Simple beam bending FEM vs. theory comparison error

I am having some trouble getting accurate values for max. stress and max. deflection for an FEM model of a simply supported I-beam in bending. I am using I-DEAS 12 NX to solve the model. I'm pretty new to finite element analysis as a whole, but I know that getting any more
than 10% error (relative to Euler beam theory) in max deflection or stress for a 60x10x8 in. should not happen. Strangely enough, using the same boundary conditions and meshing techniques for a 60x10x8 in. rectangular solid, I find the results I was expecting. Here are some more details:



-For each geometry, there are 3 models. One uses
beam elements, one uses solid (hexa) elements, and one uses shell
(quad) elements.



-The boundary conditions (pin on one side,
roller on the other) are applied to the mesh, using constraint elements
on each face, with the node in the middle of the face being the
independent node (where the b.c.'s are applied). Similarly, the applied
force of 1000 lbs was modeled as a series of point forces applied along
the neutral axis of the beam at its center.



-As for mesh
density, I modeled the rectangular prism using an element size of about
2.5 in x 2.5 in x 2 in for the hex elements, just to give an idea. The
I-beam model required a finer mesh for model "convergence" (there seem
to be singularities at the b.c.'s and loads for the I-beam,
regardless), so we're talking element sizes of about 1 in x .5 in x .5
in.



Any thoughts on what I (or possibly I-DEAS) is doing wrong?
I'm thinking Saint-Venant's principle, but the magnitude of the max
stress error and max deflection error in the I-beam model is much
bigger than the errors in the rectangular prism.



Hope this wasn't too ridiculous to read.



Thanks,



Raymond C. Singh