EBSS’ life safety risk grades are set out in the table to the right. Assigning the numbers to risk categories is obviously subjective. One person’s high risk could be a medium risk to someone else. We have taken a cautious approach in describing the grades.
The starting point was to define the boundary for the No Effective Risk grade. Here we have used odds of 1 in 1 million, which is the conventional test for no effective risk in the international risk literature.
The most sensitive boundary is the upper limit on the high risk grade. The most relevant consideration here is the risk that we find acceptable in other activities. Consider life risk on the roads. While we would prefer that the road toll were lower, as a society we do not regard driving as a ‘high risk’ activity.
The chance of dying on the roads for an average New Zealander per year is 1 in 12,000. However, we spend much less time on the roads than in buildings, so the risk per hour on the roads is higher than in a building that has a life safety risk of 1 in 12,000 (on the assumption of 40 hours per week occupancy). Assuming the average person spends one hour a day on the roads, the road safety risk figure will be on the same scale as building risk with annual odds of death of 1 to 2100.
Based on this number, we have chosen a high risk cut-off of 1 to 5,000. At that level we are also getting into the area where it makes economic sense to do the strengthening to lift the building out of the high risk category.
The EBSS Risk Calculator will be available in a few weeks and will allow you to calculate the life safety risk of different classes of buildings in different cities and towns in New Zealand