Response Spectrum

Engineers use response spectrum (modal superposition) analysis to determine the structural response to sudden forces or shocks such as an earthquake.

During an earthquake, violent shaking is transmitted into a structure at the points where it is attached to the ground. To help engineers design sound structures, response spectrum (modal superposition) analysis is used with information gathered by instruments from past earthquakes. An engineer can determine how a structure would react to a past real-world earthquake and consider it in any new design.

Response spectrum (modal superposition) analysis can also help engineers design components for nuclear power plants, such as nuclear reactor parts, pumps, valves, piping, and condensers. The response spectrum (modal superposition) analysis uses formulae recommended by the U.S. Nuclear Regulatory Commission.

Note: Response spectrum (modal superpositon) uses the results from a modal analysis and operating systems create files with different formats. So, you must perform the modal analysis and response spectrum analysis on the same operating system. (Technically, the endian determines the file format. Any combination of operating systems using the same endian can be used for both analyses.)

The general steps in performing a response spectrum analysis are as follows:

Set up the model for a Natural Frequency (Modal) analysis or Natural Frequency (Modal) with Load Stiffening. The calculated mode shapes are combined during the response spectrum analysis. The number of frequencies calculated in the modal analysis should be sufficient to excite the model in the appropriate directions. (Generally, the cumulative mass in the excited directions should be above a certain percentage, such as 80% although design codes can dictate a different minimum.)
Run the natural frequency (modal) analysis.
Copy the model to a new design scenario. (Right-click the current design scenario heading in the tree view and select Copy.)
Change the analysis type of the new design scenario to response spectrum. (Analysis Change Type Linear Response Spectrum)
Enter the spectrum and parameters in the Analysis Parameters dialog box.
1. Indicate which design scenario has the modal results.
2. Specify the spectrum and directions. The spectrum load is transmitted to the model through the appropriate boundary conditions. (Spectrum in the X direction is transmitted through any boundary conditions that have X translation fixed, and so on.)
3. Other types of loads applied to the model have no affect on the analysis.
Perform the analysis (Analysis Analysis Run Simulation).
When the analysis is complete, review the results in the Results environment. Use the Results Options Analysis Specific Resultant to toggle between the resultant and the response to each natural frequency. Use the Results Options Load Case Options Load Case. to view the response at each natural frequency. When the spectrum is assigned to multiple directions, use the Results Options Analysis Specific Spectrum Component to choose which direction to view. Keep in mind that the displacement results are relative to the ground excitation (input spectrum).