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*MODAL DYNAMIC

Keyword type: step

This procedure is used to calculate the response of a structure subject to dynamic loading. Although the deformation up to the onset of the dynamic calculation can be nonlinear, this procedure is basically linear and assumes that the response can be written as a linear combination of the lowest modes of the structure. To this end, these modes must have been calculated in a previous *FREQUENCY step (not necessarily in the same calculation). In the *MODAL DYNAMIC step the eigenfrequencies, modes and mass matrix are recovered from the file jobname.eig. The time period of the loading is characterized by its total length and the length of an increment. Within each increment the loading is assumed to be linear, in which case the solution is exact apart from modelling inaccuracies and the fact that not all eigenmodes are used. The number of eigenmodes used is taken from the previous *FREQUENCY step. The dynamic loading is the static loading at the start of the step multiplied by the amplitude history for each load as specified by the AMPLITUDE parameter on the loading card, if any. Loading histories extending beyond the amplitude time scale are extrapolated in a constant way. The absence of the AMPLITUDE parameter on a loading card leads to a constant load.

First line:

- *MODAL DYNAMIC

Second line:

- Time increment.
- Total time period.

    Example:

    *MODAL DYNAMIC
    1.E-5,1.E-4

defines a modal dynamic procedure with time increment and time period .

Example files: beamdy1, beamdy2, beamdy3, beamdy4, beamdy5, beamdy6.

Next: *MPC Up: Input deck format Previous: *MODAL DAMPING Contents