BS en 12101-5

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Design app roaches for sm oke con tro l

in at r ium bu i ld ings

G

0 Hansell*, BSc, PhD, CEng, MCIBS E, AlFireE

H P Morgan, BSc, CPhys, MlnstP, AlFireE

*Colt International Limited

Fire Research Station

Building Research Establishment

Borehamwood, H erts

WD6 2BL

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Prices

for

all available

BRE publications can be

obtained from:

BRE Bookshop

Building Research Establishment

Garston , Watford, WD2 7JR

Telephone: 0923 664444

Fax: 0923 664400

BR 258

ISBN

0 85125 615

5

0

Crown copyright 1994

First published 1994

Applications to reproduce extracts

from the text

of

this publication

should be madc

to

the Publications Manager

at the Building Research Establishment, Garston

Cover illustration by Bob Stoneman

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Contents

Chapter 1

Chapter 2

Chapter 3

Chapter 4

Chapter

5

Chapter 6

Foreword

Nomenclature

Introduction

General principles

of

smok e production, movem ent and control

Fire growth and smoke production

Pressurisation

Depressurisation

Throughflow ventilation

Design fire size

Smoke control on the floor

of

fire origin

Within the fire room

Flow of hot gases out of the room of origin into the atrium

Ventilation

of

the balcony space

Smoke layer temperature

Effects of sprinkler systems in smoke reservoirs

Flowing layer depth

Local deepening

Inlet air

Minimum number

of

extraction points

Required ventilation rate (powered exhaust)

Slit extract

False ceilings

The use of a plenum chamber above a false ceiling

Smoke ventilation within the atrium

Smoke movement in the atrium

Channelling screens

Entrainment into spill plumes rising through the atrium

Fires on the atrium floor

Throughflow ventilation rea

of

natural ventilation required

Throughflow ventilation emaining design procedures

Limitations to the use

of

throughflow ventilation

Design considerations other than throughflow ventilation

Void filling

Compartment separation

DepressurisatiEn ventilation

Principles

Natural depressurisation

Natural depressurisation and wind effects

PO

wered depressurisation

Depre ssurisatiodsm oke ventilation hybrid designs

Principles

Design procedures for hybrid systems

Mass

jZo

w based systems

Temperature based systems

Page

V

vi

1

4

4

5

6

6

7

9

9

10

13

14

14

16

17

17

18

19

19

20

20

21

21

22

23

33

33

35

35

37

37

37

37

37

38

40

41

42

42

42

42

43

(continued)

...

I11

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iv

Chapter 7 Atrium sm oke layer temperature

Chapter

8

Additional design factors

Atrium roof-mounted sprinkler systems

Sm oke detection systems in the atrium

Pressurisation

of

stairwells and lobbies

Air-conditioned atria

Chan nelling screens and hybrid systems

Wind-sensing devices and n atural de pressurisation

Appendix A Case history

Appendix

B

Users guide

to

BRE spill plume calculations

Introduction

Scenarios and assumptions

Outline of procedure

Detailed procedure

Nomenclature used in Appendix B

Acknowledgements

References

Page

44

46

46

46

46

46

46

47

48

49

49

49

50

50

54

55

55

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I

This Re po rt is the culmination of a long-running collaborative project betw een th e Fire

Research Sta t ion of the Building Resea rch Establishm ent and Colt Internation al Limited

on aspects of smo ke mov eme nt and its control in atr ium buildings. I t is based on both the

latest scientific knowledge and practical experience

of

smoke movement a nd cont ro l

systems, an d has been pr epa red u nde r the overall supervision

of

the F ire Research

Stat ion.

T h e present Rep or t is intended to serve the des igners of smok e control sys tems for a t r ium

buildings in th e same way that th e earlier Building Researc h Establishm ent Rep ort,

Design principles fo r

smoke

ventilation in enclosed shoppin g centres,

has serve d designers

of

smo ke ventilation systems in sh oppin g malls. As such, those graphs an d tables i t

conta ins which are re levant to a particular design of building can be applied directly to

that building; or th e formu lae cited can b e used

to

apply the work to

a

broader range

of

circumstances.

Th e Rep or t does no t exc lude the op t ions of using a l ternat ive metho ds where they are

appropr ia te , o r

of

using new techniq ues (such

as

com putation al f luid dynamics) as they

are developed and validated.

A P S

Ferguson

Building Regu lations Division

D e p a r t m e n t

of

the Envi ronment

July

1993

V

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Nomenclature

Note: The list of nomenclature used in Appendix B is given on page 54.

Area of the fire (m2)

Area of inlet (measured) (m2)

Area

of

exhaust ventilator (measured) (m2)

Area of opening into atrium from adjacent fire room (m2)

Specific heat

of

air (kJkg-IK-')

Coefficient of discharge for a vertical opening

Entrainment coefficient

Coefficient

of

discharge for an inlet

Wind pressure coefficient acting on an inlet

Wind pressure coefficient acting on the leeward side of building

Wind pressure coefficient acting on an exhaust ventilator

Coefficient of discharge for an exhaust ventilator

Depth of smoke beneath an extraction point (m)

Depth of a smoke layer under a balcony (m)

Depth

of

a downstand fascia (m)

Diameter of fire (m)

Design depth of a smoke layer in a reservoir (m)

Depth of a flowing smoke layer in a vertical opening (m)

Maximum depth of smoke in an atrium (m)

Acceleration due to gravity (ms-2)

Height of a vertical opening (m)

Height of a vertical opening with no upstand (m)

Height of rise

of

a thermal line plume from an opening or balcony edge to the smoke layer (m)

Height of the atrium

(m)

Height to the ceiling (m)

Channelling screen separation (m)

Mass flow rate (kgs-')

Mass flow rate from the fire (kgs-')

Mass flow rate under a balcony (kgs-')

Mass flow rate entering (leaving) a smoke layer in a reservoir (kgs-I)

Mass flow rate flowing through a vertical opening (kgs-')

Critical exhaust rate (kgs-l)

Number of exhaust points

Perimeter of fire (m)

Heat flux (kW)

Convective heat flux of fire (kW)

Convective heat flux passing through a vertical opening (or under a balcony) (kW)

Convective heat flux per unit fire area (kWm-2)

Absolute temperature of gases

(K)

Absolute temperature of gas layer under a balcony (K)

Absolute temperature of gas layer in a reservoir (K)

Absolute ambient temperature (K)

vi

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V

VI

"wind

W

WB

X

Y

P

ADB

e

@B

01

P

P O

A

' ..

7

Volumetric

flow

rate of gases (m3s-1)

Volumetric

flow

rate of gases from a reservoir (m3s-I)

Design wind velocity (ms-')

Width of vertical opening (m)

Width

of

balcony (distance from vertical opening to front edge of balcony) (m)

Height from the base of the smoke layer to the neutral pressure plane (m)

Height from the base of the fire to the smoke layer immediately above (m)

Coefficient in critical exhaust rate equation (kgrnp3)

Empirical height of virtual source

below

a balcony edge (m)

Additional smoke depth due to local deepening (m)

Temperature rise above ambient of smoky gases

( C)

Temperature rise above ambient of smoky gases under a balcony

( C)

Temperature rise above ambient of smoky gases in a reservoir ( C)

Temperature rise above ambient

of

smoky gases in a vertical opening

("C)

Density of gases (kgm-3)

Density

of

ambient air (kgmW3)

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Introduction

This Rep ort is inten ded to assist designers of smoke

ventilation systems in atrium buildings. Most of the

methods advocated are the outcome

of

research into

smoke movem ent and control a t the F ire Research

Station (FRS), but also take into account experience

gained an d ideas developed whilst the authors and their

colleague s have discussed many prop osed sc heme s with

interested parties. The primary purpose

of

the Re port is

to sum marise in a readily usable form the design advice

available f rom F RS at the t ime of i ts preparation. As

such, it d oes.no t attempt to cover installation, detailed

specification of hardware, or aspects of fire safety

engineering other than smo ke control.

Th e predominant cause

of

death in fires in the U K has

been attr ibuted to the inhalation of smok e and toxic

gases’, and the annu al num ber of f ire fatali t ies in the U K

is

approximately one thousand. How ever, the majority

of these d eath s occur in do me stic premises. Th is implies

that the life-safety m easures re quired by legislation for

most public and com mercial buildings have been

effective on the w hole.

Fire safety in buildings must in the U K con form to th e

relevant regulations (guidance for Englan d a nd Wales is

given in A pprov ed D ocum ent B2). Th e principal

objective of these regulations

is

to safeguard life by:

@ reducing th e potential for fire incidence,

Q controlling fire propagation and spread, and

8 the provision of adequate means of escape for the

building’s occupants.

Means of escape in case

of

fire was first introduced to

the Building R egulations for England and Wales in 1973.

Prior to that date , the powers

of

control in Englan d and

Wales over means of escape had been co ntained in othe r

legisIation~-‘‘,s.

Historically, th e preven tion of fire growth within (o r

between ) buildings has been achieved by th e

containment of the f ire and its products by me ans

of

compa rtmentation and /or separation. Th e design of

structural compartme ntation and separation has been

largely empirical, and the concepts gradually refined a nd

enhance d in such a way that the Building Regulations

now c over primarily life safety and th e protection of

means

of

escape.

It

is necessary

to

consider four major

aspects of buildings- urpose, s ize, separation an d

resistance to fire- o p romo te safe design.

Social an d technical c hanges have led to chang es in

building e nvironme nts which incorporate new (or

revived) building form s and th e use

of

innovative

construction techniques and new synthetic materials .

Th e buildings ad opting these changes often have

included within the ir design large sp aces or voids, often

integrated with many of the storeys. These large spaces

have b een described as malls, atr ia, arcades and

lightwells. Th e generic term for the building type te nds

to be ‘atrium’ and , by the ir very nature , atrium buildings

often run contrary to the traditional Building

Regulation s’ appro ach in terms of horizontal

compa rtmentation a nd vertical separation.

Th e original atr ium was an entrance hall in a Ro man

house and was one of the most important rooms in the

building. The conce pt

of

this space has evolved

architecturally over the past few hundred years and now

applies to structures much large r than th e typical Rom an

house. Atria today are designed as undivided volumes

within a structu re, intending to create visually and

spacially an ideal external env ironment - ndoors6.

In Rom an times the control

of

any smoke and hot gases

that may h ave issued from a fire in a room adjacent

to

the atr ium was likely to have been a simple matter .

Providing there w ere

no

adverse wind conditions (due to

local topography of adjacent structures), then the smo ke

and heat would undoubtedly vent through the open

portion of the atrium roof k nown as the ‘compluvium’

(generally used for lighting purposes).

Mode rn atrium buildings tend to contain large quantit ies

of

combustible material and o ften have open-plan

layouts which increase th e risk of the spread

of

f ire. Th e

population s within such buildings are also greater; hence

there has been a substantial increase in the nu mber of

people to be p rotected and evacuated in an emergency.

Mo dern atrium buildings are usually designed with the

atrium as a feature which can be app reciated from

within the adjacent rooms. The room/atrium boun dary is

usually eith er glazed o r completely open . Thu s when

compared to ‘conventional’ buildings, this

architecturaVaesthetic re quirem ent impo ses additional

problems

of

lifesafety during a fire, since sm oke, hot

gases and even flames may travel from one (or more )

rooms into the atrium and thence affect areas which

would have remained un affected in the ab sence of an

atrium.

In

conve ntional multi-storey structu res the re is always

the possibility of f ire-spread up th e outside of the

building, with flam es issuing from o ne roo m and

affecting the floors above. Recen t example s of this mode

of fire-spread have bee n a n office block in SiioPaulo7

an d the Villiers building in Lon don . If the e scape

facilities from th e va rious rooms ar e of a suitable

standard and segregated from other compartm ents (as

required in the U K), there should not ( in theo ry) be any

serious hazard to life safet y in this fire cond ition . It is

only when the me ans

of

escape are inadequate or the

parameters dictating their design are violated, that the

loss

of

life may occur.

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If a building has an atrium then this fire condition can

also occur internally, since ther e is generally a

maximisation of the window a rea and /or open boundary

between the rooms and the atrium. Hence there is an

increased risk t o oth er levels of the entry of smoke,

toxic gases and p ossibljl flames from a fire.

Rece nt exp erience of fires in atrium buildings in the

has shown the problem of flame travel internally

through th e atrium to be minor in comparison to that

of

hot and toxic gases accumulating and building down in

the atrium preading throughout the building and

affecting escape routes. Thu s there appe ars to b e a need

for a properly designed smoke control system in atrium

buildings.

Th e ideal option would be to prevent any smok e from a

room fire entering the atrium a t all. A n easily

understood way

of

achieving this is to ensure th at the

boundary between the room a nd the atrium is both

imperforate and fire-resisting, and that th e atrium base

has only a very restricted use. This optio n has

frequently been used, but is widely regarded as being

architecturally restrictive. Conseque ntly it is not o ften

favoured by designers. Th e concept has been labelled

the ‘Sterile tube’6.

Where the boundary between th e room and the atrium

is ope n,

i t

is sometim es feasible to provide a smoke

ventilation system within the room, to maintain smoky

fire gases above the opening to the atrium .

Unf ortun ately it is often very difficult, impracticab le, or

extremely expensive to fit a sep arat e smoke extraction

system

to

each and every room, however small.

Occasionally circumstances dictate that smoke control

dedicated to each room in this way is the most viable

option for protecting th e atrium (this can occur, for

example, when the room layout is

of

a large a rea, is

predominantly open-plan and open-fronted). There

have been several examples of this. Neverthe less it

remains generally true that this option

is

rarely found to

be a ppropriate for most atrium buildings.

An oth er possibility is that the atrium should be

pressurised to preven t smo ke moving into it from a

room. This is not usually a viable option where the

opening between the room and the atrium is large (for

example, an open-fronted room o r a room whose

glazing has fallen away in whole o r in large part). Th is is

because the air speed needed from the atrium into the

room in order

to

prevent the movement

of

smoky gases

the oth er way through the same opening, can vary

between abo ut 0.5 ms-l and approximately 4 ms-1

depending o n gas temperature, etc. All of this air must

be continuously removed from within the fire room in

ord er to maintain th e flow. Th e quantities of air-

handling plan t requir ed will exceed the size of smoke

ventilation systems for many typical atrium room

openings. Note however, that pressurising the atrium

may be a viable option w here the atrium faqade has

only relatively small leakage paths.

Wh ere smoke from a fire in a room can spread i nto the

atrium , with the possibility of rapid further spread

affecting ot he r parts of the building, there will be an

extreme threat to safe evacuation

of

occupants from

those parts. Similar thre ats will occur if there is a serious

fire in the atrium space itself. In eit her case, the threat

to means of escape which are either within the atrium or

in spaces open 1.0 the atrium, can develop rapidly unless

som e form of smo ke control is used in the atrium in

ord er to protect those means of escape. In othe r words

a smoke con trol system in the atrium is essential to

ma ke certain that escape is unhindered, by ensuring

that any large quantities

of

thermally buoyant smoky

gases can be kept sep arat e from people who may still be

using escape routes, or awaiting their turn for

evacuation. Th eref ore the role of a sm oke control

system is principally one of life safety.

It should however also be rem embered that fire-

fighting becom es both difficult and dang erou s in a

smoke -logged building. It follows that t o assist the fire

services, the smo ke control system should

be

capable

of

performing its design function for a period of time

longer than that req uired for the public to escape,

allowing a s peedier attack

on

the fire to be made after

the arrival of th e fire service.

Th ere has been no readily usable guidance available to

designers of atrium sm oke control systems within the

UK . Th ere have been a num ber of purely qualitative

papers, as well as papers on work using relatively simple

models of smoke movement within atria (see for

example References 10-12). Th e National Fire

Protection A ssociation

of

the

USA

has recently been

developing a Code13which sets out a fire engineering

approach t o the design of smok e control for atria

(termed ‘smoke management’ in the USA ). While this

code is in many ways very comprehensive and bro ade r

in purpose than the present Rep ort, some of the

approaches used differ from alternatives with which

U K

designers are more familiar, or are m ore approximate

than m ethods currently used by the Fire Research

Station. This particularly applies to sm oke entering the

atrium from adjacent rooms.

The purpose of this Repor t is to provide guidance to

assist designers of smo ke con trol systems in atrium

buildings in line with curren t knowledge. Th e guidance

is based o n results of research where possible, including

as yet unpublislied results of experiments, but a lso on

the cumulative experience

of

design features required

for regulatory purposes

of

many individual smo ke

control proposals. Many of these design features have

been evolved over a number of years by consensus

between regulatory authorities, developers and fire

scientists, rath er than by specific research. Such adv ice

has been included in this Rep ort with the intention of

giving th e fullest picture possible. It is therefor e likely

tha t some of this guidance will need to b e modified in

the future, as the results of continued research becom e

available.

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A

Co de of practiceI4 for atrium buildings is currently

being prepare d by the British Standa rds Institution

(BSI).

Th e aim of this present R epo rt is to provide

guidan ce only on design principles of sm ok e control

and it is hoped to sup port the code rather than to pre-

empt

i t .

Th e Re port cannot cover all the infinite

variations of atrium design. Instead it gives general

principles for th e design of efficient systems, with

simplified design procedures for an ideal model of an

atrium, and then further guidance on frequently

encountered practical problems. As the design

proced ures are of necessity simplified,

it

also gives their

limitations

so

that, when necessary, a more detailed

design by specialists can be carried o ut.

Such an approa ch may employ field modelling, which

exploits the new techniques of computa tional fluid

dynamics (C FD ) to deduce how, and at what rate ,

smo ke would f i l l an enclosure.

I t

do es this by avoiding

resort (as far as is currently possible) to experimental

correlation s, and returning t o first principles to solve

th e basic laws of physics for the fluid flow.

As

a

consequence, with adequate validation, this type of

modelling should h ave a wide ap plicability. Th e use of

a com pute r is necessary since the technique involves

the solution of tens

of

thousands of mathematical

equations for every ste p forward the simulation makes.

An atrium can be defined a s any space penetrating

more than o ne storey of a building whe re the spa ce is

fully or partially covered. Most atria w ithin shop ping

centres may be considered as part of the sho pping mall

and tre ate d accordingly. A BSI Cod e of practiceIs

specifically for shopping complexes has bee n pu blished,

and a lso a BR E Repor t I6giving advice on sm oke

ventilation of enclosed shopping centres. W here atria

have mixed occupancies including shops then re ference

should be made to these docu ments, or specialist

advice sought.

In orde r for a design to be achieved, i t is necessary to

identify the various ‘types’ of atrium tha t are built.

Th ese can be simply defined as follows:

(a) Th e ‘sterile tube’ atrium

Th e atrium is separated from the remainder of the

building by fire-resisting glazing (FR G) . Th e

atrium sp ace generally has no functional use

other than as a circulation are a (Figure

1).

(b) Th e closed atrium

Th e atrium is separated from the remainder of the

building by ordinary (non fire-resisting) glass.

Th e atrium space may well be functional

(cafeterias, restaurants, recreation, etc) (Figure

2).

(c) Th e partially open atrium

He re som e lower levels are open to

t h e

atrium and

t h e

remaining levels closed off by glazing (Figure

3).

(d) Th e fully open atrium

Some of the upp er levels or all of the building

levels are open t o the atrium (Figure

4).

Figure 1 ‘Stcrile tube‘ atrium

-g laz ingStandard

Figure 2

Closcd

atrium

Figure

3 Partially ope n atrium

Figure 4 Fully ope n atrium

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Fire growth and smoke production

In

most ins tances , a room (co mpartm ent) f i re may be

assumed to burn in either of two w ays:

(a) Fuel-bed control

When the ra te of combustion, heat output a nd fire

growth are depende nt upon the fuel be ing burned.

(Th e 'normal' fire condition found in most single-

stor ey buildings whilst th e fire is still small enough

for successful sm oke cont rol.)

Where the ra te of combust ion , e tc is depen dent

upon the quanti ty

of

air avai lable to the f i re

compartment (assuming any mechanical

ventilation systems are inactive).

(b) Ventilation control

Th e quan t i ty

of

smoky gases produce d (ie the mass

flow rate

of

gases) in and from the comp artment , and

the energy (heat f lux) conta ined there in a re d ifferent

for both regimes. It is therefore imp ortant to identify

the regime which applies . He nce the mass f low an d

heat flux within the smoky gases may be determ ined.

I t

is

important t o unders tand the basic mechanisms

which control the fire condition.

A

step-by-step history

of a growing fire m ay b e a s follows:

1 Th e f i re s tar ts for whatever reason, i ts ra te of

growth dep ending upon the materials involved. In

most practical comp artme nts the re is sufficient

oxygen to support com bustion in the first few

minutes, and th e f i re growth a nd sm oke product ion

are controlled by the fuel, ie, fuel-bed control.

2

Sm oke from the fire rises in a plume to th e ceiling.

As the p lume r ises , a ir is entra ined in to i t ,

increasing the volum e

of

smoke a nd reducing i ts

3

temperatu re . The sm oke spreads out radial ly

undern eath the ceiling and fo rms a layer which

deepens as the compartment begins

to

f i l l .

If the compartment is open to the a tr ium, then the

gases flow out immediately they reach the opening.

If th e compartm ent is glazed or the op ening is

below a deep downstand then the sm oke s teadi ly

deepens.

As

the layer gets deep er the re is less

height for the p lume

of

smoke to rise before

it

reaches th e smo ke layer, hence less air is being

entra ined, wit 'h the result tha t the temperatu re of

the sm oke layer increases with layer depth, even for

a steady fire. Most fires will continu e to grow larger

as the layer deepen s, reinforcing this effect.

