Variables affecting modal frequency and shape
Variables affecting modal frequency and shape
(OP)
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?






RE: Variables affecting modal frequency and shape
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.
RE: Variables affecting modal frequency and shape
This correspond to seismic loading.
If you have different loading, use load participating ratio instead.