Civil Engineering Reference
In-Depth Information
5.3.2 Service life, life-cycle cost and environmental
impact issues
Figure 5.2
illustrates the relationship between the main phases
in the life of a constructed asset (on the horizontal axis) and
the three factors denoted below relative to the particular phases
(time) in the life of a constructed asset, namely:
A.
The relative importance of decisions made in different
phases in the life of the asset.
B.
The relative influence that these decisions can have on the
life-cycle cost and environmental impacts of an asset.
C.
The potential to add value through the application of value
management processes.
Figure 5.2
shows the potentially high influence or impact of
factors A to C in the early stages of the life of a constructed
asset, and the decreasing influence/impact as the asset passes
into the later stages of its life. The potential influence and
impact of decisions upon total direct life-cycle cost and envir-
onmental impacts of a constructed asset will clearly be greatest
when the conceptual development is being undertaken; which
is when the least design information is available.
The potential beneficial effect diminishes greatly for deci-
sions made during the later stages of the life-cycle, such that
there is little that can realistically be done during the later
phases in the life-cycle of a constructed asset to change its per-
formance or influence its life-cycle cost/environmental impact.
In addition, attempts to effect changes to the performance of
an asset in the later stages of its life will be much more expen-
sive and also probably incur higher environmental impacts
from the greater resources required to effect the changes, as
portrayed in
Figure 5.3
. Furthermore, changes made at this
time will generally also affect the availability of the asset for
use and/or impair its functionality while the required works
are undertaken. In the case of 'public' assets such as bridges,
such actions may also create considerable disruption to users;
with the degree of disruption and consequential delay to users
depending on the intensity of use. Typically changes in the
later stages of the life of a constructed asset will generally pro-
vide poor value to the owner and may only achieve a marginal
operational benefit.
Birth of Asset
Operation and use
Concept
Design
Construction
Disposal
Creation of Asset
Use of Asset
Figure 5.3 Timing of service life decisions relative to (D) the level
of knowledge about performance of asset, and (E) the potential cost/
environmental impact of any change
Thus these considerations are pertinent in two circum-
stances, namely:
■
during the design and construction of a new asset, and
when through-life intervention works are carried out on an exist-
■
ing asset.
ISO 15686: Part 5 (BSI, 2008) suggests that up to 80% of the
operation, maintenance and replacement costs of a building are
influenced by the first 20% of the design process.
Figure 5.7
in
ISO 15686: Part 5 presents a curve indicating the scope for life-
cycle cost savings during the various life-cycle phases of project
for the construction of an asset. The ISO 15686: Part 5 curve
defines a similar relationship to that portrayed in
Figure 5.2
,
which is sometimes referred to as the 'opportunity curve'.
Total direct life-cycle operational costs could be an order
of magnitude (or more) larger than the construction cost,
but this depends on the nature of the asset being considered.
(
Figures 5.2
and
5.3
consider only direct costs and do not
address the potentially much larger business related expendi-
tures associated with the use of the asset, as discussed above.)
For illustrative purposes, design and through-life manage-
ment strategies for constructed assets may be summarised some-
what simplistically by two conflicting ideologies, namely:
Approach 1. Buy cheap (low initial or first cost) and pay more at
a later stage via a higher through-life operational cost
resulting in a higher life-cycle cost.
Approach 2. Pay more initially (higher initial or first cost), but gain
from a reduced through-life operational cost resulting
in a lower life-cycle cost.
However, strictly speaking neither approach is necessarily
applicable in all circumstances, as spending money on items
that do not contribute meaningfully to extending the service
life or improving functionality will simply increase cost with-
out achieving commensurate benefits. Accordingly the invest-
ment made has to be appropriate. Thus 'throwing money' at a
project by 'gold-plating' the specification is unlikely to mean
lower costs later in the life of the asset. However, discounting
future costs would confirm whether various investment options
brought worthwhile benefits.
Birth of Asset
Operation and use
Concept
Design
Construction
Disposal
Creation of Asset
Use of Asset
Figure 5.2 Timing of service life decisions relative to (A) their
potential impact upon the performance of a structure, (B) their
impact on life-cycle costs and environmental impacts, and (C) the
potential to add value through value management processes
