The modal model

Response Dynamics uses the modes calculated by the response dynamics solution as the modal model for response calculations. In Response Dynamics, the equations of motion are written in terms of the modal degrees of freedom, and the physical responses are recovered from modal responses.

Note:

This topic assumes you are using the SOL 103 - Response Dynamics solution. You can also use the SOL 103 Real Eigenvalues solution, but it does not generate the attachment and constraint modes necessary for many of the Response Dynamics features.

Sample normal mode shapes

Dynamic modes

After you solve the solution, the solved modal model may include one or more of the following modes:

• Normal (real) modes represent the deformed shapes (eigenvectors) of the structure at specific natural frequencies (eigenvalues). After you solve the solution, you can view the normal mode shapes on the Post Processing Navigator. For more information, see View the normal mode shapes.

• Constraint modes represent the static deformation due to a unit static enforced motion of a boundary degree of freedom. They are required for transient, frequency, random, and response spectrum analysis when enforced motion (displacement, velocity, or acceleration) excitations are applied. When you solve the solution, the number of constraint modes generated is based on the number of nodes and enforced degrees of freedom in the enforced motion location you defined on the model. For example, if your enforced motion location has 5 nodes and DOF2 and DOF3 are set to Enforced, 10 constraint modes are generated.

• Attachment modes represent the static deformation shapes due to unit static loads applied at locations where excitation forces are applied. Attachment modes are used with the Mode Acceleration data recovery method in transient and frequency response analysis. When you solve the solution, for each node in the model's nodal force location, the solver generates an attachment mode for each DOF set to Nodal Force. For example, if your nodal force location has 5 nodes and DOF2 and DOF3 are set to Nodal Force, 10 attachment modes are generated.Note: Frequency response analysis generally does not need attachment modes. However, using them may increase the analysis accuracy at the frequencies away from the resonant frequencies, if you calculate a conservative number of normal modes.

• Distributed attachment modes represent the structural behavior due to a load set (a distributed load or point load). They are required for calculating mode acceleration responses to distributed dynamic excitations in transient and frequency analysis. They can also be used in a Quasi-Static event to define a static excitation. When you solve the solution, a distributed attachment mode is generated for each load defined in the Dynamics subcase for the solution.Note: You can view the distributed attachment modes by right-clicking the Modal Representation node and choosing Information.

After you create the Response Dynamics solution process, the generated modes are listed in the Simulation Navigator.

Response Dynamics 1 Solution 1 Modal Representation Normal Modes [10] Constraint Modes [5] Attachment Modes [0] Sensors

The number of modes generated is shown after each mode type. You can right-click each mode node and choose Quick View to view a contour plot of each generated mode. For Normal modes, you can view the details of each mode in the Response Dynamics Details View subpanel. For more information, see Response Dynamics Details View subpanel (Simulation Navigator).

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View the normal mode shapes

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The modal model, Simcenter 3D 2021.1 Series

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