Modal participation factors

[KostaKurta]

Hi everybody!

I'm currently working on a project related to structural dynamics, which is a subject I don't know much about. Hope you can help me out. The thing is: I'm using ANSYS to do a modal analysis of a given structure and everything is right, but then when I do the spectrum analysis to observe the response of the structure to an earthquake, I get some negative modal participation factors. My question is: does it make sense that a modal participation factor has a negative sign?

I don't know if my explanation is clear enough...

Sorry for my English.

Thanks in advance.

 

[PicoStruc]

1) For Modal Participation Factor, Negative value are normal

Factor = Transpose ( Mode Shape) * Mass

So because modeshape value are positive and negative, mass positive, it's normal to get negative modal participation factor.

2) For Modal Participating Mass ratio, All values should be positive and less than 1.0 (100%)