Structural vulnerability analysis of civil infrastructure facing seismic hazards - Seismic Risk Analysis and Management of Civil Infrastructure Systems - page 206

signifi cantly to the occurrence of such major failure scenarios (see Kami-
netzky 1991 and Delatte 2009 for several examples, including Hyatt Regency
Walkways, Skyline Plaza, Alfred P. Murrah building). Thus the whole
problem of dealing with total system uncertainty requires an effective
management of structural risks anticipating not just failure scenarios that
are most likely to occur but also those which lead to catastrophic consequ-
ences. This has motivated the development of an approach, called structural
vulnerability analysis (Wu et al. 1993; Lu et al. 1999; Agarwal 2001; Agarwal
et al. 2001a, 2003). The approach has been generalized to analyse infrastruc-
ture systems which can be modelled as a network system (Agarwal et al.
2001b, 2011). Once vulnerable failure scenarios have been identifi ed, their
likelihood can be assessed by gathering the evidence available for those
failure scenarios, thus enabling a measure of risk to a structure or to a
system (Agarwal 2005).
The objective of this chapter is to present the vulnerability analysis
approach, with examples, to manage structural or system risks. The chapter
is organized as follows. Firstly, interpretations of different related terms, e.g.
vulnerability, hazards and risk, are set out. Secondly, a theory of vulnerabil-
ity of structures is introduced. It forms a basis for fi nding failure scenarios
that are later examined for the chance of failure under specifi c actions and
loads. Thirdly, natural and man-made actions having the potential to cause
failure scenarios are examined. Structural risk assessment is carried out by
considering the consequences of different vulnerable scenarios and the
probabilities of actions causing them. Finally, generalization of the vulner-
ability approach to infrastructure systems is outlined.

8.2 Vulnerability, hazard and risk

Vulnerability captures the idea of susceptibility to damage. The susceptibil-
ity arises from the form of the system within a given context. An inappro-
priate form if damaged may lead to consequences which are disproportionate
to the initial damage. For example, the Ronan Point high-rise block of fl ats
in London was a vulnerable structure, because a small domestic gas explo-
sion in one fl at caused the whole side of the building to collapse. While there
is little variation in the meaning of vulnerability across different disciplines,
the way it is measured varies. In earthquake engineering, vulnerability of a
component is its susceptibility from an earthquake of a given intensity. An
earthquake in California may produce very different consequences from
the same strength earthquake in Iran (developing country). In social
systems, adaptability and stability to change or damage determine the vul-
nerability. A probabilistic measure of vulnerability, defi ned as the ratio of
the failure probability of the damaged system to the failure probability of
the undamaged system (Lind 1995), can be applied to almost any system.