Modal Participation

Analysis > Solve > Result Request > Modal Participation

Introduction

The Modal Participation tool allows users to request and visualize the contribution of each vibration mode to the structural response at specific frequencies.

It operates in two main stages:
  • Pre-processing: specifies the necessary data required to compute modal participation during the solver analysis.
  • Post-processing: enables visualization of the computed modal participation results using 2D graphs and bar charts.

Modal Participation

In a modal frequency response analysis, the dynamic behavior of a structure is expressed as a combination of its natural modes. Each mode is characterized by:
  • A natural frequency
  • A mode shape (the deformation pattern of the structure)
  • A modal mass and stiffness

When the structure is excited by harmonic loads, the total response results from the superposition of the contributions from all modes. However, not all modes contribute equally. Some modes dominate the response within certain frequency ranges, while others have little to no influence.

Modal participation helps identify these dominant modes and quantify their relative impact on the overall response at a given frequency.

Importance of Modal Participation

By analyzing modal participation, engineers can:
  • Identify the modes that have the greatest influence on displacement, velocity, or acceleration.
  • Understand whether modes act in-phase or out-of-phase with the total response.
  • Evaluate how modal behavior evolves across the excitation frequency range.
  • Support design decisions by focusing on the most significant modes to minimize vibration and noise issues.

Computation Methods Supported in SimLab

SimLab supports two methods to compute modal participation:
  1. Magnitude and
  2. Projected

Both methods quantify how each vibration mode contributes to the total response, but they differ in how they treat the phase information.

Magnitude Method

This method measures the overall strength of each modal contribution, regardless of phase. It indicates how strongly each mode participates in the response at a given frequency.

Characteristics
  • Evaluates the total amplitude of each modal contribution.
  • Ignores the phase relationship between the mode and the total response.
  • Easy to interpret and useful for ranking modes by relative importance.
Recommended use
  • For quick comparison between modes.
  • When the interest is in identifying the most energetic or dominant modes.

Projected Method

By projecting each mode onto the total response the projected method accounts for the phase relationship between each mode and the total response. It captures whether a mode acts in-phase or out-of-phase with the total response, providing a more physically meaningful interpretation.

Characteristics
  • Includes both magnitude and phase alignment information.
  • Positive values indicate in-phase modes, while negative values indicate out-of-phase modes.
  • Gives deeper insight into how individual modes influence the actual system behavior.
Recommended use
  • To understand whether individual modes act in-phase or out-of-phase with the total structural response. This helps determine whether a mode amplifies (in-phase) or attenuates (out-of-phase) the overall vibration at specific frequencies.
  • When analyzing resonance peaks or response sign changes observed in displacement, velocity, or acceleration results, to understand how modal phase relationships influence the total response.

Modal Participation setup

Accessing the Tool

User can access the Modal Participation tool from Analysis > Requests > Modal Participation.

This tool is available only in the following situations:
  • A Modal Frequency Response solution with load cases is defined, and one of the Modal Frequency Response load cases is set as Current.

Defining the Modal Participation Setup

  1. Name

    Enter a name for the Modal Participation request. This name will be used to identify the object in the Solution Browser.

  2. Select Nodes

    Pick the nodes where modal participation will be evaluated.

  3. Specify Included Modes
    Define which modes are considered in the participation calculation:
    • All – Participation factors will be computed for all calculated structural modes.
    • User defined – Specify a fixed number of modes. The selected modes will correspond to those with the largest magnitude of modal contribution.
  4. Select Result Type
    Choose the physical quantity to evaluate:
    • Displacement
    • Velocity
    • Acceleration
  5. Select Degrees of Freedom

    Specify the degrees of freedom (DOFs) along which modal participation will be computed.

  6. Confirm the Setup

    Click OK to create the Modal Participation request.

    A new object is added under the Requests section of the Result branch in the associated load case within the Solution Browser:

Editing a Modal Participation Request

To modify the definition of an existing Modal Participation request, simply double-click the object in the Solution Browser.

The panel will reopen, allowing user to adjust any of the previously defined parameters.

Visualizing the Results

After the analysis is complete, modal participation results can be reviewed directly within SimLab.

To visualize the results double-click on the Modal Participation object.

This opens the Modal Participation panel, which contains three distinct sections:

Definition Section

This section displays the original setup of the Modal Participation request.

It corresponds to the same panel used during the definition stage and allows user to review or modify the existing definition.

If the definition is modified after the analysis is complete, SimLab will display a warning message when clicking on OK. This message informs the user that the model setup has changed and that existing results will be deleted. In this case a new analysis must then be run to review the new modal participation results.

Result Selection Section

This section allows user to specify what result to visualize during post-processing:
  • Node: the node for which participation results are displayed.
  • Degree of Freedom: the degree of freedom to evaluate.
  • Method: select between Magnitude and Projected participation computation.
  • Frequency: the excitation frequency at which to evaluate modal participation.
This section also includes a result summary table:
  • For the Magnitude method, the table lists the magnitude values of modal participation for each mode.

    Modes are sorted from the highest to the lowest participation magnitude, allowing the user to quickly identify the dominant modes.

  • For the Projected method, it lists the projected values and the percentage contribution of each mode to the total response.

    Modes are sorted by the absolute value of their projected value onto the total response, from highest to lowest.

Note: the modal participation is only given for the 10 most significant modes.

Visualization Section

The right-hand side of the panel displays graphical representations of the selected results:
  • The upper plot shows the response curve (displacement, velocity, or acceleration) for the selected node and degree of freedom.
  • The lower plot displays a bar chart representing the modal participation for the 10 most contributing modes. The horizontal axis corresponds to the mode number, while the vertical axis represents:
    • The participation magnitude for the Magnitude method.
    • The percentage contribution for the Projected method.
  • The bar with abscissa 0 represents the total response, and the other bars represent the contribution of each individual mode.
  • The modes in the bar chart are sorted using the same logic as in the Result Summary Table.

Analyze Modal Participation results

To analyze modal participation results:
  1. Select the node and degree of freedom of interest.
  2. Choose the computation method (Magnitude or Projected).
  3. Specify the frequency to evaluate:
    • users can either type the frequency value manually, or use the sliding bar located below the input field.
    • Moving the sliding bar updates the frequency interactively and move the vertical line on the response curve to indicate the selected frequency.
    • This vertical line moves dynamically as user adjust the sliding bar.
    • By default, when the Modal Participation tool is opened for the first time, the vertical line is automatically positioned at the peak of the response curve, corresponding to the maximum response amplitude.
  4. Review the response curve and bar chart to identify dominant modes and their contributions.

OptiStruct Cards Exported

Solver Supported cards
OptiStruct – Modal Frequency Response subcases PFMODE (STRUCTURE, ALL, DISP) = SETID