Learning from Earthquakes - Earthquake Engineering for Structural Design - page 98

3.5.3 ImproveQualification of Seismic Risk

Recent earthquakes have demonstrated that the behavior of some structures was
unacceptable (Gioncu and Mazzolani, 2002). This poor behavior can be explained
for the buildings erected before the elaboration of the modern codes, but it is
unacceptable for the damaged buildings which were designed and detailed in
perfect accordance with these code provisions. These are the reasons why the
structural engineers are sceptical and have many questions about today's design
philosophy (Mazzolani, 1995, Kennedy, 2000):
- Does the code give the correct loading conditions, reflecting in a proper way
the actual ground motions on the structure site?
- Does the current design methodology consider the actual behavior of
buildingsfrom the point of viewof material, structuraland non-structural
elementsas well as structure properties?
-What makes the structure more safe but, at the same time, more economic?
The last important earthquakes, such as Northridge and Kobe, gave rise to the
starting of some very important programs of research activity in the USA, Japan
and Europe, especially for steel structures. The results of this research provided
much information about the behavior of structures during exceptional earthquakes.
The main question resulting from this research is:
To what extent the lessonsof these exceptional earthquakes produced in some
special situations are valuable for other seismic areas?
This question was a challenge for the scientists and engineers, and progress in
the last period led to the understanding of the complex phenomena, associated with
an earthquake, which represents the advances in seismic design.
There are three main advances in building earthquake resisting structures, which
must be examined.
(i) Advances in the knowledge of seismic actions. The knowledge of seismic
actions is based on the last historic 300-years information and the last 50-years
instrumentally monitored seismic records. Terms like fault, hypocenter, epicenter,
tectonic plates, etc., were established only at the beginning of the 20

th

Century, but
today, due to the common efforts of seismology science together with the new
branch of engineering seismology, very important new terms have been added:
interplate and intraplate earthquakes, active and moderate seismic areas, far-field
and near-field earthquakes, etc. Although until now knowledge is poor in many
cases, due to the difficulties to obtain proper information, the basic concepts are
reliable and the following years will be devoted to complete the existing lacks.
Learning from past earthquakes is a very important way to improve the knowledge,
which must be enriched by systematic geological prospection and aerial
supervision, using modern technologies such as Geographical Positioning Systems
(GPS).
(ii) Advances in the design methodology. At the beginning of 20th Century
structures were designed against earthquakes using the same methodologies as for
wind loads. Only in the middle of this century the notions like dynamic resonance,
spectra, seismic energy dissipation, ductility, etc, were introduced in the design