Mechanical APDL Structural Analysis Guide

The SIGNIF and COMODE fields on the PSD mode combination method [PSDCOM] offer options

to reduce the number of modes to be combined (see the description of PSDCOM command).

If you want to exercise these options, it is prudent to print the modal covariance matrices in

Obtain the PSD Solution (p. 159) to first investigate the relative contributions of the modes toward

the final solution.

4. Start the solution.
   Command(s):SOLVE
   GUI: Main Menu> Solution> Solve> Current LS


5. Leave the SOLUTION processor.
   Command(s):FINISH
   GUI:Close the Solution menu.

Note

You can run multiple PSD analyses without performing the modal analysis each time.

To do so, you must activate modeReuseKey on the SPOPT command after the first

PSD analysis and for each subsequent one so that the database and necessary files are

ready for the new analysis.

6.6.3. Review the Results

Results from a random vibration analysis are written to the structural results file,Jobname.RST. They
consist of the following quantities:

1. Expanded mode shapes from the modal analysis


2. Static solution for base excitation [PFACT,,BASE]


3. The following output, if mode combinations are requested [PSDCOM] and based on the PSDRES
   setting:
   
   
   • 1 σ displacement solution (displacements, stresses, strains, and forces)
   
   
   
   

• 1 σ velocity solution (velocities, stress velocities, strain velocities, and force velocities)


• 1 σ acceleration solution (accelerations, stress accelerations, strain accelerations, and force accel-
  erations)

1 σ is the standard deviation of the response; that is, for any output value the expectation is that this
value will not be exceeded 68.3% of the time.

Only component displacement, force, stress, and strain values are 1 σ values and follow a Gaussian

or normal distribution. Combined values (e.g. USUM, SI, SEQV, S1, etc.), or component values trans-

formed into another coordinate system are not statistically meaningful, and they should be avoided.

The exception is the SEQV value, for which a special algorithm is used to compute its value, such

that multiplying it by 3 (the "3 σ" rule) yields a good approximation to its upper bound, see Equivalent

Stress Mean Square Response in the Mechanical APDL Theory Reference. SEQV is not computed for

beam and pipe elements.

You can review these results in POST1, the general postprocessor, and then calculate response PSDs in
POST26, the time-history postprocessor.

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

Spectrum Analysis