Learning from Earthquakes - Earthquake Engineering for Structural Design - page 74

“… Many engineers appear to have the idea that earthquake stresses in
framed structures can be calculated so that rational design to resist
earthquake destruction can be made, just as one may allow for dead and live
loads, or wind and impact stresses. An attempt to calculate earthquake
stresses is futile. Such calculations could lead to no practical conclusions of
values…” .
This quotation reflects, in the past time, the negative attitude of engineers
concerning seismic design.
Recently, thanks to the important progress in Seismology and Structural
Engineering, a contrary attitude has been developed, with the conviction that all
phenomena produced by an earthquake, even when very complex, can be
controlled by analyses. Indeed, in the last thirty years, the understanding of the
nature of earthquakes, of the effects of site soil characteristics and of the structural
response of structures subjected to seismic actions has made real advances. The
installation of a dense instrumentation in high earthquake risk areas provided a
great amount of information about the main characteristics of ground motions. At
the same time, due to the development of personal computers and special computer
programs, the structural analysis of earthquakes now is no more an unsolvable
problem. Therefore, today, the structural engineers are confident to have reached
the stage where the actual structural performance during strong ground motions
can be satisfactorily predicted, explained and quantified.
But, the recent important earthquakes (as 1989 Loma Prieta, 1994 Northridge,
1995 Kobe, 1999 Kocaeli, 1999 Chi-Chi) have shown that sometimes the reality is
different from this optimistic attitude. The lessons learned after these events have
shown that, despite the above-mentioned significant progress, there are still many
unsolved problems and today it is more appropriate to speak only about advances
in seismic design rather than about infallible solutions. Unfortunately, for the
future, one can expect that earthquakes will continue to produce damage and large
loss of lives. The real tragedy is that these human losses are due to the failure of
the constructions and not due to the earthquake itself, the builders being the makers
of tools for killing people.
Therefore, the mitigation of seismic risk is a moral duty of the structural
engineers. This mitigation is possible only when the problems connected to the
earthquakes are correctly approached. After Bertero (1997), the definition of
seismic risk is:
“…the probability that social and/or economic consequence of earthquakes
will equal or exceed specified values at a site, at various sites, or in an area,
during a specified exposure time…”.
After Ambraseys (2002), Grossi (2004), the seismic risk can be defined as:
(Seismic risk) = (Seismic hazard) (Vulnerability) (Losses)
where:
- Seismic hazard is the probability of occurrence, at a severity level and at a
specific period of time, for a given area, of a potential damaging
earthquake, which is beyond human control, but the knowledge of it is