Mechanical APDL Structural Analysis Guide

model. In a multi-point random vibration analysis, you specify different PSD spectra at different points
in the model.

6.1.4. Deterministic vs. Probabilistic Analyses

Response spectrum and DDAM analyses are deterministic analyses because both the input to the analyses
and output from the analyses are actual maximum values. Random vibration analysis, on the other
hand, is probabilistic in nature, because both input and output quantities represent only the probability
that they take on certain values.

6.2. Single-Point Response Spectrum (SPRS) Analysis Process

The general process for performing a single-point response spectrum analysis consists of four primary
steps:

• St ep 1: Build the Model (p. 137)


• St ep 2: Obtain the Modal Solution (p. 137)


• St ep 3: Obtain the Spectrum Solution (p. 138)


• St ep 4: Review the Results (p. 143)

The process for running multiple spectrum analyses can be found in:

• Running Multiple Spectrum Analyses (p. 145)

If you wish to perform mode selection that includes and expands only the significant modes in a single-
point response spectrum analysis, refer to Mode Selection Based on the Mode Coefficients (p. 74).

6.2.1. Step 1: Build the Model

See Building the Model in the Basic Analysis Guide. For further details, see the Modeling and Meshing
Guide.

6.2.1.1. Hints and Recommendations

• Only linear behavior is valid in a spectrum analysis. Nonlinear elements, if any, are treated as linear. If you
  include contact elements, for example, their stiffnesses are calculated based on their initial status and are
  never changed.


• Both Young's modulus (EX) (or stiffness in some form) and density (DENS) (or mass in some form) must
  be defined. Material properties can be linear, isotropic or orthotropic, and constant or temperature-de-
  pendent. Nonlinear properties, if any, are ignored.


• You can define damping using damping ratio, material damping and/or proportional damping. For more
  details about damping definition, see Damping (p. 2).

6.2.2. Step 2: Obtain the Modal Solution

The modal solution is required because the structure's mode shapes and frequencies must be available
to calculate the spectrum solution.

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Single-Point Response Spectrum (SPRS) Analysis Process