modal shape and forced response, pls. help

hi, all,

I have a question about the modal shape and the forced response of a structure.

Let say once you have done modal analysis of a linear structure, for certain mode N, you will know the relative displacement between two points in this structure. Let denotes these two points as p1, and p2.

Then for the same structure, I excited it with a harmonic force and the excitation freq. is the Nth modal freq. The damping is defined by 0.1% modal damping. So, I will know the forced displacements of p1 and p2.

I expect that the ratio of the forced displacements should be equal or very very close to the ratio I calculated based on the modal analysis only. So, for the whole struture, the pattern of the forced response of this structure excited at Nth mode should be very similar to the pattern we see from the modal shape .

Unfortunately, using a commerical FEM package ANSYS, I did not see what I expect. Is there anyting wrong with my understanding???

Pls. help.

Thanks a lot, William,