Response Spectra in different Directions

[Settler]

Hello all,

I'm looking to apply response spectra in 3 different directions, however one is not a multiple of the other. As a result I have to define two different spectra.

Do I have to run the analysis two times and the combine the results, or is there a way to define two response spectra in two different directions.

If I have to run it two times and combine, how can I do that?

Thanks

 

[Drej]

If I understand you correctly then I can give you a approach that would generally be used in seismic analysis (although be aware that variations do exist). Typically you would have orthogonal spectra for each global direction (X, Y, Z). The "horizontal" spectra (X, Y) are often the same and the vertical direction (Z) spectra are different from those of the horizontal. Carry out a spectrum analysis for each global direction (subsequent to a modal analysis) and use a modal combination method which is applicable to you, of which there are several (CQC for example) - take care to choose one which is considered applicable. Once you have this, use a suitable spatial combination method (SRSS for example) to combine the spectrum results you obtained for each direction. This will give you a single seismic loadcase. then there will be consideration needed for any missing mass. Finally, combine the seismic / missing mass loadcase results with a static 1g analysis i.e seismic +/- 1g.


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