Generally I'd use shells if the thickness was about a tenth of the overall size.  

corus

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