Mechanical APDL Structural Analysis Guide

• SNODE method (MODOPT,SNODE)


• SUBSP method (MODOPT,SUBSP)

Mechanical APDL stores the calculated residual vectors in the Jobname.MODE file and uses them in
the subsequent mode-based analysis (e.g., mode superposition harmonic, mode superposition transient,
spectrum, or random vibration (PSD) analysis).

When using the residual vector method, set FREQB on the MODOPT command to zero or blank. Residual
vectors can not characterize the high frequency range if significant modes in the low frequency range
are not extracted.

Because the residual vectors are calculated before the modes are expanded, they are based on all the
extracted modes. As a result, a partial expansion driven by the MXPAND command will not affect the
residual vectors. If you do not expand the residual vectors, they will not be available on the results file
for post-processing.

3.9.1.2. Using the Residual Vector Method

Use the following procedure to calculate residual vectors:

1. Build the model.


2. Specify the mode-extraction method (MODOPT,LANB or MODOPT,LANPCG,MODOPT,SNODE, or MOD-
   OPT,SUBSP).


3. Activate residual vector calculation (RESVEC,ON).


4. Specify pseudo-constraints (D,,,SUPPORT) if rigid body motion is present.


5. Specify the load vectors (F,BF,SF, etc.).


6. Solve the modal analysis. (Mechanical APDL generat es a Jobname.MODE file containing the residual
   vectors.)


7. Issue a FINISH command.

The residual vector calculation can be performed during a modal analysis restart. Refer to Modal

Analysis Restart in the Mechanical APDL Basic Analysis Guide for more details.

8. Set up a mode-superposition transient,mode-superposition harmonic,PSD, or spectrum analysis, and
   include the previously calculated residual vectors (RESVEC,ON).

A load vector is also generat ed in Step 6. Ensure that you do not duplicat e any loading.

9. Solve the mode-superposition analysis. Mechanical APDL includes the residual vectors in those calculations.

Specifying Pseudo-Constraints

If rigid body motion exists, specify only the minimum number of displacement constraints necessary
to prevent rigid body motion: three constraints (or fewer, depending on the element type) for 2-D
models and six (or fewer) for 3-D models.

Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information

Modal Analysis Tools for Subsequent Mode Superposition Analysis