Ground Motions and Structures - Earthquake Engineering for Structural Design - page 297

Ground Motions and Structures 297

free-field ground motions. This approach is confirmed by many observations during
some important earthquakes (Gueguen et al, 2002a,b). Numerical analyses consider the
alluvial basin located in the center of Nice (France), where city-site effects are upposed
to be significant. Five different building densities are considered: 1, 3, 7, 15 and 30
identical structures along the 2 km width of the basin (Fig. 8.3b). The influence of
building density is shown in Figure 8.3c, for the first frequency value of buildings (0.5
HZ). One can see that the amplification level of free-field ground motions is
significantly modified by the presence of buildings on site. There are three important
zones: until 500m and beyond 1500m the influence is insignificant, but in the area
between 500 and 1500m the differences are very important. For largest building
density, the amplification of ground motions is very high (three times), when compared
to the free-field case. It is possible to explain the damage during the 1995 Kobe
earthquake due to the city-site effect, because of the presence of very soft soil and large
density of buildings. The conclusion of these researches is that the modifications of the
free-field amplification due to the city-site effect lead to introduce a specific factor, the
urban field amplification, to be introduced in the design code, which is very important
especially in case of soft soils.

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(a)
Figure 8.3 (continues)