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The need for a co mprehensive guide for fire safety in design
was identified as a recommendation by the Authors in their
Report to the Department of Trade and Industry-titled Fire
and Building Regulatio+in 1990.
I n s u m m e r 1 9 9 3 th e D e p a r t m e n t o f t h e E n v i r o n m e n t
comm issioned us to produce an illustrated text on the fire safety
principles underlying current United Kingdom legislation. The
target audien ce was building d esigners, fire safety officers and
building control officers who, together with students and a w ider
audience in other disciplines, would find the guide a useful
amplification of the principles behind legislative provisions.
The current methods of prescribing technical levels for fire
safety range from broad functional requirements to detailed
technical specifications which, together with the continuing
changes in detail occasioned by developments, has led us to
concentrate on principles rather than numeric detail.
The principal contributors were:
Geoff G Connell Hon Dip Arch
Roger Jowett BSc MSc Dip Arch RIBA A CIArb
Phillip H Thomas PhD (Cantab) FIMechE FIFireE MIF S and
0 Leslie Turner OBE RIBA AIFireE
They would like to thank their support team, particularly
mentioning John Blew, Lesley Turner Dip Arch RIBA, and
Robert Biddulph, who produced the illustrations.
Foreword written by: Dr William A Allen CBE BArch LLD
RIBA HonFAIA HonFIOA, who was Chairman of the Fire
Research Advisory Comm ittee 1975-1983.
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Chapter 8: Fire safety engineering
History
Current applications
Smoke logging of an enclosure
Flames out of openings
Fire resistance
Trade off
The future of fire safety engineering
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Design principles of fire safety 203
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HistoryIn the United Kingdom, the Institution of Fire Engineers was
founded in 1924 and in the USA, the Society of Fire Protection
Engineers has existed since 1950. In 1972, the word safety was
added to fire engineering by the newly established Professor of
Fire Safety Engineering at the University of Edinb urgh because to
at least one university official fire engineering sounded like a course
in arson! Safety however impacts wider aspects of safety engineering
and has the connotation of risk management.
Fire safety engineering in short, is the provision of fire safety by
quantitative methods based on science and thus has much in comm on
with other disciplines of engineering.
Calculation methods derived from structural and heat transferengineering have long been available for assessing structural
behaviou r in fires and much of the drive for develop ing fire safety
methodology has com e from structural engineers who saw no reason
why a fire should be treated in a way d ifferent from an earthquake
say, or wind or snow or gravitational loads.
The range of engineering interest has however expanded well
beyond structural considerations. From the 1940s onwards, there
have been successful applications of the theory of fluid dynamics
to model the movement of gases and smoke. Resource managementand operational research have been used to assess fire service
deployment and modell ing is now being adopted to predict
behaviour of people in response to fire.
Building Regulations reflect the evolution of systems of rules to
meet socially and legally required levels of performance. These
are of ten i l l def ined and are inherent ly based on leve ls of
perform ance of types of solution which-historically-have been
regarded as acceptab le. Th e basic problem is the expression of what
is unacceptable. Recommendations made after disasters are oftendesigned to preve nt the repetition of an identified h azard and wh ilst
this procedure is sometimes criticised for being post hoc it is an
evo lution ary proc ess and prog ress is real-as it has been with the
aircraft industry. The goal is to identify hypothetical failures and to
forestall them by adequate design and proper risk assessment. This
may involve extrapolation from the solutions which were acceptable
in the past, exercising judgement and art, since quantitative solutions
are not necessary available for all problems. Some solutions may
involve removing one problem by design changes and replacing it
with another, e a si e r- o r cheaper-to solve.
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..... . . . I _ , ..... . ." . . . . . " . I . . " . .. . - ..". . . . I -.". ... ._ .......... . _I _. . .. .. -
Until recently, most theoretical analysis of fire behaviour was
encompassed in zone modelling. This term describes the bringing
together of theoretical descriptions of heat transfer and fluid flow
to different regions or zones in a room-or building. Som e
judgem ent o r art is required in the choice of "Zones" such as the
upper and lower parts of the gases in a room and the plume joining
them.
