Mechanical APDL Structural Analysis Guide

Table 6.3: Solution Items Available in a PSD Analysis (p. 163) shows a summary of the possible solution

sets. To limit the amount of data written to the results file, use OUTRES at the mode expansion

step.

Table 6.3: Solution Items Available in a PSD Analysis

Solution Items Form

Displacements, stresses, strains, Relative, absolute, or neither

forces

Displacement Solution (label

DISP on PSDRES)

Velocities, stress velocities, force Relative, absolute, or neither

velocities, etc.

Velocity Solution (label VELO on

PSDRES)

Accelerations, stress accl's, force Relative, absolute, or neither

accl's, etc.

Acceleration Solution (label

ACEL on PSDRES)

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


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

6.6.2. Combine the Modes

The modes can be combined in a separat e solution phase. A maximum of 10000 modes can be combined.
The procedure is as follows:

1. Enter Solution.
   Command(s):/SOLU
   GUI: Main Menu> Solution


2. Define analysis type.
   
   
   • Option: New Analysis [ANTYPE]
   
   
   
   

Choose New Analysis.

• Option: Analysis Type: Spectrum [ANTYPE]

Choose analysis type spectrum.

3. Only the PSD mode combination method is valid in a random vibration analysis. This method triggers
   calculation of the one-sigma (1 σ, the standard deviation of the response, see Review the Res-
   ults (p. 164) below) displacements, stresses, etc., in the structure. If you do not issue the PSDCOM
   command, the program does not calculate the one-sigma response of the structure. You can also
   specify the type of modal forces to be used in the combination.ForceType=STATIC (default)
   combines the modal static forces (i.e., stiffness multiplied by mode shape forces, both of which are
   stress-causing forces) while ForceType=TOTAL combines the summed modal static forces and in-
   ertia forces (i.e., stiffness and mass forces, both of which forces are seen by the supports).
   Command(s):PSDCOM
   GUI: Main Menu> Solution> Load Step Opts> Spectrum> PSD> Mode Combin

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Performing a Random Vibration (PSD) Analysis