Mechanical APDL Structural Analysis Guide

4.3.2. The Mode-Superposition Method

The mode-superposition method sums factored mode shapes (eigenvectors) from a modal analysis to
calculate the structure's response. Its advantages are:

• It is faster and less expensive than the full method for many problems.


• Element loads applied in the preceding modal analysis can be applied in the harmonic analysis via
  the LVSCALE command.


• It allows solutions to be clustered about the structure's natural frequencies. This results in a
  smoother, more accurat e tracing of the response curve.


• Prestressing effects can be included.


• It accepts modal damping (damping ratio as a function of frequency).

4.3.3. Restrictions Common to Both Methods

Both methods are subject to certain common restrictions:

• All loads must be sinusoidally time-varying.


• All loads must have the same frequency.


• No nonlinearities are permitted.


• Transient effects are not calculated.

You can overcome any of these restrictions by performing a transient dynamic analysis, with harmonic
loads expressed as time-history loading functions.Transient Dynamic Analysis (p. 107) describes the
procedure for a transient dynamic analysis.

4.4. Performing a Harmonic Analysis

We will first describe how to perform a harmonic analysis using the full method, and then list the steps
that are different for the mode-superposition method.

4.4.1. Full Harmonic Analysis

The procedure for a full harmonic analysis consists of three main steps:

1. Build the model.


2. Apply loads and obtain the solution.


3. Review the results.

4.4.2. Build the Model

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

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Performing a Harmonic Analysis