Stewart Brown - University of South Wales
A fire risk assessor in the UK inspected a seven-story office building and suspected that it might be occupied by more people than guidance recommends.
The guidance stated that the upper floors of this building should be occupied by no more than 427 people. These floors were occupied by 480 people. He was reluctant to tell the occupiers that they should lose staff so he organized an evacuation drill and he arranged to time how long it took people to evacuate each of the floors. The thinking was that if everyone could get to safety promptly then no action would be necessary, even though the building was slightly over-occupied.
The drill was held on a day when the building was nearly fully occupied. Rather shockingly, the times that it took for each floor to evacuate were much longer than the anticipated 2.5 minutes which is the general objective of UK fire safety.
The evacuation was then reproduced using Pathfinder. The model was tweaked so that repeated runs, with occupants in differing locations, averaged the observed evacuation times on each of the floors. This mainly involved adjusting individuals’ relative priorities so that they behaved as observed where flows merged in the staircase.
The model was adjusted to show a code compliant building (i.e. the population was dropped from 480 to 427) and the evacuation times were still far longer than was considered safe.
This exercise exposed the fact that evacuations from such buildings physically cannot take place in the time that it is assumed they do. And yet we do not have a history of fire deaths in office buildings in the UK.
Pathfinder can give an answer to this apparent conundrum and can clearly depict how our assumptions of what takes place during an evacuation can be wrong.