With the increasing availability of fast computation, it has becom e
possible to deal with turbulent flow of gases, taking into account
the statistical nature of turbulence (now the subject of chaos theory),
although certain basic coefficients have to be determined by
experiment. These fluid flow theories are referred to by the terms
CFD (computa t iona l f lu id dynamics)-or f i e l d m o d e l li n g .
Calculations are then based on the solution of spatial differentialequations.
Zone modelling is adequate for many purposes, but is inherently
deficient due to the lack of a theory of fluctuation and eddies and
the neglect of certain continuities between one zone and its
neighbour. There are also problems in CF D; som e sizes of eddies
are not always included and there is a problem in linking the solid
and gas phase of materials; variations in physical or chemical
properties with temperature and lack of directional uniformity in
the movement of the moisture may present problems; flammablematerials need to have a description of flamespread properties in
terms that cannot be deduced from conventional fire tests or
flamespread. Improvements in the quality of data o r application of
the method or model can be derived by inventing new tests, but it
may take some time for these to be accepted, or to be incorporated
into regulations.
Som e of these zone and field models are incorporated into com puter
programmes which are available commercially. Users however
should understand the nature of the approximations and limitationsof the physics employed. This is not to argue that the results are
necessarily defective but the results may not be as precise as the
numerical calculations imply, even if the physical basis is sound.
There are some programmes where even this is not so. However
technical progress is rapidly being made and basic fire science is
rapidly being codified into practical, useful codes of practice and
design m ethodologies.
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Current applicationsIn considering examples of the application of fire safety engineering,
only the essential principles can be referred to: any application to a
particular building will need to consider details beyond the scope
of this publication.
Smoke logging of an enclosure We can undertake experiments o r rely on past experience to show
how quickly a room can become sm oke logged, but it is also possible
to calculate the time period and to assess quantitatively ho w v arious
factors influence it.
Taking as a very basic example a room with an opening in one
wall, theory can be used to show that the pressure rises as a result
of a fire inside are relatively small. The heat source produces a
rising plume of gas, lighter than its surroundings, which entrainsair as it rises up into the hot layer, which becomes deeper. The
formulae are available by which to relate the mass flow of hot gas
to the rate of heat release, Q, and the height above the heat source, Z.
Equ ating the rate of loss of the lower lay er air to that lifted up leads
to a formula for the tim e interval (t, - ,) during which the base of
the upper layer descends from height Z , to height Z,.
This extremely simple zone model gives a formula:-
where Atloo, is thefloor area
0 is the gas density
QC is a known constant
is the rate of heat release
This formula shows:-
- Except for a room with a low level opening, the height,
Z, never becomes zero in a finite time, so in practice it is
usual to take a final value for Z, as say 1.5 to 2 metres.
- If Z, is taken as 1.5 metres and if Z, , the height of the
room , is muc h greater than this, then the time period do es
not depend muc h on the height of the room-only on the
floor area. The reason for the lack of importance of the
enclosure height may not at first be apparent. A greater
mass of air is entrained as the height of the plume
increases. This has the effect of accelerating the descent
of the base of the hot layer so the height of the room
doesn't have a marked effect on the time taken to descend
to a critical level.
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Variations in this type of calculation have been u sed for 30 years in
roof venting design and have been shown by experiment to be
consistent with general experience. The method is now being
combined with formula for the movement of people to develop
calculation m ethods for safer egress.
Flames out of openings
Fire resistance
Th e impetus for development in fire safety engineering methodology
may in some cases derive from the need to solve quite unusual
problems-the dat a and techniques then finding much wider
application. A classic exam ple of this is a study in the early 1960s
of whether flames from a group of bu rning buildings could threaten
a l a rge s t ee l s t ruc tu re abo ve them-the Tok yo Tower . A
mathe matical analysis-not repeated here-was ma de of the rising
hot plume and experiments devised on a small and large scale to
check parts of the theory and also scaling laws, similar to thoseused in roof venting and current studies of smok e movement. Th ese
studies helped to determine the effects of changing the ratio of
window width to height and to develop formulae for determining
the horizontal balcony widths and vertical separations necessary
for safety.