6 mm plate glass may sha tter when exposed t o gases

as little as

100

K warm er than ambient . Thus once

th is temperatu re

is

passed, there is an increasing

likelihood that t he glass will fracture. If the

compartment is spr inklered and the w ater spray h i ts

th e glass, the localised heating of the glass

by

radiation from the fire and by the g as layer,

combined with sudden cooling due t o the water

spray will increase th e likelihood of the glass

breaking. Th e smok e an d hot gases wil l then f low

external ly to a tmosphere , or enter the a tr ium, or

both , depending upon the nature of t h e

com partme nt a nd its relative position in the

building, the size and position

of

the fire in the

compartment , and the s trength of differing glazing

systems. If th e fire can be accidentally or

deliberately vented externally then the threat to

othe r levels via th e atriu m is greatly reduced .

Th er e will, howev er, be instances wh en a fire will

vent all

its

effluent gas into the atrium , and this is

generally the worst design scenario (Figure 5).

Area A,

Perimeter P

Figure 5

Smoke en ter ing an at r ium from

a

fuel-bed-controlled fire in an adjacent room

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. ._

The re is so much m ixing of ambient air into t he

plum e that , except close

to

the fire itself, the hot

smoky gases can be regard ed as consisting of

warm ed air when calculating the quantity (mass

flow rate) being produc ed in the compartment .

Initially this mass flow rate of sm oke will be

controlled by the fuel-bed, as mentioned above.

However , the geom etry of the opening on to the

atrium has

a

crucial effect.

A s

the fire grows large in

comparison to th e area of the opening, the air

supply to the fire is 'thro ttled ', cau sing it

to

burn

inefficiently.

Th is leads to the situation w here t he inability of t h e

com partme nt t o vent the gases effectively d ue to the

restricted are a available causes the layer to dee pen

furth er which, combined with th e increasing fire

area , causes the layer temp eratu re to rise. Once th e

layer tem perature reaches approximate ly 600 C,

then

in

most compartments the dow nward radiat ion

from the gas layer is sufficient to caus e auto-ignition

of

t he

remaining combustible materials in the

compartment (F igure

6).

W here there is sufficient

fuel within the com partmen t for the ent i re

com partme nt to becom e involved, the layer

tem pera ture will rapidly rise to flame tem pera ture,

very approximately

1200

K (930 C). The ra te of

burning, heat ou tput an d mass Qpw leaving the

compartment are now strongly dependen t upon the

geom etry of th e openin g, ie ventilation contro l

(Figure 7).

The transition from the fuel-bed-controlled fire

with

a

layer at

600 C

to the ventilation-controlled

condition is very rapid, and may tak e only seconds.

Th is condition is often known as 'flashover'.

Th ere may be an interme diate situation where t h e

com partme nt has flashed o ver or the fire simply

grown to encompass the ent ire width of the

compar tmen t , bu t w here t he quanti ty of air now

rq9uired t o maintain combustion is adeq uate , even

though the only surface available for air

entrainm ent is the width of the op ening (as

opposed

to

the fire perimeter for

a

fuel-bed-

controlled fire). Th is condition is known as the

'fully-involved, large-opening fire'".

8

The re are many factors which determ ine the

prevailing condition, including the type

and

disposition of the fuel, the dimen sion of the

enclosure and the dimensions of the ventilation

opening . The y can however be reduced to two

principal param eters for most compa rtmen ts:

A,VH

This is the are a

of

the opening in to

t he

atrium A, , multiplied by the square root

of its height

H.

The area of the fire.

r

Note: Both t he fully-involved large-opening fire

and ventilation-controlled fire conditions will

almost certainly produce flames from the

opening in to the a tr ium.

9 T he presence of sprinklers will usually serve to

prevent fire growth proceeding to full involvement,

and will usually maintain the fire in a fuel-bed-

controlled s tat e where extinguishm ent is not

achieved.

Air is introduced into

a n

escape route (usually a

stairway) a t a rate sufficient to hold back any smo ke

trying to pass on to tha t route . The pressure difference

across any small open ing on to the rou te must be large

enough to offset adverse pressures caused by wind,

building stack effect and fire buoyancy. It must also b e

low enoug h to allow the esca pe doors

to

be opened

with relative ease. The air supply must also be large

enough to produce a velocity sufficient

to

hold

back smo ke at any large opening on to

the

pressurised

space. Exp erie nce of pressurisation designs suggests

that the technique is well-suited to the protection of

T+

873K

/

Heat radiat ion

Figure

6 T h c onset of flashover

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stairways used as escape route s

in

tall buildings,

though it can also be useful in oth er circumstances.

Depressurisation

This is a special case of pressurisation, wh ere gases are

removed from the smoke-affected space in a way that

maintains the desired pressure differences and/o r air

spee ds across leakage openings between th at space

and adjacent spaces. No te that depressurisation d oes

not protect the smoke-affected space in any way;

instead it protects the adjacent spaces. In the

circumstances of an atrium it is sometimes possible to

use the buoyancy of the smoky gases themselves to

create the desired depressurisation effects. This is

explained

in

more detai l in Ch apter 5.

Throughflowventilation

Air m ixes into the fire plume as it rises, giving a larg er

volume of smoky gases. These flow outw ards below

the ceiling until they reach a barrier (eg the walls, or a

downstand). Th e gases then form a d eepe ning layer,

whose buoyancy can drive them through n atural

ventilators (or alternatively smoky gases can be

remo ved using fans). For an y given size of fire, an

equilibrium can be reached where th e quantity of

gases being removed eq uals the quantity ente ring the

layer

in

the fire plume

o

significant mixing of air

occurs upwards into the base of the buoy ant smok e

layer. Sufficient air must e nte r the space below the

layer to replace the gases being removed from the

layer, otherwise th e s mo ke ventilation system will

not

work.

Sm oke ventilation (throughflow ventilation) is used

when th e fire is in the sam e space as the people,

contents or escape routes being protected , without i t

filling that space. Th e intention is to keep the smo ke in

the upp er reaches

of

the building, leaving clean air

near th e floor to allow people t o move freely. This

stratification or layering of the sm ok e is mad e possible

by the buoyancy of hot smoky gases produc ed by the

fire, and it follows that t o be m ost successful t he high-

level smok e layer must remain warm. Smoke

ventilation is theref ore only suitable for atria where

fires can cause smo ke to en ter the atrium space. Such

fires can eithe r be fuel-bed-controlled fires at the base

of the atrium , or fires in adjacent spaces (rooms) which

allow smoky gases

to

ent er the atrium . Much of this

Report is concerned with the calculation of design

para me ters for smok e ventilation systems tailored to

the circumstances found in various typ es of at ria. First

though, it is worth reviewing the underlying principles

of smoke ventila tion and the general approach need ed

for successful design.

I

Figure

7

A ful ly-involved vent i lat ion-control led fi re

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.

.

-

-

,

,,

:,

.

.-. ~ .__,_--_,.-

Chapter 2

Design

fire size

The

calculation of the quantity of smo ke and hea t

produced by a fire requ ires a knowledge of its size in

te rms of are a, perim eter and heat flux developed pe r

unit area o r from

t he

fire as a whole. Whe n designing

sm oke ventilation o r depressurisation systems, the

mass flow rate and heat flux developed in the room a re

major parameters

in

the calculation of the system

requirements, changes in which can substantially affect

all

of the sub sequ ent smoke flow conditions.

Th e prefer red choice of design fire would be a time-

depe nden t growing fire, to which the mean s of escape

and evacuation time for the particular building

occupancy could be relate d, allowing th e increasing

threat to occupants to be calculated as time progresses.

Unfortun ately there is no available research, at th e

time of pr epar ing this Repo rt, which allows assessment

of t he probability distribution describing the variation

of fire growth curves for are as typically associated with

atria. Clearly, on e doe s not want an ‘average’ fire for

safety design, since typically half

of

all fires would

grow faster. I t is much sim pler to assess the m aximum

size a fire can reasonably be expected to reach during

the escape period, and to design the system to cope

with t hat. Such assessments can som etimes be b ased

upon available statistics o n fire damaged areas , but

may have to depend upon experienced judgem ent

based on the anticipated fire load where

a

more

rigorous approach is not feasible.

‘W ork on design guidance for s mo ke ventilation

systems

in

shopping cen tres used the principle

of

selecting a fixed size of fire tha t would ca ter for almost

all of t h e fire sizes likely to be fou nd in th at class of

occupancy, and t h e n deduc ing a pessimistic heat

ou tpu t from that fire16,18.his procedure has been

ado pted for occupancies oth er than retail which are

also comm only associated with atrium buildings

-

offices and hotel bedrooms1y*20 .

The’proc edure has no t ime dependen cy and d oes not

reveal any information regarding the growth

characte ristics of the fire. It is the ref ore

usual

to

assum e tha t the fire is at ‘steady-state’. This

assumption allows the smok e control system to cater

for

all

fires within th e accep ted design fire size, and by

not

considering the growth phase of the fire,

introdu ces a significant margin of safety

to

the system

design.

It follows from t h e foregoing that ther e is a strongly

subjective element in assessing wh at fire size is

acceptably infrequent for safety design purposes.

Various design fires have been suggested for

occupancies associated with atria. A wide range of

. .

. .

fires may potentially be a dop ted. I n

t he

present work

th e following, in term s of fire are a and convective heat

flux, ar e used to illustrate the calculation procedures

adopted:

(a) RetailI6 (sprinklered shops)

10

m2,

12 m perimeter .

(b) Offices” (sprinklere d) Offices”(unsprink1ered)

16

m2,

14 m perimeter . 47 m2, 24

m

perimeter.

(c) Ho tel bedrooms2”

Floor area of the largest bedr oom .

It should be no ted t hat th e design fire size for (a) was

originally chosen by the Ho me Office and Scottish

Ho me and Health Departm ent, and that for those of

(b) an d (c) the re is no official ‘approved ’ choice.

Afte r considering the heat losses to the stru cture of the

room , and any losses to sprinkler sprays, etc,

t he

commonly used heat outputs are approximatelyl’:

Sprinklered office 1 MW

Unsprinklered office 6 M W

Hote l bedroom* 1 MW

Gas es flowing into

the

atrium from a fire deep inside

a

large-area office with op eratin g sprinklers may be

cooler than is assum ed in the ‘sprinklered office’

design fire above. T he m ass flow rate

of

gases entering

th e final reservoir will b e less than would be calculated

using the value given above . Even for this scenario

therefore, the above value should er r on t he side of

safety. Designers w ishing

to

tak e sprinkler cooling in

the fire compa rtme nt more rigorously into account

should ado pt a fully fire-engineered approach

appropriate

to

their specific circumstances, for

example by using the methods described in the section

‘Effects of sprinkle r systems

in

smo ke reservoirs’

(page 14) to assess the effect of sprin kler cooling on

the outflowing gases.

Th e hotel bed room fire represents a fully-involved

unsprinklered fire2”. W here sprinklers are present this

will be clearly unrealistic and a value of 500 kW (for a

6 m perim eter f ire) may be more a ppropriate for

designers wishing to adopt a fire-engineering approach

to a design.

This R ep ort will however assum e the fully-involved

value for design purposes as this will introd uce a large

safety margin to the design, in particular to the

calculation of the mass flow rate leaving the room

through the opening.

*

Assuming a

floor area of approximatcly 20 m2

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Th e use of the bedroo m floor for the hotel bedroom

design fire reflects the situation where there are no

sprinklers present. Unpublished research o n

sprinklered bed fires (P

G

Smith and

J

V Murrell, Fire

Research Station; private communication, 1986),

where the low heat out put per unit area was

com para ble to values for hotel bedroom s, suggests that

the much lower fuel load (compared

to

an office)

expected

in

a hotel b edro om utilising conventional

sprinklers should make it possible for the smoky gases

to be co oled sufficiently t o be retained within the room

of origin (assuming the w indow is not op en). Th e

ope ration of sprinklers is likely to cool any smo ke

from a fire and suppress that fire to such an exte nt that

the glazing to the bed room will probably remain intact.

This is particularly tru e for double-glazed window s.

Th e same research indicates tha t the use of

conventional sprinklers in a residential environment

may not however allow conditions within the room to

remain tenable, and it may be inferred that the

presence of an open window to the room could

prod uce hazardous conditions in the at rium , at least

above

t h e

floor of fire origin. Since the re is no

statistical dat a available on fires in sprinklered hotel

bedroom s in the UK, any ch oice of design fire size will

be subjective. Should a designer wish to exam ine the

effect of a plume em anati ng from an o pen window in a

sprinklered hotel bedro om , it would not seem

unreaso nable to use a value of 6 m per imeter

(equivalent to a single bed ) with a convective he at

outp ut of around

500

kW as

the

design fire.

Research in to the use

of

fast-response sprinklers

in

a

residential environment21,22 as clearly shown that at

the time of ope ration of these sprinklers the conditions

inside the rooms w ere still tenable, ie the re was no life-

safety risk from t he sm ok e, even w ith excessive ceiling

level temp eratu res. This indicates tha t for any gases

flowing into the atrium (eg through an op en window)

the furth er entra inm ent induced by the rising smok e

plume will ensu re that con ditions within the atrium

must be tenable, regardless of the smo ke tempe rature

or sm oke production rate in the room . While i t is

possible that this may also be true for cellular offices

employing fast-response sprinklers, there is no

evidence (experimental or emp irical) to validate this,

and so,

to e r r on

t he

side of safety, this Rep ort will

regard sprinklered offices employing fast-response

sprinklers

in

the sam e way a s offices using

conventional sprinklers. Fu rthe r research and

statistical data are desirable in this area.

As men tioned in the Introduction the retail atrium is

considered separately in the Rep ort on covered

shopping complexesI6, and will not be considered

further in this Repo rt. T he fire sizes on the previous

page (excluding (a)) will be those used thro ugh out.

Furt herm ore, when considering an unsprinklered

office occupancy, the re exists the pote ntial for

flashover to occur and for the en tire floor to become

involved in fire. Even

if

the building geometry can

accommo date this fire condition, the destructive power

of a fully-involved office room fire is such that sm ok e

control systems cannot usually be designed to

satisfactorily protect means of escape

in

this situation,

excep t for fires in small rooms. An accurate

assessment of the mass flow rate an d heat flux from a

room fire will allow the potential for flashover to b e

estimated, and thence whether additional

precautionary m easures are re quire d, eg sprinklers.

This R epor t will only provide guidance for the design

of smo ke control systems for a fuel-bed-controlled fire

in an office, and a fully-involved fire in a hotel

bedroom.

Should a different design fire be considered for

whatever reason, the eq uatio ns, figures, etc given h ere

may no longer apply, and advice should be sough t

from experts.

8

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. ..;

i

.

.

,

. *

_. _.

. .

,,

. , . h -...,-.- L C .

~. ... .

Chapter

3

Smoke control on

the

floor of

fire

origin

Within

the

fire

room

In any situation involving the potential m oveme nt of

smo ke into escape routes, i t is always preferable to

control the smok e

in

the fire room and hen ce prevent

its passage to otherwise unaffected area s. Ventilation

of the fire room may be achieved by either a dedicated

smo ke exhaust system or by adap ting and boosting an

air-conditioning o r ventilating system. If the

compartment is open to the atr ium, then

i t

must have

either a dow nstand barrie r to create a reservoir within

the com partm ent, or a high-powered exhaust slot at

the boundary edge to achieve a similar effect (Figure 8).

Th e minimum height of the sm oke layer base in the

room must be compatible with the openings on to the

atrium , with the layer dep th being no lower than th e

soffit of th e op ening (Figure 9). Where no dow nstand

exists and an ex haust

slot

is used instead, the exh aust

capacity provided will need to b e compatible with the

layer depth (Figure 10). See the section on exhaust

slots ('Slit extract') o n page 19.

(a)

Use of

a

downstand to c rea te a smok e reservoir

Exhaust from

compartment

(b)

Use of a 'slot exhaust' to prevent smoke entering the atrium

Exhaust from

compartment

Boundary edge

exhaust

slot

Exhaust fr omf

ComDartment

*Volumetric flo w rate suffic iently great to prevent

smoke

spillage beneath downstand for height

of rise

Y

Figure

9 Plume height and layer depth with

a

downstand

Having established the clear layer height in the room,

the mass flow rat e

of

smo ke can then be calculated.

Re cen t work by Hansel123drawing on work by

Zukow ski e t a124and Quintiere et a125has shown that

the rate

of

air entrainmen t into a plume of smoke

rising above a fire

( M , )

may be ob tained by using the

equation:

Mr

=

C, P

Y 3'2

kgs-' ...(1)

where

C, =

0.188 for large-space room s such as

auditoria, stadia, large o pen-plan offices,

atrium floors, etc wh ere t he ceiling is

well abov e the fire.

C, = 0.210 for large-space room s, such as

open-plan offices, where the ceiling is

close to the fire.

Note:

As

the two values are approxim ately similar and

the demarcation between them uncertain, then the

value for all large-space rooms is taken to be 0.188 for

the purposes of design.

C, = 0.337

for small-space rooms such a s unit

sho ps, cellular offices, hotel bedroom s*,

etc with ventilation open ings

predominantly to on e side of the fire (eg

from a n office window in on e wall only).

Most small rooms will therefo re take

this value.

Perimeter

of

the fire (m ).

Height from th e base of the fire to

t h e

smo ke layer (m)

P =

Y

=

Figure

8 Smoke ventilation within

a

compar tmcnt

* Prior to flashover

or

full involveme nt

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Exhaust f rom*

compar tmen t

B o u n d a r y e d g g

exhaust slot

U

*

To ta l vo lumet r i c f low r a te su f f i c ien t l y g rea t

to p reven t smoke sp i l lage beyond th e exhaus t

s lo t for he ight o f r ise Y

Figure

10 Plume heigh t and layer dep th wi th a

slot

exhaust

Equat ion

1

has been validated experimentally26 or

values

of

Y up to 10 times U A f , or fires in large

spaces, for values of the hea t release ra te betwee n

200 and 750 kWm-*.

Th ere is

no

information available to show how

Equat ion

1

or any c urrent alternatives) should be

modified t o allow for the effects

of

sprinkler spray

interactions. Consequently, it is used her e unm odified.

Th e quantity

of

smoke enter ing

a

ceiling reservoir o r

flowing layer given by Eq uati on

1

s shown graphically

in Figures

11

and 12 for both cellular and open-plan

offices

(C, =

0.337 and C,

= 0.188

respectively) and for

sprinklered and unsprinklered offices

( P

=

14 m and

P =

24 m respectively). Fo r furth er discussion

on

the

criteria for selecting

a

value of

C,,

see the section

‘Flow of hot gases out of the roo m of origin into th e

atrium’ th at follows.

Whilst Figures

11

and 12 show the mass flow

production curves for cellular offices, many such

configurations will not in practice have a fixed wall

construction with a good eno ugh fire resistance, or

have a large eno ugh open ing to sustain the

replacem ent air supply neede d for such large fires (see

the section ‘Inlet air’ o n page 17).

Figure 12 also has

a

‘cut-off‘ below which th e

tem per atur e of the gas layer will exceed 600 “C and

flashover of the room will almost certainly have

occurred. Th e mechanism of flashover may well start

to occur pr ior to this critical point, and gas

tem perature s in excess of

500 “C

may be considered a

conservative lower limit for flashover poten tial27.T h e

‘danger-zone’ is shown shaded

on

Figure 12.

Mass

flow rates should be a bove this shaded zone for

the sm oke control systems to ope rate safely.

Flow of hot gases out of the room of

origin into the atrium

Th e mass flow rate of smoky gases passing through a

vertical open ing

( M , )

may be found from23:

L

L

where

W

=

Width of openin g (m )

h

=

Height of the opening above the floor (m)

Cd

=

Effective coefficient of d ischarge for the

opening

W here the s mo ke flow directly approa ches a ‘spill

edge’ with

no

dow nstand (eg wher e the ceiling is flush

with the to p of the op ening),

Cd=

1.0.For o the r

scenarios the following proced ure may be ado pted:

W here the smoke f lows beyond

a

downstand o r lower

ceiling level in the form

of a

plume

of

height

D d

(Figure 13(a)

arid

13(b)), i t has been show n23 hat th e

height of rise of the plume has a n effect

on

the ra te of

flow of sm oke leaving the opening. This effect can be

expressed

as

a modification to the coefficient of

discharge as follows:

Cd

=

0.65

4

32

30

I8-

2 6 -

f

24-

VI

VI

h

2 2 -

; 0-

o

18-

e

6 12

I

...(

)pen -plan off ices

:,=0.188

Height of smoke base (m)

Figure 11 R a t e of product ion of hot sm oky gases

sprinkle red offices

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where D , = Flowing layer dep th in the plane of the

Dd =

D e p th

of

downstand or height

of

rise

of

opening (m).

plume beyond the op ening (m).

Where Dd2 1.0 then D d may be taken as 1.0 for most

openings of practical interest. For a plain ope ning with

no downstand obstruction (Figure

14),

Ddcan be

conside red as the rise of the plume beyond the balcony

edge. Th e flowing layer dept h 0,)may be found from:

.*.(4)

A simple proced ure for calculating the mass flow rate,

e tc is as follows:

(a) Set

C ,

to

0.65.

(b) Calculate mass flow rate from:

C,

P

W h 312

Mw=

3/2

[ 213+;[ g3

(c) Calculate buoyant layer de pth from:

213

D w = ~ [ ]

,

2w m

(d) Calculate discharge coefficient from:

113

Cd=0.65[ y]W + Dd

(e ) Use the new value of Cdand repeat from step (b) ,

until th e difference between the currently

calculated value of Mw (M w(n )) nd the

previously calculated value

of

Mw (Mw (n- l ) ) is

less than 0.1

Y0.

ie:

x 100

<

0.1

Mw(n)

-

Mw(n

-

1 )

MW(n)

This proce dure usually converges after ab out five

iterations and will therefore quickly yield

M,,

C ,

and D,.

Figures 15 and 16 give t h e mass flow values in

graphical form for various opening heights and widths,

using t h e above procedure.

A

ceiling and projecting

balcony 4 m above t h e floor have been assumed. I t

should be noted that the sh aded areas on the graphs

represent

t h e

onset

of

flashover (calculated using

M,

and

Q,

appropriate t o the example i l lustrated and a

layer tempera ture of approximately 550 "C), and

values of mass flow lying within this band should b e

regarded with caution.

