Modal Analysis

 

Modal analysis is used for the study of vibration modes of a structure. Using this analysis type, the responses of a structure exposed to dynamic actions (seism or other), such as resonance frequencies or modal deformations, are obtained.

When a seism family is created, a modal analysis is generated along with the Modes case.

Note

For the definition of seismic actions, the parameters of the seism family, seismic load cases, and the modal case, (available in their properties window) must be defined.

The modal analysis parameters are defined in the properties window of the Modal load case.

For a dynamic study of the structure, the mass must be defined prior to the generation of the modal analysis. The mass options are also configured in the modal load case properties window.

 

How to create a modal analysis

At the creation of a seismic family, a modal analysis family and a modal case are automatically generated in the Project Browser, under the Settings group.

Except when a seismic action is defined, a modal analysis is created as follows:

 A modal analysis family and a modal case are created in the Project Browser, under Settings.

Modal analysis

Modal Analysis family

 

How to define the static load combinations for the modal analysis

  1. In the properties window of the Modal load case, Masses category, Definition drop-down list, select Masses obtained by combining static loads.

  2. In the field below, click Select the cases > to open the Combinations dialog box and define the static load combinations for the modal analysis.

Static load combinations

Modal analysis - Combinations

 

  1. From the Available analyses list, select the cases to add.

Tip

  1. Enter the coefficient value for the selected items in the Coefficient field.

  2. Click Add to place the selected cases in the Selected analyses list.

  3. Click OK to apply and close.

 

How to set custom damping values

It is possible to set custom damping values.

  1. In the modal load case properties window, Damping for seism category, disable the automatic calculation.

  2. Click the Imposed damping cell and click to open the Damping analysis dialog box.

Damping analysis

Damping values

 

  1. Define the damping values for all modes or only for the selected mode, using one of the following methods:

    • Directly input the custom values for each mode in the Damping (%) column (press <Enter> to validate).

    • Click the Modify button to open the Modify Analysis dialog box and input the damping values for the selected mode, or for all modes.

Modify analysis

 

Selected modes: when enabled, the damping value is applied only on the selected modes.

All modes: when enabled, the damping value is applied on all modes.

Damping: defines the damping value.

 

How to delete a modal analysis

 

The selected analysis is deleted.

 

 

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