Variables affecting modal frequency and shape

[alannakate]

I have modeled a building, applied a horizongal sinusoidal load to the top floor causing the building to vibrate, and am trying to make sense of my results (fundamental frequency and mode shape in particular). I want to make sure that I have the relationships between different variables correct. I know that a structure's frequency depends upon its modal mass and stiffness. I know that the mode shape depends on the frequency. I am a bit hazy when it comes to determining a building's modal mass. I know that the modal mass depends on the participation factor which describes how much a single mode influences the building's movement but does that have anything to do with how the mass is distributed throughout the building?

 

[JoshPlumSE]

Not sure if I can give a answer in English. I just look at modal participation based on math:

R_m = sum(Phi_im*M_i)

R_m = participation factor for mode m (note: this is not equal to the mass participation factor given in many programs)

M_i = mass at Joint i
Phi_im = mass normalized mode shape for Joint i for mode m.

This only works for mass normalized mode shapes. But, I would think most programs would give you that already.

 

[PicoStruc]

Participating mass is based on unit acceleration load on each DOF.

This correspond to seismic loading.

If you have different loading, use load participating ratio instead.