Civil Engineering Reference
In-Depth Information
12.10.2 Hazards
Some of the hazards that should be considered in a Class 3 risk
assessment are listed in
Table 12.2
. The list is illustrative only
and is not exhaustive. Indeed, it is strongly preferable for the
designer to give fundamental consideration to the hazards that
might occur in each circumstance, rather than to use a check-
list or prescribed list of hazards.
The basic assumption underlying a Class 3 systematic
risk assessment is that the buildings categorised as Class 3
are such either because the hazard consequences (usually
in terms of potential loss of life but sometimes also safety
or financial loss) or the likelihood of one or more hazards
occurring are greater than in Class 2. The classification of
the building as Class 3 is itself a decision that results from a
high-level risk assessment on this premise, and therefore the
systematic risk assessment may be focused on the physical
effects of the hazard in terms of the extent of the structure at
risk of collapse.
A key difficulty in a systematic risk assessment is the treat-
ment of low likelihood/high consequence events for which
a quantitative assessment is often meaningless. However,
for such events the consequences are often so onerous that
should the hazard materialise it will not be deemed a tol-
erable event. Here, therefore, the designer should consider
whether it is foreseeable that the hazard will materialise, and
if so focus on determining measures through which the risk
can be mitigated so far as reasonably practicable. In doing
so, the risk assessment for such hazards effectively becomes
a conditional sum used to determine mitigation measures
necessary, given that there is a finite likelihood that hazard
could materialise and the consequences would automatically
be disproportionate.
Design and construction
Calculation error
■
Robustness during construction (or demolition/alteration)
■
Sub-standard construction
■
Gross construction error
■
Material defects
■
Sub-standard components, for example, due to counterfeiting of
■
Quality Control markings/certification
Dropped object
■
Unauthorised alteration
■
Permanent, imposed and environmental actions
Wind, snow, rainwater ponding, flooding■■
■
Excessive floor loading
■
Earthquake
■
Fire
■
Structural deformation/movement
■
Subsidence/ground movement
■
Groundwater level change
■
Undermining of foundations
■
Fatigue
■
Corrosion/rot
■
Accidental actions
Vehicle impact
■
Gas explosion
■
Malicious actions
Safety-critical vandalism
■
Explosive terrorist attack
■
Combined hazards*
12.10.3 Uncertainty
The risk assessment should also consider the uncertainty of the
assumptions made. This uncertainty can substantially affect the
determination of the risk, a sensitivity which increases expo-
nentially as likelihood and consequence move into the 'tail' of
the distribution. As such, it is necessary to undertake a sensitiv-
ity analysis as part of the risk assessment, particularly for low
likelihood/high consequence events where the uncertainty and
sensitivity are likely to be greatest. The sensitivity of the risk
to the underlying assumptions can result in the need for more
extensive mitigation over that suggested by the 'central' values
of the likelihood and consequence. For low likelihood/high con-
sequence events, a 'cliff-edge' analysis may sometimes be war-
ranted, the purpose of such an analysis being to demonstrate that
there is a gradual change in the structural behaviour beyond the
design value. Where a so-called cliff edge is identified, measures
should be implemented either to change the structural behaviour
so that the cliff edge is removed, or to push the cliff edge to a
point sufficiently above the design value that the risk associated
with the uncertainty of the assumptions is mitigated.
* A combination of hazards is not necessarily as straightforward as multiplication
of the two independent risks. After some events a second hazard can become
highly likely, for example, vehicle impact or sudden ground movement leading
to flooding or to a gas release and potential gas explosion or fire. Equally the
consequences of a second otherwise independent hazard may increase, for
example, where fire protection is lost in a vehicle impact such that structural
collapse could occur if a fire does happen. The World Trade Center exhibited
remarkable robustness under the aircraft impact with the loss of several columns,
but collapse was eventually due to the loss of fire protection in the initial impact
and the ensuing fire.
Table 12.2
Examples of hazards
12.10.4 Determination of risk
The central part of a systematic risk assessment is the deter-
mination of the risk for each hazard through consideration of
the two constituent parts, likelihood and consequence. This is
followed by consideration of what is necessary to mitigate the
hazard so far as reasonably practicable as described above.
The risk assessment may take any form appropriate to the par-
ticular case under consideration. This may be qualitative, semi-
quantitative or in some cases fully quantified (
Table 12.3
).
