A powerful substructuring feature is the ability to use nested superelements (one superelement containing another). When you generate a superelement, one of the elements in the generation pass may be a previously generated superelement.

For example, suppose that you have a superelement named PISTON. You can generate another superelement named CYLINDER which contains the superelement PISTON. Now, for a complete analysis of the cylinder and its piston, you will need to perform one use pass and two expansion passes. The use pass calculates the reduced solution for the master DOF in the superelement CYLINDER. The first expansion pass calculates the complete solution for CYLINDER and the reduced solution for PISTON. The second expansion pass then gives you the complete solution for PISTON.

You can also generate transformed nested superelements (using SETRAN or SESYMM commands). It is important in this case to perform the expansion pass in the exact reverse order used for the generation pass.