4 1

5

>

E

6 30-

r

r

1 5 t

/

/

Height of smoke base m)

Figure 12 Rate of production of hot smoky gases

unsprinklcred offices

Figure

13

Flow out of an opening

4- U

(a)

With downstand and projecting balcony

(b) With high balcony

:ellular offices

:,=0,337

)pen-pla Jffices

:e

= 0.188

Increasing

potential for

flashover

11

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---- -

--

- -

.

ii.

l D i

Figure 14 Plain opening Cd= 0.8

2 5 2

= 4 '0m

h= 3.5m

i

.20

i

W h= 3.0m

Y

h= 2.5m

5

h= 2.0m

10

h= 3.5m

i

.20

i

W h= 3.0m

Y

h= 2.5m

5

h= 2.0m

10

h=4,0m

h= 3.5m

h= 3.0m

h= 2.5m

h= 2.0m

10

I I I

I 1 1 1

0 5

10 15

20 25 30 35 40

W (m)

Figure

15 Mass flow rate through

a

vertical opening -open-plan offices

-

I

v

Y

2

3 5 t

301

5

0

0

5 10 15

20

2

Sprinklered

W

(m)

3 5 7 7 = 4'0m

5t

-

0 5 10 15 20 25

W

(m)

Unsprinklered

Figure 16

Mass

flow

rate through a vertical opening ellular offices

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Th e demarcation between a cellular room and an

open -plan layout is dete rmi ned by the ability

of

the

incoming air to flow into the rising plume from all

sides. Th e narro we r the room becomes, the less easily'

the air can flow behind the plume. In this Repo rt

cellular offices ar e considered to b e those in which the

maximum room d imension is less than or equa l to

five times the diam eter

of

the design fire size, and the

incoming air can only ente r from on e direction

(Figure

17).

This demarcation dim ension was chosen

arbitrarily and has no theoretical derivation. Research

in this area is highly desirable.

W i dt h < 5 x Df

Restr ic ted air f lo w Cel lular roo m

to f i re and plume

..

' Opening

4

Figure

17

L i m i t i n g s iz e of a c e l lu l a r r o o m

Ventilation of the balcony space

If the sm oke canno t be contained within the room of

1

origin because:

the roo ms have d emo untable partit ions,

insufficient replacement air can be provided, or

the engineering implications ar e too costly or

then the s mok e and hot gases will be able

to

travel

from the r oom of origin into the space beyond,

including the atrium.

difficult to app ly,

Som e atria are designed with balconies around the

perim eter of the void, serving all the room s at that

level (Figure 18 ). Figure 19 illustrates in schem atic

form a n atrium with floors (two levels only are shown

in bo th th e figures) which have balconies that leave a

considerable are a for pedestrians. O n each level there

is a large area situated below each balcony. If screens

(activated by sm oke detectors or as permanen t

features) are hung down from the balcony edges, the

region below each balcony can be turne d into a ceiling

reservoir. This is similar to the proc edur e used in

multi-storey shopping complexesI6.

This balcony reservoir can then be provided with its

own extraction system. Oth er screens can be

position ed across th e balcony to limit the size of this

reservoir to ensur e that the sm oke retains its

buoyancy. Each reservoir should be limited to an area

no t exceeding 1300 m2, with a maximum length of

60

m by analogy with shopp ing maIlsI6. T he screens

aro un d the balconies w ill, in general, be fairly close to

potential fire com partm ents (eg offices). Being close,

sm oke issuing from such a com partm ent will dee pen

locally on m eeting a transverse barrier. T he d epth of

these screens should tak e into account local deepening

(see page 17). Sm oke removed from these lower level

reservoirs should usually be ducted

to

outside the

building bu t can b e ducted into the ceiling reservoir of

the atrium (Figure

20).

T he mass flow rate of smoke

to

be exhausted from th e atrium roof will then be tha t

calculated for the under-balcony condition28.

Ea rly experiments with smok e flow

in

shopping malls29

and unpublished workI7 at FR S (also

N

R Marshall,

Fire Research Station; private comm unication, 1984)

have shown tha t the sm oke flowing from a room with a

deep downstand and then under a balcony beyond the

open ing becomes turbu lent with increasing mixing of

air. This subsequent evidence suggests that for the

purpose of enginee ring design the m ass flow rate of

sm oke entering the balcony reservoir

(MB )

can be

taken t o be approximately doub le the amou nt given by

Eq uatio n 2, ie:

C o m m o n

bal cony

space

A t r i u m

Common

bal cony

space

Figure 18

Schemat i c sec t ion

of

a n a t r i u m w i t h b a l c o n i es

U

Exhaust from

balcony reservoir

Figure 19 An u n d e r - b a l c o n y s m o k e r e s er v o i r

...(5 )

13

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I

Figure 20 Under-balcony sm oke reservoir venting into a n atrium

smok e reservoir

Entra inment in to smok e f lows f rom compartm ents is

being studied23.Th e purpose

of

this is to determine

m ore accu rately the influence of such factors as

com partm ent opening geometry, the presence of a

dow nstand fascia and balconyldownstand

combinations. I t follows that Equ ation

5

may be

superseded in d ue course.

Sm oke layer temperature

Th e mean tem perature r ise

of

the smok e layer above

ambient 0) an be calculated from:

QW

__ "C ...(

6)

Mc

where

Qw=

He at output at window

or

exit point (kW )

M

=

The

mass flow of smoke (eg M , or MB)(kgs- ')

c

=

Specitic heat capacity

of

the gases (kJkg-'K-')

Tables

1

and

2

give the tem perature rise 0) or

a

1

M W a n d

6

MW fire, taking into account the cooling

processes mentioned

in

Chapter

2.

In unsprinklered fire situations a high sm oke layer

tem pera ture will result in intense heat radiation which

may cau se difficulties for peo ple escaping along a

balcony bene ath the s mo ke layer, especially

if

the

balconies form a major escape rou te. The m aximum

sm oke layer tem pera ture which will allow safe

evacuation w ithout un due stress is

in

the o rder of

200 C. If this gas tem perature (or lower) canno t be

achieved then co nsideration should be given to:

alternative escape routes,

shorter escape paths along the balcony, and

the installation

of

sprinklers to cool the gases

fur ther .

Effects of sprinkler systems in

smoke reservoirs

Offices, shops, assembly, industrial and stor age or

othe r non-residential purpo se groups are now

expected to have sprinklers if they have a floor mo re

than 30 me tres ab ove g round level. Multi-storey

buildings

in

the assembly, shop, industrial o r storage

purpose groups will also be fitted with sprinklers

if

individual un com partm ented floors exceed a given

size. Sprinklers may also be requ ired in oth er

circumstances for insurance pu rposes.

T he action of a sprinkler system in an office

o n

the

cooling of gases flowing from th at office to the atrium

is accounted for in the derivation of the

1

MW hea t

output17.W here the sm oke layer is contained wholly

within the room

of

origin by a sm oke con trol system

and has a large area, the sprinklers will cool the sm oke

layer furt her. Similarly, w here s m oke is collected

within a balcony reservoir adjacent to sprinklered

offices, ope ratio n of sprinklers und er th e balconies will

lead to increased heat loss reducing the buoyancy

of

sm oke, which in tu rn can c ontribute to a progressive

loss of visibility unde r the smok y layer. How ever,

gases sufficiently h ot eno ugh t o set off sprinklers will

remain initially as a thermally buoyant layer unde r the

balcony ceiling, and will not be pulled out of the layer

by the sprinkler sprays.

When the fire occurs in an office, the o peration of

sprinklers un der the balcony w ill not assist in

controlling it. If too many sprinklers operate d u nder

the balcony, sprinklers in the office could becom e less

effective as the available water supply app roa che d its

limits.

It follows th erefore tha t sprinklers nee d only be

installed

in

a sm oke reservoir

i f

the sm oke layer tempe rature is l ikely to exceed

200

"C and thus produ ce sufficient radiation to

impede escape, o r

if ther e is the likelihood of sufficient

com bustibles being pre sen t to pose a significant

threat

of

excessive fire-spread.

A powered extract system, to a reasonable

approximation. removes a fixed volume of smo ke

irrespective

of

temperature . Therefore

if

the extent of

sprinkler coolirig is overe stim ated , the system could

be underdesigned.

A system using natural ventilators d epen ds o n the

buoyancy

of

the hot gases to expel smo ke through the

14

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Table 1 Volu me flow rate and temperature o f gases from a

1 MW

fire (including cooling within room o f origin)

Mass f low rate Temperature

of

gases

Volume rate

of

extraction

(Mass rate

of

extraction) above ambient . (at maximum temperature)

(kgs-9

( C)

(m s- )

4

6

8

10

12

15

20

25

30

35

250

I67

125

100

83

67

50

42

33

28

6.0

8.0

9.5

11.0

12.5

15.0

19.5

22.5

27.5

32.0

40 25 36.0

50

20 44.5

60 17 53.0

ventilators. In this case the system w ould be

underdesigned if the sprinkler cooling were

underest imated.

Th e heat loss from sm oky gases to sprinklers is

currently the subject of research, although da ta

suitable for design application a re not yet available.

Nevertheless, an appro xima te estima te can be

obt aine d as follows:

If the sm oke passing a sprinkler is hot ter than the

sprinkler op erating tem perature , that sprinkler will

eventually be set off and its spray will cool the sm oke .

If th e sm oke is still hot enough the next sprinkler will

ope rate, cooling the sm oke further. A stage will be

reached when the sm oke temp erature is insufficient to

set off further sprinklers. Th e sm oke layer

temperature can thereafter be assumed to be

approximately e qual to the sprinkler operating

temp erature beyond the radius of operation

of

the

sprinklers. This radius is generally n ot kn own.

In the absence of better information, it may be

reasonable to assume that no mo re sprinklers will

ope rate than are assumed when calculating the design

of sprinkler systems and the ir water supply (eg 18heads

for Ordinary Hazard Gr oup

3).

Fo r powered extract systems the cooling effect of

sprinklers can be ignored in determining t he volume

extract ra te required. This will err

on

the side of safety.

Alternatively, this further cooling and th e consequent

contraction

of

smoky gases can be ap proximately

estimated o n the basis of an average value between th e

sprinkler operating tempera ture and t h e calculated

initial smo ke tempera ture. Wh ere the fan exhaust

openings a re sufficiently well separ ated it can be

assumed that one opening may be close to the fire, and

.

Table

2

Volu me flow rate and temperature of gases from a 6 MW

fire

(including cooling within room o f origin)

Mass f low rate Temperature

of

gases Volume rate of extraction

(Mass rate

of

extraction) above ambient (at maximum temperature)

(k g s -9

( C)

1113s-

)

10

12

15

20

25

30

35

40

50

60

75

90

I10

130

150

200

300

400

600

500

400

333

240

200

171

150

120

100

80

67

54

46

40

30

20

15

25.5

27.0

29.5

32.0

38.0

41 .5

46.5

50.5

59.0

67.5

80.0

92.5

107.0

123.5

140.0

181.0

263.0

345.0

15

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will extract gases at th e

full

initial temperature given

by Equ ation 6. T he oth er openings in these

circumstances can be assumed to b e outside the zone

of ope ratin g sprinklers, and will extract gases at the

sprinklers’ effective operating temperature.

T he n um ber of potential ‘hot’ and ‘cool’ intakes m ust

be assessed when calculating the average tem perature

of

extracted gases.

If the sprinkler operating temp erature is above about

140 “C, or above the calculated sm oke layer

temp erature, then sprinkler cooling can be ignored for

natural ventilators.

No te tha t the effect of sprinkler cooling is to reduce

the h eat flux (Q,) without significantly changing t he

mass flux. It follows th at on ce a new value of

0

has

bee n estimated, th e new heat flux can be found using

Equ ation 6.

Flowing layer depth

Sm oke ente ring a ceiling reservoir will flow from the

point

of

entr y towards th e exhaust points. This flow is

driven by the buoyancy of the sm oke. Even if th ere is a

very large ventilation area downstream (eg

if

the

ceiling downstream were t o be rem oved), this flowing

layer would still have a d epth related to the w idth

available un der th e remaining ceiling (which can now

be considered a balcony), the tem perature of the

smo ke and the mass flow rate of smo ke. W ork by

Morgan30 has shown that this depth can b e calculated

for unidirectional flow as follows:

...(

)

where

DB Flowing smok e layer depth und er

MB

=

Mass flow rate u nde r the balcony (kgs-I)

WB

=

Balcony channel width (m)

the balcony (m )

c d

= Coefficient

of

discharge

Note: Values of

Cd

will vary fo r differing flow

geometries. However, for the p urpose of engineering

design c d can be taken to be 0 .6 if a dee p downstand is

present at r ight angles to the flow, or 1.0 in the absence

of a down stand.

A t the t ime

of

writing, v alues of

Cd

for in termedia te

depth downstands canno t be stated with confidence for

the wide range of geometries to be found in practice. I t

is suggested th at eith er of th e extre m e values should

be ad opted in seeking a conservative design approach .

T he resulting values of layer depth f or different balcony

reservoir widths and m ass flow rates of sm oke are

.

.

.. . ~. _.. .

.

shown in Table

3

for layer tempe ratures 0B 5 OCl7.

This ignores the effects of cooling (See th e section

‘Smoke layer temperature’ on page 14). Each depth

shown in this table is the m inimum possible regardless

of the smo ke extraction me thod employed

downstream; consequently

i t

represents the minimum

dep th for that reservoir.

T he dep th must be measured below th e lowest

transverse downstand obstacle

to

the flow (eg

structural beams or ductwork) rather than the true

ceiling. Whe re such structu res exist and are an

appreciable fraction of the overall layer depth, the

dep th below the obstacle should be found using

Table 3(b) ra ther than 3(a) .

Table

3

Minimum reservoir depths or minimum

channelling screen depths required und er

balconies for both 1 MW and

6

MW

convective heat output

(a) Unimpeded

Itlow

Mass flow rate

entering

reservoir

Width

of

reservoir W

or chann elling screen width

L

(m)*

-

(kgs-’1

4

6

8 10 12 15

10

15

20

25

30

40

50

70

90

110

130

150

1.1 0.8 0.7 0.6 0.5 0.5

1.4 1.1 0.9 0.8 0.7 0.6

1.7

1.3 1.1 0.9

0.8 0.7

2.0 1.5 1.2 1.1 1.0 0.8

2.3 1.7 1.4 1.2 1.1 0.9

2.8 2.2 1.8 1.5 1.4 1.2

3.4 2.6 2.1 1.8 1.6 1.4

4.5 3.4

2.8 2.4

2.2 1.9

5.6 4.3

3.5 3.1

2.7 2.3

6.7 5.1 4.2 3.6 3.3 2.8

7.8 6.0 4.9 4.2 3.8 3.2

9.0

6.8 5.6

4.9 4.3 3.7

b) Impeded

f low

eep downstand

Mass flow rate

entering

reservoir -

Width of reservoir

W

or

channelling screen width

L

(m)*

(kgs-’) 4

6

8 10 12 15

10

15

20

25

30

40

50

70

90

110

130

150

1.8 1.4 1.2 1.0

0.9

0.8

2.3 1.8

1.5 1.3 1.1 1.0

2.8 2.2 1.8

1.5 1.4 1.2

3.3 2.5

2.1

1.8 1.6

1.4

3.8 2.9 2.4 2.1 1.8 1.6

4.7 3.6

3.0 2.6 2.3 2.0

5.7 4.3 3.6

3.1 2.7

2.4

7.5 5.8

4.8 4.1 3.6 3.1

9.4 7.2 6.0 5.1 4.5 3.9

11.2 8.6 7.1 6.1 5.4 4.7

13.1 10.0

8.2 7.1 6.3

5.4

15.0 11.5 9.5 8.2 7.2

6.2

*

For

bi-directional

flow of

smoky gases this should be twice the

actual reservoir width

16

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- .. ...

.. -

(

Local deepening

Whe re a buoyant layer of hot sm oke f lows a long

beneath a ceiling an d meets a transverse barrier,

it

deep ens locally against that barrier3 ' and, as the gases ,

are brought to a hal t , the k inetic energy of the

approaching layer

is

converted to buoyant potentia l

energy against the barrier.

Wh en designing a smoke ventilation system for atria,

in which t he balconies ar e acting as reservoirs, it is

of ten necessary t o contro l the path of smo ke flow

using downstand smo ke curtains. These are typically

installed around the edge of the voids to prevent

smo ke flowing up through t he voids. If the void edg e

is

close to the room this local deepening could cause

smoke to underspi ll the sm oke curtain and f low up

through the void, possibly affecting escape from o ther

s toreys . Clear ly, the void edge screens must be dee p

enough to contain not only the established layer, but

also the additional local deepen ing outside the room

on fire.

Th e ex ten t of local deepening can be found from

Figure 21 . Th e depth of the established layer (DB in

Figure 21) under the balcony immedia te ly downstream

of the local deepening m ust first be found using the

design proced ure given in the p receding sections.

Usually this means in the channel formed betwe en th e

void edge screen and the room faqade. T he additional

depth ADBcan then be fou nd by inspection of Figure 21,

allowing the necessary m inimum overall dept h (DB

+

ADB) of the void edge screen to be found.

It can be shown that th e following scale-ind epende nt

formula can b e used to approx imate Figure 21:

..

,

...(

8)

where

ADB=

the addi t ional deepening a t the

H ,

=

the floor-to-ceiling height (m )

DB the established flowing layer depth ( m)

WB

= dis tance between the opening and the

transverse barr ier (m)

I

transverse barrier (ie balcony depth) ( m)

Inlet air

Th ere must be ade quate replacement a ir for the

efficient operation

of

a sm oke ventilation system.

Wh en ventilating comp artmen ts directly,

if

the fa iade

is normally sealed then facilities should be provided

for the necessary quantity

of

replacement a ir to be

supplied to the fire room automatically. This

requirem ent often make s the provision of smoke

ventilation to the room of origin prohibitive or .

undesirable. Th e provision of replacement a ir to a

system employing balcony reservoirs is far easier,

provided the balconies are op en to the a tr ium.

If the area available for inlet becomes to o restricted,

incoming airflow through escape doors may be a t

too

high a velocity for easy escape. Such air inflows

through door s in public buildings could hinder

I

I I

1.0 1.5 2.0

Dg

fm)

Figure

21

Local deepening at a transverse barrier

17

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. ...-

escapees. Recent research32 nto the ability of people

to

move th rough an exit against an opposing airflow

has shown that mov emen t is not impeded for airspeeds

below

5

ms-', and is not seriously imped ed below

10 ms- ' (a l though some discomfort was reported a t

these higher airspeeds). This suggests that inflow

airspeeds should not usually exceed

5

ms-I.

Ot her values may be appropria te for o ther

circumstances. For exam ple, in buildings where the

populatio n is largely familiar with the es cape routes;

where the incoming a ir is enter ing the f i re room

directly, or where (in the instance of the inlet air being

supplied via the atrium) the major escape route s are

away from the a tr ium; then a less onerous parameter

can be applied. Cur rent advice regarding

pressurisation system design33 ec omm ends a

maximum pressure dr op across a door

of

60 Pa. This

accord s to a face velocity acro ss a rectang ular inlet

opening of about 6 ms-I . Th e pressure drop cr iter ion

may be increased i f the popula t ion of the building is

adu lt an d physically fit , to perhaps 100 Pa (8 ms-I).

A fan-driven inlet air supply may be employ ed, but

can give proble ms when mechanical extraction is used

(th e building will usually be fairly well sealed in such

circumstances). This is because the w arme d air taken

out

will have a greater volume than

the

inlet air.

As

the fire grows and declines, the mismatch in volume

between the in le t a ir and the extrac ted f ire-warmed a ir

will also chang e. This can result in significant pressu re

differences appearin g across any door s on the escape

routes. For this reason simp le 'push-pull ' systems

should be avoided.

Minimum number of extraction points

T h e n u m b e r of extraction points within the reservoir is

impo rtant since, for any specified layer depth, ther e is

a maximum ra te a t which smoky gases can enter any

individual extraction point. Any further at tem pt to

increase the ra te of extraction through that point

merely serves to draw air in to the or if ice f rom below

the smoke layer . This is sometimes known as 'plug-

holing'. It follows that, for efficient extraction, the

number

of

extraction points must be chosen

to

ensure

that no air is drawn up in this way. Tab le 4, which is

based

on

experim ental subsequently modified

( A J

M

Heseld en, Fire Research Station; private

comm unication, 1976), lists the m inimum numb ers

needed for different reservoir conditions and for a

variety of mass flow rates being extracted from t he

ventilators in the reservoir. Table 4 strictly applies to

ventilators which ar e small comp ared to the layer

depth below the vent i la tors (eg where the d iameter is

much less than th e depth

of

the layer).

