In the Selection of Modes tab, you can specify which mode shapes are relevant for the response spectrum analysis of the selected load case. Since the analysis is based on a multimodal response spectrum method, you can select as many shapes as are required for the design.
Info
If there are no results of the modal analysis, the table is empty. You can then use the
button to determine the shapes without leaving the dialog box tab.
Mode Activation
The "Mode Activation" table lists the natural periods T with the corresponding accelerations Sa of the response spectrum and the
effective modal mass factors
fme that exist in the X, Y, and Z directions. The response spectrum of the selected table row is shown as a diagram in the section below. The information in the table and in the diagram are preset in relation to the natural period (the
button in the diagram).
In the table, select the mode shapes to be used for the response spectrum analysis. All shapes are preset
. You can define your selection manually by activating or deactivating the check box of a mode shape.
The buttons below the table facilitate the selection of the mode shapes. The buttons have the following functions:
| Button | Function |
|---|---|
|
Select all mode shapes |
|
Deactivate all mode shapes |
|
Deactivate all mode shapes with an effective modal mass factor less than the value that can be specified in the text box on the right |
In the chapter Natural Frequencies , you can find more information on how to determine the effective modal mass factors.
Diagram
The lower section shows the response spectrum as a diagram. If you select a table row, the associated values are displayed in the graph. This way, you can quickly check in which area of the response spectrum the mode shape is.
To switch between the diagrams of the excitation directions of excitation in X, Y, and Z, use the buttons to the right of the graphic, as described in the chapter Response Spectrum .
Click the
button to switch the diagram to the display of the natural frequencies f (see the chapter
Response Spectrum
). This also affects the table.
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The values Sa in the table and in the diagram are affected by the scaling factor entered in the "Response Spectrum" for each excitation direction. If the response spectrum is rotated by an angle α, the directions X and Y given in the "Mode Activation" table refer to the corresponding rotated local directions.
Damping for CQC Combination Rule
For the CQC rule, the Lehr's damping values Di are required, which can be defined identically (constant) or differently for each mode shape of the model. If you have specified in the Spectral Analysis Settings that the damping values are "different for each mode", it is necessary to specify them separately in the "Damping" column.
