# QUAD4 Shell Elements: Offsetting the Midplane

Entered by Paul Curry
2-18-05

Useful for analyses where a shell thickness may be a design variable. Update is very quick.

Note that if one constructs a model of shells and solids, there is an important item that must be understood. The midplane of the shell may be modeled coincident to the solids. This is useful if one wishes to connect the shell to the solid by nodal equivalencing. However, one must then apply an offset to the midplane so that the shell is defined as it is in the actual structure. Note the direction of the element normals. They must all be in the same direction. The normal is the Z axis of the element coordinate system. The X and Y axes are defined by how the element is created (the order of node selection). Thus the node csys may differ in a given plane of shell elements, but the normals need to be in the same direction.

The shell property is modified by selecting the “Property” form, and then “Modify”, then select the set to modify. Input the following data in addition to the shell thickness:

Zoffs: the Z offset of the midplane, in the element coordinate system (offset from the existing nodes)
Z1: the distance from the element midplane to the TOP of the shell (the shell top is in the +Z direction from the shell midplane). NOTE Z1 is POSITIVE.
Z2: the distance from the element midplane to the BOTTOM of the shell (the shell bottom is in the –Z direction from the shell midplane). NOTE Z2 is NEGATIVE.

Post processing tip: Select the global or other appropriate coordinate transformation by using the Results tools. This will allow you to see the shell stress components aligned with the Global csys. Very helpful!

Troubleshooting tip: If you see stresses of exactly the same magnitude (for Von Mises stress) on top (Z1 layer) and the bottom (Z2 layer), then there is an error in the Property set (such as Z1 and Z2 having the same sign!). Also, when posting the stress components to see the membrane and bending stress combined, the signs will be opposite on Z1 and Z2 layers (consider a plate in bending, there will be tensile stress on one side and compressive stress on the other side).