W here sprinklers ar e installed and additional cooling

of the sm oke layer needs to be accounted for , the

number of extraction points re quired will differ from

those shown in Table 4 . Th e num ber can be

determ ined by calculating the critical exhaust rat e for

an opening

(A J M

Heselden, Fire Research Station;

private com munication, 1976), beyond which air will

be draw n through the layer. This critical exhaus t rate

(MCR IT) ay be found from:

where

p = 1.3 for a vent near a w a l l ( k g ~ n - ~ )

o r

p

=

1.8

for a vent distant from

a

wall (kgm-')

g = Accelera t ion d ue to gravi ty (ms-2)

D

= Depth

of

smoke layer below the

To = Abso lu te ambien t t empera tu re (K)

0 = Excess tempera ture of smoke layer ("C)

T

=

To + 0 ( K )

extraction point (m)

The requ i red number of extrac t vents (N) is then given

by:

M

N2 ...(

10)

MCR T

where M = Th e mass f low ra te enter ing the layer

(ie Mf o r

Ms)

kgs-I)

Table 4 Minimlim number

of

extraction p oints

needed in a smoke reservoir

(a)

1

MW heat output

Total mass

rate of

extraction

(kgs- ) 0.5 0.75 1.0 1.25 1.5 1.75 2.0

Depth o f layer b elow extraction point (m)

9

12

15

18

20

25

30

40

21-28

2 9 4 0

39-54

50-68

57-79

77-1 07

99-137

149-205

8-11 4-5 3-3 2-2 1-2 1 -1

11-15 6-8

3-5 2-3 2-2 1-2

14-20 7-10 4-6 3- 4

2-3 2-2

18-25 9-13 5-7 4-5

3-3 2-3

21-29 10- 14 6-8 4-6

3-4

2-3

28-39 14-19 8-11 5-7 4-5 3 4

36-50 18-25 10 -14 7-9

5-6 4-5

54-75 27-37 15-21 10-14 7-9 5-7

(b)

6

MW heat output

Total mass

rate of

extraction

kgs-I)

0.5 0.75 1.0 1.25 1.5 1.75 2.0

Depth o f layer below extraction point ( m)

12 26-35 10- 13

5-7 3-4 2-3 2-2 1-2

15 31-43 12-16 6-8

4-5

2-3 2-2 1-2

18

37-51 14-19 7-9

4-6 3-4

2-3 2-2

20 41-56

15-21

8-10 5-6 3 4 2-3 2-2

2.5 51-70 19-26

9-13

6-8

4-5 3- 4 2-3

30 62-85 23-31 11-15 7-9

.

4-6

3-4

2-3

40 85-118 31 -43 15-21 9-12 6-8 4-6 3-4

Note:

In both table s the first num ber

of

each pair denotes extraction

points wcl l

away

from the walls, and th c second is for those

close to

the walls.

18

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W here very large or physically extensive ventilators

are used (eg a long intak e grill in the side

of

a

horizontal duct) an alternative method is possible. For

this case, Table 3(a ) or 3(b) can be used w ith the

‘width

of

reservoir’ being take n as the total horizontal

accessible perim eter of all the ven tilators within the

reservoir (eg the total length of intak e grilles in the

example abo ve) and the ‘minimum reservoir depth’

corresponds to the depth of the smoke layer beneath

the to p edg e of the i ntak e orifice. In practice, for

a

given mass flow rate and layer dep th, Table 3(a), 3(b)

o r Equa t ion

7

can be used to find the m inimum value

of

accessible p erim eter.

F i re room

Interm ediate size intake s (ie where th e ventilator size

is comparable to layer depth ) cannot be t rea ted so

simply and it is recom me nded that T able 4 be used

since

i t

errs on the side of safety.

I /I

Required ventilation rate

(powered exhaust)

A

powered smo ke exhaust system consists of fans and

associated ductwork designed to rem ove the m ass flow

rate

,of

smok e entering the smoke reservoir, and to be

capable of withstanding the anticipated smoke

temperatures.

Th e controls and wiring should of course be pro tected,

to m aintain the electrical supply to the fans during a fire.

Th e mass flow ra te of smo ke determined f rom the

previous sections can be converted to th e

corresponding volumetric flow rate and tem perature ,

using Ta bles

1

o r

2

or th e fol lowing equat ion for

selection of the a pprop riate fans:

M T

v =

1313s-1

P O To

...(11)

where VI

=

Volumetric exhaust rate required in the

M

= Mf

r M B determined f rom the

reservoir (m3s-I)

previous section (kgs-I)

T = e + T , (K)

po = Density of ambient a i r (kgmP 3)

Slit extract

When removing the smoke from a common balcony

reservoir, and ther e is no possibility

of

using

dow nstand screens to prevent th e passage of smoke

into the atrium , a slit extract system may be employed

over th e length

of

the f low path to supplem ent the

underbalcony exhaust system and replace the screens

(Figure

22).

Similarly, a slit extract system can b e used

across a room’s openings t o prevent any outflow of

smoke .

Such a system is

likely

to work best with further

extraction distributed w ithin th e fire roo m , which for a

sprinklered room may possibly be provided by the

normal ventilation extraction system (the normal

ventilation input an d recirculation of air being

stoppe d) or, for an unsprinklered room , by a partial

sm oke exhaust system. Whilst this system is designed

to prevent smo ke entering the atrium void,

i t

will not

necessarily maintain a clear layer within the room

itself, and an y balcony m ay becom e ‘fogged’. T he

extraction should be provided very close to the

op enin g from a continuous slit which may be situated

in the pla ne of th e false ceiling.

It has been show n that pow ered ex haust from a slit at

right angles to a layer flow could comp letely prevent

sm oke passing that slit, provided t hat th e extraction

rate at the slit was at least 5/3 times th e flow in th e

horizontal layer flowing towards the slit

(H G

H Wraight, Fire Research Station; private

com mun ication, 1984). This allows a useful general

m eth od for sizing such an extract:

(a)

First calculate th e flow rate of gases approaching

the opening (or gap in the balcony edg e screens)

using

as

app rop riate the following sections:

I I

Balcony reservoir

Drop screen

creen

i

F i re room

I

Slit

I ,

Atr ium void

Drop screen

Balcony reservoi r

screen

Plan

’////// ///////////////////////////

F i re room

Sect ion

Figure

22

Slit extract ion

19

7/24/2019 BS en 12101-5


‘Flow of hot gases out of the ro om of origin into

the atrium’ (page 10)

‘Ventilation of the balcony space’ (page 13),and

‘Smoke layer temperatu re’ (page 14).

(b) Multiply this mass flow rate by 5/3 .

(c) Using the known layer convective heat flux (and

allowing for sprinkler cooling, referring

if

app rop riate to ‘Effects of sprinkler systems in

sm oke reservoirs’

on

page 14), calculate the

volumetric exhaust rate req uired from the sli t ,

using Equa tion

6

to calculate the m ean gas

temp erature drawn through the fan, and

Equat ion

11to calculate th e required fan capacity.

False ceilings

W here th ere is an unbro ken false ceiling in the fire

room or balcony it must be treated as the top of the

sm oke layer. If t he false ceiling is por ou s to sm oke , ie

if

i t has an appreciable fre e area, any sm oke screens

forming the smo ke reservoir must be continued above

the ceiling. If the pr opo rtion

of

free a rea is large

enou gh, the reservoir and its screens may even b e

totally abo ve the false ceiling. Th e perm eab le ceiling

oug ht not t o interf ere appreciably with th e flow of

sm oke from the fire to the sm oke ventilation openings

abo ve the false ceiling.

It h as b ee n s ho wn e ~ p e r i m e n t a l l y ~ ~hat a minimum

free are a of

25%

can be used a s a rule of thum b value

for allowing safe escape. For balcony reservoirs cool

sm oke can be expected t o affect some nearby rooms

und er so m e circumstances, but would no t significantly

hinde r safe escape. Fre e areas

of

less than

25%

a re

possible in som e circumstances, but ex pert advice

should be soug ht w her e this possibility is felt desirable.

The use of a plenum chamber above a

false ceiling

Som e designs have been seen in which th e space above

the m ainly solid false ceiling in a roof or a bove a

balcony

is

used for the extraction of air for normal

ventilation purposes.

A

fan extracting air from this

space (effectively a plenum cha m ber ) reduces its

pressure and

so

draws air from th e space below

through a nu m ber of openings in the false ceiling.

In

the event

of

a fire , a fan of suitably larger capacity

starts up and draws smoky gases into the cham ber in a

similar way.

.

A potentially v aluable bonus of such a system in a

sprinklered building is that the sprinklers which are

normally required in the space ab ove th e false ceiling

will cool the smoky gases befo re they reach th e fan.

Furthe rmo re, i t can be desirable to leave the false

ceiling below th e extraction po ints ‘solid’ (ie not ab le

to pass sm oke) to preven t air being drawn up through

the sm oke layer.

A

sufficiently extensive area of solid

’

20

. -

L J .

false ceiling will ensure th at the sm oke passes through

at least one sprirtkler spray en rout e to th e extract.

Th e plenum cham ber should not be larger in area than

its associated sm oke reservoir. Larger cha mb ers

should be subdivided by sm oke screens extending the

full height of the ch am ber and below the false ceiling

to form a complete smo ke reservoir below. Th e

minimum num ber of openings through the false ceiling

required within

a

single subdivision can be fo und from

Tab le 4. Th e total area of such openings per reservoir

should be decided by consideration of the design

pressure differences between ch am ber and sm oke

layer, and of the flow imped ance

of

the openings

concerned.

A system of reasonably wide (perh aps one-

or two-m etre) slots surro und ing a region of false

ceiling could per hap s be used instead of screens below

the false ceiling.

7/24/2019 BS en 12101-5


Chapter

4

Sm oke ventilation within the atrium

Smoke movement in the atrium

When the sm oke and h eat cann ot, for various reasons,

be confined and removed from the room

of

origin or

associated balcony space, the use of 'throughflow' or

steady-state ventilation from the atrium itself is usually

considered.

This form of sm oke control is that m ost readily

unders tood by most a s 'smoke ventilation' and is based

upon a defined bu oyan t smok e layer being established

at som e point within th e struc ture , with a 'clean' layer

of air benea th. Th e mass flow of gases ente ring this

layer is equivalent to th at flowing ou t through t he

exhaust system (Figure 23).

T he base of such a layer is usually at a height cho sen

for safety reasons or to avoid breaching the practical

'cut-off' limits outlined in the section 'Lim itations to

the use of throughflow ventilation' on page 35. On c e

the height of this layer base is chosen fo r a lowest-level

fire, the height abo ve the top of the op ening (or abov e

the edg e of the projecting canopy or balcony over the

opening where relevant) must be established where

the fire is in an adjacen t roo m .

No te that w hen the fire is on the floor

of

the atrium

and is directly below the sm oke layer that forms un der

the atrium ceiling, entrainm ent into th e rising plume is

different to e ntra inm ent into spill plumes. This special

case of fires on the atrium floor is discussed later

on

page 33.

In gene ral however, the worst condition to be c atered

for is a fire in an ad jacent ro om on th e lowest level, as

this results in the most en train m ent

in

the rising smoke

plume and lience the largest quantity of smoky gas

entering the buoyant layer.

T he fire condition in the com partm ent (the design fire)

should be specified, and th e mass flux leaving through

the com partment opening and any entra inment under

the projecting balcony o r canopy can be calculated as

described in the first three sections of Ch apte r 3.

As

the sm oke flows through the room opening into the

atrium space

i t

will eithe r:

ro ta te upwards around the top edge of the opening

and pass directly into the atrium spa ce as a plume, o r

flow unde r a horizontal projection such as a balcony

beyond the opening, pass to the ed ge

of

the

projection and rise upwards into t he atrium space

as a plume.

Such plumes a re often re ferred to as 'spill ' plumes, or

as 'therm al ' l ine plumes. Th e term ' l ine' denote s that

the base

of

the plum e imm ediately following rotation

is long and relatively narrow.

Line plumes may take one of two forms:

1

Adhered plumes,

where the sm oky gases project

directly from a co mp artme nt opening, and the

plume attaches to the vertical surface abov e the

opening whilst rising upwards. This will also occur

when there is a vertical surface immed iately abo ve

any rotation point into the void. T he surface of the

plume in contact with the ambient atm osphere

in

the atriu m will cause additional air to b e entraine d

,

.......................

....................

.

....................

.....................

.........................

___Y.....

'r I

I

-

nlet

Figure23 Throughflow ventilat ion

of

the a t r i u m

' 2 1

7/24/2019 BS en 12101-5


into it (Figure 24(a)). This type of plum e is

also

known variously

as

a single-sided, attac hed or w all

plume.

2 Free

plumes, where the smoky gases project into

space beyond a horizontal projection, eg a balcony,

thus allowing the forming plume to rise upwards

unhindered. This creates

a

large surface area for

entrainm ent on both sides of the plume along its

spill width (Figure 24(b)), for which reason they are

also known as double-sided plumes.

Th e degree of entrainm ent into the rising plume, and

hence the total quantity of gases entering the sm oke

layer forming un der the ceiling of th e atrium space, is

Reductions in the m ass flow rate

of

smoke enter ing the

sm oke layer can usually be effected by change s to (c)

and (d). In practice t he height of rise of the plume is

usually chosen to permit safe evacuation, leaving only

a

dependency on the length

of

the l ine plume.

Channelling screens

Wh en the a t r ium has a p lane f a p d e with no hor izonta l

projections, the length of the plum e is dete rm ined by

the width of the opening throug h which the sm oke is

passing. When , however, sm oke is able to flow

unrestricted u nde r a horizontal projection, eg a

balcony, it will flow forwards tow ards th e balcony

edg e, and laterally sideways. It will continue to flow

sideways until it meets an ob struction o r loses

(a ) Adhered plume

(b)

Free plume

sufficient energy to becom e sta gna nt, and will the n rise

into the atrium space as

a

very long line plume

(Figure 25(a)). This results in large quantities of air

being entrained and hence

a

very large mass flow rat e

of sm oke entering the layer in the a trium roof.

This excessive ent rain m ent can be reduced by

restricting th e sideways travel of the smo ke und er the

balcony and hence redu cing the length of the

line

plume. Th e devices used to achieve this are commonly

known

as

channelling screens, an d literally ‘channel’

the smoke f rom the

exit

of the room

to

the balcony

edg e (Figure 25(b)). This concept is used in smok e

control systems in multi-storey sh opping cen tresI6.

Th e minimum depth required for a pair

of

these

screens to channel all the sm oke is depen dent on their

separation at the void edge

( L ) .

om e values for 1

MW

and 6 M W fires in offices are given in Ta ble 3 (a) for

a

balcony with no do wnstand obstruction at the void

edge (un imp eded flow), and Table 3(b ) for a balcony

with a downstand at the void edg e, which is deep

relative to the approaching layer (impeded flow).

Alternatively the minimum channelling screen dep th

may be calculated using Equation

7,

where

WB

will be

the channelling screen sepa ration width L. Whichever

proced ure is chosen, the resulting dep th is the

maximum flowing smoke layer dep th and hence

minimum screen depth. Go od practice suggests that

a

safety margin should be considered. A n additional

dep th of 0.25 m would not seem un reasona ble.

Screens may be fixed o r may descend upon s mo ke

detection. A s described abo ve, the final mass flow

ente ring the laye r is a function of fo ur initial

param eters, one of which is the plume width at the

balcony edge. The narro we r the plume at i ts base, the

less the mass flow enterin g the layer.

Thu s the closer the screens may be installed to each

othe r, the m ore the sm oke base may be allowed to rise

for the sa me heat flux.

Figure

24 Line plumes with in the atr ium

22

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Figure 25(a)

Smoke sp read ing s ideways beneath

a

projecting

canopy

or

balcony

These screens must , of course , meet the wall

of

a

compa rtment where i t meets the balcony. Any screen

fixed midway across a compa rtmen t o pening will serve

no purp ose, since sm oke will flow on b oth sides

simultaneously.

Rec ent research37 uggests that channelling screens

may b e unnecessary if the balcony projec ts no m ore

than 1.5 m beyond the f i re room. This research has

also shown that balconies which a re shallow (<2 m)

will cause th e rising plum e

to

curl inwards towards the

structure (Figures 26(a) and 26(b)). If the re ar e higher

ba1conies.a vortex will be cre ated betwe en them,

smoke-logging the balcony levels above the fire floor.

(a)

De ep balcony projection

Figure 25(b)

Smoke confined to

a

compa ct spill plume by

channelling screens

Entrainment into spill plum es rising

through the atrium

Calculations of entra inment in to the sm oke f lows

rota ting around the openinghalcony edge and in to the

subsequent rising plume follow the procedures

of

M organ and M a r ~ h a l l ~ ~ ? ~ ~or free plumes, using the

modifications introduced by M organ and Hanselli7.

Entrain ment into smo ke flows rotating into a rising

adhered plume can b e calculated using the similar

method

of

Mor gan a nd Hansell17, althou gh

it

should be

noted that the entrainment constant appropriate

to

an

adhered line plume is about half that for a free

p l ~ m e ~ ~ . ~ ' .value of

0.077

has been used for the

pre sen t calculations37.

This calculation method is outline d in detail in

'

Appendix

B.

Th e algorithm described can be used

either directly, or as the basis for a compu ter program.

(b)

Shallow balcony projection

Figure 26

Th e effect

of

balcony depth on plum e trajectory

23

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O nce the desired height of the layer base (hb )has been

chosen, the opening width established, or th e

channelling screens separation

L

(and hence also

channelling screen depth using Tables 3(a), 3(b) or

Equation 7) chosen

on

practicality grounds, eg such

that th e screens contact the w alls separating the

rooms, the m ass flow rate of sm oke entering the layer

forming th e ceiling space of the atrium can be found.

Results of calculations of sm oke production d ue to

entrainm ent into the rising plume are shown

graphically in Figures 27-58. H ea t outp uts of 1MW

and

6

MW are considered. Dow nstand fascia dep ths of

0

m, 0.5 m, 1.0 m and 1.5 m ar e used with an overall

com partm ent height of aro und 4 m. Opening widths or

channelling screen separations

of 5

m, 10m, 20 m and

40 m a re shown for both cellular and op en-plan offices.

A choice is provided for either adh ered o r free plumes.

T he results given in Figures 27-58 are rep resentative of

typical room/atrium geometries, with a nominal slab-

to-slab height of 4 m. In practice th e room o pening

geometry, the presence and absence of a deep

downstand fascia and a balcony beyond, and different

floor-to-flo or heights will affect the m ass flow ra te

of

smoke. For exam ple, som e atria may have u pper levels

set back above th e rooms on the storey below with no

balcony projection beyond the lower room front. In

such designs the room walls themselves act as

channelling screens, ie in this case the width of th e

room opening W is identical with the line plume length

L. W here such rooms have a downstand fascia, the

plume rise (hb ) hould be measured from the bott om of

the dow nstand. Figures 27-58 can again be used to

estimate the e ntrainmen t into the plume, but a more

precise calculation for this case is feasible, using th e

procedures of Appendix B.

This fire engineering approa ch is of necessity more

complicated and needs individual consideration.

An alternative method

of

calculating the entra inm ent

into the line plume is due to Thomas40. This treats th e

plume in a 'far plume' approximation apparently rising

from a line source of z ero thickness so me distance

below th e void edge.

T he relevant formula is:

...(12)

QwL2

M I

= 0 . 5 8 ~

y i b

+ A)[ 1+

L

where

M I

= Mass flow of smoky gases en tering the

smo ke layer at height hb (kgs-I)

p

Q,

=

Convective heat flux

in

gases (kW )

=

Density

of

warm gases at height

hb

(kgmP3)

i

-

- -

- - - - _ _

L =

Length

of

void edg e past which gases spill (m)

c

=

Specific heat of air (kJkg-'K-l)

To

=

Absolute ambient temperature (K )

A = Empirical height of virtual source

below

void edge (m )

h b = Height of rise of thermal plume above

void edge (m )

It should be realised th at the derivation of Equati on 12

limits its application to scenarios where smoky gases

issue directly from th e com partm ent on fire, with a

balcony projecting beyond. With ap prop riate changes

to the value

of

A to cater for changes in room/opening

geometry, and hence the mass flow un der the balcony

(see the second and third sections of C hap ter 3), the

Equ ation 12 and Figures 43-58 should give broadly

similar results, since although both m ethod s use

different empirical approaches, these constants are

obta ined by fitting

to

the sam e data36. t should also be

realised that Equation 12 only describes a free o r

double-sided plume and cannot be ad opted for an

adhered or single-sided plume. For a free plume, the

appropriate changes to

A

may be inferred by

inspection of the graphs and comp arison with the

calculated results, eg for a free plume from an op en-

plan office with

a

1.0 m downstand, A

=

0.83H.

A n analysis by Morgan4' for a shopping centre

geometry suggests that for practical purposes it can b e

taken that A

=

0.3 t imes the height of the com partmen t

opening

H,

e

A

=:

0.3H, but n ote that this result strictly

applies for the specific exp erim enta l geometry and for

the entrainm ent abov e the electric convector heaters

in the 'fire com partm ent' used to simulate the fire.

Rec ent research3' h as suggested that the entrai nm ent

into a high-temperature spill plume might be lower

than into a thermal plume. T he effect is not sufficiently

well studied to allow quantitative advice to be given,

beyond the statem ent that the effect becomes ap paren t

for values of

eB

o r

of

8 , where the compartment

opening has no projecting canop y) greater than

approximately 300

C.

W here the entrained mass is the

critical design param eter (eg for estimating the

capacity of powered smoke exhaust ventilators) it is

recommended that the same calculation p rocedures be

followed as for lower te mp erature therm al plumes,

since this will resiilt in an ove res tim ate of the exha ust

capacity and an und erestimate

of

the smoke-

reservoir's layer tem pera ture iving an extra margin

of

safety. It is expected tha t all the calculation

proc edu res for spill plumes described in this Rep ort

will give sufficiently er ron eou s results for flam e

plumes (eg typically for

eB

or e,) greater than ab out

550

"C) that they should not be employed. A t the t ime

of writing

i t

is not clear what calculation procedures

can be a dop ted for flame spill plumes.

24

7/24/2019 BS en 12101-5


N = 4 0

N=

20

N =10

NE5

120

0 1 2 3 4 5 6 7 8 9 1 0

Height of r ise ( m )

-

-

, Figure

27 Adhere d p lum e from open-p lan sp r ink lered o ff ice

H ea t o u tp u t : 1 M W

Downstand dep th at open ing : 0 m

120

l3I

/

V.40

w=20

d=10

J = 5


Figure

28 Adhere d p lum e from open-p lan sp r ink lered o ff ice

H ea t o u tp u t :

1

M W

Downstand dep th at open ing : 0.5 m

Y = 2 0

Y-10

Y =

5

I I I I l l l l I

0 1 2 3 4 5 6 7 8 9 1

Height of r ise (in)

Figure 29

Adhere d p lume from open-p lan sp r ink lered o ff ice

Heat ou tpu t :

1

M W

D o w n s tan d d ep th a t opening:

1.0

m

Figure 30

Adhere d p lum e from open-p lan sp r ink lered o ff ice

Heat ou tpu t :

1

M W

D o w n s tan d d ep th

a t

open ing :

1.5

r

25

7/24/2019 BS en 12101-5


460

420

7

300-

n

Y

2

260-

m

m

-

VI

r

-

c

2 220-

-

VI

-

r

180-

-

140

-

-

-

-

100

L

I

60 1

W=40

W = 2 0

Y.10

v. 5

U

2 4

6

8 10 12 14 16 18

Height of r ise

(m)

Figure 31 Adh ered plume from open-plan unsprinklered office

Heat output : 6 M W

Downstand depth

a t

opening: 0 m

V-40

v.

