Civil Engineering Reference
In-Depth Information
1.
Durability critical:
where failure of a component part would
result in a significant and adverse affect upon the function-
ality of the asset, such that it is unable to provide the desired
minimum level of performance. Durability failure therefore
has the potential to cause major disruption and unacceptably
high costs. These costs would typically comprise direct costs
(i.e. to undertake the repair) and consequential costs (i.e.
those due to disruption/loss of service). Durability critical
component parts of an asset are therefore required to last its
expected lifetime without an intervention being made.
For example, the failure of a series of bridge beams may
be durability critical where their failure causes unacceptable
disruption to the functioning of a roadway underneath. Also
if works to maintain a component part are expected to have
an unacceptable impact on the functionality of the bridge,
then the component part would need to be classified as dur-
ability critical. This would of course depend on the owner's
requirements for continuity of the function of the bridge.
Durability critical assets or component parts may require
additional protection measures to reduce the risk of durabil-
ity failure or to minimise the need for intervention within
the required service life, thereby achieving a satisfactory
through-life performance.
2.
Durability sensitive:
where failure of a component part
would adversely affect upon the functionality of the asset,
but not to such an extent that it is unable to provide the
desired minimum level of performance. Although the effi-
ciency of operation or the functionality of the asset might
be reduced, maintenance, remedial or replacement works
could be carried out for an acceptable cost and associated
environmental impact.
However, if the necessary remedial or other works were
delayed excessively or not undertaken for some reason, it
is conceivable that the functionality of the asset could be
adversely affected in the longer term. For example, if the
joint sealants in the cladding of a tall reinforced building
were to experience deterioration it is unlikely that the struc-
tural functionality of the building would be immediately
impaired. However, this situation might permit the ingress
of moisture that would potentially cause corrosion and the
risk of falling debris, which could pose a hazard to pedes-
trians passing the building. This is likely to be deemed to
be an unacceptable hazard and would therefore potentially
affect the functionality/use of the building and/or the zone
around the building concerned.
3.
Maintainable:
where deterioration of a component part
does not impact upon the performance of the asset such that
the functionality of the asset is not adversely affected. Thus
maintenance, remedial or replacement works can be carried
out without undue inconvenience or disruption and for an
acceptable cost and associated environmental impact.
In making this categorisation, all component parts should be
considered carefully and the designer should be mindful of the
consequences of a lack of performance or physical failure of
seemingly unimportant items.
The designer should also consider the level of redundancy
in the structure (e.g. simply supported structures are likely to
have lower redundancy than a structure utilising continuous
beams). The smaller the degree of redundancy in a structure
the more critical durability failure tends to become. However,
the designer should also be aware that unchecked deterioration
of a component part of a structure with a high degree of redun-
dancy, while possibly taking longer to fail than a similar com-
ponent in a structure with a lower degree of redundancy, could
eventually result in a more catastrophic and/or widespread
failure.
Box 5.4
What is the required service life?
BS EN 1990
- Basis of structural design
defines design working life
(service life) as:
The assumed period for which a structure or part of it is to be
used for its intended purpose with anticipated maintenance but
without major repair being necessary
.
BS EN1990 gives the following examples:
Design
working life
category
Indicative design
working life
(years)
Examples
1 10 Temporary structures (see
Note 1)
2 10-25 Replaceable structural
parts (e.g. gantry girders,
bearings)
3 15-30 Agricultural and similar
structures
4 50 Building structures and
other common structures
5 100 Monumental building
structures, bridges and
other civil engineering
structures
Note 1: Structures or parts of structures that can be dismantled with a view
to being reused should not be considered as temporary.
Some owners have been known to specify service lives of perhaps
300 years or more on some forms of very long-life infrastructure. The
actual end of use can be determined by a number of factors includ-
ing changes of use and economics, as well as technical failure/non-
compliance of the structure or its parts, as is discussed in the main text.
There are many factors in the design and construction process
that can determine whether an asset will meet its design service life.
These problems highlight the need for:
A holistic approach to design embracing the entire construction
■
process and explicitly addressing service life.
Whole life costing in assessing the cost performance of con-
■
structed work and in deciding between alternative means of
achieving the owner's objectives.
