When using Newmark Combinations in seismic analysis do I still use signed combinations?
No, signed combinations shown in your screen capture are used only together with quadratic combinations - when checking "Signed..." it automatically activates "Quadratic..." in this dialogue.
Signed quadratic combinations are described there:
Defining dominating (main) modes for specific seismic load cases is the method to restore sign of results lost when calculating quadratic combinations between modes. This sign is inherited also by Newmark combinations containing such seismic cases - see the attached screen capture.
In case of not using Newmark combinations between seismic cases but using quadratic combinations between them and indending to restore sign of results from dominating modes it is necessary to additionally use signed quadratic combinations mentioned by you.
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I up this old but intersting topic.
I just need one precision :
When I choose the main mode on the "sign combination" panel and "CQC by default", does it automaticly sign all combinations including seismic cases or the "signed combination" box must be checked as well for each combination.
I guess the answer is already given by Pawel Pulak. I just need to be sure because don't understand english very well.
In case of Newmark combinations it is not necessary - these combinations "inherit" sign from the sign of seismic load cases.
In case of quadratic combinations it is necessary to change them into signed ones too. Generally in the dialogue below it is impossible to activate "Signed combination" without "Quadratic combination"
When planning to use signed guadratic combinations between seismic directions it is recommended to activate it in direction parameters of seismic analysis.In such case more then one such combination will be automatically generated - so it will not be necessary to manually define additional ones: