New Challenges in Seismic Design - Earthquake Engineering for Structural Design - page 18

consequently, they may suffer local damage, due to the loss of resistance in some
members. But a properly designed structure must preserve its general integrity,
which consists of the quality of being able to avoid local damage, the structure
remaining stable as a whole. This goal can be achieved by arranging structural
elements, which gives stability to the entire structural system, and by assuring a
sufficient ductility to the members and connections (Gioncu and Mazzolani, 2002).
The above criteria are assumed as a basis of the modern design philosophy,
according to which the design lateral force is obtained by dividing the maximum
force that a structure will experience by a reduction factor q (Fig. 1.6a), which is
specified by seismic design codes. The use of this factor is possible provided that
the structure can stably withstand a structural damage in the range of plastic
deformations without collapse due to loss of strength. This property is called
ductility, which represents the ability of the structure to undergo plastic
deformations without any significant reduction of strength. There are three
ductility types : good (high), medium and poor (low) ductility (Fig. 1.6b).
But it must be recognized that, beyond these considerations, there is a very
important incertitude in determining the seismic actions. This incertitude is due to
uncontrolled and unpredictable aspects, many of them being not well understood
from the physical standpoint. Reliable statistical analyses of earthquakes are rather
difficult since existing recorded data cover only too short time intervals as
compared with the geological period responsible for the seismic processes.
Finally, an important aspect of seismic design is the realization of the important
role of the quality of conception and construction. Earthquake does not respect
theories and calculations in case of a poor quality project and execution (ESDEP,
2005).
Therefore, it becomes clear that the strategies for insuring reliability against
such events could be based on modeling these phenomena in the most simple and
conservative way to compensate the lack of actual information. There are some
fundamental theories, developed only in the last period, which can be used to
understand the phenomenon of earthquake generation and to use the actual
knowledge for a proper seismic structure design. These new branches of the
earthquake science will be presented in the following chapters.
1.5.3 TasksofEarthquakeEngineering
The general field of Structural Engineering deals with the investigation of factors
influencing structural behavior and in the analysis of structures under these factors.
A complete structural analysis requires the evaluation of the interaction of the
structure with soil, foundations and non-structural elements. The spatiality of the
structural schemes, the material properties and the action characteristics must be
taken into account. The complexity of the analysis implies many difficulties, which
today are largely overcome by using computer aids.