I have a model of a tall, slender structure that I am investigating using both shell and $3D$ frame elements.  The shell elements are type MITC4, $4$-node membrane elements.  The frame elements are the basic line ($1D$) elements found in any introductory structural analysis / stiffness method book - they include axial, bending, and torsion, but no shear deformation.Both analysis are linear-elastic (no geometric or material non-linearties), small-deformations using the direct-stiffness formulation.As a test case, I loaded the structure with a unit load at the top node (or nodes, for the shell element model), once in a direction perpendicular to the long axis of the structure ($X$-axis), and once down the axis of the structure for pure axial loading ($Z$-axis).  I neglected all self-weight for this test case.[I also compared $X$ and $Y$ direction bending, which matched exactly as expected, since the structure is symmetric.]I compared these results with those found by simple hand computations for a linear-elastic beam:Axial deformation ($Z$-axis) = $PL/(AE)$Bending deformation ($X$-axis) = $PL^3 / (3EI)$For the frame element model, these results matched exactly.For the shell element model, the axial value was nearly identical (which I take as a good sign that the area is correct).The bending deformation for the shell element model were somewhat smaller (~8%).What I would like to know is, is this expected behavior?  Should I expect the shell element model to be stiffer than the frame element in general?I understand that the choice of Poisson's ratio in the shell element has an effect here as well.  I'm using $0.30$ for steel.Regards, Madeleine.
Shell vs frame element model stiffness differences
finite element