Square Root of the Sum of the Squares (SRSS)

Complete Quadratic Combination (CQC)

Rosenblueth Double Sum Combination (ROSE)

The response spectrum is calculated based on modal responses. A modal analysis is therefore a prerequisite.

If response strain/stress is of interest, then the modal strain and the modal stress need to be determined in the modal analysis.

The excitation is applied in the form of a response spectrum. The response spectrum can have displacement, velocity, or acceleration units. For each spectrum value, there is one corresponding frequency.

The excitation must be applied at fixed degrees of freedom.

Because a new solve is required for each requested output (displacement, velocity, and acceleration), the content of a Commands (APDL) object is limited to solution (SOLU) commands.

The results from the Mechanical APDL solver are displayed as the model’s contour plot. The results are in terms of the maximum response.

Supported boundary condition types include Fixed Support, Displacement, Remote Displacement and body-to-ground Spring. If one or more fixed supports are defined in the model, the input excitation response can be applied to all fixed supports.

A Remote Displacement cannot coexist with other boundary condition types, such as a Fixed Support or a Displacement, at the same location (scoping) as the excitation. The Remote Displacement will be ignored due to conflict with other boundary conditions.

Note that the All boundary condition types for Single Point Response Spectrum only includes those fixed degree of freedoms defined using Fixed Support, Displacement, Remote Displacement and Body-to-Ground Spring. To apply an RS load to All boundary condition types for Single Point Response Spectrum, at least one allowed boundary condition must be defined.

For a Single Point spectrum type, input excitation spectrums are applied to all boundary condition types defined in the model. For Multiple Points however, each input excitation spectrum is associated to only one boundary condition type.

Three types of input excitation spectrum are supported: displacement input excitation (RS Displacement), velocity input excitation (RS Velocity) and acceleration input excitation (RS Acceleration). See RS Base Excitation for further details.

The input excitation spectrum direction is defined in the global coordinate system for Single Point spectrum analysis. For Multiple Points spectrum analysis, however, the input excitation is defined in the nodal coordinate systems (if any) attached to the constrained nodes.

More than one input excitation, with any different combination of spectrum types, is allowed for the response spectrum analysis.

Specify option to include or not include contribution of high frequency modes in the total response calculation by setting Missing Mass Effect to Yes or No. The option for including the modes is normally required for nuclear power plant design.

Specify option to include or not include rigid responses to the total response calculation by setting Rigid Response Effect to Yes or No. The rigid responses normally occur in the frequency range that is lower than that of missing mass responses, but is higher than that of periodic responses.

Missing Mass Effect is only applicable to RS Acceleration excitation. See the RS Base Excitation section of the Help for more information.

For a Single Point spectrum type, the entire table of input excitation spectrum can be scaled using the Scale Factor setting. The factor must be greater than 0.0. The default is 1.0.

To view strain/stress results, a selection must be made in Output Controls of the Modal analysis. By default, only Deformation drop-down menu results are available.

Applicable Deformation results are Total, Directional (X/Y/Z), Directional Velocity and Directional Acceleration. If strain/stress are requested, applicable results are normal strain and stress, shear strain and stress, and equivalent stress.

Equivalent stress is a derived stress calculated using component stresses.

Results are displayed as a contour plot on the model.

For contour results, the Information category includes the following properties. These properties are calculated after the associated result is evaluated and they refer to results contained in the results (file.rst) file.

Example Details Properties

Property Output Descriptions

You can also see the Review the Results topic in the Performing a Random Vibration (PSD) Analysis section in the Mechanical APDL Structural Analysis Guide for additional information.

If you set the Store Modal Result property of the Output Controls to No in the upstream Modal analysis, the application creates additional auxiliary files in addition to standard files. The application automatically creates the Displacement.mcom file. And, if you set the Calculate Velocity and Calculate Acceleration properties (also Output Controls) of the downstream spectrum analysis to Yes, then the application also creates the files Velocity.mcom and/or Acceleration.mcom. These files contain combination instructions as well as mode coefficients.

Force Reaction and Moment Reaction probes can be scoped to a Remote Displacement, Fixed Support, or Displacement boundary conditions to view Reactions Results.

When the Missing Mass Effect property is set to Yes, the Deformation results that include the data from property in their result calculation are the Directional (Deformation/Displacement) and Directional Acceleration results. Note that the application supports the Directional Velocity result; however, it does not incorporate Missing Mass Effect conditions for its calculation.

The Directional (Deformation/Displacement) result is a relative result whereas Directional Velocity and Directional Acceleration are absolute results.

A Directional Acceleration result at or near fixed supports scoped to an RS Acceleration are correctly calculated by setting the Missing Mass Effect property and the Rigid Response Effect property to Yes. Corresponding properties display and require definition when these two properties are included.