Structural vulnerability analysis of civil infrastructure facing seismic hazards - Seismic Risk Analysis and Management of Civil Infrastructure Systems - page 207

Whatever the measure is, vulnerability is a property of a system and it
concerns consequences in relation to the initiating damage events.
The core idea of hazard is potential for danger. It is a state of the system
or process with a potential for harm. For example, a wind load is potential
for damage to property; extreme events such as hurricanes and earthquakes
are a potential for large loss of life and damage to infrastructure. Hazards
can be managed by removing them if at all possible, by reducing them, by
modifying the system or by remedying them so that the risk is acceptable.
Hazards can incubate over a period of time (Turner and Pidgeon 1998).
Blockley (1992) has used a balloon analogy to illustrate this. The air pres-
sure is analogous to the proneness to failure of the system. Hazardous
events increase the predisposition to failure and the balloon grows in size.
Letting out some of the air parallels the effect of risk management decisions
that remove some of the predisposing events and mitigate the proneness to
failure. An overstretched balloon represents 'an accident waiting to happen'.
The distinction between hazard and vulnerability is a subtle one. Physical
scientists view vulnerability in terms of likelihood of occurrence and impact
of physical events such as natural hazards. Social scientists tend to view
vulnerability as the socio-economic factors that determine people's ability
to cope with stress or change (e.g. Boulding 1989; Blaikie et al . 1994). The
broad distinction, however, is made based on whether vulnerability is com-
prehended as potential damage to a system or as a state that exists within
a system before it encounters a hazard event. The latter view indicates that
vulnerability exists independently of external hazards. Social vulnerabilities
of this type might include poverty and inequality, access to insurance and
housing quality. Material defects or lack of corrosion resistance also fall in
the latter category. Whatever the defi nition or measure, one of the impor-
tant ways of managing risks is to reduce vulnerability. In an earthquake,
potential for catastrophic disasters does depend on vulnerability of the
infrastructure and so in that sense, vulnerability is a particular form of
hazard - a hazard which is internal to the system. The concept of robustness
is directly related to vulnerability. Robustness is the ability of a system to
fulfi l its function in the face of uncertain and adverse conditions such as the
loss of a component or abnormal changes in the demands. A system which
is vulnerable in any one way cannot be robust. There are several ways to
increase the robustness of a structure but it is diffi cult to quantify it. Fragility
is another term that has developed a specialist meaning. The word implies
easily damaged or broken, but it is often used to describe the probability
of a stated level of damage for a specifi c hazard, e.g. an earthquake.
Risk is commonly defi ned as the combination of chance and conse-
quences of an event. Kaplan et al . (1981) defi ned risk by a triplet ( h, p, c )
where h if

it does happen, what are the consequences? Generally, hazard analysis is

= what can go wrong, p = how likely is that to happen, and c =