Advances in Seismic Design Methodologies - Earthquake Engineering for Structural Design - page 387

Advances in Seismic Design Methodologies 387

The recent decades have seen a dramatic rise in economic losses related to
earthquakes. In the period 1988-1997 (including Loma Prieta, Northridge and Kobe
earthquakes, which cannot be considered catastrophic from the point of view of
magnitude) the economic losses were estimated as twenty times larger than in the
previous 30 years (see Section 3.1). These losses are due to several factors, which
include:
- Dense population of buildings located in seismically active regions.
- Stock of aged and non-engineering buildings, which were designed or constructed
without respecting any anti-seismic rules or based on inadequate concepts.
- Deficiencies in knowledge of regional seismic hazard, behavior of structural
materials and structural systems under dynamic loads.
- Increasing cost of business interruption, due to the loss of building functions.
- Large amount of damage in non-structural elements and contents.
Contrary, the structures, which were designed in conformance with the modern
seismic design, performed as expected, the loss of lives being minimal, in agreement
with the intent of the adopted methodology. However, there is a misperception from
many owners, insurers and government agencies about the expected performance of
seismic designed buildings (FEMA 349, 2000). The economic losses of the past
earthquakes raised many questions regarding the adequacy of the current seismic
design rules to prevent these losses.
So, in the recent years, a new philosophy for the seismic design of constructions has
been discussed within the engineering community.

9.1.2 After NorthridgeandKobe: Appropriate PerformanceLevels

The main characteristic of the new seismic design is the participation of the owners and
users for establishing target and appropriate performance levels. They are determined
not only in terms of structural design, but from the demands of owners, users and
society. As a consequence, the new methodology must be very explicit to be easily
understandable also for people ignoring structural design (Aoki et al, 2000).
The bases of the new seismic design started in the United States, with the Vision
2000 Document (SEAOC, 1995), which provides the main concepts of this approach
(Bertero, 1996a,b). The goal of this design philosophy is to produce structures, that
have predictable seismic performance under multiple levels of earthquake intensity.
Performance - based seismic design is defined as:
“… consisting of selection of design criteria and structural systems such that at the
specified levels of ground motion and with defined levels of reliability, the structure
will not be damaged beyond certain limiting states or other useful limits “ (Bertero and
Bertero, 2000).
It means that a comprehensive performance-based seismic design involves several
steps (Gioncu and Mazzolani, 2002):
- Selection of performance objectives and definition of the acceptable damage level.