Floor response spectra generation - FFT or DFT?


Floor response spectra generation is based on FFT (fast fourier transform) or DFT (Discrete fourier transform)? 

i am doing a multiple story building, try to find out each floor natural frequency. 

This is the result i got from floor spectrum,


1. can i determine the natural frequency from the peak value? say, the natural frequency is 9.95 Hz.

2. how i consider the input load if i just want to determine the frequency? i noticed that the peak will shift if i change the load.