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One approach might be to develop the entire model, save it on a named database file, and select only the portion to be substructured for the generation pass. In the use pass then, you can RESUME (Utility Menu> File> Resume from) from the named database file, unselect the portion that was substructured and replace it with the superelement matrix.

10.2.1.2. Applying Loads and Creating the Superelement Matrices

The "solution" from a substructure generation pass consists of the superelement matrix (or matrices).

As with any other analysis, you define the analysis type and options, apply loads, specify load step options, and initiate the solution. Details of how to do these tasks are explained below.

Enter SOLUTION using either of these methods

Command(s): /SOLU

GUI: Main Menu> Solution

Define the analysis type and analysis options The applicable options are explained below.

Analysis Type - Choose a substructure generation using one of these methods:

Command(s): ANTYPE

GUI: Main Menu> Solution> Analysis Type> New Analysis

New analysis or restart - If you are starting a new analysis, choosing the analysis type (as described above) is all you need to do. However, if the run is a restart, you must also indicate this by setting

STATUS = REST on the ANTYPE command (Main Menu> Solution> Analysis Type> Restart). A restart

is applicable if you need to generate additional load vectors. (The files Jobname.EMAT, Jobname.ESAV, and Jobname.DB from the initial run must be available for the restart.)

Note

Restarting a substructuring analysis is valid only if the backsubstitution method is chosen. You cannot restart a run if the full resolve option is selected using the SEOPT command.

Name of the superelement matrix file - Specify the name (Sename) to be assigned to the superelement matrix file. The program will automatically assign the extension SUB, so the complete file name will be Sename.SUB. The default is to use the jobname [/FILNAME]. To specify the name of the superelement matrix file:

Command(s): SEOPT

GUI: Main Menu> Solution> Analysis Type> Analysis Options

Equation Solver - The SPARSE solver is the only solver available for the generation pass of the substructure analysis. To specify an equation solver:

Command(s): EQSLV

GUI: Main Menu> Solution> Analysis Type> Analysis Options

Matrices to be generated - You can request generation of just the stiffness matrix (or conductivity matrix, magnetic coefficient matrix, etc.); stiffness and mass matrices (or specific heat, etc.); or stiffness, mass, and damping matrices. The mass matrix is required if the use pass is a structural dynamic analysis or if you need to apply inertia loads in the use pass. For the thermal case, the specific heat matrix is required only if the use pass is a transient thermal analysis. Similar considerations apply to other disciplines and to the damping matrix. To make your request, use the SEOPT command as described above.

Matrices to be printed - This option allows you to print out the superelement matrices. You can request listing of both matrices and load vectors, or just the load vectors. The default is not to print any matrices. To print out the matrices, use the SEOPT command:

Expansion Pass Method - Allows you to select the expansion pass method you plan to use during subsequent expansion passes with this superelement. The backsubstitution method (default) saves the factorized matrix files needed to perform a backsubstitution of the master DOF solution during the expansion pass. The full resolve method does not save any factorized matrix files. The factorized matrix files are named Sename.LNxx for the sparse solver.

Note

Factorized matrix files can become very large as the problem size increases, but are not needed if the full resolve method is chosen during the expansion pass.

During the expansion pass, the full resolve method reforms the elements used to create the superelement, reassembles the global stiffness matrix, and applies the master DOF solution as displacement boundary conditions internally. These displacement boundary conditions are deleted upon finishing the expansion pass solution.

Note

You cannot restart a substructure analysis with the full resolve expansion pass method chosen.

Mass matrix formulation - Applicable only if you want the mass matrix to be generated. You can choose between the default formulation (which depends on the element type) and a lumped mass approximation. We recommend the default formulation for most applications. However, for dynamic analyses involving "skinny" structures, such as slender beams or very thin shells, the lumped mass approximation has been shown to give better results. To specify a lumped mass approximation, use one of these methods:

Command(s): LUMPM

GUI: Main Menu> Solution> Analysis Type> Analysis Options

Modes to be used - For superelements being used in a subsequent dynamic analysis [ANTYPE,MODAL, HARMONIC, or TRANSIENT], you may include mode shapes as extra degrees of freedom to obtain better accuracy [CMSOPT]. See the chapter on Component Mode Synthesis for more information.