20

v=10

U- 5

601

I '

0

2 4

6

8 10 12 14

16

18

Heigh t

of

r ise

lrn)

Figure 32 Adh ered plum e from open-plan unsprinklered office

Hea t output : 6 M W

Down stand depth a t opening: 0.5 m

420461

380

-

~

340

-

x

5 300-

n

Y -

260-

VI

r

m

?

c

2

220-

-

r

VI

5 180-

140

-

-

100

-

60

-

L l I I I I 1 I

0

2 4

6

8 10 12 14 16 18

2

Height of r ise (rn)

Figure 33

360

320

m

Y

280

240

W=40

iN. 20

w.10

JV 5

Adhe red p lume from open-plan unsprink lered office

Heat output :

6

M W

Downstand depth a t opening: 1.0 m

N.40

Y = 2 0

v=10

Y = 5

Heigh t

o f

r ise (m)

Figure 34 Adhe red p lume from open-plan unsprinklered office

Heat output : 6 M W

Downstand depth a t opening: 1.5 m

26

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W = 4 0

t

l F

I

20

i-

w=20

w=10

w=5

100

l 0 I

-

-

-

2 90-

VI

VI

-

6 8 0 -

m

c 70-

a,

c

-

VI

2

-

60

-

50

-

Heigh t o f r i se [m)

Figure

35

Adhered plume from cellular sprinklered office

Heat output : 1 M W

Downstand d epth at opening:

0

m

Height of r ise (m)

Figure 37 Adhere d plume from cel lular sprinklered office

Heat output: 1 M W

Downstand dep th at opening: 1.0m

W=40

N - 2 0

Y = 1 0

Y = 5

H e i g h t of r ise (m)

Figure 36 Adhcrc d plume from cel lular sprinklcrcd office

Heat output: 1 M W

Downstand depth at opening:

0.5

m

I -

0

1 2

4 5

6

7

8

9

10

Height of r ise (m)

Figure 38 Adhcrc d plume from ccl lular sprinklcrcd office

Heat output:

1

M W

Downstand dep th at opening:

I .5 m

27

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480

-

-

440-

400

-

360-

320-

m

-L

YI

YI

B 280-

m

m

E 240-

r

VI

-

r

-

2

200-

160

-

-

120

-

-

80

-

W.40

w - 2 0

w - 10

i N - 5

0

2

4

6

8 10 12 14 16 18 20

Height of rise (m)

Figure

39 Adhered plume from cellular unsprinklered office

Heat output: 6 MW


0

m

/

N= 40

N=20

N = 10

N =

5

0

2

4

6

8 10 12 14 16 18

Height of r ise (m)

Figure 40

Adhered plume from cellular unsprinklered office

Heat output:

6

MW

Downstand depth at opening:0.5 m

28

-

460

-

-

420

-

380

-

340

-

-

L

5

300-

m

Y

260-

YI

-

3

r

a

-

+

E

g 220-

r

180-

=

YI

-

-

140

-

-

Heigh t of r ise (m)

Figure

4

N

=

20

w =10

w =

5

Adhered plume from cellular unsprinklered office

Heat output:

6

MW


1.0

m

N =40

Y = 2 0

M : 10

Y.5

Figure 42

Adher ed plume from cellular unsprinklered office

Heat output: 6 MW


1.5

m

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240-

2 2 0

-

200

-

180

-

.= 20

I0

= 5

240

-

220-

200

-

180

-

=

160-

w -

140-

U

U

-

W

r

a

*

e 120-

-

-

P

r 100-

r

-

80

-

Height of r ise

(m)

Figure

43

Free plum e from open-plan sprinklered office

Heat output :

1

M W

Downstand d epth at opening: 0 m

Height of r ise (m)

Figure

45

Free plum e from open-plan sprinklered office

Heat output : 1 MW

Downstand de pth at opening: I .O m

L-40

L

.20

L=10

L =

5

L = 4 0

L:20

. 10

. I 5

I I I I I I I I I

0 1 2 3 4 5 6 7 8 9 1 0

Height of r ise (m)

Figure 44 Free plume from open-plan sprinklered office

Heat output : 1 MW

Downstand d epth at opening: 0.5 m

Heigh t of r ise

(m)

Figure

46

Free p lume from o pen-plan sprinklered office

Heat output: 1 M W

Downstand de pth at opening: 1.5 m

29

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-

550

-

-

500

-

-

450

-

-

400

-

-

& 350-

Y

VI

VI

m

-

- 300-

m

2

250-

al

c

-

E

VI

s

-

200

-

-

150-

-

I10

. = 5

0

2

4 6 8

10 12 14 16 18 :

Height

of

rise

(m)

Figure 47

Free plume from ope n-plan unsprinklered office

Heat ou tpu t : 6 M W

Downstand depth at opening: 0 m

.

=

10

.=5

0 2 4 6 8 10 12 14 16 18 20

50'

I I I

Height of rise (m)

Figure 48 Free

plume fro m open-plan unsprinkle red office

Heat ou tpu t :

6

M W

Downstand depth at opening: 0.5 m

= 10

L E 5

Figure

49 Free: plume from ope n-plan unsprinklered office

Heat output: 6 M W

Downstand dep th at open ing :

1.0

m

600-

550

-

-

500

-

450

-

-

400

-

-

-

VI

2 350-

VIl

m

-

300-

al

c

m

r

250-

-

U

s

-

200

-

150-

-

100

-

-

-110

_ =

5

Height of rise (m)

Figure 50

Free plum e from open-plan uns prinklered office

Hear ou tpu t : 6 M W

Downstand dep th

a t

open ing : 1.5 m

30

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t

. 2 0

10

.=5


Figure

51 Free p lume from cellular sprinklered office

Heat ou tpu t :

1

M W

Downstand dep th at open ing : 0 m

. 10

.

= 5

-

VI

m

1

VI

t

m

r

a

E

2

s

=

m

Height o f r ise

(m)

Figure 53

Free plu me from cellular sprinklered office

Heat ou tpu t :

1

M W

Downs tand dep th at opening: 1.0 m

-

VI

m

1

m

VI,

.-

a

m

L

P

VI

f

t

. = 1 0

I

.

= 20

.=10

..5

0 1 2 3 4 5 6 7 8 9 1 0

He igh t

of

r ise

(m)

Figure 52

Free plum e from cellular sprinklere d office

Heat ou tpu t :

I

M W

Downstand dep th a t open ing : 0 .5 m

1

0 1 2 3 4 5 6 7 8 9 1 0

He igh t

of

r ise

(m)

Figure

54

Free plum e from cellular sprinklered office

Heat ou tpu t :

I M W

Downstancl dept h at opening: 1.5 m

31

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50; ; 6 b 1 1 lk 16 18 :

H e i g h t of r ise (m)

Figure

55 Free plume from cellular unsprinklered office

Heat output: 6 MW

Downstand depth at opening: 0 m

0

2 4

6

8 10 12 14 16 18

Height of r ise (rn)

Figure 56

Free plume from cellular unsprinklered office

Heat output: 6 MW


32

. =

10

. = 5

L = 5

L = 4 0 L =

20

600-

~

550

-

-

5 0 0

-

4 5 0

-

4 0 0

-

I I I I I I I I I

0 2 4 6 8 10 12 14 16 18 2

H e i g h t

of

r i s e

(rn)

Figure 57

Free plume from cellular unsprinklered office

He;3t output: 6 MW

Downstand depth at opening: 1.0rn

5 0

0 2 4 6 8

10

12 14 16 18

2

Height of r ise (m)

Figure 58

Frec plume from cellular unsprinklered office

Heat output:

6

MW


. 10

I

i 10

. = 5

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I 1

4

- ,

,

I . * ?

\.

_ -

Fires on the atrium floor

This relatively simple case can be tre ated in the sam e

way as a fire in a single-storey space , where the plum e

can rise un hind ered from th e fire directly into the base

of the layer. The design fire can be specified in terms

of area ( A f ) nd perimeter ( P ) ,based on expert

assessment of th e fire load at th e atrium floor (which

can vary from trees to cars, from fur niture to

exhibition s). If kn own , the calorific value of th e likely

fuel can be used to estim ate th e heat flux in the rising

gases

(Qf).

Examples of known h eat fluxes may be:

(a) A grou p of four easy chairs clustered togeth er,

forming a perim eter

of

around 6 m, with a hea t

flux of 2 MW.

(b) A sprinklered office environment (providing the

sprinklers can op erate o ver the f ire area) with a

total convective heat flux

(qf)

of about

115

k W m P 2

of fire.

Note: if

the atrium ceiling is high, special

provisions may have to be made t o ensure

effective sprinkler ope ration .

convective heat flux

(qf)

of about

185

k Wm P 2of fire.

(d) A vehicle (car) with a fire perim eter of 12 m and a

(c) An unsprinklered office environment with a total

total convective heat flux

(qf)

of

2.5

MW.

If

t h e

heat flux is not known for the predicted fuel

load, a convective he at

f lux qf

of

0.5

M W per m2 of fire

are a is a usefully pessimistic rule of thu m b covering

many cases.

Th e mass f low rate in the plume as i t enters the smok e

layer may be established from Figure

59

or Equa t ion

1

This procedure can be used for

Y < 10.0d A f .

For

larger values of Y i t would be be tter t o seek specialist

advice on the use of 'small fire' plume theories.

Throughflow ventilation- rea of

natural ventilation required

A n atura l ventilation system uses the buoyancy of the

smoke to provide the driving force for extraction. The

rate of extraction is largely depen den t upon the dep th

and temperature of

the

smoke. The ad vantage of a

natural ventilation system is that it is very simple an d

reliable, and can cope with a wid e.rang e of fire

conditions. Should for any reason the fire grow larger

than the design fire size, a greater de pth an d

temperature of smoke leads to an increased extraction

.rate, so to an ex tent a natural ventilation system has a

self-compensating mechanism.

The precise relationship between t he m ass flow rate

extracted, the ventila tor area, the inlet area and the

smo ke layer is42:

where

A , =

Measured throat area of ventilators (m2)

A i = Total area of all inlets (m2 )

C,

=

Coefficient of disch arge (usually

between

0.5

and

0.7)

about

0.6)

Ci = En try coefficient for inlets (typically

M I

= Mass flow rate of sm oke to be extracted

(kgs-

1

p

= Am bient air density (kgm P3)

g

=

Acceleration due t o gravity (ms-*)

D B

=

Dep th of sm oke beneath ventila tor (m )

€4 = Tem peratu re r ise of smoke layer above

Tl = Absolute tem perature of smoke layer

(K)

To = Absolute temperature of ambient air ( K )

ambient ("C)

Height of smo ke base (m)

I I I I I I I I I I

1 2 3 4 5 6 7 8 9 1 0 1 1

P = 1 2 m

P = 6 m

Figure

59 R a t e

of

p r o d u c t i o n

of

hot

smoky gases

f rom

a

fire

o n t h e a t r i u m f l o o r

33

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Table 5 gives the minimum fr ee area of ventilation

req uir ed , ignoring the effect of any inlet restriction (ie

assuming an infinite are a of inlet ventilation ).

Th e effect of limited fresh air inlets can be allowed for ,

using the following approximations:

If th e inlet area to th e atrium is twice the exhaust

ventilation ar ea given by Table

5,

the indicated

ventilation area an d the inlet area should both b e

increased by approximately 10%.

If the inlet area is equal to the exhaust v entilation

are a, the indicated ventilation are a and the inlet

area should both b e increased by approximately 35%.

If the inlet area is half the exhaust ventilation area,

the indicated ventilation area and t he inlet are a

should both be increased by approximately 125%.

When natural venti lators are used for smoke

extraction, it is imp ortan t that they a re positioned

where they will not be adversely affected by extern al

wind conditions. A positive wind pressure can b e much

greater than the pressure head developed by a smoke

layer. Should this occur the ventilator may act as an

inlet rathe r than as an extract. However,

if

sited in an

area of negative wind pressure, the resultant suction

force on a natural ventilator would assist sm oke

extraction.

Tall buildings or taller a reas of the sam e building (such

as rooftop plant rooms, etc) can crea te a positive wind

pressure

o n

nearby lower roofs. Steeply pitched roofs,

ie roofs over 30 pitch, may also have a positive wind

pressure

o n

the windward slope.

A

suggestion sometimes advanced for offsetting wind

overpressure is to increase the total area of natural

ventilation per reservoir. Since the o verpressure is, by

definition, force pe r unit a re a, this will usually no t

work and indeed could exac erba te the problem by

allowing even greater quantities of air to b e driven

through the venti la tor to mix into the sm oke.

In so me cases it may be possible

to

retain natural

ventilation op enings in a vertical plane by arranging

them t o face inwards to either a region sheltered from

wind action, or wh ere the wind will always produ ce a

suction. In oth er cases the erection of suitably

designed screens or wind baffles (outside the vertical

wall or window holding th e ventilators) can overcome

wind interference and may even b e able t o convert

a n

overpressure into a suction. Th ere is also the possiblity

of

selectively openin g ventilators in respon se to signals

from a wind direction sensor. Expert advice should be

sough t for such designs.

Du e to th e complexity of wind-induced air flow over

som e atrium buildings and t he surrounding buildings,

i t may sometimes be desirable to carry out bo undary

layer wind tunnel s tudies to establish the wind

pressure ov er the building's envelope. On ce areas

of

overp ressure ancl suction have be en identified for all

possible wind d irections, design of ventilators or fans

can proceed as before.

A

powered extract system should be used where

positive wind pressures a re likely to be a problem, or

where i t is necessary to extract smoke via an extensive

ductwork system.

Table

5

Minimum total ventilation area

m2)

eeded

for a smoke reservoir

(from

Equation 13 with C

=

0.6)

( a ) Q = 1 M W

Mass flow rate

(exhaust rate) ~

Smoke depth beneath ventilators

(m)

(kgs-I) 1.5

2

3 4 5 7

10

4 2.1 1.8 1.5 1.3

1.1

1.0 0.8

6 3.2 2.8 2.3 2.0 1.7 1.5 1.2

8 4.5

3.9

3.2 2.7 2.4 2.1 1.7

10 5.9 5.1 4.1 3.6 3.2 2.7 2.3

12 7.4 6.4 5.2 4.5 4.0 3.4 2.9

15 9.9 8.5

7.0 6.0 5.4

4.6 3.8

20 14.5 12.5 10.2 8.9

7.9 6.7

5.6

25 19.6

17.0

13.9 12.0

10.8 9.1

7.6

30 25.3 21.9 17.9 15.5 13.9

11.7 9.8

35 31.4 27.2

22.2 19.2 17.2

14.5 12.2

40 37.9 32.9 26.8 23.2 20.8 17.6 14.7

50 52.2 45.2 36.9 32.0 28.6 24.2 20.2

60 67.9 58.8 48.0 41.5 37.2 31.4 26.3

( b ) Q = 6 M W

Mass flow rate

(exhaust rate)

Smoke depth beneath ventilators

(m)

(kgs-I)

1.5

2

3 4 5

7

10

10 5.5 4.7 3.9 3.3 3.0

2.5 2.1

12 6.4 5.5 4.5 3.9 3.5

2.9 2.5

15 7.8

6.7

5.5 4.8 4.3

3.6 3.0

20 10.2 8.9 7.2 6.3 5.6

4.7 4.0

25 12.9

11.1

9.2 7.9

7.0 6.0

5.0

30 15.6 13.5 11.1

9.6 8.6 7.2 6.1

35 18.6 16.1 13.1 11.4 10.2 8.6 7.2

40 21.6 18.7 15.3 13.2 11.8 10.0

8.4

50

28.2 24.4 19.9 17.2 15.4

13.0 10.9

60 35.2 30.5

24.9 21.6 19.3

16.3 13.6

75 46.7

40.4

33.0 28.6 25.6 21.6 18.1

90 59.2 51.2

41.8 36.2 32.4

27.4 22.9

110

77.2

66.9 54.6 47.3

42.3 35.8

29.9

130 96.8 83.8

68.5 59.3 53.0

44.8 37.5

150 117.8

102.0

83.3 72.1 64.5

54.5 45.6

200 175.9 152.3 124.4 107.7 96.3

81.4 68.1

300

313.1 271.1

221.4 191.7

171.5 144.9

121.2

400 474.2

410.7

335.3 290.4 259.7

219.5 183.7

Note:

To

account for the restriction im posed by the inlet area, add

the following

to

both inlet and exhaust venti lat ion areas:

10% if the inlet area is twice the venti lat ion are a,

35% i f the in le t a rea i s equal to the vent i la tion area , and

125%

if

the inlet area is half the venti lat ion are a.

34

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y-

Throughflow ventilation- emaining

design procedures

Ot her design ca lcula tions are essent ial ly the same as

descr ibed in the previous chapter , eg mean smo ke

layer temperatu re (page 14), flowing layer dept h

(page 16 ), inlet air (page

17),

minimum number

of

extraction points (page 18) an d required ventilation

ra te of powered exhaust vent i la tors (page 19).

Limitations to

the use

of throughflow

ventilation

As

may be se en from the gra phs in Figures 27-58, the

mass flow ra te genera ted by the entra inm ent in to the

rising plum e is very large, and hence the plum e cools

quickly with height. This larg e increase in mass flow

with increases in height tend s to suggest there m ay be

som e cut-off poin t in the rise of the plum e, above which

it might becom e economically impracticable in terms of

a sm oke control system. Experience suggests that this is

often true for flows larger than 150-200 kgs-I.

An othe r effective limit may occur

if

the t empera tu re of

the sm oky gas layer forming in the roof void is too low.

If internal day-to-day heat gains (solar, plant, etc) ar e

allowed to accumulate with in the a tr ium roofspace (eg

passive solar atria) then high-level air tem pera tures

within the atrium m ay be very high. Roofs pace

temperatu res have been recorded a t or above 50

“C.

Smok e spreading in to an a tr ium during the inc ip ient

stages of a fire will naturally be very cool, and th e

entra inm ent processes will draw in the surround ing

ambient a ir as the plum e rises. In most instances this

ambient a ir will be a t or near 20 “C ( due e i the r to

ventilation or air-conditioning), producing a plum e

tempera ture which may b e considerably lower than the

air within the roofspace.

Unless the hot air can be rem oved sufficiently quickly,

i t will result in the initial smo ke layer forming at a

point lower down in the building than may be

desirable. This process is known as early (or

prem ature ) stratification (Figure 60).

As

the fire is

probably growing, the p lume tempera ture will

progressively rise with time. This may result in hotter

smoke ‘punching’ i ts way through the cooler smok e

layer and forming another w armer layer above. Th e

process may continu e until th e smo ke ‘strata’ have

becom e sufficiently mixed to rise up as a single bulk of

; I

{,

. * I

. . . .

. A

..

.

smoke. This problem of early stratification can to some

extent be overcome by providing smoke detec tors a t

many heights within the atrium or located to ensu re

detec t ion

of

smoke c lose to the f i re . Once a forming

smoke s tra tum is detec ted an d the smoke venti lat ion

system caused to opera te , the hot tes t (and therefore

highest) gases will be remov ed first, allowing any

cooler strata to r ise to ta ke their p lace . Hence sm oky

gases will reach the ventilators and the sm oke

ventilation system should settle into

its

‘design’ state .

T he timescale for this process is uncertain and he nce

early detection of smoke

in

these circumstances

is

essential.

A

further problem which could be encountered may be

mo re problematical during cooler weather. Atria with

large areas of extern al glazing will presen t a large surfac e

area to the sm oke layer, which can lead t o considerable

heat losses from it. Most throughflow sm oke control

systems are designed with an arbitrary limitation

to

the

ceiling reservoir of between 1000-3000 m2

(see for example References 1 6,36 and 42), on e reason

being to preve nt excessive energy

loss

from the buoyant

sm oke layer. Many atria c annot physically or

architecturally ado pt such reservoir formations and, if

larger than th e areas me ntioned ab ove, will cause

additional energy to be lost from the layer.

This energy loss will increase with distance, the further

the smok e has to travel from the fire source, and will

manifest itself as a loss of buoyancy within the flowing

layer. This in turn can cause the layer to deep en

beyond t he desired design depth , perhaps

considerably

so.

Cool

sm oke will also b e sensitive to airflow

movements , such as a ir currents (draughts) due

to

ventilation, air conditioning or weathe r conditions.

Furt herm ore, experim ental evidence3’ has shown that

excessive air move ment (such as that which m ay occur

d u e to

an

arbitrary air change rate) into a cool but

otherwise stable sm oke layer can cause it to become

unstable, spreading further throughou t the building.

Th e fo rma t ion of a smoke layer depends upon

buoyancy for the maintenance

of

stability. Smoke

layers which ha ve tempera tures (and hence densities)

approaching that

of

the incoming replacement air

supply will have a tendency to ‘mix’ with this air, rathe r

than ‘float’ above it. This process is known as dilution

ventilation and

is

frequently used in industry to reduce

contam ination levels in buildings (eg welding shops).

Figure 60 Early (or premature) stratif ication

35

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Th e mechanisms involved in dilution ventilation can

easily induce downward mixing of a sm oke layer to the

exten t that, with sufficient air movem ent, com plete

smoke-logging of an atrium can occur. It follows

therefore that the atrium smok e layer should be at a

tem pera ture com patible with stable stratification.

Th ere is little inform ation available on the'

destabilisation of cool buoyant iayers, so a precise

limiting tem pera ture beyond which the abov e effects

will lessen can not be given. Fu rthe r research is

desirable in this area:Experience and experim ental

observation however indicate that these effects may be

severe in terms

of

smo ke control, perhaps leading to

sm oke spreading to otherwise unaffected escape

routes.

