The definition of materials for application to shells is identical to the process of defining materials for use with members. In fact the dialog is the same. The only difference is in some of the ways that the materials can be assigned.
Materials consist of the properties necessary for structural analysis: Young's Modulus, Poisson's Ratio, Density and Coefficient of Thermal Expansion or Contraction. Note that these are not material strengths, which are related to code checking. Those get entered in a different location.
Materials can be defined and assigned using the Materials table, which can be accessed by clicking:
•Create (or Modify) > Member Properties > Materials
•Create (or Modify) > Shell Properties > Materials
•or by clicking Tables > Materials.
Regardless of how the table is accessed, it allows the user to create material definitions and to assign them to currently selected shells.
Note that there is a Standard Material button on this table that allows for the quick definition of many standard steel and concrete materials.