> For the complete documentation index, see [llms.txt](https://docs.enercalc.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.enercalc.com/e3d-user-manual/analysis/response-spectrum-analysis2.md).

# Response Spectrum Analysis

How to perform response spectrum analysis with CQC, SRSS, or ABSSUM modal combination in ENERCALC 3D.

Analysis > Response Spectrum Analysis prompts you with the dialog shown in the screen capture below.

![](https://560001759-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FWNe8U6EBBBdUJHdb2X9G%2Fuploads%2Fgit-blob-927bb484b4bbf8a6afff8a8301eb0be4d5643ed7%2FResponse%20Spectrum%20Analysis.jpg?alt=media)

It allows you to perform response spectrum analysis in global X, Y and/or Z directions. There are three mode combination methods available in the program: CQC (complete quadratic combination), SRSS (Square root of sum of squares) and ABSSUM (absolute sum). CQC method for modal combination is applicable to a wider class of structures and is therefore recommended method. Critical damping ratio (0 <= damp < 1.0) affects CQC results. When critical damping ratio is 0, CQC method is the same as SRSS method.

You must first run from Analysis | Frequency Analysis prior to running this command. Response spectrums can be defined from Loads | Response Spectra Library or Input Data | Response Spectra Library. Inertia forces in global direction X, Y or/and Z from response spectrum analysis will be calculated and then converted to nodal loads. These nodal forces will be placed in respective load cases such as INERTIA\_LOADCASE\_X\_MODE\_1, INERTIA\_LOADCASE\_X\_MODE\_2 etc. Existing loads in these load cases will be deleted prior to the load conversion. In addition, response spectrum load combinations INERTIA\_LOADCOMB\_X\_MODE\_1, INERTIA\_LOADCOMB\_X\_MODE\_2 etc. will be created or recreated. Static analysis will be performed on spectrum load combinations (as well as normal user-defined load combinations) automatically. Modal combinations will be subsequently calculated for results such as displacements, forces and stresses etc. using CQC, SRSS or ABSSUM on the response spectrum load combinations. Normally, modal combination results are all positive due to the sign lost during SRSS, CQC and ABSSUM procedures. However, you can choose to use signage for modal combination results based on the dominant mode (with maximum participation factor) in each global direction.

Modal combination results is done in each global direction first. Using directional factors, these directional modal results will be combined into final modal combination results, which can be added to any user-defined load combination results if non-zero response spectrum load factor is specified in the load combination definition (see Create > Loads > Load Combinations).


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