In the absence of the necessary experimental da ta, as a

result of practical experience this Re po rt will adopt a

tem pera ture band of between 15-20 C above ambient

as the critical layer tempe rature below which

undesirable effects may occur. This tem pera ture rise

should be regarded as that which the layer will have

after suffering heat losses to th e str uctur e containing it

(see Chap ter 7).

T he practical lim itations to th e use

of

throughflow

ventilation ar e therefore a maximum mass flow rate

of

150-200 kgs-' ,and/or a minim um s m ok e layer

temperature of 15-20 C above ambient.

Which limit is reache d first will depe nd up on the

situation being considered, ie on the type of fire, the

construction of th e com partmen t, the geometry

of

the

atrium, etc.

Exp erien ce (and Figures 27-58) suggests that one or

oth er limit is usually reached when the height

of

rise

abov e the fire room opening exceeds 8-12 m. It

follows that it do es not usually appe ar to b e

practicable to design a throughflow ventilation system

requiring mo re than three t o four storeys (sometimes

less) to be k ept free

of

smok e, regardless of whether i t

is powered or natur al smok e ventilation.

This limitation would ap pear to pose a serious threat

to th e design

of

interesting atr ia, and indeed building

codes and fire safety stan dard s in the USA are

apparently reflecting this43.Th e large , open a t r ium

designs for which American architects have become

renow ned are no longer widely regarded as acceptable,

and new atrium designs are sometimes restricted t o a

maxim um of thr ee open (no n fire-separated) floor

levels, o ne of which must be the ground floor of the

atrium . Ho we ver, various m etho ds exist whereby this

limitation may be overcome to a certain extent.

36

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- - -_

--

1-

Chapter 5

Design considerations other than throughflow ventilation

Void filling

Some atria provide large available volumes

in

which

any smoke from a fire could be contained, such that

smoke control/ventilation may be unnecessary.

This approach is usually based upon the assumption

that a fire

will

grow at a predictable rate, and that the

quantity of smoke generated can be contained safely

in

the roof void during the evacuation period without

prejudicing the evacuation of occupants of the space.

This relies upon quantitative predictions

of

both fire

growth and personnel escape times. Fire growth is

difficult to predict during the very early stages of

development and can therefore, at best, be only a

rough estimate. Similarly, actual times needed for

evacuation are also extremely difficult to determine.

buildings can vary from 10 minutes for a 15-storey

building, where escapees were ‘caught’

o n

the

thirteenth floor for

5

minutes before being able to

descend, to

31

minutes for a 21-storey structure where

escapees were ‘caught’ on the twelth floor for

20 minutes. A similar exercise in a public complex in

the UK (St David’s Centre, Cardiff) has shown a total

evacuation time

in

excess of 30 minutes.

has shown that escape periods

in

multi-storey

I t

should also be remembered that in the UK it is

customary for the fire service to search buildings for

trapped or lost escapees. There will be some designs

where the evacuation times

will

be shorter than the

time for smoke to endanger the escape routes. It

follows that the smoke control option of ‘doing

nothing’ should not be ruled out completely, but

should only be accepted when supported by fire

engineering calculations embodying appropriate

safety margins.

Compartmegt separation

One approach that may be considered for the

protection of the atrium from fires

in

adjacent rooms

(or vice versa) is the concept of the ‘sterile tube’,

which is outlined

in

the Introduction.

I n this instance the atrium is glazed throughout

with

fire-resisting glass or its engineered equivalent. Thus

there is no opportunity for hot smoky gases to enter

rooms adjacent to the atrium, and the building fire

safety precautions revert to those found in the absence

of an atrium. The obvious advantage is simplicity.

The technique has several disadvantages. It is rather

restrictive for building designers, as the atrium cannot

be utilised as a functional space, and generally there

must only be limited quantities of combustible

material contained upon the atrium floor. There can

be no areas ’of public movement

within

the atrium

space other than at ground-floor level.

Since there is the potential for the atrium to be

wholly

full of smoke, the facade should be well sealed. If the

gases in the atrium become hot, as they often may

locally

to

the fire, the facade materials and

construction, and the sealing techniques used must be

able to withstand these higher temperatures.

Such atria may be fitted with means of removing

smoke for fire service use. These systems are often

provided

o n

an arbitrary design basis, usually

comprising an air change rate

if

powered ventilation is

used, or a percentage of the atrium floor

if

natural

ventilation is used. These systems are purely for fire

service use only , for clearing of residual smoke

(usually post-extinction) and must not be regarded as

life-safety systems.

Depressurisation ventilation

Principles

Greater architectural freedom becomes possible if the

atrium facade need not be sealed, but can be allowed

to be leaky, even if the upper atrium is filled with

smoke. Examples of such ‘leaky faqade’ designs might

include:

Hotel bedrooms having doors on to ‘decorative’

balconies overlooking the atrium (ie not access or

escape routes), small enough to be evacuated

through the doors in a few seconds.

Where unsealed windows are used for simplicity

and cheapness.

Where small ventilation openings allow air to

circulate between the accommodation spaces and

the atrium. Clearly there must be no escape routes

open to the upper atrium.

If such doors and other such leakage paths do not have

tight seals, smoke from the atrium may enter many

adjacent rooms o n many levels, causing a loss of

visibility in those rooms and possibly affecting escape

routes away from the atrium (Figure

61).

This might happen simultaneously on many floors,

requiring the simultaneous evacuation of all affected

floors, thus adding to the pressure of use on escape

routes elsewhere in the building. This is likely to be a

particular problem where there is a ‘sleeping risk’, eg

atrium hotels. It

will

also be a problem for firefighters,

since they may feel the need to search all

accommodation on all the affected floors to ensure

that no-one remains at risk. Such a search would be

37

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. . . . . . . . . . . 1 *. . . . . ~ . , .

. .

much quicker if all accommod ation were kep t clear

of smoke.

Hence smok e must be preve nted from passing in

appreciable quantities through these small leakage

openings. On e way of achieving this may be by

depressurising the a t r i ~ m ~ ~ . ~ ~ .

Natural depressurisation

In

any struc ture with natu ral ventilation openings at

high and low level, and with a quantity of heat trapp ed

inside, a ventilation rate will be cre ated d ue to th e

‘stack effect’.

In ord er for air to move o ut through the high-level

opening, the pressure at high level inside must b e

great er than the external pressure otherwise there

would be

no

air mov ement. Similarly, for air t o flow

inwards at low level the p ressure at low level inside

must be less than th at outside. Thus there must b e a

position within the stru cture where the pressure inside

is equa l to that outside. This is known as the ‘neutral

pressure plane’

(NPP).

Any openings si tuated at the

neutr al pressure plan e will have no airflow through

them , as there will be n o pressure differential at

that point.

In

buildings where a throughflow ven tilation system is

installed, where the inlet are a is equal t o the exhaust

vent a rea , then the neut ral pressure plane will exist

approximately midway within the sm oke layer

(Figure

62).

If th e inlet vent a rea is smaller than the

exhaust vent a rea, then th e neutr al pressure p lane will

move upw ards (Figure 63).

An y openings abov e the neutral pressure plane will be

..............

Figure 61 Smoke. logging in a ‘leaky’ closed a trium

und er a positive pressure (defined positive outw ards

from th e atriu m). Thu s there will be a flow of smoke

from the atr ium into rooms abov e the neutral pressure

plan e through any leakage path which may exist.

However, careful manipulation of the ne utral pressure

plan e can raise i t to a safe height abo ve sensitive

levels, whe re th ere is little or

no

threat from the

positive pressure abov e (Figure 64). The pressure in

the atrium below the neutral pressure pl ane will be at

a pressure lower than am bien t, thus any airflow will be

from the room into the atrium. Hen ce t he levels below

the neutral pressure plane a re protected from heat and

sm oke contamination.

Appendix A gives a descriptio n of a fire that occurred

in the

I M F

Building in Washington. It start ed

on

the

tenth floor of a 13-storey atrium , and by th e time the

fire service arrived (16 minutes lat er) the sm oke level

had descended below the tenth floor.

A n interesting aspect

of

this fire was that, despite the

presence of a natural ventilation system in the roof,

the atrium became completely smoke-logged at one

point. This app are nt failure of th e venting system was

attrib uted to th e use of natur al ventilation in a ‘tall’

building, where the smo ke had insufficient buoyancy

to reach the vents.

However, th e fire occurred on the tenth f loor, and for

all practical purposes, when the fire bro ke ou t

i t

was

effectively in a three-storey building with a de ep

base me nt. Natural ventilation works extremely well in

‘shallow’ buildin,gs, and t her efo re the re m ust ha ve

been s om e othe r mechanism in action affecting the

operation of the v entilation system.

...........

Open

ven t i l a t o rs

38

Figure

62

Neutral p ressure p lane

throughflow ventilation

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Th e atrium had no app are nt inlet facility and

accordingly, instead of the ventilators providing a

throughflow ventilation effect, the atrium b ecame

depressurised

in

t h e ’m a n n e rdescribed abo ve. This in

turn prevented s mo ke from spreading beyond the

atrium , despite being smoke-logged

to

ground-floor

level at o ne stage.

Th e neutral pressure plane will lie somew here within

the de pth of the smok e layer in

the atrium depen ding

upon factors such as inle then t area rat io , gas

tem perat ures, wind pressures, etc. It is

not ,

and should

not be con fused with, the actual base of the smoke

layer.

The

equa tion describing the a bov e relationship, in the

absence of wind e ffects is45.46:

...(

14)

where

X = Th e height from the base of the smoke

layer to the desired position of the

NPP

(m)

D,,,

= Maximum de pth of smoke layer from

the cen tre line of the exhaust

ventilator (m)

This equat ion is represen ted graphically

in

Figure

65.

Figure 63

Neutral pressurc

planc

v e n t

largcr than

inlet

Equation 14 represents the condition where the atrium

has

a

single, dominan t inlet leakage path from the

exterior (eg access doors) bu t smaller leakage paths

between t h e atrium and accommodation and the

exterior (Figure 66).

With the technique

as

described ab ove it is quite possible

for the atrium to be entirely filled with smoke (see

Appendix A- M F Building), in which case

D,,,,

will

approach the height of the atrium (H: ,) , g

D,;,, + H,.

I t

is a straightforw ard task to calculate

t he

ventilation

requi rements for a ‘pure’ depressurisation system

using Equation

14

or Figure

65

where

the

smoke layer

tem pera ture is known or can be determ ined as shown

in Chapter 6.

I f

t he

neutral pressure plane were to descend below

the desired design d epth then som e of

the

higher

storeys may become en dan gered . This can arise from

a n increase in the actual inlet leakage area available,

for example, where the fire brigade have ope ned

access doo rs to th e atrium to investigate the severity of

the fire. A successful depressurisa tion design should be

able to prevent smo ke infiltration into adjacent spaces

on the higher floors even in this condition.

In addition i t is possible th at

t h e

fire may cause

windows to break on both the ex ternal f a p d e and t h e

atr ium f a p d e of the f ire room. I n this case the broken

areas can act

as

a ‘dominant’ leakage path fr om ,the

exterior.

Figure 64

Ncutral pressure planc

abovc

highest

‘leaky’ storey

39

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00

00

60

10

30 - O C

I 1.5 2.0

X

Figure

65 Solu t ion to neu tral p ressure p lane equat ion (14)

Th us all potential inlet leakage path s must be assessed

when using Equ ation 14 o r Figure 65.

I t should be noted that the simple approach set out

her e will be invalid wh ere the leakage p aths across the

atr ium bound ary have appreciable areas on several

storeys (although all leakage areas below th e smok e

layer’s base can be aggregated an d reg arded as being

at th e layer’s base for calculation purposes w hen using

Equation 14 or Figure 65). Where there a re

appreciable significant leakage paths o n several

storeys above the layer’s base the sam e

depressurisation principle can be em ployed, but a

mo re complicated ‘flow network’ calculation must b e

used. This is best left t o specialists in th e field.

It is difficult to give a simp le gene ral rule t o identify

when a building can be regar ded as having a single

dom inant inlet. Nevertheless, i t may be sufficient to

ado pt a guideline from the related field of ‘air

infiltration’, so that on e can assume a dom inant inlet

if

the to ta l a rea of all openin gs below the layer base is

mor e than twice the total area of all openings abov e

the layer base (excluding the area of the ventilators

th e m ~ e lv e s )~ ’ .

Natural depressurisation and wind effects

Th e neut ral pressure plan e is sensitive to the effects of

wind, and ‘adverse’ wind pressures might cause th e

NPP to fall to a lower position o n the leeward side of

th e building, po jsibly contam inating th e topmo st

leeward storeys. It follows that the depressurisation

design proc edur e must tak e wind force into account.

T o assess the efficiency of oper ation of a

depressurisation system a knowledge of th e wind

pressure coefficients acting upon a building will be

necessary. The se are a well established way of relating

the wind pressure anyw here on a building to th e wind

velocity at roof level.

Wind pressure coefficients have often been measured

so that structural wind-loading can be calculated.

Th ere is a considerable body of da ta in existence.

W her e complete certainty is requ ired for a novel or

complicated building, wind-tunnel observations using

scale mode ls will yield usable results. In g eneral

however it should often be possible to obtain

reasonable values for the wind pressure coefficients

need ed for sm oke contro l calculations from available

l i terature (see Referen ce 47 for example).

Figure 67(a) shows t he typical three-dimensional

com plicated pat tern of wind pressure coefficients over

a tall tower

to identify th e most pessimistic values for each storey,

in which case the problem can be simplified to two-

dimensional as shown in Figure 67 (b).

In practice it would be necessary

en

vent i la tors

40

Figure

66

Neutral p ressure p lane -dominan t in let

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(a)

Three-dimensional distribution for

typical tower block

(b) Two-dimensional distribution for

typical tower block

Figure 67

Wind pressure coefficients

around buildings

With these d at a established for any specific building,

the design procedure for checking on the performance

of

a natu ral depressurisation system is fairly simple

where there is a single domin ant opening.

To prevent smok e leakage in to the top leeward s toreys

for all wind speeds4?

( A - I ) C , v - A C , L + C , i ]

I 0

.

15 )

where C,,

=

Wind pressure coefficient at the vent

CpL W ind pressure coeff ic ient a t the

CPi

=W in d pressure coeffic ient a t the in le t

topmost leeward s torey of the building

To

7i

a n d A =

AVCV

2

+ 1 .

.

1

6)

Providing the requirements of Equation 15a r e

satisfied then natural ventilation will work at all wind

speeds. This implies that the

roof

ventilation system

should be subjected

to suc t ion

wind pressures a t all

times. However,

if

i t

is impossible to emp loy a natu ral

ventilator on a particular building, fans can be used

instead.

PO

ered depressurisation

Th e necessary capacity is a little harder

to

calculate,

and the best fan is on e which is not affected by wind

pressures on its exhaust. With a fan however, a

maximum w ind speed must a lways be assumed for

design purposes. T he requ ired volumetric flow rate

may be calculated from4?

where

VI

= Fan capacity requir ed (m3s-I

V,i,,d = Design wind velocity (ms-I)

I12

..

(17)

A natural sm oke control system will be affected by the

wind pressures op erating against all the openings in

the structure ; thus pressure differentials vary with

wind d irec t ion an d opening posi t ion, and the

throughflow of air will vary w ith wind velocity.

How ever when t he hole in the roof is replaced by a

fan, the pressure differentials within the building now

have to be ch anged by mechanically altering the

throughflow

of

air. Therefore th e system must be

design ed with a m axim um design wind velocity to

cater for all conditions.

Furt her sophistication may be achieved by the use

of

an anemomete r and by having ‘groups’of fans, each

grou p operatin g at a different wind velocity. So if the

wind was l ight, one group would opera te and, if the

wind speed increased, fur ther groups might be

activated a s necessary.

41

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Chapter

6

Depressurisatiodsmoke ventilation hybrid designs

Principles

In Chapters 2 to 4 we have indicated how sm oke

ventilation can only kee p a limited num ber of lower

storeys clear of sm oke below the buoyant smok e layer

formed in the atrium . Th e technique do es in principle,

however, allow those lower storeys to have adjacent

spaces - nd their escape routes- pen t o the atrium.

Th e section ‘Depressurisation ventilation’

on

page 37

shows that it is often possible to design a

depressurisation system where clean air is drawn

through all significant leakage openings on the atrium

faqade immersed in the sm oke layer.

Depressurisation does not however protect any large

leakage openings on any storey abo ve the layer base in

the atrium, n or will it protect any escape routes

on

that

storey open to the atrium. In this context a large opening

is on e where th e openin g in the atriu m faqade is larger

than the sum of openings further along the same leakage

path away from the atrium (eg if the atrium faqade

opening is larger than openings in the external wall).

But i t will often be th e case tha t architects will want to

maxim ise use of the atriu m space, and an obvious way is

to combine the smo ke ventilation approach of C hapters 2

to 4, allowing grea ter freedom of design on the lowest

storeys, with the lesser freedom of ‘leaky faGades’

allowed by the depressurisation technique set o ut in

Chapter 5. In this ‘hybrid’ design the ratio of vent a rea t o

fresh-air inlet area will be determ ined by Eq uati on 15 ,

whereas the actual values

of

these areas must be

consistent with the necessary smo ke extraction

require men t as defined in the two sections on

throughflow ventilation design in Ch apte r 4 (pages 33

and 35). It should be apprec iated th at in such a hybrid

design the sm oke layer tempe rature in the atrium

required for the dep ressurisation calculations is a

natural outcome of the plume entrainment calculations

needed for the smo ke extract calculation. Note that

hybrid designs are similarly possible wh ere po wered

ventilators a re used for atrium smoke exhaust.

Hybrid designs usually follow one of two approaches:

1

Mass

flow based, where the atrium is designed with a

num ber of open levels above the atrium floor, requiring

a plume

of

a specific height. Th e maximum num ber of

levels will be de termine d by either the magnitude of the

mass flow rate entering the layer, or the s mok e layer

tempe rature falling below the m inimum value of

15-20 “C (see Chapter 4).

2 Temperature based, in ord er to cool a potentially hot

smo ke layer by the deliberate entrainm ent of ambient

air into the rising plume. This may e nable the use of

faqade materials that cann ot withstand high

tem pera tures (eg float glass).

Design procedures for hybrid systems

Massf low

based systems

(see Figure 68)

(a) Dete rmin e the height of rise of the smo ke plume

required to clear the open levels (hb), ith the

design fire (from C hap ter 2) chosen o n the lowest

op en level. This will also yield th e sm ok e layer

dep th D), easured from the centr e line of the

ventilator.

(b) Fro m Figures 27-58 or by detailed calculation and

with the desired channelling screen separ ation ( L )

or opening w idth

(W) ,

eterm ine the mass flow

rate

( M , )

enter ing the base of t he layer. If th e fire

is on the atr ium floor, determine M I using the

section ‘Fires

on

the a tr ium floor’ on page 33.

(c) Calculate the total surface area of the sm oke layer

(the atrium surface area in contact with the

smo ke layer plus the area of the layer base), and

determine the l ikely smoke layer temperature,

using Chap ter 7. If the s mo ke layer temper ature is

below 15-20 “C above ambien t then the num ber

of open levels may nee d to be reconsidered, or

som e (or all) of the lower levels vented

independently from the atriu m, using the

procedures set out in Chapter

3 .

Neutral pressure plan

Ai

Figure 68 Principles of hybrid smoke venti lat ion system -

mass f l o w based

42

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* .

. . . . . --.~

.

...

...

.-

(d) Set the neutral pressure plane height ( X ) o that

required above the base of the sm oke layer , and

determine the value of (A,CvIA,C,)2 rom

Equation 14 or F igure 65.

(e) With these values

of

(A,Cv/A,C,)2,

DB,

Iand

01 alculate the ventilation area required from

Equation 13.

(f) With the values

of

(A$, /A ,C J2 an d A,C,

calculate the quantity

of

inlet ventilation required.

In

the eve nt that t he actual inlet area available is

greater than that require d by calculation, then t he

ventila t ion a rea should be increased to mainta in

the ra t io

of

(A,C,/A,C,).

(g) Using Equat ions 15 and 16 and the appropria te

wind pressu re coefficients, check the system

operation with regard t o wind effects.

(h) I n the even t that th e wind effects may adversely

affect the operatio n of a natura l ventilation

system, calculate the fan capacity require d using

Equa t ion 17, with an appropria te value of design

wind velocity.

(i) Check that the anticipated suction pressure and/or

air inflow velocities do not in themselves enda nger

the safe use of any escape routes away from the

atrium (see the section 'Inlet air '

o n

page

17).

Temperature based systems

(see Figure 69)

(a) Decide upon a smoke layer tempera ture r ise 0,)

compatib le with the faqade mater ia l employed.

For f loa t

glass

a tempera ture r ise

of

70

C

above

amb ient will give a reasonable safety margin to t he

system design. Tou ghen ed glass may be capable

of

withstanding higher tem pera ture rises ( eg 200

C).

(b) Calculate the total surface area

of

the sm oke layer

( the a tr ium surface area in contact with the sm oke

layer plus the are a

of

the layer base) , and

determin e the m ass flow ra te required to g ive the

desired tempera ture r ise , using Chapter 7. As a

simplification incorporating a margin of safety,

th is s te p can be omit ted and the mass flow ra te

calculated using Equ ation 6.

' . . . l

' I

.

.

-4

1

. .

.......

...............

.............

................

..................

..................

..................

................

...................

.................

................

...............

..........

.........

....................

iii/ijjiir

Centre l ine

of

vent i lators AvCv

Neutral pressure plyne

A- -

- q - - -

(c) From Figures 27-58 or by detailed calculation and

with t he channelling screen separation ( L )o r

opening width

(W),

determine the height of rise

( h b ) to the base of the layer, necessary to give the

required mass flow rate.

