plate and Hierarchical Finite Element

Hi All

I have problem in choosing shape funcction for modelling vibration of plate. Vibration mode of plate is not symmetric. Then I don't know that Can I suppose that plate deflection(for example W(x,y,t) ) is seperable? i.e. Can I Write W(x,y,t)=w(x,y)*u(t) where d2u/dt2=-a2*u where a is frequency of vibration.

Please Help....

Thanks