Sempre le FEMA NEHRP 2009, sulla normalizzazione per frequenze

Quote:
Another ground motion scaling method involves transforming the time-acceleration data into the
frequency domain (such as by means of the fast Fourier transform), making adjustments (to match exactly
the target spectrum at multiple, specific frequencies) and transforming back into the time domain. This
method affects amplitude, frequency content and phasing (and tends to increase the total input energy).
This method makes it possible to estimate mean response with fewer ground motions, but may obscure
somewhat the potential variability of response. Use of this method is permitted by the Provisions, but the
same number of records is required as for time-domain scaling. Given the jaggedness of individual
response spectra, the process of spectral matching (which produces smoother spectra) requires scale
factors that can be considerably smaller or larger than those used in time-domain scaling. Since this
method applies numerous scale factors to differing frequencies of each ground motion component in order
to match spectral ordinates, there is no requirement that the two components be scaled identically. As the
spectral ordinates of frequency-domain scaled records may fall below the target spectrum at frequencies other than those used for matching, a second round of (minor) scaling is needed to satisfy the Provisions
requirements.

"Data speak for themselves" -Reverend Thomas Bayes 1702-1761
P(Ai|E)=(P(E|Ai)P(Ai))/P(E)