(d) With t he design fire at th e lowest level and taking

into account t he necessary height of rise (hb) or

cooling purposes, de termine th e maximum smoke

layer depth

(Dmax).

et the neutra l pressure p lane

height ( X ) o that required above the base

of

this

smoke layer depth , and determ ine the value of

(A,Cv/A,C,)2 rom Equ ation 1 4 or F igure 65.

(e) With the required value of hb,determine the

shallowest smok e layer dep th Ds) , ompatib le

with t he depressurisation concept (this is often the

second level beneath the

NPP).

(f) With this value

of

(A,Cv/A,C,)2,

DB,M I

n d €4

calculate the ventilation area requ ired from

Equa t ion

13.

I n the event tha t the actual inlet area

available is grea ter than that required by

calculation, then th e ventilation area should be

increased

to

mainta in th e ra t io

of

(A$, /A,C,).

(8) Using Equat ions 15 and 16 and the appropria te

wind pre ssure coefficients, check th e system

operatio n with regard t o wind effects.

(h) In the eve nt that th e wind effects may adversely

affect the operation

of

a natural ventilation

system, calculate the fan capacity requ ired using

Equat ion 17, with the a ppropria te value of design

wind velocity.

(i) Check that the anticipated suction pressure and/or

air inflow velocities do not in themselves endang er

the safe use of any escape routes away from the

atrium (see the section 'Inlet air '

on

page 17).

Figure

69

Pr inc ip l es

of

h y b r i d s m o k e v e n t i l a t io n s y s te m

t c m p c r a t u r e b a s ed

43

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. .̂

Chapter

7

Atrium smoke layer temperature

In most sm oke con trol system designs no account is

taken of th e heat losses to the struc ture. It is assumed

there is conservation of he at an d tha t all of the heat

flux entering a smo ke reservoir is contained in, and

remains in, the smok e. Experimental work in the past

has shown th at for relatively small smo ke reservoirs

with medium to high thermal resistance, or for high

mass flow rates of s mo ke, this assumption holds good.

When considering atria however, the assumption can

no longer be considered entirely valid.

An

atrium

generally h as a large surface a rea , which is

predom inantly glazed in most cases, thus providing a

good h ea t sink. Th er e will be a passage of heat en ergy

from the smoke layer into the structu re, and

accordingly the sm ok e layer will suffer a reduction in

temperature.

Figure 70 shows the heat balance in a n atrium. This

model was used to d etermine the loss of energy from

the sm oke layer, based upo n 'worst case' assumptions

for th e f a ~ a d e ~ ~ .he fasade fabric

is

assumed to be

thin glazing, with n o appa rent delay in the transfer of

the en ergy from the layer. The results of using this

model are shown graphically in Figures 71,72 and 73

for 1MW,

5

MW a n d 6 MW fires respectively. A s can

be seen from the graphs espite the fact that many

differing atrium geometries were considered, with

different values of ext ernal exposure - he resultant

calculation points may be plotted comfortably as single

curves for each value of mass flow rate.

A t high values of mass flow rate there is little change

in the atr ium sm oke layer temperature fo r wide

variat i0ns.h smoke layer surface area. This is due to

the gas flow being the prim e mover of energy, and

tends to justify th e assumption th at loss of heat t o the

structure of a building may be ignored fo r relatively

small contact area s.

Heat carried by

exhaust gases

Open vent i lators

or powered extract uni ts

Heat

l os s

th rough roof

Figure 70 Heat

balance

in

an a t r ium

44

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140

130

I I

I

0;

4 6

8 b

h

1L

,b

2b

d2

Q = l M W

24 Figure 71

The atr ium layer

,o

M i =

100

kgs-

Total surface area of smoke layer ( including base ) ( m 2

x

1000)

Q =5MW

40

-

I I I I I I I I 1 I

4 6

8

10 12 14 16

18 20 22

:

Total surface area of smoke layer (including base)

( m z

x l000)

6o

40

Me= 100

kgs-

-

0

2

8

10 12 14 16

18 20 2 2

Total surface area of smoke layer (incl udi ng base ) ( m 2 x

1000

)

Figure

72 The

atr ium layer

tempera tu re

(Q

= 5 M W )

Figure 73 Thc atr ium laycr

tempera tu re (Q =

6

M W )

45

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Chapter

8

Additional design factors

Atrium roof-mounted sprinkler systems

Conventional sprinklers mou nted in the roof of an

atrium will only be of potential benefit if there is a fire

in the atrium floor itself. How ever , du e

to

the height

that these sprinklers will generally be m ount ed above

the f loor, the fire will be of considerable proportions

before th e sprinklers activate and they will therefo re

be of limited benefit. Mor e importantly perha ps,

if a

smo ke layer is just above the ope rating temp eratu re of

the spr inklers, it will be reasona bly stab le (e, 50

“C).

Th e action of all the roof sprinklers operatin g almost

simultaneously (as can be inferred from work by

H i r ~ k l e y ~ ~ )ay rapidly cool the layer and cause it to

become unstable. This can occur if a fire is not a t the

atrium floor, and is there fore highly undesirable.

If t he re is a likelihood th at a fire load will be presen t

on the atrium floor, the sm oke control system designed

for a fire in an adjacent room will generally be able to

cope with the pro ducts of a much la rger fire directly

und er the roof17.Thu s with good housekeeping and

prop er managem ent to restrict the use of fuel

assemblies, such a fire could feasibly exist and bu rn

itself out before becoming a problem to othe r areas

outside the atr ium.

Sm oke detect ion sysftems

in the

atrium

Th e opera tion of any sm oke control system in an

atrium is generally depe nde nt upon a smo ke detecto r

operatin g. If the atrium is particularly tall,

stratification of the smo ke layer before reaching th e

ceiling is probable- specially in atr ia which are air-

conditioned in the low er portion only, or have a high

proportion of roof glazing (see the section ‘Lim itations

to the use of throughflow ventilation’ on page 35).

Thus

if

the only detection system present is roof-

moun ted i t may not oper ate , or a t least i ts operation

may be con siderably delayed. This problem can b e

overcome by th e installation of smo ke detection t o the

various rooms o r by interm ediate detection zones at

different heights in the atr ium , possibly using bea m

detectors .

1

Pressurisation of stairwells an

In som e atr ium buildings there may be a requirem ent

or desire to pressurise the escape stairs and associated

lobbies. If th e atrium em ploys a depressurisation o r

hybrid sm oke control system and the glazing between

the f ire room and the a tr ium has cracked or shattered,

th e pressure within the fire room will, of necessity, be

lower than th e outside ambient pressure. This

reduction in pressure will act as though a n extract fan

were fitted t o the fire room , increasing the p ressure

differential developed across the escape lobby doors.

This increased pressure differential will increase the

air flow through the leakage paths of the lobby, thus

enhancing the efficiency of the pressurisation system in

preventing the passage of sm oke into the escape route.

Air-conditioned atria

In som e hot countries it is common practice to totally

air-condition an at rium , so that its internal amb ient

temperature is lower than the external ambient

tem pera ture. In this case the use of a natural

ventilation system would cause a reverse stack effect

(ie vents acting as inlets) and cou ld cause proble ms of

cool sm oke spreading downwards towards the escape

doors. Once th e cool atrium air has bee n flushed o ut

by the warmer ambient air entering through th e vents,

the system will reverse its direction

of

flow.

This problem

of

an initial downward movement of

smo ke may be alleviated by the use of smo ke detecto rs

in the rooms, rather than the atr ium , causing the

ventilation system to o pe rate and creating a balance

between the internal and external temperatures prior

to the smoke entering the atrium in quantity, or by

using powered ventilators.

ling screens and hybrid system s

It has been shown in Ch apte r 4 (‘Channelling screens’,

page

22)

that when sm oke passes under a balcony to

rise into the roof void above , the quantity of smo ke

entering the sm oke layer

in

the rising sm oke plume

can be reduced by restricting the width of the plume as

it passes the balcony edg e, by the use of channelling

screens.

T he need fo r plume width restriction is necessary for

any smo ke conlrol design where a clear layer

of

air is

requ ired above a balcony projection beyond th e fire

room (eg for escape purposes), and so will apply t o a

hybrid sm ok e control system w hen th e height of rise of

the sm oke plume is fixed by escape requirem ents

(mass flow based system s).

As

described in the sub-section ‘Tem pera ture based

systems’

on

pag,e 43, an alter native use of a hybrid

system is to cool the sm oke layer for some purp ose (eg

to prevent glazing from cracking) by deliber ate

entrainmen t of air into the smoke plume (tem perature

based systems). When designing a natural ventilation

system for this purpose a knowledge of t he de pth of

the sm oke layer in the atrium is necessary to calculate

the vent are a req uired . This in turn implies a

knowledge of the height of rise of th e smo ke plum e.

The refore an estimate of the plume width leaving the

room is desirable to determ ine the height of rise

required for cooling purposes, and this estimate should

46

7/24/2019 BS en 12101-5


be reasonably narrow (usually not mo re than 10-20 m ).

However, variations between th e mass flow entering,

and that be ing vented from, the a tr ium sm oke layer

with this type of hybrid system will be immediately

compensated for by

a

change in the d epth of the

smo ke layer.

Thus

the actual width of plume achieved

is irrelevant t o the satisfactory operatio n of the system.

He nce there is no practical ad van tage in physically

reducing the width

of

th e rising plum e for this form of

hybrid system. The refo re in the design

of

t empera tu re

based h ybrid systems, channelling screens are

unnecessary.

Wind-sensing devices and natural

depressurisation

T he sub-section 'Natura l depressurisation and wind

effects ' on pag e 40

detailed the effects

of

wind

pressures aroun d an atrium and concluded that roof

vents should operate in areas of high suction pressure

for

all

wind directions. In certain instances it may be

likely that roof vents may expe rience an adverse

pressure effect (eg vertically mo unte d vents), in which

case the ven tilation system should be controlled by a

wind direction indicator, an d the

required

amoun t of

ventilation should opera te in a leeward zo ne only.

47

7/24/2019 BS en 12101-5


ry (based on

Reference

9)

Building

1 3 - s to r e y s q u a r e - s h a p e d r e i n f o r c e d concrete o f fi c e b u i l d in g w i t h p e n t h o u s e , b a s e m e n t a n d

4 - s to r e y underground g a r a g e .

A

c e n t ra l l y s i t u a te d e n c l o s e d c o u r t y a r d c r e a t e d t h e a t r i u m .

The

w i n d o w s

of

t h e

offices

facing t h e a t r ium were of 6 .35 m m plate glass.

Date

of

fire

13

M a y

1977.

Atrium

Tw o ventilation systems recirculated air at th e to p of

the atr ium , and at its base there was an air handling

unit. Smoke detec tors were provided at the fans of the

air handling unit and were arran ged to shut down the

fans when the de tector s activated. Th e units could be

manually restarted and put o n exhaust. The general

office area was fed by pent hou se air handling units

that could go into

a

‘smoke-purge mode’

if

they were

running when a fire occurred. None of the above

systems was in operation at the t ime of the fire.

Th e roof of the atr ium was made of clear plastic

panels. Six custom-made smo ke ventilators were

provided in the atrium’s roof and com prised clear

plastic panels

o n

hinges equip ped with springs and

release mechanisms. Th e release device was operated

by one sm oke detec tor located in the atrium roof.

Fusible links on individual ventilators were a lso fitted.

Sprinklers were provided at roof level in the atriu m,

and th e building was equip ped with man ual fire alarm

points and hydrant valves on each floor.

fire difficult. Thick black s mo ke issuing from the office

had built down from th e roof of the atrium to below

the tenth floor.

Although the smo ke detector had operated , only two

of the six smo ke ventilators had ope ned. The oth er

fou r had rele ased but t he springs had lost sufficient

strength to ope n them fully. The se units had to be

manually opened from outside. Smok e however did

not vent effectively and at on e stage the atrium was

completely smoke-logged.

Sm oke extractors could not be connected t o the smoke

ventilators and so firemen used large extracto rs

pointed upw ard from the atrium g roun d floor to pull

fresh air from the front door s and push smok e upward

and ou t through the ventilators. N o building

engineering staff were available to advise firemen on

the

H V A C

smoke purge capability until much later. I t

took 2-3 hours to finally remove th e smo ke from the

atr ium.

1 Th e fire was confined to th e room of origin by the

closed office door and the wall construction.

2 Windows facing the atrium ab ove the fire floor

were cracked by heat but fire and smo ke had not

penetrated other f loors.

3 Th e temperature of the gas layer in the atrium was

insufficient t o activate the sprinkler s in the atrium

roof.

4 Du e to an insufficiency of replacement air the

existing ventilation system design was inapp ropria te

for clearance of smoke from the atr ium, and t h e

‘dilution’ ventilation ap proa ch used by th e fire

brigade took m any hours to clear the smo ke.

5 If this had been a n atrium with balconies providing

access to escape-ways, the sm oke may well have

caused serious escape problems from upp er floors.

6 Des pite the l’act there were unp rotec ted openings

on

to the a tr ium, and that a t on e point the atr ium

was totally smoke-logged, smoke did n ot m igrate

to other parts

of

the building. This indicates that

t he

existing ventilation arr ange men ts apparently

‘depressurised’ the atrium .

he fire

At 18.45 h a w orker discovered

a

fire in

a

small office

(3 m X 4.6 m) on th e tenth floor;

a

plan of this floor is

shown in Figure

A l .

Th e fire brigade received the

alarm at 19.01 h. On arrival firemen foun d fire venting

from th e office window into the atrium . Th e fire floor

was hot and sm oky and this, coupled with the fact that

the fire involved an in ner office, made locating of th e

Figure

A1 In terna t iona l Monetary Fund Bui lding .

Plan

of

the ten th

floor,

showing loca t ion

of the

office involved

48

7/24/2019 BS en 12101-5


IInndrodnnction

Th e Fire Research Sta t ion has carr ied out a number

of s tudies in to the mo vement of smo ke in buildings.

Pa r t

of

th is work has resul ted in th e development

of

a

theory by Mo rgan and Marshal136 o estimate th e

a m o u n t of air . en tra ined in to f ree (or double-s ided)

thermal sp il l p lumes (see Figure 24(b)) . This

calcula t ion me thod is important f or smoke contro l

design in that

it

enables the designer to ca lcula te the

required fan capaci ty or vent are a for a smoke

ventilation system for large undivided volum e

buildings (eg multilevel shopping malls and atria ).

A

number of s tudies have s ince been carr ied o u t which

have resulted in the modification of the original

theory to inc lude more recent work

on

thermally-

buoyant horizonta l f lowsI7 and adhered (or a t tache d,

o r wall, o r single-sided) p l ~ m e s ' ~ . ~ ~see Figure 24(a)).

This appendix presents the modif ied theory in a form

which th e designe r can follow mor e easily than in the

ear l ier research papers .

Th e ca lcula t ions can be d one using an e lec tronic

calculator having full scientific functions. This

however may be time-consuming, particularly where

the designer wishes to look a t

a

number of geometr ies

or conditions . Th e ca lculat ions can be incorpora ted in

a compu ter program wh ere f requent ca lculat ions ar e

required .

An

al ternat ive method to Figure

B1

is given

later in this appendix (page 53) in or der t o fac i l ita te

such programming.

Many of th e variables used in equations in this

appendix do not app ear in the main body of the

Repor t . To avoid unnecessary complications for the

reader w ho does not wish to use this calculation

procedure , the appendix

is

provide d with

a

separa te

list of nomencla ture (page

54).

Scenarios and

assnmptioms

Th e calculation method strictly only applies to fire

scenarios where

a

horizontally-flowing thermally-

buoyant layer of smoky gases approaches

a

void,

through which those gases then rise. More specifically,

the fo llowing assumptions are made:

o This approa ch flow is assum ed to be beneath a flat

ceiling (or a downstand) a t the edge

of

the void .

Q

It is channelle d by downstands (which may be

eithe r walls or channelling screens).

0 T he flow has flow-lines which are every where

parallel and which approach the edge of the void at

a right angle.

o

Th e approach f low is assumed

to

be fully

developed.

o

T h e r e

is no

imm ersed ceiling jet.

o

It is assumed that the velocity

of

the clear air

below the sm oke layer has a velocity much smalle r

than t he velocity

of

the layer itself.

5

M od i f i ed d i s t anc e abov e v i r t ua l s ou r c e

I ~ ( v

Figure B l Graphica l representa t ion ( f rom Rcfcrcnce 50) of the theore t ica l

solutio n for a plume issuing from

a

rcs t ra ined source

F < 1)

49

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Fortunately these assumptions correspond to m any

practical scenarios of interest to designers.

It should further be noted that experimental evidence”

suggests that th e calculation procedu re which is the

subject of this guide shou ld not be used for app roach

flow layer tempe ratures higher than ab out 350 “C.

Accurate m ethods for higher temperatures

do

not yet

exist . The present method signif icantly overpredicts

the mixing

of

air into th e rising hot gases for higher

temperatures.

In

practice the designer will have arrived at th e key

param eters of the app roac h flow by som e calculation

procedure independent of the present guide. For

example , Equa t ions 6 and 7 of Reference 17 could be

used t o calculate the flow of smoky gases passing from

a room into an atr ium void. Ano ther examp le is

where a single-storey mall allows smoke to rise

thr ou gh the void of a two-storey mall: here the flow in

the single-storey mall can b e calculated in the usual

way using, for exam ple, Refe rence

16,

which will give

the designer values for both th e mass flow rate and

the h eat flux.

T he calculation procee ds in discrete stages:

-

1

Th e designer must know:

(a)

the in terna l geom etry of his building,

(b) at least two of the key param eters of the

including relevant c han nel widths, and

app roach flow. Useful pairs are:

mass flow/heat flux

m ass f l o w h e a n l a y er t e m p e ra tu re

mass flowkeil ing temp eratur e

h e a t f l u x h e a n l a ye r t em p e ra tu re

heat flux/layer depth

layer depth /mea n layer tempera ture

layer depth/ceil ing temp eratur e

.

hea t f luxke i ling tempera ture

2

Using the known pa rameters for the approach f low,

calculate the remaining param eters of the flow.

3

Using the results from the preceding stage,

calculate the entrainm ent into the f low as i t rotates

arou nd the void edg e, ie as the sm oky gases change

. from a horizontally-moving flow to a vertically-

moving flow. By the en d of this stage the key

pa ram ete rs of th e vertically-moving gases will be

known a t the horizontal plane passing through th e

ceil inghoid edge. These param eters are the heat

flux, th e vertically-moving mass flux, and th e kinetic

energy of the gases (th is last is based on ly o n the

vertical component of velocity).

4

Th e plume at g reater heights behaves as

if

it rises

from an infinitely wide source located in the

horizontal plane passing through the ceil ingh oid

edg e, whe re that source has horizontal profiles of

both buoyancy and (the vertical compo nen t of)

velocity which can be described by Gaussian

function s. This sourc e is, of course, virtual. We have

fo llow ed L ee a nd E m m ~ n s ’ ~n using this source,

and indeed in the m ethod of calculating the plume

above the source. ‘We ollow Lee and Em mo ns in

calling this source an ‘Equivalent Gaussian source’.

Calculate the key param eters of the E quivalent

Gaussian source by ensuring that the th ree key

parameters from Stage

3

above keep

t h e

same

values.

5 Knowing

t h e

height above the ceil ingh oid edge (for

example, this is likely to be chosen to be equa l to

the sm oke layer base in the reservoir ab ove the

void), calculate the entra inm ent into th e spill

plume. This calculation trea ts the plume as a perfect

two-dimensional plume having a length equa l to the

width of the channel of the app roac h flow.

6 Calculate the addit ional en trainment into the free

ends of the plume. This assumes that th e bulk

of

the

plum e is relatively unaffected by th ese en d effects

- easonable for plume heights typically smaller

than or com parable to the plume length”.

Stage

1

Co mp lete all necessary pre-calculations to derive th e

key parameters, of the approa ch flow described in l b

on this page.

Stage 2

Select from th e following equationsI7 to d eterm ine the

remaining param eters for the approac h f low from the

initial known parameters:

Calculate the mean layer temperature (e,)

-

...(B

1

Calculate the mass flow rate

(M,)

at the opening given

by30:

.

(B2)

d;I2 KM

1/2 WPO

M , = d3/* (2g0,, T o )

3

T,,“

where

po

=

1.22

kgmp3 or an ambien t tempera ture

T o of

288

K

Cd = 0.6 for opening with a deep downstand

o r

1.0

for n o downstand

g = 9.81 ms-2

KM=

1.3 for most typical flowing layers

50

7/24/2019 BS en 12101-5


The depth of the layer (d,) at the opening is then

given by30:

If

the line plume is single-sided, go to Stage 7 of this

procedure, after completion of Stage 3.

Stage 4

Calculate the Equivalent Gaussian Source:

]

...(

B3)

3 M ”

Tc,

~ c ~ ~ ‘ ~ K ,

p0(2g T ~ ) ~ ’ ~

First convert Q and My into the corresponding

parameters per unit length of plume (ie divide by the

channel width (W) to give Q, and

A .

The mass-weighted average temperature

layer is30 :

of the gas

Then solve the following equations36

.

(B4)

1

t = [ A E l

...(BlO)

where

Note the importance of knowing whether there is a

downstand running along the edge of the void (and

thus at right angles to the direction of the flow),

because this changes the value of Cd.

KQ =

0.95 or most typical flowing layers.

Q , -

T0ch

A + -

[

“.I1

...(B11)

Greater accuracy can be achieved by calculating the

values of the profile correction factor KM nd KQ using

the temperature dependent formulae

in

Reference 30,

although this is usually unnecessary for most practical

designs.

The layer’s characteristic velocity

( v )

is given byI7:

where the empirical thermal constant5” A) = 0.9

2B

h .

.(

B

2)

I

13

C d K ~ g Qw Tcw

K Q ” ~ c

PO

wTo

v=0.96-

2 ]

...(B5)

..

