SUMMARY OF GEOTECHNICAL EARTHQUAKE ENGINEERING - Geotechnical Earthquake Engineering - page 487

15.3 SUMMARY OF PEAK GROUND
ACCELERATION

In order to perform geotechnical earthquake engineering analyses, the peak ground accel-
eration (referred to as a max or PGA) for the design earthquake must be determined. Section
5.6 presents a discussion of the methods used to determine the peak ground acceleration.
When determining the peak ground acceleration, the local soil and geologic conditions
must be included in the analysis. For example, if the site has thick deposits of soft soil, then
the peak ground acceleration and duration of shaking will likely be greater than adjacent
hard rock sites (Sec. 4.6.1).

15.3.1 MCE G Peak Ground Acceleration
If the 2012 edition of the International Building Code governs the design of the project, then
the first step in the analysis is to determine the Seismic Design Category of the structure. As
discussed in Sec. 14.2.8, the structural engineer is the best individual to determine the
seismic design category. If the structure is assigned to Seismic Design Category D, E,
or F, then the more stringent method of using the MCE G peak ground acceleration for
studies of liquefaction, slope movement, etc. will be required. The engineering geologist
and geotechnical engineer will need to determine the probabilistic and deterministic
geometric mean peak ground accelerations. The probabilistic geometric mean peak
ground acceleration has a 2 percent probability of exceedance within a 50-year period.
The deterministic geometric mean peak ground acceleration is the largest 84 th per-
centile geometric mean peak ground acceleration for characteristic earthquakes on all
known active faults within the site region. Usually these values are determined by the
engineering geologist and then the procedures outlined in Sec. 14.2.8 are used to deter-
mine the MCE G peak ground acceleration.
As discussed in Sec. 14.2.8, for structures assigned to Seismic Design Category D, E,
or F, using the MCE G peak ground acceleration means that sites that were previously not
liquefiable could now be liquefiable. In addition, sites where liquefaction occurred to a
limited extent could now undergo more liquefaction in terms of depth and lateral extent
because of the higher value of the MCE G peak ground acceleration. Other analyses related
to the development of liquefaction, such as lateral spreading and flow slides, could increase
in severity.
In order to determine the MCE G peak ground acceleration, the site class must be known.
For example, as indicated in Table 14.12, the value of the site coefficient F PGA to be used
in Eq. (14.2) is dependent on the site class. It is possible that the initial site class selection
will turn out to be incorrect once the liquefaction analyses are complete. In this case, the
determination of the MCE G peak ground acceleration will have to be repeated for the new
site class.

15.4 SUMMARY OF ENGINEERING ANALYSES

Regarding engineering analyses, Terzaghi (1936) states:

“…the earth in its natural state is never uniform… Its properties are too complicated for
rigorous theoretical treatment …Even an approximate mathematical solution of some of the
most common problems is extremely difficult.”