Civil Engineering Reference
In-Depth Information
8.5.2 Transport networks
A road network can be represented as a directed, weighted graph. There is
traffi c demand between any two nodes that drives the fl ow of traffi c along
links in some way. Usually the shortest paths between any pair of nodes or
the paths with the shortest travel times are identifi ed. However, real net-
works are far more complex and the consequences of the failure of one or
more links need to be analysed with respect to the whole system. It is not
uncommon to see 'knock-on' effects of the failure of a major road on many
neighbouring roads at least in the short term (Goodwin
et al.
1998; Clegg
2007).
Application of vulnerability concepts to road network (Agarwal
et al.
2011) is based on a measure of transmittance which depends on capacity
speed and length, and orientation is used to calculate the well-formedness.
The clustering process starts with the identifi cation of loops with the best
well-formedness and higher-level clusters are formed by including the
neighbouring loops so as to increase the well-formedness of the cluster. If
there is no increase in the well-formedness, a new loop is started and the
process continued until the whole network is one cluster. A search through
the clustering hierarchy results in the identifi cation of different vulnerable
scenarios which can be used for managing risks.
8.6
Advantages of vulnerability analysis
Vulnerability analysis, as presented here, can be used to analyse risks to
lifelines and to plan for recovery following an earthquake. An analysis of
chance is one part of risk assessment. For low-probability high-consequence
events, numerical results based on traditional probability have to be set in
a broader picture (van Breugel 1997). The Kobe earthquake in 1995 has
brought a qualitative change in the way risk analysis should be done and
used. Probability of failure alone is not suffi cient to exclude a failure sce-
nario - we must also consider the consequences of such failure scenarios.
Vulnerability analysis provides an approach to assess the form of a system
so that disproportionate consequences could be minimized.
It has its own place amongst the popular quantitative techniques used
for the analysis of failures. For example, cut set analysis gives only those
scenarios that are obtained as total failure scenarios in vulnerability analy-
sis. Fault tree analysis starts with a predefi ned failure whereas vulnerability
analysis aims to explore unforeseen but catastrophic failure scenarios. In
vulnerability analysis, the effect of a deterioration event becomes apparent
through a clustering and unzipping procedure rather than by traversing all
the paths as in event tree analysis. In structural reliability analysis, it is
necessary to have a system model of loading. In the vulnerability approach,
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