Mechanical APDL Structural Analysis Guide

1. Analyze a simpler model first. A model of beams, masses, and springs can provide good insight into
   the problem at minimal cost. This simpler model may be all you need to determine the dynamic
   response of the structure.


2. If you are including nonlinearities, try to understand how they affect the structure's response by
   doing a static analysis first. In some cases, nonlinearities need not be included in the dynamic ana-
   lysis.


3. Understand the dynamics of the problem. By doing a modal analysis, which calculates the natural
   frequencies and mode shapes, you can learn how the structure responds when those modes are
   excited. The natural frequencies are also useful for calculating the correct integration time step.


4. For a nonlinear problem, consider substructuring the linear portions of the model to reduce analysis
   costs. Substructuring is described in the Advanced Analysis Guide.

5.2. Two Solution Methods

Two methods are available to do a transient dynamic analysis:full and mode-superposition. The ANSYS
Professional program allows only the mode-superposition method. Before we study the details of how
to implement each of these methods, we will examine the advantages and disadvantages of each.

5.2.1. Full Method

The full method uses the full system matrices to calculate the transient response (no matrix reduction).
It is the more general method because it allows all types of nonlinearities to be included (plasticity,
large deflections, large strain, and so on).

Note

If you do not want to include any nonlinearities, you should consider using the mode-super-

position method because the full method is also the more expensive of the two.

The advantages of the full method are:

• It is easy to use, because you do not have to worry about choosing mode shapes.


• It allows all types of nonlinearities.


• It uses full matrices, so no mass matrix approximation is involved.


• All displacements and stresses are calculated in a single pass.


• It accepts all types of loads: nodal forces, imposed (nonzero) displacements (although not recommen-
  ded), and element loads (pressures and temperatures) and allows tabular boundary condition specific-
  ation via TABLE type arra y parameters.


• It allows effective use of solid-model loads.

The main disadvantage of the full method is that it is more expensive than the mode-superposition
method.

For procedural information about using the full method, see Performing a Full Transient Dynamic Ana-
lysis (p. 109).

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Transient Dynamic Analysis