Mechanical APDL Structural Analysis Guide

Command(s):/SOLU

GUI: Main Menu> Solution

2. Define the analysis type and analysis options:
   
   
   • For spectrum type [SPOPT], choose Power Spectral Density (PSD).
   
   
   
   

• If you are interested in element results and reaction forces, specify YES for Elcalc on the SPOPT
  command. Element results and reaction forces caused by the spectrum are calculated only if they were
  also requested during the modal expansion pass. Note that you must have asked for element results
  during the modal analysis as well (MXPAND)

3. Specify load step options. The following options are available for a random vibration analysis:
   
   
   • Spectrum Data
     
     
     • Type of PSD
       Command(s):PSDUNIT
       GUI: Main Menu> Solution> Load Step Opts> Spectrum> PSD> Settings
     
     
     
     
   
   
   
   

The PSD type can be displacement, velocity, force, pressure, or acceleration. Whether it is a

base excitation or a nodal excitation is specified in Steps 4 and 5. If a pressure PSD is to be

applied, the pressures should be applied in the modal analysis itself.

• PSD-versus-frequency table

Define a piecewise-linear (in log-log scale) PSD versus frequency table. Since a curve-fitting

polynomial is used for the closed-form integration of the curve, you should graph the input,

which is overlaid with the fitted curve to ensure a good fit. If the fit is not good, you should

add one or more intermediate points to the table until you obtain a good fit. For a good fit,

the PSD values between consecutive points should not change by more than an order of

magnitude.

Command(s): PSDFRQ,PSDVAL,PSDGRAPH

GUI: Main Menu> Solution> Load Step Opts> Spectrum> PSD> PSD vs Freq

Main Menu> Solution> Load Step Opts> Spectrum> PSD> Graph PSD Tables

PSDFRQ and PSDVAL are used to define the PSD-versus-frequency table. Step 6 describes how

to apply additional PSD excitations (if any). The maximum number of tables is 200.

You can issue STAT to list PSD tables and issue PSDGRAPH to graph them.

• Damping (Dynamics Options)

The following forms of damping are available:ALPHAD,BETAD, and MDAMP result in a frequency-

dependent damping ratio, whereas DMPRAT specifies a constant damping ratio to be used at all

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

Spectrum Analysis