The thickness properties of a structure are defined through the section property definitions in the case of 2D shell elements, whereas it is defined from the geometry of the element nodes in the case of 3D shell elements. 2D shell elements have 5 or 6 degrees of freedom per node (i.e 3 displacements and 2 or 3 rotations), whereas 3D shell elements have 3 displacement-only degrees of freedom per node, just like brick elements. The selection between 2D or 3D shell elements depends on the relative thickness of the structure, compared to its other two dimensions. For very small thickness, I would prefer 2D shells, whereas for larger values of thickness I would prefer 3D shells.

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Dear Dongxin Gao , The video tutorial (the link attached below) will be so beneficial, please also check the other link.

https://www.youtube.com/watch?v=ajZ1cOU4708

http://mechdocsblog.blogspot.com/2013/02/what-is-difference-between-shell.html

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I agree with George Papazafeiropoulos Sir. Also, since 3D shell elements have 3 dofs per node just like solid elements, it is easy to combine 3D shell and solid elements in a mesh. The same cannot be done in a straightforward manner when using 2D shell elements since the dofs at the nodes are different for a 2D shell and a solid element.

I wish you all the best I apologize that this is not my specialty