Probabilistic seismic risk assessment of civil infrastructures has been at
tracting attention
in Japan,
especially after recent
mega
-
earthquakes
with
a long
-
lasting series of aftershocks capable of accumulating building damage; e.g., the 2011 Tohoku
earthquake. To this aim
, it is valuable to
be able to
assess
the
failure probabil
ity
of a particular structure and its
evolution
in
time
due to sequential earthquake events, which
may
cause a difficulty for stakeholders
to perform
consistent decision
making
to warrant
business
continuity.
This kind of risk analysis may require state
-
de
pendent fragility curves, which in the
study were developed
for
a
Japanese steel frame
.
To construct
the curves
, a numerical model of
a
t
hree
-
story
steel moment
-
resisting frame
was
first constructed
and
calibrated
acc
ording to the results of shake
table te
st
s for
a
typical
Japanese steel
structure
. This model
was subsequently
transformed in an
equivalent single degree of freedom
(ESDOF) system
,
based on the results of the nonlinear static
(pushover) analysis. The probabilistic damage model
was then
construc
ted
via
nonlinear dynamic
analys
i
s
of the ESDOF
system
.
The spectral acceleration of the elastic period of the ESDOF system was selected as
the
ground motion intensity
measure while the drift angle was
selected as
response measure.
All
the records
used in
the analysis were
selected from the
Japanese strong
-
motion network, K
-
N
et.
Finally
, the state
-
dependent fragility curves
were
developed for
five
levels of
damage:
As
-
New
(AN),
Immediate Occupancy
(IO),
Life Safety
(LS),
Collapse Prevention
(CP) and
Failure
(
F
). The limit
state value for each damage state
(DS)
was set
in compliance with the
results of the
shake table
tes
t
s
.
After computing the
damage state
probability due to the mainshock, t
he time
-
variant aftershock risk of the
steel
structure
was
quantifie
d
integrating
the developed state
-
dependent fragility curves with the seismic hazard,
followin
g a
Markov chain
model
already
available in the literature,
which makes use of aftershock probabilistic seismic hazard analysis
(APSHA)
.
Hazard
was computed
assum
ing that the structure
was
located in
Osaka,
a site that may be affected
by
a
mega
-
earthquake at
the
Nankai
Trough subduction
-
zone
.
In particular,
in the considered exercise,
the most probable damage state due to the
considered
mainshock
scenario
was found
to be
IO,
followed, in probability terms, by
LS, F, AN, and CP.
Given the
probability distribution of the mainshock
-
induced damage,
the daily evolution of aftershock damage was computed, and it
was found that
the
most likely
DS after two m
onths since the
mainshock was
still
IO
followed by F, LS, CP and AN