(B 3)

For a deep downstand, where Cd= 0.6,his becomes:

uG

=Jr

. (B6)

and:

With no downstand at the opening, Cd= 1.0,and:

. (B 4)

I

/ 3

g

Qw

Tc,

c P O WTO

v

= 1.27

[

..(B7)

where

[” ]

G , U G and

b

are parameters of the

Calculate the horizontal flux

( B )

of vertical buoyant

potential e n e r g ~ ~ ~ . ~ ~relative to the void edge): Equivalent Gaussian Source.

Stage 5

Calculate the entrainment into the rising plume.

The Source Froude number (F) for the line plume is36

(B8)

112

Stage 3 a

Calculate the mass flux

( M y )

rising past the void edge17:

F = [ t ] ’ ” [mG]

. (B

5)

g bG)1‘2

I

2

2

3 where

a

=

0.16 or double-sidedsOand 0.077 for

My

=

o W a’ [2g d,:’*

+ M, ...(B9)

single-sided line plumes3’. Calculate the transformed

parameter ( V G ) for the Equivalent Gaussian Source:

here the entrainment constant

(a’)=

1.1

51

7/24/2019 BS en 12101-5


1

Z)G =

. B16)

Determ ine the value of

I , ( z ) G)

by using the following

procedure o r an a l ternat ive method se t out in la ter in

this append ix on page 53.

uG

epresents a value o n the vertical axis

of

Figure

B1.

Look across

to

the m iddle solid curve and find the

corresponding value

of

I 1 ( u ~ )

n

the oth er axis.

Calculate the t ransformed height parameter of

x’

corresponding to the desired p lume height

(x):

Next calculate A I l (u) :

.

B17)

X’

A I, u) = ...(B lS )

[ P 1

- P ) ]

and

Determ ine values of b’,p’ and LC‘ corresponding to the

calculated value of I l (u ) using the following procedu re

or the a l ternat ive method

on

page 53.

I l ( u )

=

Il(uG)+

A

I l ( u )

.

B

19)

I l (u) represents a value on the horizontal axis

of

Figure

B1.

Using this value find the corresponding

values (from all three curves) for U”,

p”

and b”. T h e n

use the fo llowing equat ions to determine U ’ , p’ a n d b’,

Calculate the mass flow per unit plume length

(nz,)

passing the chosen height36

x:

r .

Convert to the total mass flow in line plume (ignoring

en d effects) by multiplying Equ ation

B25

by the

channel width (ie

rn,W).

Stage

6

Calculate3x he entrainment 6 M , in to the f ree ends of

the l ine p lume. Th e width of the l ine p lume (and a lso

its axial velocity) can be taken as being approxim ately

constant for most

of

i ts height as a first orde r

approximation. and equal to the mean of the values at

the Equivalent Gaussian Source and a t the chosen

height (x).

Th e en t ra inmen t 6M,. nto both ends

of

the l ine p lume

is then38:

6 M r = 4 b L c axp, ...(B26)

where:

b

=

(b(; b)

2

...(

B27)

-

U

=

( U ( ; + U )

/

2

..

B28)

Ad d th is

to

the p lume entra inment resul t f rom Stage

5

to obtain t he total m ass flow M,of smoky gases rising

past th e specified height

(x).

where:

ie: M,= w, ,W+ 6 M ,

.

B29)

.

B20)

..(B22)

Next de term ine the characteristic half-width

(b)of

the

line plume36at height x:

b = b’ bG .

B23)

Th en calculate the axial vertical velocity com pone nt

( U ) of the gases a t he ight

x:

U’

U G

U = ~

F

. .

B24)

I t should be noted that w here both ends

of

a p lume a re

bou nded by side walls (eg, as in a shaft) then

6 M ,

=

0.

Stage

7

Modif icat ions to the above procedure for s ingle-

sided17,37,51or a dhered) l ine p lumes.

Convert both the E quivalent Gaussian Source and the

plume in to a composi te of a real an d an imaginary half,

so that the cen tre line of the com posite lies along the

vertical wall to which th e plum e is adhering . This is

do ne by doubling values for

B , M y

(and hence A ) , and

Q f rom Stage 3 before re turning to S tages 4 t o 6

above. Note tha t experiment^^^ show that the value of

a

needed in Stages 4 to 6 should change from 0.16

(valid for a f ree or double-s ided p lum e) to

0.077

for

the adhered p lume.

On completing Stage 6, halve the final value of mass

flow M , rising past the desired plum e height

(x).

52

7/24/2019 BS en 12101-5


..-

.

.- .

-.,

.

.

..

.~

..

_

/

.

. . .

.

. . .

c

Alternative method fo r determination of I ,

UC)

If uG is greater

than

1.549 then /,(uG)= (uG- 0.75)/0.9607

If uG is less than or equal to 1.549 and UG is greater

than 1.242 then II(uG) (uc - 0.843)/0.8594

If uG is less than or equal

to

1.242 and uG s greater

than 1.059 then

/ , (uG)

= (uG - 0.9429)/0.6243

If

uG

s

less than 1.069 then

/,(uG)

=

(vG

-

1.0)/0.3714

Alternative method f o r calculating value

of

b p' and

U'

(a) Determination of U

If /,(U) is greater than 1.896 then U

=

1.0

I f /, U) s greater than 0.786 and

I

U ) is less than or

equal to 1.896 then

U

= 0.0908 I U ) + 0.821.

I f

/, U)

s less than or equal

to

0.786 then

U

=

II

u)0,35

(b) Determination ofp

If

/,(U)

s greater than 0.832 then p" = 0.9607 I , (u) +

0.75

If

/,(U) is greater than 0.464 and less than or equal to

0.832 then p" = 0.8594

/, U)

+ 0.8429

If /,(U)

is

greater than 0.186 and /,(U)

is

less than or

equal

to

0.464 then

p"

= 0.6243

/,(U) +

0.9429

I f

/, U)

s less than or equal to 0.186 then

p" = 0.3714 /,(U) + 1.0

(c) Determination of 6

I f

/,(U) is greater than 2.161 then b = 0.938 /, U)+ 0.82

If /,(U) is less than or equal to 2.161 and /, U) s

greater than 1.296 then b

=

0.89

/,(U)

+ 0.95.

If /, U) s less than or equal to 1.296 and /,(U) is

greater than 0.896 then b

=

0.81 /,(U) + 1.071.

If /,(U)

is

less than or equal to 0.896 and /,(U)

is

greater than 0.65 then

b =

0.619 /,(U)

+

1.214.

I f

/,(U)

s less than or equal to 0.65 and

/,(U)

s greater

than 0.543 then b = 0.331 Il(u) + 1.414.

If /,(U) is less than or equal to 0.543 and /,(U) is

greater than 0.421 then

b

= 0.0627

/, U)

+ 1.55.

If

/, U) s less than or equal to 0.421 and /, U) s

greater

t h a n

0.348 then b = 1.821

-

0.6 /, U)

I f /, U)

is less than or equal to 0.348 then

b = / , u ) - O . ~

N o w calculate U', p' and b' f rom Equations B20, B21

and B22 in Stage

5.

53

7/24/2019 BS en 12101-5


_--- __

._

Nom enclature used in A ppendix B

Note:

The list of nomenclature used in the main text of this Report is given on page vi.

A

b

b

b

B

Cd

C

d

F

g

11

m

6m

M

6 M

P'

P

Q

QO

T

U f

U f f

W

x f

a'

P

e

KM

KQ

h

U

V

X

a

2)

5

5

Upward mass flow rate per metre across the horizontal plane through the balcony (kgs-lm-l)

Characteristic half-width of line plume at height

x (m)

Dimensionless half-width of line plume

Transformed dimensionless half width of line plume

Potential energy flux per metre of horizontal gas stream below corridor ceiling edge (Wm-l)

Coefficient of discharge

Specific heat at constant pressure of gas (k.lkg-' C-')

Depth of gas stream beneath ceiling (m)

Source Froude number (for line plume)

Acceleration due to gravity (mss2)

Transformed height (dimensionless)

Mass flow rate per unit width of gas stream (kgss'm-')

Mass per second per metre of air entrained into hot gas stream at corridor ceiling edge (kgm-Is-')

Mass flow rate of gases (kgs-I)

Mass per second of air entrained into free ends of plume (kgs-l)

Dimensionless buoyancy o n plume axis

Transformed dimensionless buoyancy

on

plume axis

Heat flux in the gas (kW)

Heat flux per second per unit width of gas flow (kWm-')

Absolute gas temperature (K)

Vertical gas velocity at height

x

(ms-')

Dimensionless vertical gas velocity

Transformed dimensionless vertical gas velocity

Horizontal velocity component of gas (ms-')

Width of gas flow (m)

Height of clear layer above fire compartment/balcony edge (m)

Dimensionless variable

Entrainment constant for plume (= 0.077 and 0.16 for single- and double-sided plume respectively)

Entrainment constant for air mixing into gases rotating around a horizontal edge

Gas density (kgmP3)

Excess temperature

of

gases above ambient temperature ( C)

Profile correction factor for mass flow (approx. 1.3)

Profile correction factor for heat flux (approx.

0.95)

An

empirical thermal plume constant

( h=

0.9)

Transformed reciprocal of buoyancy (dimensionless)

Function defined in text (Equation B12)

Function defined in text (Equation

BlO)

Subscripts

0 An ambient property

C

G

r

W

Y

54

Variable evaluated at highest point in a flow (but outside any boundary layer)

A property of the equivalent Gaussian source

Base of ceiling smoke reservoir

Variable evaluated in the horizontal flow at opening

Variable evaluated in vertical flow past top of opening

7/24/2019 BS en 12101-5


. .

.

R.

. . . .. I-

.

Acknowledgements

References

We would like to thank Mr N R Marshall of the Fire

Research Station

for

his help in preparing Appendix B.

The Fire Research Station is grateful to the Task

Group of the Chartered Institution of Building

Services Engineers engaged

in

preparing a draft

CIBSE guidance document on smoke control in atria,

for its comments

o n

a draft of of this Report. These

have as far as possible been taken

i n to

account.

Butcher

E

G and Parnell A C.

Sm ok e cont ro l in

fire safety'design. London, E & F N Spon Limited,

1979.

Department

of

the Environment and the Welsh

Of ic e .

Th e Building Regulations

1991.

A p p r o v e d

D o c u m e n t

B .

Fire safety

(1992 edition). London,

HMSO. 1991.

House

of

Commons. Public Health Acts 1936 and

1961. London, HMSO.

House

of

Commons.

Factories Act 1962.

London, HMSO.

House

of

Commons. Offices, Shops and Railway

Premises Act 1963. London, HMSO.

Saxon R.

At r iu m bui ldings . Development and

design.

London, The Architectural Press, 1983.

The Andraeus Building fire in

SBo

Paulo, Brazil.

Fire Prevention,

1973,97 (January) 37.

Sharry

J

A .

An atrium fire.

Fire Jou rnal,

1973,67

(6) 39-41.

Lathrop J

K. Atrium fire proves difficult to

ventilate.

Fire Jo urnal,

1979,73 (1) 30-31.

10

Robinson P.

Atrium buildings: a fire service view.

Fire Surveyo r,

1982,11(4) 42-47.

11

Degenkolb J G.

Atriums. Th e Building Official

and Cod e Adminis trator , 1983, XVIl (6) 18-22.

12

Parnell A C and Butcher

E G. Smoke movement in

atria. Fire Protection (South Africa), 1984, U(3) 4 6.

13 National Fire Protection Association.

S m o k e

managemen t sys tems

in

malls, atria and large areas

92B. Quincy MA, NFPA, 1991.

14

British Standards Institution.

Fire precautions in

the design, construction and use of buildings.

Part 7. Code of practice for atrium buildings.

British Standard

BS

5588:Part 7 . London, BSI.

To be published.

15

British S tandards Institution.

Fire precautions in

the design, construction and use of buildings.

Part

10.

Code of practice for shopping complexes.

British Standard BS 5588:Part

10:

1991.

London, BSI, 1991.

16

Morgan H P and Gardner

J

P.

Design principles

fo r smo ke ventilation in enclosed shop ping centres.

Building Research Establishment Report (BRE

Bookshop ref BR186). Garston, BRE, 1990.

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17

Morgan

M

P and Hansell G Ca Atriu m buildings:

calculating smo ke flows in atria for sm oke control

design. Fire Sa fety Journal,

1987,U

(1)

9-35.

18 Morgan

M

P and Chandler S E.

Fire sizes and

sprinkle r effectiveness in shopp ing complexes and

retail premises. Fire Surveyor,

1981,lO

( 5 )

23-28.

19 Morgan MP and Hansell G 0 Fire sizes and

sprinkler effectiveness in offices mplications

for sm oke control design. Fire Safety Journal,

1985,

8

(3) 187-198.

U) Hansell G 0 and Morgan

H

B.

Fire sizes in hotel

bedrooms

-

mplications for sm oke control

design. Fire Safety J ournal,

19 85 ,s (3) 177-186.

21 Vincent B G, Kung

PI

C and W i l l E E.

Residential

side wall sprinkler fire tests with limited w ater

supply. Fire Science and Technology,

1988,s (2)

41-53.

22 Cote

A

E.

Highlights of a field test of a retrofit

sprinkler system.

Fire Journal, 1983,77 (3) 93-103.

23

Hansell G 0.He at and m ass transfer process

affecting smok e con trol in atrium buildings.

Ph D Thes is . Lon don , South Bank University,

1993.

24

Zukowski

E E

Kubota

T

and Cetegen B.

Entra inm ent in fire plumes.

Fire Safe ty Journal,

1981,3 (2/3) 107.

25

Quintiere

d G,

Rinkinen

W

J

and Jones

W

W.

Th e effects of room openings on fire plumes

entrainment.

Combustion Science and Technology,

1981,26 (5/6) 193-201.

26 Hinkley P L. Rate s of production of hot gases in

roof venting experiments. Fire Safety Jo urnal,

1986,lO

(1)

57-65.

27

McCaffrey B J, Quintiere 9 G and Harkleroad M F.

Estimating room tem pera tures and the likelihood

of flashover using fire test da ta correlation s.

Fire Technology,

1981,17 (2) 98-119.

28

Morgan H P and Marshall N R. Sm oke hazards in

covered multi-level shopping malls: a me thod of

extracting smo ke from each level separately.

Building Research Establishment Current Paper

CP19/78.

Gars ton , BR E,

1978.

29 Heselden A J M. Fire problems of pedestrian

precincts. Part 1.Th e smo ke production of various

materials. Fire Research Station Fire Research No te

856. FRS, Borehamwood, 1971.

30

Morgan H P. Th e horizontal flow of buo yant gases

toward an opening.

Fire Sa fety Journal, 1986,11

(3) 193-200.

, -

31 Morgan H P and Marshall

N

R. The dep th of void-

edge screens

in

: ,loj-ping malls. Fire En gineers

Journal, 1989,44

\

152) 7-9.

32 Bosley

K.

‘The effects of wind spee d o n escai’e

behaviour through em ergency exits. Summary

Repor t . FR D C Research Repor t Number

53.

Londo n, Hom e Office,

1992.

33

British Standards Institution.

Fire precautions

in

the design

of

buildings. Part

4.

Smok e control

in

protected t:scape rou tes using pressurization.

British Standard BS 5588:Part

4:1978.

Lond on, BSI,

1978.

34

Spratt

D

and Heselden A

J

M.

Efficient extraction

of smok e from a thin layer un der a ceiling.

Fire Research Station Fire Research Not e 1001.

FRS,

Borehamwood, 1974.

35 Marshall RI

R,

Feng S Q and Morgan H P.

Th e influence of a perfo rated false ceiling

o n

the

performance of smo ke ventilation systems.

Fire Safety Jo urnal, 198.5,8 (3) 227-237.

36

Morgan

W B

and Marshall

N R. Sm oke hazards in

covere d multi-level shopping malls: an

experimen tally-based theory for smo ke

production. Building Research Establishment

Current Paper

CP48/75.

G a rs to n , B R E ,

197.5.

37

Wansell G 0 arshall N R and Morgan M P.

Sm oke flow experiments in a model atriu m.

Building Research Establishment Occasional Paper

OP55. Gars ton , BR E, 1993.

38

Morgan

H P

and Marshall N R.

Smok e control

measures in a covered two-storey shopping mall

having balconies as pedestrian walkways. Building

Research Establishment Current Paper CPl

1

/79.

Gars ton , BR E,

1979.

39 Grella J

9

and Faeth

G

M.

Mea surements in a two-

dimensional therm al plume along a vertical

adiabatic wall. Journhl of Fluid Mechanics,

1975,

71

(4)

701-710.

40

Thomas

P

H. On

the upward movement of smoke

and related shopping mall problems. Fire Safefy

Journal, 19 87 ,U (3) 191-203.

41 Morgan H

P. Comments on

‘A

note on smoke

plum es from fires in multi-level shoppin g malls’.

Fire Safety Journ al,

1987,U

(1)

83-84.

42 Thomas

P .H,

Hinkley

P L,

Theobald

C

R and

Simms D

B,. Investig ations into the flow of hot

gases in roof venting. Fire Research Technical

Paper N o 5‘. L o n d o n, H MS O ,

1963.

56

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43

44

45

46

47

48

49

50

51

Boehmer

D

9.

Atrium fire engineering- orth

American experience. Proc eeding s of seminrir

‘Atriu m Engineering’ held by Environm ental

Energy Group of the Institution

of

Mechanical

Engineers, 26 June 1990. London, I Mech E,

1990.

P a d s 9. Calculating evacuation times for tall

buildings. Proceedings of symposium ‘Qurmtitative

Methods fo r Li fe Safety Analysis’ held by the

Society

of

Fire Protection Engineers, University

of

Maryla nd, March 1986. Boston MA, SFPE, 1986.

Hansell

6 0

and Morgan

H P. Smoke control in

atrium buildings using depressurisation. Part

1:

Design principles. Fire Science and Techno logy,

1990,lO

(1

&

2)

11-26.

Nansell G 0 and Morgan

W P. Smoke control

in

atrium buildings using depressurisation. Part 2:

Considerations affecting practical design. Fire

Science ancl Tech nolo gy,

1990,lO

(1 &

2) 27-41.

Building Research Establishment. The assessment

of

wind loads. Part

8:

Internal pressures.

B R E Digest

346.

Garston, BRE,

1990.

Douglas-Baines

W. Effects of velocity distribution

on

wind loads and flow pattern o n buildings.

Proceedings

of

Symp osium N o 16 ‘Wind Eff ec ts

on

Buildings and Structures’ held b y the National

Physical Lab ora tory , Tedclington , 26-28 Ju ne 1963.

London, HMSO,

1965.

Hinkley

P

L.

The effect of smoke venting

on

the

operation of sprinklers subsequent to the first.

Fire Safety Jou rnal,

198 9,14 (4) 221-240.

Shao-Lin Lee and Emmons H

W . A study of

natural convection above a line fire. Journal of

Fluid Mechanics,

1 9 6 1 ,l l (3) 353-368.

Marshall N R.

Air entrainment into smoke and hot

gases

in

an open shaft. Fire Safety Jour nal,

1986,lO

(1)

37-46.

Printed in the

UK

for HMSO. Dd.8392663, 1/94, CIO, 38938

57

7/24/2019 BS en 12101-5


Also

available

f rom BFIE

Design principles or smoke venti lation in

enclosed shop ping centres

H P Morgan and J P Gardner

Ref BR186 €28 BRE Report 1990

This Report is intended o assist designers of smoke

ventilation systems who are faced w ith many

difficulties when evolving a smoke control system that

will ensure safe escape from fire in an enclosed

shopping complex. A serious fire in most shops will

usually penetrate the shop front and cause hot smoky

gases to spread quickly along the malls, leading to

rapid loss of visibility unless preventive measures are

taken. Work at FRS has shown that these gases can

be confined to a ceiling reservoir formed by screens,

downstands or other features p rovided that the gases

are then removed by some m eans and that an equal

amount of replacement air is allowed

to

enter the

main smoke layer. The latera l spread of smoke on the

lower level of a mu lti-storey mall must be con trolled

o

reduce the size of the smoke plume rising through the

upper level. This minimises the quantity of gases

entering the ceiling reservoir. Such systems for

controlling smoke movement provide a smoke-free

zone below the m ain layer and allow people to

escape.

A simplif ied approach to sm oke-venti lation

calculations

H P Morgan

BREInformation Paper lP19 /85 f3.50

Outlines the calculations or either fan capacities or

vent areas for single-storey, large undivided-volume

buildings where the fire is directly below the smoke

reservoir.

It

is not intended or use with more

complicated building geometries such as shopping

malls. Will be of pa rticular value to building control

officers, fire officers and others who have

to

check

smoke ve ntilation proposals or give approvals.

Prices current at January 1994 but subject to change

Please send order, enclosing cheque, to:

BRE Bookshop , Bu lldlng Research Establishment,

Smoke contro l in large stores: an extended

calculation method for sl i t extraction design

N

R Marshall

BRE

Occasional Paper OP51

(Free from

N R Marshall at BRE’s Fire Research Station)

Extends the existing calculation method for slit

extraction (which is one of three methods used

to

prevent cool smoky gases flowing in bulk from large

stores on to enclosed shopping m alls)

to

include

stores having one or more openings of differing

heights and widths. It also updates the existing

method to include the effects of differen t geometry

openings between store and mall.

Experiments at the Muit i functioneel

Trainingc entrum , Ghent, on th e interaction

between sprinklers and sm oke venting

P L H inkley, G 0 Hansell, N R Marshall and R Harrison

Ref

6R224 €25

BFlE

Report

1992

Describes a series of experiments carried out to validate

a mathematical model of the effect of roof venting on

the operation of sprinklers (see BR213 below).

Sprinkler operation and the effect of venting:

studies u sing a zone mo del

P L Hinkley

Ref

BR213 f 3 0

BRE Report

1992

This Report describes a mathematical zone model

developed as a joint project by the Fire Research

Station and Colt International Limited

to

remedy

deficiencies in current knowledge about the interaction

between roof venting and sprinklers.

BS en 12101-5

Documents

Transcript of BS en 12101-5