Analysis and modelling of the seismic behaviour of high ... - Ingegneria
Analysis and modelling of the seismic behaviour of high ... - Ingegneria
Analysis and modelling of the seismic behaviour of high ... - Ingegneria
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
2. DUCTILITY AND SEISMIC RESPONSE OF STRUCTURES<br />
interplate or intraplate fault, etc. Ductility dem<strong>and</strong>s vary depending on each<br />
ground motion type.<br />
ii. Seismic micro-zonation, which considers <strong>the</strong> possible earthquake sources<br />
at <strong>the</strong> level <strong>of</strong> region or town, on <strong>the</strong> basis <strong>of</strong> common local investigation.<br />
The result <strong>of</strong> <strong>the</strong>se studies is a local map indicating <strong>the</strong> position <strong>and</strong> <strong>the</strong><br />
characteristic <strong>of</strong> <strong>the</strong> sources, general information about <strong>the</strong> soil conditions<br />
<strong>and</strong> design spectra. It is very useful to accompany <strong>the</strong> time-history<br />
accelerograms with very precise indications about <strong>the</strong> place where <strong>the</strong>y<br />
have been recorded (directions, distance from epicentre, soil conditions,<br />
etc.). Recordings such as magnitude, distance from source, attenuation<br />
<strong>and</strong> duration are directly involved in ductility dem<strong>and</strong>.<br />
iii. Site condition, established through <strong>the</strong> examination <strong>of</strong> <strong>the</strong> stratification<br />
under <strong>the</strong> proposed structure site. This is a very important step, because<br />
dramatic changes <strong>of</strong> earthquake characteristic within a few hundred meter<br />
distance are not unusual during an earthquake. These differences are<br />
mainly caused by <strong>the</strong> difference in soil conditions. For s<strong>of</strong>t soil <strong>the</strong> ductility<br />
dem<strong>and</strong> is more important than for rigid soil.<br />
Differently from <strong>the</strong> past, when <strong>the</strong> developed design methods was based on<br />
simple hypo<strong>the</strong>ses because <strong>the</strong> reduced number <strong>of</strong> records during severe<br />
earthquakes, today several measurements <strong>of</strong> ground motions for different distance<br />
from <strong>the</strong> sources <strong>and</strong> on different site conditions are available thanks to a large<br />
network <strong>of</strong> instrumentation all over <strong>the</strong> world,. This situation allows to underline a<br />
new very important aspect, which was previously neglected in <strong>the</strong> current concept:<br />
<strong>the</strong> difference in ground motion between near-field <strong>and</strong> far-field earthquakes. The<br />
near-field region <strong>of</strong> an earthquake is <strong>the</strong> area, which extends for several kilometres<br />
from <strong>the</strong> projection on <strong>the</strong> ground surface <strong>of</strong> <strong>the</strong> fault rupture zone. Since in <strong>the</strong><br />
past <strong>the</strong> majority <strong>of</strong> ground motions were recorded in <strong>the</strong> far-field region, <strong>the</strong><br />
current concept refers to this earthquake type only. The great amount <strong>of</strong> damage<br />
during <strong>the</strong> Northridge <strong>and</strong> Kobe earthquakes is due to <strong>the</strong> fact that <strong>the</strong>se towns are<br />
situated in a near-field region. Thus, ground motion recorded in far-field regions<br />
cannot be used to describe in proper manner <strong>the</strong> earthquake action in near-field<br />
regions. The differences, as presented in Figure 2.5, consist in:<br />
• <strong>the</strong> direction <strong>of</strong> <strong>the</strong> propagation <strong>of</strong> <strong>the</strong> fault rupture has <strong>the</strong> main influence<br />
for near-field regions, <strong>the</strong> local site stratification having a minor<br />
consequence. On <strong>the</strong> contrary, for far-field regions, soil stratification for<br />
travelling waves <strong>and</strong> site conditions are <strong>of</strong> first importance;<br />
• in near-field regions, <strong>the</strong> ground motion has a distinct low-frequency pulse<br />
in acceleration time history <strong>and</strong> a pronounced coherent pulse in velocity<br />
21