Spectrum Scaling of Modes

When a modal dynamic analysis is carried out the results are normalised to either a unit displacement or unit modal mass depending on the selection made by the user when creating the task. For response spectrum analysis these mode shapes are scaled to predict displacement values but the CQC combination procedure then combines these results across all the modes. It is useful to be able to recover directly the scaled modal values. This dialog creates a set of combination cases that correspond to the modal case scaled by the modal multiplier.

$$\alpha_i = f_{part_i}\frac{a_{spectral}}{(2\pi f)^2}$$

So that for each mode there is a combination case of the form $α_iAm$.

Response spectrum

The response spectrum is used to define the spectral acceleration. This may be different for horizontal and vertical excitation, and will be affected by the participation factor, so the direction must also be specified.

Modal task

Any dynamic modal (or Ritz) task with results is made available. The process will then generate a set of combination cases which scale the modal values according to the response spectrum and direction.