Linear Dynamic Analysis (LDA) - Modal Combination

[ponczek]

Hi,

I would like to ask how the modal responses are combined and if I can control the combination. I have not found any setting in SeismoBuild for this.

I am mainly interested in two aspects:

(1) Combination rule (SRSS, CQC, ABSSUM) - what is used and how can I change it?

(2) Modes used for combination - Are all modes used? Or only those with effective masses greater than 5%? How can I select the modes to be considered?

Thanks in advance

[huffte]

It seems that you want to run a dynamic response spectrum analysis, as opposed to a dynamic response history analysis. SeismoStruct may be the better choice for your analysis, if that is indeed the case. SeismoStruct will enable you to combine modes using ABSUM, SRSS, or CQC. SeismoStruct will also enable you to specify the minimum mass percentage required for your analysis and will incorporate enough modes to achieve that specified minimum.

[ponczek]

Exactly, I want to do Response Spectrum Analysis: i.e. "Linear Dynamic Procedure" (SeismoSoft naming convention) = "Modal Response Spectrum Analysis" (Eurocode 8 naming convention). Thank you for your answer. I would still be interested in how SeismoBuild approaches these two points. And if it can be changed manually.

[seismosoft]

No, unfortunately such settings cannot be changed manually in SeismoBuild. This can be done only in SeismoStruct.
Seismosoft Support

[ponczek]

Thanks for the answer. Ok, it cannot be changed manually in SeismoBuild - understood. But how does the software handle this? I couldn't find any hints in the documentation about these two issues.

[seismosoft]

The modal combination rule employed is CQC. Regarding the modes used in the combination, SeismoBuild considers adequate modes, so that the sum of the effective modal masses is greater 90% of the total.
Seismosoft Support