Mechanical APDL Structural Analysis Guide

3.4.2.4. Option: Number of Modes to Extract (MODOPT)

This option is required for all mode-extraction methods.

For the unsymmetric and damped methods, requesting a larger number of modes than necessary reduces
the possibility of missed modes, but results in more solution time.

3.4.2.5. Option: Number of Modes to Expand (MXPAND)

The MXPAND command specifies which extracted modes to expand (i.e., which extracted modes to
write to the results file) and how to compute element quantities (e.g., stress, strains, forces, energies,
etc.) for subsequent postprocessing or for downstream mode-superposition analyses (e.g., spectrum,
PSD, transient, or harmonic).

You can specify the following methods of mode expansion using the MXPAND command:

• Input the number of modes to expand (NMODE).


• Select the modes using a table (input using NMODE).


• Select the modes based on their modal mass or mode coefficient (input as ModeSelMethod). Refer to
  Using Mode Selection (p. 71) for more information.


• Specify a frequency range to extract (FREQB,FREQE).

In a mode-superposition analysis, the DOF results can either be combined in the expansion pass and
the element results calculated from the combined DOF solution, or the stresses can be combined directly
during the expansion pass to reduce computation time. This behavior is controlled by MSUPkey at the
modal analysis level.

If MSUPkey is set to YES (MXPAND,ALL,,,YES,,YES), the element results are written to the mode file and
then directly combined during the expansion pass. This option is the default setting. It will reduce the
computation time, but it has the following limitations for the downstream mode-superposition analyses:

• Strain and kinetic energies are not available.


• Equivalent strains are evaluated using EFFNU=0.0. Use AVPRIN to set to the appropriate Poission's ratio
  value.


• Thermal loads are not supported.


• NMISC results are not available.


• Superelements are not supported (Mechanical APDL automatically sets MSUPkey to NO).


• To include the damping and inertia contributions for reactions in a harmonic analysis, the element nodal
  loads must be output (OUTRES,NLOAD,ALL). Otherwise the reactions will only reflect the static contributions.


• The damping element forces and reaction forces are not available in a mode superposition harmonic
  analysis based on QRDAMP eigensolver.


• Strains and stresses of PIPE elements are not supported when pressure load is applied.

If MSUPkey is set to NO (MXPAND,ALL,,,YES,,NO), the element results are not written to the mode file.
For subsequent downstream mode-superposition analysis, the DOF results are combined in the expansion

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Applying Loads and Obtaining the Solution