but you can specify any number of solutions from 2 through 20. (Any value over this range defaults
to 10 and any value below this range defaults to 4.)- Damping in some form should be specified; otherwise, the response will be infinity at the resonant
frequencies.ALPHAD and BETAD result in a frequency-dependent damping ratio, whereas DMPRAT
specifies a constant damping ratio to be used at all frequencies.MDAMP specifies a damping ratio
for each individual mode.DMPSTR specifies a constant structural damping coefficient. See Damp-
ing (p. 2) for further details.
- By default, if you used the Block Lanczos, PCG Lanczos, Supernode, Subspace, or unsymmetric option
for the modal analysis (MODOPT,LANB; LANPCG; SNODE; SUBSP; or UNSYM), the modal coordinates (the
factors to multiply each mode by) are written to the file Jobname.RFRQ and no output controls apply.
If however you explicitly request not to write the element results to the Jobname.MODE file (MX-
PAND,,,,,,NO), the actual nodal displacements are written to the .RFRQ file. In that case, you may use a
nodal component with the OUTRES,NSOL command to limit the displacement data written to the reduced
displacement file Jobname.RFRQ. The expansion pass will only produce valid results for those nodes
and for those elements in which all of the nodes of the elements have been written to the .RFRQ file.
To use this option, first suppress all writing by invoking OUTRES,NSOL,NONE, then specify the item(s)
of interest by invoking OUTRES,NSOL,ALL,component. Repeat the OUTRES command for any additional
nodal components that you want to write to the .RFRQ file. - Save a copy of the database.
Command(s):SAVE
GUI: Utility Menu> File> Save as - Start solution calculations.
Command(s):SOLVE
GUI: Main Menu> Solution> Solve> Current LS - Repeat steps 3 to 7 for any additional loads and frequency ranges (that is, for additional load steps). If
you plan to do time-history postprocessing (POST26), the frequency ranges should not overlap from one
load step to the next. - Leave SOLUTION.
Command(s):FINISH
GUI:Close the Solution menu.
The mode-superposition harmonic solution is written to the reduced displacement file,Jobname.RFRQ,
regardless of whether the Block Lanczos, PCG Lanczos, Supernode, Subspace, QR damped, or unsymmetric
method was used for the modal solution.You will therefore need to expand the solution if you are inter-
ested in stress results.
4.8.3. Expand the Mode-Superposition Solution
The expansion pass starts with the harmonic solution on Jobname.RFRQ and calculates the displace-
ment, stress, and force solution.These calculations are done only at frequencies and phase angles that
you specify. Therefore, before you begin the expansion pass, you should review the results of the har-
monic solution (using POST26) and identify the critical frequencies and phase angles.
An expansion pass is not always required. For instance, if you are interested mainly in displacements
at specific points on the structure, then the displacement solution could satisfy your requirements.
However, if you want to determine the stress or force solution, then you must perform an expansion
pass.
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