Civil Engineering Reference
In-Depth Information
technical criteria (Penelis and Kappos 1997). The general criteria are: con-
sideration of cost, durability, availability of workmanship, possibility for
appropriate quality control, occupancy, aesthetics, preservation of architec-
tural identity, and duration of work. Whereas the technical criteria can be
broadly categorized as: enhancement of system ductility, enhancement of
stiffness and strength, and enhancement of ductility, stiffness, and strength.
In Section 6.4.3, the primer of the CBA and LCC is provided and illustrated
with an example. Subsequent to that, an application of MCDM for retrofi t
selection is illustrated with an example.
6.4.3 CBA and LCC
CBA and LCC are the essential tools for effi cient risk management of
structures and infrastructure that are subjected to environmental distur-
bances, such as deterioration and natural hazards (Rosenblueth 1976; Rack-
witz 2000; Wen 2001). They can take into account all relevant costs arising
during the service life of a structure, such as initial construction cost, main-
tenance and operation cost, and damage cost due to external hazards. It is
important to note that the timing of occurrence of various costs differ; initial
cost is incurred at the beginning; the maintenance and operation cost occurs
continuously and regularly over the service period; whereas the earthquake
damage cost, for instance, only occurs infrequently but can be very costly.
For maintenance/operation and earthquake damage costs, adequate dis-
counting to the present time is necessary. Once all relevant costs are taken
into account and adequate discounting is applied, CBA and LCC produce
a cost estimate of an engineered structure over its lifetime for a given
design/retrofi tting/intervention alternative. Comparison of such estimates
for different alternatives is a rational approach to identify the most eco-
nomically effi cient and viable option.
Suppose that a structure is to be designed and constructed for a service
in a seismic region. Focusing on earthquake risk, the LCC of the structure
consists of initial construction cost, earthquake damage cost, and mainte-
nance/operation cost. The objective function
O
(
X
,
t
) of the structure defi ned
as the benefi t at the present value,
B
(
X
,
t
), minus the LCC at the present
value can be expressed as (Rosenblueth 1976; Rackwitz 2000; Wen 2001):
()
∑
⎡
⎢
Nt
k
⎤
∑
(
)
=
(
)
−
()
−
(
)
(
)
OXt
,
BXt
,
C X
E
C
exp
−
γ
t P Xt
,
⎥
0
j
i
ij
i
i
=
1
j
=
1
t
∫
()
(
)
−
CX
exp
−
γτ
d
τ
[6.12]
m
0
where
X
is the structural design parameter, such as the design base shear
coeffi cient;
C
0
(
X
) is the initial construction cost;
N
(
t
) is the number of
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