Civil Engineering Reference
In-Depth Information
Occupancy
Travel distances
Evacuation time
Means of detection,
giving warning and
suppressing fire
MEANS OF
ESCAPE
Evacuation procedures
Escape exits
Emergency lighting signage
Travel routes
External escape routes
Figure 9.21
Factors affecting means of escape
➤
Travel distances
➤
Escape exits
➤
Travel routes
➤
escape time (ASET). The difference between RSET and
ASET is the margin of safety between the time when
evacuation is complete and conditions within the building
become untenable.
The available safe escape time is therefore the time
from ignition to complete evacuation of the building.
The time it actually takes a person to move from any
occupied part of the building to a place of ultimate
safety will vary considerably due to the physical
condition of the occupants as well and the distance they
have to travel before reaching safety.
It is sometimes useful to consider the ASET in four
simple phases:
External escape routes
➤
Emergency lighting signage
➤
Means of detection, giving warning and suppress-
ing fi re.
It is important to understand how quickly it is necessary
to evacuate people from a building, not least because it
allows a realistic calculation of the numbers and width
of exits required to achieve evacuation within the time
available.
In published document PD7974 the British Standard
Institute suggests that evacuation time can be seen as
the sum of a number of key elements including the time
taken from ignition to detection and from detection to
the sounding of an alarm. When planning buildings it
is important for architects to understand how quickly
they need to evacuate people in case of fi re. PD7974
describes this as the required safe escape time (RSET).
The RSET includes time taken to:
Phase 1 - alert time from fi re initiation to detection/
recognition
Phase 2 - pre-movement time taken by behaviour that
diverts an individual from the escape route(s)
Phase 3 - travel time to physically get to an exit
Phase 4 - fl ow time, i.e. how long it takes for the occu-
pants to move through the various stages of the
escape route. Doorways are invariably the least effi -
cient element with the longest fl ow time and restric-
tion on the route.
➤
Detect the fi re
➤
Sound the alarm
➤
Occupants to recognise the alarm
➤
Occupants to respond to the alarm
➤
Travel time.
Research has shown that Phase 1 can be as long as two-
thirds of the total evacuation time required. The most
important variable to consider when assessing means of
escape provision is the construction of the building. If the
building is less likely to burn or allow rapid fi re spread the
occupants will have more time to evacuate and therefore
a better chance of escape. (Chapter 10 explores reasons
why people may delay reacting to a fi re alarm.)
It can be seen from Figure 9.22 that the safe evacuation
time is divided into free movement time and travel time.
The overall available time for occupants of any
building to escape is described as the available safe
