Substructure (Superelement)

A substructure (also called superelement in some coded) is an element characterized by a number of nodes and a stiffness (and optionally a mass matrix) describing the relationship between forces and displacements in these nodes. The matrices may have been obtained by another code. The element definition is triggered by a *MATRIX ASSEMBLE card (instead of an *ELEMENT card), the name of the file containing the stiffness matrix and, optionally, the name of the file containing the mass matrix. Right now, both have to be symmetric and only the upper triangle (including the diagonal) or the lower triangle (including the diagonal) should be given in the form

row node, row dof, column node, column dof, value

for each entry of the matrix, one per line (the order is irrelevant). A substructure can right now contain at most 256 nodes and only translational degrees of freedom (=dof) are allowed (dof 1 for the global x-axis, dof 2 for the global y-axis and dof 3 for the global z-axis). In addition, the matrix should receive a name containing at most 4 characters. Names of existing user elements exluding the “U” in front should not be used, such as “S3”. The nodes belonging to the element may be subject to Single Point Constraints, Multiple Point Constraints or point loads, but not to distributed loading (neither facial nor volumetric). Since the location of any integration points is not known, no element values such as stresses or strains are calculated. Only nodal information can be stored to file using *NODE PRINT and similar keyword cards.

A substructure is basically a linear construct. Right now, it can be used in a *STATIC, *DYNAMIC or *FREQUENCY procedure, including cyclic symmetry conditions.