Spread can occur up the outside of buildings, even if the facade is
itself nonflammable, because the flame may cause ignition through
openings in rooms above. Imposing.a vertical separation reduces
the hazard and becau se it is possible to calculate the flame lengths,minim um vertical se parations can be calculated as can the balcony
projection required to prevent spread. It has sometimes been
overlooked however that fire on a w ide front cannot be stopped by
a balcony becau se the flame tends to adhere to the wall if mo re air
is entrained into the flames than can get behind them. T his work
has been incorporated as one ingredient in the safe design of external
structures, but is also fundamental to the eng ineering design of atria.
For several decades efforts have been directed to relating the
performance of a structural element in a fire test (for example BS476) to that of the whole structure in a fire. Most attention has been
directed at the first step-the relationship between the fire test and
the fire for one element. The second step, relating the fire test to
the performance of the whole structure in a fire is a continuing
subject of study.
From work dating from as far back as the 1920s it had been
established that in a simple enclosure with a ventilation opening,
the fire resistance required was proportional to the fire load per
unit floor area. This was later modified by identification of theeffects of the geometry of the ventilation opening.
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Trade off
Formulae are now available that express required fire resistance
time in terms of the fire load and the various geom etrical properties
of the room and the opening (or for one effective equivalen t if there
are several openings). The relevant properties are the ventilation
opening a rea, the opening heigh t, the area of the walls, ceiling andfloor. Allowance has to be made for the thermal properties of the
enclosing walls, ceiling and floor.
Th e theory is still incom plete, it assum es the temperature is uniform
throughout the space and this may not be so. The formulae also rest
on assumed correlations, unsupported by physically based theory,
of experimental burning rates. Continuing research is attempting
to overcom e these deficiencies-but it is impo rtant to be aware of
current limitations.
There are many occasions in design when consideration may be
given to an over provision of one aspect of fire safety in order to
effect a reduction or the removal of another. A commo n examp le is
seeking to reduce the fire resistance level required for structural
elements by employing sprinklers. What are the principles to be
employed in such an exchange or “trade of f ’ as it is commonly
called?
Fund amen tal to this discussion-in the absence of any generally
recognised quantitative assessmen t of risk-is the determ inationof “equivalence” of the level of safety in the likely scenar ios of fire
occurrence and consequences between the proposal and what
historically may have been regarded as acceptable.
In addition to a general statemen t of purpose-to provide a safe
building-it is necessary to understand the potential benefits and
drawbacks of a particular fire protection system. The historical
purpose of fire resistance was to limit structural damage and in so
doing to provide protection for the routes used for escape and for
the access by the fire brigade. Most fire safety engineeringcalculations are based on a requirement to survive a burnout.
However, in a class of occupancies, there is a statistical variation
in the fire load and the associated fire resistance required.
Introducin g sprinklers reduces the probability that the structure will
be exposed to a fire which threatens it. Hence fire resistance
requirements can be relaxed while still maintaining the same overall
probability of structu ral failure in that occupancy. Risk is a product
of the probability and the consequence of a hazardous event. For
monetary loss there are insurance criteria, but for life safety the
relevance of the concepts employed may be less sure.
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- . . ..".. .. .. . . ... . . . . . -- .
Fire Safety (IFS) has a professionally qualified membership and
historically close links with the SFPE and with the Institution of
Fire Engineers-an organisatio n based originally on technically
qualified fire service officers but now w idening its membership.
Th e first undergraduate course available in the UK was at what is
now The University of the South Bank in London and there are
many Masters courses, the oldest being at Ed inburgh University.
At these institutions links have been found between departments,
particularly those of Civil, Mechanical and C hemical Engineering,
but links with Departments of Architecture could well develop.
There is much international contact and a m odel syllabus has been
published by an international group of academics.
Th e future developm ent of fire safety enginee ring is not in questionin the Industrial Sector. The question at issue is its future in the
Building Sector. Fire safety engineering will be most in demand
for problems beyond the conventional ones for which prescriptive
measu res may continu e to prove adequate-although even in those
cases, the potential for the application of engineering methods to
achieve the same performance more efficiently or to improve
performance at the same cost should not be underestimated.
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