Page 10 - Advanced Design Examples of Seismic Retrofit of Structures

2 Advanced Design Examples of Seismic Retrofit of Structures

essentially about the economic and political policies to implement and enforce
design requirements.
However, old buildings constructed with almost no seismic considerations
are considered to be the chief source of seismic risk threatening the most com-
munities. These buildings must be identified and evaluated to determine their
level of seismic risk, and then appropriate risk management solutions should
be selected and implemented for them. This chapter presents the overall steps
of procedures for risk assessment and retrofit of individual buildings as risk
assessment; readers can find more detailed information describing these meth-
odologies elsewhere [2].
The seismic vulnerability evaluation of the existing buildings covers almost
every aspect of earthquake engineering and construction techniques. On the
other hand, retrofit of individual buildings is the main part of the evaluation
and risk reduction strategies. It is worth noting that there are a variety of
research topics that will not be presented in detail here. In the next sections,
a brief review on these fields of study is described.

1.2 SEISMIC RISK REDUCTION STRATEGIES
Seismic codes and standards traditionally have not been developed on the basis
of reliability methods such as load and resistance factor design (LRFD), because
of the lack of a variety of unknown parameters that must have been defined in a
reliable manner in this design methodology. The need to know the probability of
failure of code-compatible buildings has been recognized since the provision of
the associated commentary of ATC 3-06 [3]. Efforts through quantifying the
mentioned problem has been made by the SAC Steel Project [4]. To propose
a performance-based framework for the design of steel moment frames, the first
methodology was developed to estimate the probability of collapse of a building
under excitation by different ground motions [5]. The proposed approach tried
to alleviate the variation in maximum demands of structures under scaled-to-
same-level time histories as well as other sources of uncertainties. Although
the method focuses on the quantification of the collapse as a target performance
level, the framework provided a basis that could be extended to other perfor-
mance targets. In the 1967 edition of the Blue Book [3], a clear performance-
related set of criteria were defined for buildings designed to its provisions:
(1) resist minor earthquakes without damage; (2) resist moderate earthquakes
without structural damage, but with some non-structural damage; and (3) resist
major earthquakes, of the intensity of severity of the strongest experienced in
California, without collapse, but with some structural as well as nonstructural
damage.
Analysis of responses of the vast majority of buildings for the effect of seis-
mic ground motion requires consideration of nonlinear structural behavior [3].
It is not economically efficient to keep structural systems in the elastic range
under strong earthquakes, and it should be borne in mind that an elastic super-
structure response would severely increase displacement demands. Therefore,