Advances in Seismic Design Methodologies - Earthquake Engineering for Structural Design - page 408

408 Earthquake Engineering for Structural Design

Ground motions are time process affected by a high degree of uncertainty, deriving
from many source types and site conditions. Therefore, the modeling of seismic
excitation turns out to be a random process. So, the average value and the standard
deviation can be determined from the processing of the recorded data. The proposed
values, generally admitted, are the average value plus one standard deviation (Fig.
9.13a). The parameters defining the spectra are the peak ground acceleration, velocity
or displacement (PGA, PGV and PGD), as well as the effective peak acceleration and
velocity (EPA and EPV). EPA is the average of the maximum ordinates in the period
range from 0.1 to 0.5 sec, divided by a mean value of 2.5. EPV is the same average of
the maximum ordinates of velocity in the period range of 0.8 to 1.2, divided by the
same mean value of 2.5.
Separate plots for accelerations, velocities and displacements can be obtained by
using the same procedure but the most common formats for spectra are (Fig. 9.14):
- Spectral acceleration vs. period (frequency).

- Spectral displacement vs. period (frequency).

Response spectra S a S d

T b T c T d

Figure 9.14 Elastic and displacement spectra (Gioncu and Mazzolani, 2002)