Civil Engineering Reference
In-Depth Information
engineering. They do not think that they are taking any risk in
respect of structural safety or building serviceability.
Clearly there are exceptions; an authority commissioning
major infrastructure in a seismic or flood prone area should
be thinking explicitly about risk. It may be crucial that bridges
and hospitals survive, in good working condition, events that
cause extensive damage around them and, in such cases, the
engineer needs to agree with the client whether the social need
dictates an unusually low 'risk appetite' and therefore greater
robustness in the structure.
Box 3.1
Individual risk criteria
The figures given by HSE for the scale of Individual Risk, from 'broadly
acceptable' to 'unacceptable' were arrived at by considering the risks
to which people are exposed in normal circumstances.
Individual Risks (of fatality) averaged over the UK population, are
approximately:
All accidents 1 in 4200 per year
Road traffic accidents 1 in 10 000 per year
Fall, etc. at home 1 in 8000 per year
Fire at home 1 in 70 000 per year
Accident at work 1 in 150 000 per year
Note: this varies greatly between occupations
Lightning strike 1 in 10 000 000 per year
3.4 CDM - construction, maintenance,
refurbishment, demolition
The Construction Design & Management (CDM) Regulations
(HSE, 2007) actively require the (UK) professional and man-
agement teams involved in construction to consider risk. The
headline risks are to do with the construction process and
the supply chain, but full treatment needs to consider main-
tenance, refurbishment (plant end-of-life replacements) and
demolition.
The basic methodology of identification of scenarios and
causes, evaluation of likelihoods and consequences and assess-
ing the acceptability or otherwise of risks is set out later in this
chapter.
However, the details of exercising risk management on the
issues covered by CDM are covered in specialist publications
at greater length than can be included in this manual. Further
reading is given at the end of this chapter.
On a recent project, the question was raised of what the
appropriate risk criterion might be for the assessment of ship
impact against the tower of a major suspension bridge over a
navigable waterway. The basic individual safety criterion on
its own would not provide an answer as it would be unlikely
that any one individual would be so constantly exposed to the
risk from ship impact that the risk to that person would be the
determinant. Risk to occupants of vehicles on the bridge at
the time of impact would be relevant and would inform the
ALARP assessment - which would assess whether the cost to
reduce the risk would be 'greatly disproportionate' to the value
of the reduction.
The project, however, was big enough for the regional eco-
nomic loss, in the event the bridge was unusable for a number
of years, to be a major factor in the assessment of impact risk
and prevention measures. It became a matter of the business
case for the bridge itself and the risk appetite of the owner.
A more detailed discussion of structural failure risk, and the
example of ship impact, is set out in Duckett (2004).
3.5 Construction time cost and buildability
The expression that 'time is money' and the triangle of 'time,
cost, quality' are commonplace. The completion date and
budget are major risk elements to any client, be they domestic,
developer or public authority. Failure to open and start gener-
ating revenue can have massive effect on the client's finances
and credibility - as Eurotunnel found when the opening of
revenue services through the Channel Tunnel was delayed by
a year and the extra debt almost crippled the company. The
design team should therefore be aware of the priority that the
client is putting on time and budget constraints.
Risk may arise in site operations or far back in the supply
chain. Clearly adverse weather at the site can reduce the rate of
construction - and the client needs to know whether the form
of contract includes that risk or transfers it to the contractor.
For a long-running project, fluctuations in the world price of
cement, steel or indeed labour can all affect the outturn cost
and, again, the client needs to know where the risk is being
carried.
While the likelihood of each potential adverse event may
be estimated with a 'spot value', such as the probability of
occurrence during the contract, the 'potential loss' item in the
risk assessment has two measures - duration and cost. Risk
will need to be evaluated and quoted in both ways, so care is
needed to keep the risk assessment even-handed.
3.3 Acceptability of risk - appetite vs. aversion
In a commercial environment, it is conventional to treat risk
in terms of willingness to take more risk with the prospect of
more gain, or reluctance to take risk in favour of greater cer-
tainty of continuity. The shorthand term is 'risk appetite'. A
company which favours lower risk and accepts lower profits is
said to be 'risk averse'.
After the 2008 banking crisis, it was claimed that the banks
were 'risk averse' when in fact they had been trapped by their
'risk appetite' exceeding their ability to quantify and manage
the risks. Whether a company considers it has a high or a low
risk appetite, ignorance of how its actual risk compares with its
appetite is itself a risk.
In construction, the client is usually taking a substantial
commercial risk in terms of developing a building for a future
market. But most clients are risk averse in respect of structural
