design principles of fire safety. part 8 - fire safety engineering. (9 of 14)

7/27/2019 Design Principles of Fire Safety. Part 8 - Fire Safety Engineering. (9 of 14)

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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.



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

205

207

207

208

208

20 9

210

Design principles of fire safety 203



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.



..... . . . 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.

206



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.



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.



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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