Define master degrees of freedom using one of these methods

Command(s): M

GUI: Main Menu> Solution> Master DOFs> User Selected> Define

In a substructure, master DOFs serve three purposes:

•They serve as the interface between the superelement and other elements. Be sure to define master DOFs at all nodes that connect to nonsuperelements, as shown in Figure 10.2: Example of a Substructuring Ap- plication (p. 264). All degrees of freedom at these nodes should be defined as master DOFs (Lab = ALL on the M command). Master DOFs must be defined even if you plan to have no elements in the model other than a superelement.

•If the superelement is to be used in a dynamic analysis, master DOFs characterize the dynamic behavior of the structure if the Component Mode Synthesis method [CMSOPT] is not used. See Modal Analysis in the Structural Analysis Guide for guidelines.

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•If constraints [D] or force loads [F] are to be applied in the use pass, master DOFs must be defined at those locations with the M command.

If this superelement is to be transformed [SETRAN] later in the use pass or used in a large deflection analysis [NLGEOM,ON], then all nodes that have master DOFs must have all six DOFs (UX, UY, UZ, ROTX, ROTY, ROTZ) defined and active.

For large deflections, master DOFs are typically defined at the joints of the flexible body and are at the nodes connected to a joint element (MPC184), another rigid or flexible body node, or ground. At least two master DOFs must be defined for each substructure, as the average rotation of the superelement is computed from the average rotation of its master DOF. If only one node is a joint node, then another must be chosen at the free end. See the Multibody Analysis Guide for more details.

10.2.1.2.1. Applicable Loads in a Substructure Analysis

You can apply all types of loads in a substructure generation pass. Some types of loading involve certain considerations, as follows:

•The program generates a load vector that includes the effect of all applied loads. One load vector per

load step is written to the superelement matrix file. This load vector is the equivalent load for the combined loads in the load step. A maximum of 31 load vectors applies.

•Nonzero degree-of-freedom constraints can be used in the generation pass and will become part of the load vector.

In the expansion pass, if the load step being expanded contains nonzero degree-of-freedom constraints, the database must have matching degree-of-freedom values. If it does not, the degree-of-freedom constraints must be specified [D] again in the expansion pass.

•Application of constraints [D] or force loads [F] can be postponed until the use pass, but a master degree- of-freedom must be defined at those locations with the M command or corresponding GUI path.

If a mass matrix is generated, apply the degree of freedom constraints in the use pass at the master degree-of-freedom (defined in the generation pass) to ensure that all mass is accounted for in the substructure. For analyses with acceleration loadings, the load should be applied in the generation pass and used in the use pass for greater accuracy, rather than apply the acceleration load on the reduced mass matrix.

•Similarly, application of linear and angular accelerations can be postponed until the use pass, but only if a mass matrix is generated. A postponement is desirable if you plan to rotate the superelement in the use pass, because load vector directions are "frozen" and rotated with the superelement.

•The Maxwell force flag (MXWF label on the SF family of commands) is normally used in a magnetic analysis to flag element surfaces on which the magnetic force distribution is to be calculated. The flag has no effect (and therefore should not be used) for a superelement in a magnetic analysis.

•If you intend to create an imaginary force vector, generate it as a real load vector, then use it as an imaginary load vector in the use pass (SFE,,,,KVAL = 2) and expansion pass (SEEXP,,,ImagKy = ON).

•When a load vector exists for a thermal superelement, it must be applied and have a scale factor of 1 (SFE,,,,,SELV,,1).

•For large-rotation analyses, do not apply constraints to the model in this pass, as you will apply constraints for large rotation analyses in the use pass.

Table 10.1: Substructure Analysis Loads

1.The menu path used to access each command in the GUI will vary depending on the engineering discipline of the analysis (structural, magnetic, etc.). For a list of menu paths, see the description of individual commands in the Command Reference.

Specify load step options The only options valid for the substructure generation pass are dynamics options (damping).

Damping (Dynamics Options) - Applicable only if the damping matrix is to be generated.

To specify damping in the form of alpha (mass) damping:

Command(s): ALPHAD

GUI: Main Menu> Solution> Load Step Opts> Time/Frequenc> Damping

To specify damping in the form of beta (stiffness) damping:

Command(s): BETAD

GUI: Main Menu> Solution> Load Step Opts> Time/Frequenc> Damping

To specify damping in the form of material-dependent alpha damping:

Command(s): MP,ALPD

GUI: Not accessible from the GUI.

To specify damping in the form of material-dependent beta damping:

Command(s): MP,BETD

GUI: Not accessible from the GUI.

Save a backup copy of the database on a named file Doing this is required because you need to work with the same database in the expansion pass. To save a backup copy, use one of these methods: