# Generating substructures

 This section describes how individual substructures are defined. See Using substructures for information regarding how they are used in a model. Substructures are defined using the substructure generation procedure. The substructure creation and usage cannot be included in the same analysis. Multiple substructures can be generated in an analysis. Any substructure can consist of one or more other substructures; if this is the case, the nested-level substructures must be defined first. The substructure library is not organized in terms of part instances; therefore, substructures cannot be generated from models that have an assembly defined. None of the substructure options are supported in models that have an assembly defined. To define a typical substructure generation step, do the following: Invoke the substructure generation procedure. Define the nodes and degrees of freedom that are to be retained as external degrees of freedom when the substructure is used. Optionally, retain extra dynamic modes to improve the dynamic behavior of the substructure during usage. Optionally, specify substructure load cases. Optionally, write the recovery matrix, substructure's stiffness matrix, mass matrix, and/or load case vectors to a file. The following topics are discussed:
 Related Topics Using substructures In Other Guides *RETAINED NODAL DOFS *SELECT EIGENMODES *SUBSTRUCTURE COPY *SUBSTRUCTURE DELETE *SUBSTRUCTURE DIRECTORY *SUBSTRUCTURE GENERATE *SUBSTRUCTURE LOAD CASE *SUBSTRUCTURE MATRIX OUTPUT *MATRIX CHECK *FLEXIBLE BODY

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