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thermore, even if the ground motion values for collapse-
prevention-level earthquakes were accurately defined using
a seismic hazard analysis, the collapse risk of building
structures still would not be geographically uniform throug-
hout the nation because the hazard of a mega-earthquake
(i.e., an earthquake more intensive than the collapse-pre-
vention-level earthquake) [1] is different for different re-
gions.
Before 1997, the seismic zonation of the United States
was mapped based on the earthquakes with an exceedance
probability of 10% in 50 years. Since 1997, the maximum
considered earthquake (MCE), corresponding to an ex-
ceedance probability of 2% in 50 years, has been defined as
the baseline for the seismic zonation of the United States.
Recently, some adjustments have been applied to MCE
ground motions to achieve a uniform collapse risk of 1%
collapse probability in 50 years [7]. The development of the
current seismic zonation map of the United States provides
a good model for future seismic zonation mapping of China.
This paper first presents the basic concept of IDA-based
collapse fragility analysis. Second, three cities with the
same seismic fortification intensity but different seismic
hazards are selected for analysis. The collapse risks of a
given reinforced concrete (RC) frame structure located in
each of the three cities are predicted and compared. The
result shows that though the three cities have the same
seismic fortification intensity, the collapse risks of the same
building in the three cities are quite different. Additional
research is required to develop the uniform-risk-targeted
seismic design approach proposed in this paper.
2 Collapse fragility analysis based on IDA
2.1 Basic process
Collapse fragility analysis [3–5] based on IDA [6] involves
the following four steps: (a) subjecting a structural model to
a set of Ntotal earthquake ground motion records, (b) in-
creasingly scaling each ground motion to multiple levels of intensity, (c) implementing nonlinear time-history analyses,
and (d) obtaining the collapse probabilities versus intensity
levels for further statistical analysis. At a certain intensity
level, the number of ground motions that will result in
structural collapse is referred to as Ncollapse. The collapse
probability at this intensity level is estimated as Pcollapse=
Ncollapse/Ntotal. By step-by-step scaling of the intensity level,
the complete sequence of the building’s structural behavior
can be investigated, from elasticity to yielding to collapse.
Meanwhile, the serial collapse probabilities versus incre-
mental intensity levels are obtained (see the data points
shown in Figure 1(a)). By assuming a rational probability
distribution (e.g., a lognormal distribution [4, 5]), the cu-
mulative distribution function of the selected intensity
measure (IM) corresponding to structural collapse, referred
to as the collapse fragility curve as shown in Figure 1(a), is
obtained by statistical methods. The collapse fragility curve
is a rational representation of the structure’s collapse re-
sistant capacity, and its reliability depends on the selection
and the total number of the ground motion records adopted
in the IDA [11]. The FEMA 695 report proposes that the
number of ground motion records should be larger than 20
to reflect the random nature of earthquakes [11].
2.2 Probabilistic significance of the collapse fragility
curve
The collapse fragility curve represents the conditional col-
lapse probability at given values of IM, denoted as P (col-
lapse|IM) (see Figure 1(a)) [12]. For a given ground motion