Mechanical APDL Structural Analysis Guide

• For pressure excitation (where the pressure distribution was provided in the modal analysis), bring in
  the load vectors from the modal analysis (LVSCALE). You can use the scale factor to scale the particip-
  ation factors.

Note

You can apply base excitations only at nodes that were constrained in the modal ana-

lysis. If you applied the constraints using solid model constraints (DK), you must use

the same solid model commands in defining the MPRS excitation. Any loads applied

during the preceding modal analysis must be removed by deleting or zeroing them.

Command(s):D (or DK, or,DL, or DA), or SED for base excitation F (or FK) for nodal excitation LVSCALE

for pressure excitation

GUI: Main Menu> Solution> Define Loads> Apply> Structural> Spectrum> Base PSD Excit> On

Nodes

6. Begin participation factor calculations for the above MPRS excitation.

Use the TBLNO field to indicat e which spectrum table to use, and Excit to specify whether the

calculations are for a base or nodal excitation.

Command(s):PFACT

GUI: Main Menu> Solution> Load Step Opts> Spectrum> PSD> Calculate PF

If you need to apply multiple MPRS excitations on the same model, repeat steps 4, 5, and 6 above

for each additional spectrum table.

6.8.2. Step 5: Combine the Modes

This step is the same as step 3 described in Single-Point Response Spectrum (SPRS) Analysis Pro-
cess (p. 137); however, the absolute sum method (AbsSumKey=yes on the SRSS command) acts as an
additional combination method.

Note

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

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

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

ready for the new analysis.

6.8.3. Step 6: Review the Results

This step is the same as step 6 described in Single-Point Response Spectrum (SPRS) Analysis Pro-
cess (p. 137).

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

1. Expanded mode shapes from the modal analysis (loadstep 1)


2. Static solutions for base excitation (PFACT,,BASE) (loadstep 2)


3. Missing mass responses if requested (MMASS) (loadstep 3)

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Spectrum Analysis