4. Death.
For each of these four possible outcomes, the expert assesses its
likelihood relative to the other possible outcomes, as a percentage
part, so that the cumulative likelihood of all the possible outcomes
for a single event is 100%. Two distributions are assessed for each
type; one assuming use of personal safety gear and the other
assuming absence of the appropriate gear. The assessment is based
on the assessor’s professional experience.
The distributions are used in the CHASTE approach to calculate
a single severity level, which is multiplied by the likelihood of
loss-of-control and the exposure levels for each event in order to
calculate an overall risk level estimate. To transform the severity
distributions into single weighted values, a set of weighting factors
must be applied to the severity levels, which express the relative
importance a risk assessor attributes to death, say, in relation to
other injuries. This is a value judgment, which must be made by
the end user. An example of four possible weight values can be
seen in the second column, ‘‘Severity weight”, of Table 2. In this
example, the user determined a scale for severity from 1 (for the
lowest level) to 100 (for the highest), and set intermediate values
of 5 and 25 for levels 2 and 3 respectively.
The example provided in Table 2 illustrates the overall procedure
used for setting the expected severity level for the case of
an accident scenario ‘‘falling from over 5 m height” while ‘‘casting
concrete for exterior walls using industrialized forms”. The values
that derive from step 2 of the CJSA survey appear in the ‘‘Expected
occurrence” column. Two distributions are provided, reflecting the
likelihood of occurrence of each outcome dependent on the use or
non-use of personal safety gear (a safety harness in this case). The
likelihood of use is 33%, and of non-use is 67%. The resulting
weighted severity level is 52.6 (out of maximum possible value
of 100).
3. Trial implementation
The CJSA method was developed and first applied in practice
within the framework of the CHASTE research project. The scope
for this implementation covered 14 common construction activities
from all phases of a typical multi-story building project.
3.1. Step 1 – Identification
In step 1, the knowledge was elicited in a series of workshops
with safety experts and senior site managers, who are legally
responsible for site-safety. Each expert was asked to analyze a single
construction activity according to his or her experience.
Table 3 lists the range of activities covered with details of the
number of work stages and loss-of-control events that were identified
for each activity. After sorting, filtering and dismissing overlapping
data, 699 different possible loss-of-control events were
defined for 14 construction activities, out of the 875 loss-of-control
events enumerated in Table 3.