Toward Performance-based Engineering
The earthquake demands are determined from the National Seismic Hazard Maps developed by USGS. A basic response spectrum is defined, which has spectral ordinates at 5% damping for different probabilities of occurence and soil conditions at T= 0.2 seconds and T=1 second. Spectral values at all other periods are determined using the empirical relations used to define the spectrum.
The approach in FEMA 273 is displacement-based, and subjective factors are used to assess the uncertainty. For example, the roof drift is given by:
droof = C0C1C2C3C4Sdelastic
Where C0C1, etc. are the subjective factors used to address uncertainty and Sdelastic is the elastic spectral displacement.
273 also defines nonlinear dynamic and static pushover analysis methods
in addition to the conventional elastic methods.