Environmental Engineering Reference
In-Depth Information
Site intensity, magnitudes, or estimates of
g
are thus found for rock excitation.
Evaluations obtained by these foregoing procedures are, at best, only approximations
applicable to shallow-focus shocks.
Selection of Ground Motion for Rock Sites
Strong-motion records
or
response spectra
for an earthquake of the design magnitude for vari-
ous ground conditions in the site area would provide the most reliable data on acceleration
and frequency content, but as yet such data are not available for many areas of the world.
Response spectra are frequently being updated and there are a number of approaches
In many cases it is necessary to estimate site ground motion from correlations.
Horizontal acceleration for rock excitation
may be estimated roughly from intensity or mag-
nitude values such as given in
Figure 11.13
or Equation 11.10, or from
Table 11.14,
devel-
oped from an evaluation of strong-motion records.
Vertical acceleration for rock excitation
has been estimated as 0.5 PHGA or higher if the site
is close to a capable fault. Hall and Newmark (1977) recommend using two thirds of the
values given in Table 11.14.
Frequency content
is estimated from
Figure 11.30,
giving the predominant period vs.
M
vs. distance from the causative fault, or estimated from Table 11.14, giving design velocity
vs.
M
. For long-distance earthquakes, of the order of 100 km or more, the possibility of
sway in high-rise structures due to long-period waves is considered.
Duration of shaking
is estimated on the basis of the bracketed duration in terms of distance
and magnitude from
Table 11.5
,
which provides these data for an acceleration
0.05
g
and
a frequency
2 Hz, or is developed from strong-motion records. Consideration is given to
the fact that the duration of large earthquakes depends largely upon the length of faulting.
Selection of Ground Motion for Soil Sites
Soil conditions
usually influence acceleration by amplification. Maximum accelerations
from the same earthquakes occur at different periods for different soil types and thick-
nesses. Damage is selective and varies with building height and period and other factors
(see
Section 11.4.3)
.
Soil amplification factor
may be estimated from
Figure 11.20,
which
gives general relationships between acceleration in terms of
g
, period, and foundation con-
ditions. Attenuation relationships and response spectra developed by numerous investi-
gators for soil sites are listed in
Tables 3-1
and
3-2
of EM 1110-2-6050 (USAEC, 1999). The
alternative procedure for evaluating soil response is by SSI analysis employing dynamic
soil properties (see
Section 11.4.7)
.
The prerequisite for selecting ground-motion relationships for soil sites is a subsurface
investigation (
Section 11.5.3)
.
In situ
and laboratory testing is performed to obtain data on
the soils for evaluations of the liquefaction potential, and for the amplification factor to
determine “
g
” for SSI analysis, and dynamic soil properties for the SSI analysis.
Synopsis
Site acceleration can be given for design in several ways:
Maximum peak or effective peak acceleration
●
Acceleration at a given period
●
Real or synthetic time motion that provides for structural periods and damping
●
Continuous spectrum of time motion based on actual recorded events
●
