Civil Engineering Reference
In-Depth Information
Chapter1
NewChallengesinSeismicDesign
1.1 AFTERTHE LASTSTRONGEARTHQUAKES
1.1.1ImpactofNorthridge andKobe EarthquakesonSeismic Design
Earthquakes represent the largest potential source of casualties and damage for
inhabited areas due to natural hazard. Although the location varies, the pattern is
the same: an earthquake strikes without warning, leaving cities in rubble and
killing tens to hundreds of thousands of people. Worldwide during the 20
th
Century, there were ten earthquakes killing more than 50,000 people and over 100
earthquakes killing more than 1000 people (FEMA 383, 2003). Every year,
something like five thousand to ten thousand people die during earthquakes
worldwide. The 1976 Tangshan-China (magnitude M 8.0), the worst earthquake in
recent times, killed over 600,000. Among these terrifying data, the moderate 1994
Northridge in Los Angeles (magnitude M 6.7), which killed 60 people, and 1995
Kobe in Japan (magnitude M 6.9), which killed 5600 people, seemed to be
relatively insignificant. Nevertheless, these two earthquakes have changed the
direction of earthquake engineering research throughout the World (Blakeborough,
2002). Two main reasons produced this crucial change.
The first reason lies not in the number of dead, but in their economic costs.
Each event was a direct hit by a moderate earthquake on a dense built-up area. In
Northridge, around 15,000 buildings had to be demolished, resulting in a total loss
ranging from $15bn to $40bn. In Kobe, 180,000 buildings were destroyed or
seriously damaged, the repair costs being estimated in the range of $90bn to
$150bn. Each earthquake set a record loss for natural disasters both for the USA
and Japan, respectively. Following these earthquakes, it was immediately apparent
that the old principles for seismic design had to change. Whereas the previous
principles had been primarily oriented to safeguard buildings against collapse, the
new and more refined rules are devoted to reduce the damage costs, by keeping the
non-structural elements and the structures in an acceptable damage level. So, the
principles of
Performance Based Seismic Design
were set up.
The second reason lies in the fact that both earthquakes struck very densely
populated areas. Only looking to these experiences, it is sufficient to appreciate the
catastrophic potential of even a moderate earthquake when its epicenter is situated
in an urban area. The differences in damage costs can be justified in the fact that
the epicenter of the Kobe earthquake was located beneath a highly urbanized
region, while the one of the Northridge earthquake was positioned beneath the
Northern edge of Los Angeles. But this enormous damage can be considered minor
in comparison with the potential losses in big cities situated in seismic areas, like
Mexico City, San Francisco, Tokyo, etc. A recent calculation model predicted
