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In-Depth Information
Improving Design Practice
wind, earthquake, etc. In this chapter, we will learn
how to find the performance level of a structure.
To that end, since some methods of performance-
based designs require nonlinear analysis, a brief
introduction to nonlinear analysis procedures is
also provided to complete the discussion.
In the traditional design of structures, attention was
mostly paid to strength of structure and the defor-
mation control was often a secondary check. The
designer aimed to design a structure in such a way
that it withstands the applied loads with sufficient
reserved resistance capacity. The structure was in
fact designed for amplified loads and checked for
deflection or side-sway. It is not too far that the phi-
losophy of design changed to load and resistance
factored design (LRFD) in which reduced ultimate
strengths of structural elements are compared to
the corresponding amplified internal forces. In this
method, different loads are amplified differently
based on the reliability of their evaluation; and
the resistance is decreased differently for bending
moment, shear, etc. for similar reasoning. After
California earthquakes, including the 1989 Loma
Prieta and 1994 Northridge events, the need for a
better control on performance of structure became
more serious. Seismic-related optimization was
supposed to address this problem. Alternatively,
Performance-Based Design (PBD) of structures
that aims to design a structure for required duc-
tility and targeted displacement in expected risk
levels was proposed to satisfy this need. This
latter new design philosophy is so attractive that
has the potential of being the next generation of
design philosophy.
In this new design philosophy, the structure
is expected to be such designed that it behaves
nonlinearly under severe loadings while it behaves
linearly under service loads, small wind and minor
earthquake effects. The design has to have enough
ductility to tolerate specified drift for severe wind
and earthquakes. Depending on behaviour of mate-
rial used in the structure, being ductile or brittle,
the importance of the structure, the risk level and
the severity of loading, different design criteria
may apply. In other words, different performances
may be expected from a structure with different
material and different load intensities including
Optimizing the Design
for Seismic Effects
The desire of optimum design has a long history
and goes back to the time of Galileo and even be-
fore. However, the new era of structural optimiza-
tion starts with the time of emergence of electronic
computers. Considerable research works can be
found in the literature that have focused on the
structural optimization under static loading. Many
of them have suggested efficient algorithms with
relatively good degrees of success.
As the design practice evolved and the de-
mand for structural design under earthquake
effects increased, the research on optimizing the
structural design for seismic effects increased.
The primary works in this field were devoted
to structural optimization under dynamic loads.
Most of research works in the field considered
the elastic behaviour of structures. The degree
of success was not fully satisfactory because of
the complexity of the problem, objectives to be
optimized, mathematical formulation, and the
solution schemes for nonlinear optimization
problems. Among these research works some
attempted to suggest a mathematical model for
a better solution of optimization problems and
speed up of the solution process; some tried to
change the dynamic behaviour of the structure.
Among the others, some recent papers by Masson
et al. (2002), Besset and Jezequel(2007), Chen,
et al. (2002), and Mills-Curran (1985), may be
consulted in this field. Park et al. (2003, 2005
& 2010) suggested an equivalent static loading
procedure that generates the same response field
in linear static analysis that nonlinear dynamic
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