Environmental Engineering Reference
In-Depth Information
Anchorage
(1964,
M
8.6): High-magnitude event was similar to many historical
events except for unusual duration of 3 min, which resulted in the liquefaction
failure of many natural, previously stable, slopes.
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Soil Condition Effects
Ground Amplification Factor
Bedrock excitation accelerations generally increase in magnitude as soil thickness
increases and soil stiffness decreases. The maximum ground amplification factor generally
ranges between 1 and 2 for strong motions, and is a function of period
(Section 11.2.6).
Effects on Frequency
Local soil conditions filter the motion so as to amplify those frequencies that are at or near
the fundamental frequency of the soil profile (Whitman and Protonotarios, 1977), but fre-
quencies are diminished by attenuation.
Various earthquakes may have the same peak acceleration but if they occur with dif-
fering periods, the ground response will differ and structural damage may be selective.
For example, in San Francisco (1957), in stiff soils, acceleration peaks occurred at low val-
ues of the fundamental period (0.4 - 0.5 s); therefore, maximum accelerations would tend
to be induced in relatively stiff structures 5 to 6 stories in height, rather than in high-rise
buildings. In deep deposits of soft soils, however, peak acceleration occurred at interme-
diate values of the fundamental period (1.5 - 2.6 s), which would induce maximum accel-
eration in multistory buildings of 20 to 30 stories, leaving lower, stiffer buildings
unaffected (Seed, 1975).
Other Factors
Depth effects
: accelerations at foundation level can be substantially lower than at
the surface, as discussed in
Section 11.2.6.
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Subsidence and liquefaction are discussed in
Section 11.3.3.
●
Slope failures are discussed in
Section 11.3.4.
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Microzonation Maps
Geologic conditions and ground response factors presented as microzonation maps have
been prepared for a few urban locations. They are useful for planning and preliminary
design, and emphasize hazardous areas.
11.4.4
Response Spectra
Description
A
response spectrum
is a plot of the maximum values of acceleration, velocity, and displace-
ment response of an infinite series of single-degree-of-freedom systems subject to a time-
dependent dynamic excitation, such as but not limited to ground motion. The maximum
response values are expressed as a function of undamped natural period for a given damp-
ing. Approximate response spectrum acceleration, velocity, and displacement values may
be calculated from each other assuming a sinusoidal relationship. The calculated values are
sometimes referred to as pseudo-acceleration, pseudo-relative velocity, or pseudo-relative
displacement response spectrum values. (USACE, 1999). An example of a response spec-
trum showing maximum displacements, maximum pseudo-velocities, and maximum
pseudo-accelerations presented on a logarithmic tripartite graph is given in
Figure 11.43.
