Page 12 - Advanced Design Examples of Seismic Retrofit of Structures

4 Advanced Design Examples of Seismic Retrofit of Structures

1.3 PERFORMANCE-BASED EARTHQUAKE ENGINEERING
Typically, seismic evaluations, and consequently building retrofits, are
intended to ensure preservation of life or to control the risk of casualties in
an earthquake. As was mentioned earlier, reducing economic losses from dam-
age, either due to repair costs or lost functionality of the building, is the other
goal of seismic design. The cost of retrofit, often over 25% of the value of the
building [3], has also caused interest in costs and benefits analyses of several
retrofit options (all satisfying expected performance). This interest in predicting
the level of damage expected in a building before and/or after retrofit has moti-
vated the provision of frameworks for performance-based seismic engineering.
This development is gradually moving toward procedures already available for
performance-based design codes that have been implemented in other disci-
plines, such as fire protection [1]. Detailed analytical risk assessment and ret-
rofit as discussed in this book are within a framework of performance-based
engineering in accordance with up-to-date guidelines such as ASCE 41-13
[7]. The theoretical background of PBEE is on the basis of probabilistic seismic
demand analysis. The Pacific Earthquake Engineering Research (PEER) center
framework is a popular methodology in order to estimate the mean annual fre-
quency (MAF) of exceedance of a particular limit state (LS), as expressed math-
ematically in Eq. (1.1):
Z Z
j
ð
j
MAF LSÞ ¼ G LS EDPÞ dG EDP IMÞj: dλ IMÞj (1.1)
ð
ð
ð
j
j
IM EDP
where EDP is the engineering demand parameter, for example, maximum inter
story drift ratio; IM is the intensity measure, for example, spectral acceleration
(S a ) at the first period of structure and a given damping ratio; G(LSjEDP)
denotes the probability of exceeding LS conditioned on the value of EDP;
and G(EDPjIM) denotes the probability of exceeding EDP conditioned on
the value of IM. It is assumed that the seismic hazard is characterized by an
elastic response spectrum at a minimum. Some nonlinear analysis methods
may also require inelastic design spectra or time histories representing the site
seismic hazard. There are many issues associated with determination of seismic
hazard, both concerning the technical methodologies used and the policy deci-
sions regarding the level of risk. The state of the art in these areas is documented
elsewhere. An issue that needs improvement in earthquake engineering is the
interaction between structural analysts and ground motion specialists to
advance the characterization of ground motions to include potentially more use-
ful parameters, such as duration and convenient measurement of near-fault
pulses.
The current methodology used in retrofit standard and guidelines that is
sometimes called the first generation of PBEE satisfies performance objec-
tives by providing sufficient system performance at a given hazard level, as
is schematically presented in Fig. 1.2. The next generation would utilize