- If you do not want to use rigid body (0 frequency) modes, use MINMODE on the TRNOPT command
to skip over them.
- Nonlinear options (NLGEOM,NROPT) are not available.
- Define gap conditions, if any. They can only be defined between two nodal degrees of freedom
(DOF) or between a nodal DOF and ground.If you used the QR damped mode-extraction method, gap
conditions are not supported. More details about gap conditions are presented in the GP command
description.
Command(s):GP
GUI: Main Menu> Solution> Dynamic Gap Cond> Define
- Apply loads to the model. The following loading restrictions apply in a mode-superposition transient
dynamic analysis:
- Only nodal forces (F) and accelerations applied via the ACEL command are available.
Note
For consistent reaction forces, apply accelerations in the modal analysis rather
than in the transient analysis.
- A load vector created in the modal analysis can be included via the LVSCALE command
(Main Menu> Solution> Define Loads> Apply> Load Vector> For Mode Super) to apply the
load vector from the modal solution. Use a zero scale factor on the LVSCALE command to suppress
a load vector at a particular load step. You can use such a load vector to apply element loads
(pressures, temperatures, and so on) on the model. If you use LVSCALE, ensure that all nodal
forces (F) defined in the modal analysis solution are removed in the transient analysis. Generally,
you should apply nodal forces in the transient part of the analysis.
- If the transient excitation is coming from support motion and if you requested the pseudo-static
modes in the modal analysis (MODCONT command), you can use the DVAL command to specify
the enforced displacements and accelerations. Imposed nonzero displacements (D command) are
ignored.
Multiple load steps are usually required to specify the load history in a transient analysis. The
first load step is used to establish initial conditions, and second and subsequent load steps are
used for the transient loading, as explained next.
- Establish initial conditions. In modal superposition transient analyses, a first solution is done at TIME
= 0.This establishes the initial condition and time step size for the entire transient analysis. Generally,
the only load applicable for the first load step is initializing nodal forces. For this pseudo-static
analysis, the mode-superposition method may yield poor results at TIME = 0 if nonzero loads are
applied.
The following load step options are available for the first load step:
Table 5.2: Options for the First Load Step: Mode-Superposition Analysis
Option Command GUI Path
Dynamics Options
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Performing a Mode-Superposition Transient Dynamic Analysis
