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Modal analysis within Sumo

A modal analysis is performed to determine a structure’s natural modes of vibration. A modal analysis considers the effect of vertical loads only. No other loads than those selected to contribute to the dynamic mass are considered during the analysis. You can specify the number of mode shapes to be calculated, as well as the loadcases which will contribute to the dynamic mass of the model.

Within the Loadcase Input Table, the Mass Case tick boxes are used to indicate which of your loadcases will contribute to the total dynamic mass – as seen within Figure 1 below. Only the selected mass cases will influence the analysis. The loadcases that are not selected as mass cases, will therefore, not form part of your modal analysis.

Loadcase Input Table

Within the analysis settings, further parameters can be specified for the modal analysis, as seen within Figure 2 below:

Figure 2 : Modal Analysis Settings

The number of mode shapes specified will improve the accuracy of the analysis up to a certain point, whereafter an increase in the amount of mode shapes, won’t have a meaningful impact on the results.

An analysis is deemed to have converged once the total result of two sequential iterations differs by less than the specified solution tolerance.

Axial forces in members, reduces the effective stiffness of those members. Enable Second-order effects to consider the reduced stiffness of the model.

With the solution of the eigen value problem, some modes include beam elements that rotate about their longitudinal axis. This is important in certain types of structures, but in most building structures those modes are not desired. The inclusion of torsional modelling applies for beam elements. If included in the analysis, torsional modes of beam elements will be included in the analysis and results.

Sumo determines the dynamic mass of the structure by considering the weight of all structural members, as well as the vertical components of all mass cases. All the mass cases selected within the Loadcase Input Table, will be displayed within this dialog box, as seen within Figure 3 below. this table lists the load cases that are converted to mass. More mass tends to decrease the frequencies of the natural mode shapes.

Figure 3 : Load Cases Contributing to Mass

The output that will be received from the modal analysis, will consist of the various mode shapes, indicating the analyzed structure’s behavior under natural vibration.

Typical Workflow

  1. Set analysis type to “Modal”, whereafter the loadcases which contributes to the total dynamic mass of the analysis, should be specified within the Loadcase Input Table.
  2. Set the analysis settings and requirements as specified above, according to the design requirements and run the analysis.
  3. The analysis will use the specified mass cases and combine all vertical components into a dynamic mass. The analysis will give the resultant frequency of the calculated mode shapes for the structure.