Two methods are available for performing a transient dynamic analysis: full and mode-superposition.
The full method uses the full system matrices to calculate the transient response (no matrix reduction). It is the more general method because it enables 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-superposition method because the full method is also the most 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 accommodates 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 recommended), and element loads (pressures and temperatures) and accommodates tabular boundary condition specification via TABLE type array parameters.
It enables effective use of solid-model loads.
The main disadvantage of the full method is that it is more computationally intensive than the mode-superposition method.
For procedural information about using the full method, see Performing a Full Transient Dynamic Analysis.
The mode-superposition method sums factored mode shapes (eigenvectors) from a modal analysis to calculate the transient 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 transient dynamic analysis via the LVSCALE command.
It accepts modal damping (damping ratio as a function of mode number).
The disadvantages of the mode-superposition method are:
The time step must remain constant throughout the transient, so automatic time stepping is not allowed.
Nonlinearities are not allowed.
It does not accept imposed (nonzero) displacements.
The calculation of the element results in a mode-superposition transient analysis for large models (large number of modes and/or large number of degrees of freedoms) and possibly having a large number of time steps can be time consuming. The most effective method is to combine the modal element results directly during the expansion pass (MXPAND,ALL,,,YES,,YES). For details, see Option: Number of Modes to Expand (MXPAND).
For procedural information about using the mode-superposition method, see Performing a Mode-Superposition Transient Dynamic Analysis.