You must be careful when you directly join elements that have differing degrees of freedom (DOFs), because there will be inconsistencies at the interface. When elements are not consistent with each other, the solution may not transfer appropriate forces or moments between different elements.
To be consistent, two elements must have the same DOFs. For example, they must both have the same number and type of displacement DOFs and the same number and type of rotational DOFs. Furthermore, the DOFs must overlay (be tied to) each other. That is, they must be continuous across the element boundaries at the interface.
Consider three examples of the use of inconsistent elements:
Elements having a different number of DOFs are inconsistent.
SHELL181 has three displacement and three rotational DOFs per node. SOLID185 elements have three displacement DOFs per node, but lack rotational DOFs. If a SOLID185 element is joined to a SHELL181 element, the nodal forces corresponding to displacement DOFs are transmitted to the solid element. However, the nodal moments corresponding to the rotational DOFs of the SHELL181 element are not transmitted to the SOLID185 element.
Elements having the same number of DOFs may nevertheless be inconsistent.
For example, a beam element and a shell element may both have three DOFs per node. However, the shell element may have three displacement DOFs (UX, UY and UZ) while the beam element has only two (UX and UY), so the UZ result reflects the stiffness of the shell element only. Furthermore, the shell element may not have the rotational DOF (ROTZ) that the beam element has, so the nodal moment corresponding to the beam element's rotational DOF is not transmitted to the shell element. The interface behaves as if the beam were "pinned."
Both 3D beam elements and 3D shell elements have six DOFs per node, but may be joined in a manner that is inconsistent.
The ROTZ degree of freedom of the shell element (the drilling mode) is associated with the in-plane rotational stiffness. This is normally a fictitious stiffness (that is, it is not the result of a mathematical calculation of the true stiffness). Thus, the ROTZ degree of freedom of the shell element is not a true DOF. It is therefore inconsistent to connect only one node of a 3D beam element to a 3D shell element such that a rotational DOF of the beam element corresponds to the ROTZ of the shell element. Beams should not be joined to shells in such a manner.
Similar inconsistencies may exist between other elements with differing number and/or types of DOFs.
Such problems may not invalidate your analysis, but you should at least be aware of the conditions at the interface between two different element types.