Elastic and design spectrum and how the damping is taken into account
In SCIA, every spectrum introduced is taken into account as if the damping is 5%.
To have an elastic spectrum, q=1 should be used. To verify it, let’s go through the analysis of the damping factor used in both the elastic and the design spectra.
When applying the CQC, a damping factor can be assigned, which modifies and recalculates the spectrum to adapt it to the chosen factor.
Example: these are the modes of the structure:
Let’s create a spectrum:
And two seismic cases are created, one with damping 5% and one with 4%:
In the results, we can see that there the damping coefficient is different, and the Sax value is multiplied by it:
Now to confirm it, let’s add a spectrum calculated with a damping factor of 4%:
A load cases with CQC and damping factor of 5% is added:
Look at the results, and these are comparable to the analysis done for the load case with CQC with damping factor of 4% and a spectrum with damping factor 5%:
In conclusion, to have an elastic spectrum, it is sufficient to use q=1.