Analysis and modelling of the seismic behaviour of high ... - Ingegneria

2. DUCTILITY AND SEISMIC RESPONSE OF STRUCTURES
interplate or intraplate fault, etc. Ductility demands vary depending on each
ground motion type.
ii. Seismic micro-zonation, which considers the possible earthquake sources
at the level of region or town, on the basis of common local investigation.
The result of these studies is a local map indicating the position and the
characteristic of the sources, general information about the soil conditions
and design spectra. It is very useful to accompany the time-history
accelerograms with very precise indications about the place where they
have been recorded (directions, distance from epicentre, soil conditions,
etc.). Recordings such as magnitude, distance from source, attenuation
and duration are directly involved in ductility demand.
iii. Site condition, established through the examination of the stratification
under the proposed structure site. This is a very important step, because
dramatic changes of earthquake characteristic within a few hundred meter
distance are not unusual during an earthquake. These differences are
mainly caused by the difference in soil conditions. For soft soil the ductility
demand is more important than for rigid soil.
Differently from the past, when the developed design methods was based on
simple hypotheses because the reduced number of records during severe
earthquakes, today several measurements of ground motions for different distance
from the sources and on different site conditions are available thanks to a large
network of instrumentation all over the world,. This situation allows to underline a
new very important aspect, which was previously neglected in the current concept:
the difference in ground motion between near-field and far-field earthquakes. The
near-field region of an earthquake is the area, which extends for several kilometres
from the projection on the ground surface of the fault rupture zone. Since in the
past the majority of ground motions were recorded in the far-field region, the
current concept refers to this earthquake type only. The great amount of damage
during the Northridge and Kobe earthquakes is due to the fact that these towns are
situated in a near-field region. Thus, ground motion recorded in far-field regions
cannot be used to describe in proper manner the earthquake action in near-field
regions. The differences, as presented in Figure 2.5, consist in:
• the direction of the propagation of the fault rupture has the main influence
for near-field regions, the local site stratification having a minor
consequence. On the contrary, for far-field regions, soil stratification for
travelling waves and site conditions are of first importance;
• in near-field regions, the ground motion has a distinct low-frequency pulse
in acceleration time history and a pronounced coherent pulse in velocity
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