Civil Engineering Reference
In-Depth Information
Calculating exit capacity
Where two or more exits are provided from either a
storey exit or on ground fl oor, it should be assumed that
one of the exits may be compromised by fi re thus pre-
venting the occupants from using it. It is therefore essen-
tial that the remaining exit or exits are of suffi cient width
to allow all persons to escape in the available time.
Therefore when considering the adequacy of the
means of escape for a building it is necessary always to
discount the largest available exit in any given situation.
For example, if a social club room is expected
to accommodate 150 people it will need to have two
separate exits of a minimum width of 1050 mm each, or
three exits of 850 mm each. It can be seen in this example
that the capacity of the exits after one of the largest exits
has been discounted is still suffi cient to comply with the
guidance in the ADB regarding exit widths.
fl oor of origin, vertical evacuation may still be necessary
if the fi re is not brought under control.
The advantage of Stage 3 travel is that those
escaping should by this stage have reached a position
where they are protected from fi re for a period of 30
minutes or more. They should not encounter further fi re
hazards en route to ultimate safety.
For most of the occupants vertical evacuation will be
by staircases. Normal accommodation access lifts should
never be used in a fi re situation as the occupants may
become trapped or be taken to a fl oor that is at risk from
the fi re itself.
For disabled people it may only be possible to
evacuate using a specially designed lift on an independ-
ent electrical supply, with the potential for fi re warden
control and communications between the lift car and the
evacuation control point.
Alternative escape routes - the 45º rule
When considering the adequacy of the exit capacity it is
also necessary to understand the term 'separate exits'.
Exits can only be regarded as separate if, from any point
in the room or space in question, they are at least 45º
apart.
Stage 4 Travel - fi nal escape at ground level
Stage 4 travel is from the foot of the staircase to the out-
side. The stairs should not all converge into one com-
mon area at ground level, otherwise a single incident
can block all escape routes.
The fi nal exit and external design of a building
also have to be considered in escape planning. It must
be possible to leave the building and to get to a safe
distance. The volume of people that may escape from
a building governs their need for a readily identifi able
assembly point or transfer area. Where large numbers of
people may be involved, it will be necessary to plan these
areas, so that access for the emergency services is not
compromised.
B
E
A
45 or
more
D
less than 45
C
9.5.4 Escape exits
Figure 9.34
The 45º rule applied in rectangular space
Many assessments of rate of fl ow of persons through
exits have been made following tests and simulation,
particularly those using Paris fi remen in 1938 and 1945
and on the London underground system.
The generally accepted rate is 40 persons per
minute per unit (0.75 m) exit width. This is an average
fi gure and the actual fi gures varied between 20 and 170.
Exit widths - the width of one unit corresponded
to the average shoulder width and was determined
to be 525 mm. Two units of 525 (i.e. 1050 mm) was
the requirement for two people to travel through,
shoulder-to-shoulder.
However, there will be overlapping between groups
of people and so further units of width required need
only be an additional 450 mm.
Hence, three units of exit width
Alternative escape routes need to be sited to minimise
the possibility of all of them being unavailable at the same
time. Alternative escape routes should be either:
(a) 45º or more apart (see Options 1 and 2 below) or
(b) If less than 45º apart, separated from each other by
fi re resisting construction.
Option 1
- alternative routes are available from C
because angle ACB is 45º or more, and therefore CA or
CB (whichever is the less) should be no more than the
maximum distance for travel given for alternative routes.
Alternative routes are not available from D because
angle ADB is less than 45º. There is also no alternative
route from E.
1500 mm and four units would be 525 525 450
450 1950 mm.
525
525
450
