SEISMIC ANALYSIS - sign a mode

[antoine.carimalo]

I have a problem with some modal analysis models

Sometimes It is not possible to sign the main mode for one direction, depending on the global modal partcipation I guess.

When I choose the mode with the biggest participation mass in the pannel "combination sign", it doesn't affect de selected direction.

In the example here attached, I can't sign Z direction if I run the analysis with 40 modes. I can if I run it with 100 modes. Somebody knows why?

Thanks by advance

[antoine.carimalo]

Sorry, I correct my sentence :

In the example here attached, I can't sign Z with the main mode (mode 31 : biggest modal participation in Z direction) if I run the analysis with 40 modes. I can if I run it with 100 modes (now mode 31 can be affected as the main mode for Z). Somebody knows why?

[Artur.Kosakowski]

This is because you filtered out some modes in definitions of seismic cases therefore you no longer have 31 of them (e.g. for the Z direction only 24 are available) . After switching off these filters (or increasing number of modes to be calculated) and calculating the model you will be able to set mode 31 as the main one again.

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[antoine.carimalo]

Thanks, You're right.

I make the mistake from a long times because I used to sign with the modal combination number even when using filters.

I try to understand the big formula on the help but it is very difficult.

What's happen if we don't choose the right mode as the main mode?

Cordially

[Artur.Kosakowski]

The signs of values of results may not be as you expect them to be. Sort of similar situation to the one when you do not use signed combinations at all.