Civil Engineering Reference
In-Depth Information
However, we should not deduce from the previous observations (or other similar
ones made in Mexico) that alluvial soils systematically amplify seismic
accelerations. If we again consider the San Francisco sites, it seems that at the time
of the 1957 earthquake, which also originated on San Andreas's fault but nearer the
city (between 15 and 20 km), the accelerations registered on rocky sites were also
about 0.10 g, as the earthquake had a lower magnitude (5.3 instead of 7.1). Yet at
the surface of the alluvial sites, the recorded accelerations for the earthquake were
1.5 to 2 lower than those on the rock (from 0.05 g to 0.07 g).
To be able to estimate such differences, it is necessary to have a thorough
knowledge of soil behavior under cyclic loading so that it can be integrated into
elaborate calculation models. At the present time, even if many aspects remain to be
clarified, our knowledge of soil behavior has progressed and the calculations models
have developed sufficiently to allow an evaluation of the phenomena that will satisfy
engineers.
Ground maximum
acceleration
Station
Stratigraphy
1957
1989
Golden Gate Park
Rock
0.13
Market/Guerrero St
Rock
0.12
State Building
Sand
+ clayed sand (60 m)
0.10
Mason/Pine St
Rock
0.10
Alexander Building
Clayed silt + sand (45 m)
0.07
0.17
Southern Pacific B.
Soft clay
0.05
0.20
Rincon Hill
Rock
0.10
0.09
Oakland City Hall
Clay, sand (30 m)
+ stiff clay (270 m)
0.04
0.26
Table 4.1.
Maximum acceleration in San Francisco (from [COL 90])
On the basis of these observations, the various paraseismic building codes
acknowledge the necessity of taking into account the geological nature of the soil in
the definition of seismic stresses. This is expressed in the way response spectra are
expressed differently according to the nature of the ground, which is characterized
by the average propagation speed of shear waves in the 30 uppermost meters of the
ground layer [COL 02].
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