Civil Engineering Reference
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the water outage area will be and how the customer demands are satisfi ed,
given the earthquake-induced damage scenario at the component level.
A special hydraulic analysis computer program, Graphical Iterative
Response Analysis of Flow Following Earthquakes (GIRAFFE), has been
developed at Cornell University for hydraulic network analysis of damaged
water supply systems (Shi and O'Rourke 2008; Wang and O'Rourke 2008).
GIRAFFE is able to compensate for the limitations of commercially avail-
able hydraulic analysis software in predicting unrealistic negative pressures
in a heavily damaged system by eliminating portions of the network con-
taining negative pressures. GIRAFFE simulates trunk line damage explic-
itly by a Poisson process to model the occurrence of pipe damage and
hydraulic models for both pipe break and leak to model the water outfl ow
from the system. In addition, damage to distribution lines is simulated
implicitly by adjusting the nodal demands in the trunk line system.
GIRAFFE is capable of performing Monte Carlo simulation and compiling
the simulation results in a format that can be used in GIS. The LADWP
water supply system performance during the 1994 Northridge earthquake
was modeled using GIRAFFE, and it was found that GIRAFFE provided
simulation results consistent with the observed water outage and the
recorded fl ows in the system after the earthquake. The favorable agreement
between simulation results and system measurements shows that GIRAFFE
is capable of modeling severely damaged water supply systems, and pro-
vides confi dence in applying GIRAFFE.
24.6.1 LADWP system response
GIRAFFE has been used to assess the seismic system performance of the
LADWP water system. Monte Carlo simulation was performed for each of
the 59 scenario earthquakes. In each simulation, GIRAFFE simulated the
trunk line damage explicitly by implementing the hydraulic models for pipe
breaks and leaks, and modeled the damage to local distribution system
implicitly by adjusting the nodal demands according to the repair rates in
the local distribution systems. Then, GIRAFFE utilized the EPANET
hydraulic engine to check network connectivity, perform hydraulic analysis,
and eliminate negative node pressures until no negative node pressures are
present in the system. Finally, GIRAFFE compiled the results from each
simulation and provided performance index, such as mean of system ser-
viceability index (SSI), which is defi ned as
n
i
∑
Q
i
i
=
1
[24.1]
SSI
=
n
i
0
∑
Q
i
i
=
1
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