Modeling a box section using Shell Elements

[slickdeals]

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).

 

[271828]

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.

 

[BAretired]

slickdeals,

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

Best regards,

BA

 

[slickdeals]

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.

 

[kslee1000]

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.

 

[corus]

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