Release 11.0 Documentation for ANSYS
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In its strictest sense, the term "expansion" means expanding the reduced solution to the full DOF set. The "reduced solution" is usually in terms of master DOF. In a modal analysis, however, the term "expansion" means writing mode shapes to the results file. That is, "expanding the modes" applies not just to reduced mode shapes from the reduced mode-extraction method, but to full mode shapes from the other mode-extraction methods as well. Thus, if you want to review mode shapes in the postprocessor, you must expand them (that is, write them to the results file).
The full extraction methods also write the mode shapes to the results file by default during the modal analysis irrespective of the MXPAND command. To not write the mode shapes with MODOPT,LANB only, use MXPAND,-1.
Expanded modes are also required for subsequent spectrum analyses. In the single point response spectrum (SPOPT,SPRS) and dynamic design analysis method (SPOPT,DDAM), the modal expansion can be performed after the spectrum analysis, based on the significance factor SIGNIF on the MXPAND command. If you want to perform modal expansion after the spectrum analysis use MXPAND,-1. No expansion is necessary for subsequent mode superposition analyses.
The mode shape file (Jobname.MODE), and the Jobname.EMAT, Jobname.ESAV, and Jobname.TRI (if reduced method) files must be available.
The database must contain the same model for which the modal solution was calculated.
Reenter the ANSYS solution processor.
| Command(s): | /SOLU |
| GUI: | Main Menu> Solution |
Activate the expansion pass and its options. ANSYS offers these options for the expansion pass:
Table 3.5 Expansion Pass Options
| Option | Command | GUI Path |
|---|---|---|
| Expansion Pass On/Off | EXPASS | Main Menu> Solution> Analysis Type> ExpansionPass |
| No. of Modes to Expand | MXPAND | Main Menu> Solution> Load Step Opts> ExpansionPass> Single Expand> Expand Modes |
| Freq. Range for Expansion | MXPAND | Main Menu> Solution> Load Step Opts> ExpansionPass> Single Expand> Expand Modes |
| Stress Calc. On/Off | MXPAND | Main Menu> Solution> Load Step Opts> ExpansionPass> Single Expand> Expand Modes |
Each of these options is explained in detail below.
Choose ON.
EXPASS is not valid in a Distributed ANSYS analysis.
Specify the number. Remember that only expanded modes can be reviewed in the postprocessor. Default is no modes expanded.
This is another way to control the number of modes expanded. If you specify a frequency range, only modes within that range are expanded.
Choose ON only if you plan to do a subsequent spectrum analysis and are interested in stresses or forces to do the spectrum. "Stresses" from a modal analysis do not represent actual stresses in the structure, but give you an idea of the relative stress distributions for each mode. Default is no stresses calculated.
In a Distributed ANSYS analysis, you must use the MXPAND command at the same time that the mode and mode shapes are computed if you want to expand the modes. In a Distributed ANSYS run, MXPAND is not supported during an expansion pass (EXPASS).
Specify load step options. The only options valid in a modal expansion pass are output controls:
Printed output
Use this option to include any results data (expanded mode shapes, stresses, and forces) on the printed output file (Jobname.OUT).
| Command(s): | OUTPR |
| GUI: | Main Menu> Solution> Load Step Opts> Output Ctrls> Solu Printout |
Database and results file output
Use this option to control the data on the results file (Jobname.RST). The FREQ field on OUTRES can be only ALL or NONE; that is, the data are written for all modes or no modes. For example, you cannot write information for every other mode.
| Command(s): | OUTRES |
| GUI: | Main Menu> Solution> Load Step Opts> Output Ctrls> DB/Results File |
Start expansion pass calculations.
The output consists of expanded mode shapes and, if requested, relative stress distributions for each mode.
| Command(s): | SOLVE |
| GUI: | Main Menu> Solution> Solve> Current LS |
Repeat steps 2, 3, and 4 for additional modes to be expanded (in different frequency ranges, for example). Each expansion pass is stored as a separate load step on the results file.
Spectrum analyses expect all expanded modes to be in one load step. In the single point response spectrum (SPOPT,SPRS) and Dynamic Design analysis method (SPOPT,DDAM), the modal expansion can be performed after the spectrum analysis, based on the significance factor SIGNIF on the MXPAND command. If you want to perform modal expansion after the spectrum analysis use MXPAND,-1..
Leave SOLUTION. You can now review results in the postprocessor.
| Command(s): | FINISH |
| GUI: | Close the Solution menu. |
The expansion pass has been presented here as a separate step. However, if you include the MXPAND command in the modal solution step, the program not only extracts the eigenvalues and eigenvectors, but also expands the specified mode shapes.