Mechanical APDL Structural Analysis Guide

3.9.6.3. Mode Selection Based on the Mode Coefficients

The mode selection is performed during a modal analysis restart. It is applicable to real eigensolvers
only. This mode selection method is used when a Single Point Response Spectrum (SPRS) analysis is
performed following the modal analysis.

You can select the modes using a significance threshold (SIGNIF on the MXPAND command). The
significance level of a mode is equal to the mode coefficient divided by the maximum mode coefficients
(of all modes per direction). The threshold is the same for all directions.

A sample input listing of the procedure follows:

! Build the Model

/filnam, ...

/title, ...

/prep7

...

finish

! Obtain the Modal Solution

/solu

antype, MODAL

modopt, LANB, ...! Block Lanczos

solve

finish

! Obtain the SPRS Mode Coefficients

/solu

antype, SPECTRUM

spot, SPRS

svtyp, 2, 386.4! Input spectrum type

sed, 1, 0, 0! Input spectrum direction

freq, 1, 2, 5, 15, 30! Input spectrum frequencies

sv,, 0.1, 0.2, 0.2, 0.05, 0.05! Input spectrum values

solve

finish

! Perform the modal selection and expansion

/solu

antype, MODAL, RESTART

mxpand,,,, yes, 1e-2,, MODC! Mode selection is based on the mode coefficients, the significance

! threshold is 0.01 and element results are requested

solve

finish

! Perform a spectrum analysis (combination step)

/solu

antype, SPECTRUM

spopt, SPRS! Single Point Response Spectrum analysis

srss! Square Root of Sum of Square combination

solve

finish

For multiple input spectra in a Single Point Response Spectrum, to obtain the mode coefficients in the
spectrum analysis, the spectrum input definitions are requested, and each one will be followed by a
SOLVE command.

Note

This procedure is not supported for Multiple Point Response Spectrum (MPRS) and Power

Spectrum Density (PSD) analyses. However, you can use a dummy SPRS input spectrum to

perform the mode selection for the subsequent MPRS and PSD analysis, or else use the mode

selection based on the modal effective mass.

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Modal Analysis