Floor response spectra generation - FFT or DFT?

Question

Hello, 
 
 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, 
 
 question: 
 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.

DSANJU · Answer

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DSANJU · Answer

The first time history analysis is for the building or the frame structure on which the equipment will be sitting on. The program solves for the values of acceleration of all the nodes constituting the floor as a result of the time history analysis. Thus we get the acceleration response in all directions over a range of time. 
 From the above data, the average acceleration of all the nodes in a particular direction over a range of time is obtained. This forms the term a(t), which is considered as the base acceleration of the equipment in a particular direction. 
 STAAD then performs a secondary time history analysis by solving the following base motion equation: 
 ϋ + 2&beta;&omega;ύ + &omega;2 u = -a(t) 
 Here, &beta; is the damping ratio and &omega; is the natural frequency of the SDOF system REPRESENTATIVE OF THE EQUIPMENT. 
 Solving the above equation by varying the values of natural frequency and/or modal damping, we obtain the values of acceleration of the SDOF mass for each values of frequency against time for one or multiple damping values. Thus, we obtain the maximum absolute value of acceleration in a particular direction for a particular value of frequency for a time range. The frequency-acceleration pair forms the floor response spectra data for a particular damping value.