Br258 Design Approaches For Smoke Control In Atrium Buildings

Design approaches for smoke control in atrium buildings G O Hansell and H P Morgan

Related titles from IHS BRE Press Design principles for smoke ventilation in enclosed shopping centres BR186, 1990 Smoke control in large stores: an extended calculation method for silt extraction design OP51 A simplified approach to smoke-ventilation calculations IP19/85, 1985 Experiments at the Multifunctioneel Trainingcentrum, Ghent, on the interaction between sprinklers and smoke venting BR224, 1992 Sprinkler operation and the effect of venting: studies using a zone model BR213, 1992

IHS BRE Press, Willoughby Road Bracknell, Berkshire RG12 8FB www.ihsbrepress.com BR 258

DESIGN APPROACHES FOR SMOKE CONTROL IN ATRIUM BUILDINGS G O Hansell and H P Morgan

BRE is the UK’s leading centre of expertise on the built environment, construction, energy use in buildings, fire prevention and control, and risk management. BRE Global is a part of thr BRE Group, a world leading research, consultancy, training, testing, and certification organisation, delivering sustaibability and innovation across the built environment and beyond. The BRE Group is wholly owned by the BRE Trust, a registered charity aiming to advance knowledge, innovation and communication in all matters concerning the built environment for the benefit of all. All BRE Group profits are passed to the BRE Trust to promote its charitable objectives. BRE is committed to providing impartial and authoritative information on all aspects of the built environment. We make every effort to ensure the accuracy and quality of information and guidance when it is published. However, we can take no responsibility for the subsequent use of this information, nor for any errors or omissions it may contain. BRE, Garston, Watford WD25 9XX Tel: 01923 664000 [email protected] www.bre.co.uk BRE publications are available from www.brebookshop.com or IHS BRE Press Willoughby Road Bracknell RG12 8FB Tel: 01344 328038 Fax: 01344 328005 Email: [email protected] Requests to copy any part of this publication should be made to the publisher: IHS BRE Press Garston, Watford WD25 9XX Tel: 01923 664761 Email: [email protected]

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Contents Page Foreword

v

Nomenclature

vi

Introduction

1

Chapter 1

General principles of smoke production, movement and control Fire growth and smoke production Pressurisation Depressurisation Throughflow ventilation

4 4 5 6 6

Chapter 2

Design fire size

7

Chapter 3

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

9 9 10 13 14 14 16 17 17 18 19 19 20 20

Chapter 4

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 — area of natural ventilation required Throughflow ventilation — remaining design procedures Limitations to the use of throughflow ventilation

21 21 22 23 33 33 35 35

Chapter 5

Design considerations other than throughflow ventilation Void filling Compartment separation Depressurisation ventilation Principles Natural depressurisation Natural depressurisation and wind effects Powered depressurisation

37 37 37 37 37 38 40 41

Chapter 6

Depressurisation/smoke ventilation hybrid designs Principles Design procedures for hybrid systems Mass flow based systems Temperature based systems

42 42 42 42 43 (continued) iii

Page Chapter 7

Atrium smoke layer temperature

44

Chapter 8

Additional design factors Atrium roof-mounted sprinkler systems Smoke detection systems in the atrium Pressurisation of stairwells and lobbies Air-conditioned atria Channelling screens and hybrid systems Wind-sensing devices and natural depressurisation

46 46 46 46 46 46 47

Appendix A Case history

48

Appendix B Users guide to BRE spill plume calculations Introduction Scenarios and assumptions Outline of procedure Detailed procedure

49 49 49 50 50

Nomenclature used in Appendix B

54

Acknowledgements

55

References

55

iv

Foreword This Report is the culmination of a long-running collaborative project between the Fire Research Station of the Building Research Establishment and Colt International Limited on aspects of smoke movement and its control in atrium buildings. It is based on both the latest scientific knowledge and practical experience of smoke movement and control systems, and has been prepared under the overall supervision of the Fire Research Station. The present Report is intended to serve the designers of smoke control systems for atrium buildings in the same way that the earlier Building Research Establishment Report, Design principles for smoke ventilation in enclosed shopping centres, has served designers of smoke ventilation systems in shopping malls. As such, those graphs and tables it contains which are relevant to a particular design of building can be applied directly to that building; or the formulae cited can be used to apply the work to a broader range of circumstances. The Report does not exclude the options of using alternative methods where they are appropriate, or of using new techniques (such as computational fluid dynamics) as they are developed and validated.

A P S Ferguson Building Regulations Division Department of the Environment July 1993

v

Nomenclature Note: The list of nomenclature used in Appendix B is given on page 54. Af Ai Av Aw c Cd Ce Ci Cpi CpL Cpv Cv D DB Dd Df Dl Dw Dmax g H h hb Ha Hc L M Mf MB Ml Mw MCRIT N P Q Qf Qw qf T TB Tl To

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 (kJkg21K21) 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 (ms22) 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 (kgs21) Mass flow rate from the fire (kgs21) Mass flow rate under a balcony (kgs21) Mass flow rate entering (leaving) a smoke layer in a reservoir (kgs21) Mass flow rate flowing through a vertical opening (kgs21) Critical exhaust rate (kgs21) 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 (kWm22) 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

V Vl vwind W WB X Y β ∆ ∆DB U UB Ul Uw ρ ρo

Volumetric flow rate of gases (m3s21) Volumetric flow rate of gases from a reservoir (m3s21) Design wind velocity (ms21) 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 (kgm23) 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 (kgm23) Density of ambient air (kgm23)

vii

Introduction This Report is intended to assist designers of smoke ventilation systems in atrium buildings. Most of the methods advocated are the outcome of research into smoke movement and control at the Fire Research Station (FRS), but also take into account experience gained and ideas developed whilst the authors and their colleagues have discussed many proposed schemes with interested parties. The primary purpose of the Report is to summarise in a readily usable form the design advice available from FRS at the time of its preparation. As such, it does not attempt to cover installation, detailed specification of hardware, or aspects of fire safety engineering other than smoke control. The predominant cause of death in fires in the UK has been attributed to the inhalation of smoke and toxic gases1, and the annual number of fire fatalities in the UK is approximately one thousand. However, the majority of these deaths occur in domestic premises. This implies that the life-safety measures required by legislation for most public and commercial buildings have been effective on the whole. Fire safety in buildings must in the UK conform to the relevant regulations (guidance for England and Wales is given in Approved Document B2). The principal objective of these regulations is to safeguard life by: ●

reducing the potential for fire incidence,

●

controlling fire propagation and spread, and

●

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 Regulations for England and Wales in 1973. Prior to that date, the powers of control in England and Wales over means of escape had been contained in other legislation3,4,5. Historically, the prevention of fire growth within (or between) buildings has been achieved by the containment of the fire and its products by means of compartmentation and/or separation. The design of structural compartmentation and separation has been largely empirical, and the concepts gradually refined and enhanced in such a way that the Building Regulations now cover primarily life safety and the protection of means of escape. It is necessary to consider four major aspects of buildings — purpose, size, separation and resistance to fire — to promote safe design. Social and technical changes have led to changes in building environments which incorporate new (or revived) building forms and the use of innovative construction techniques and new synthetic materials. The buildings adopting these changes often have included within their design large spaces or voids, often integrated with many of the storeys. These large spaces

have been described as malls, atria, arcades and lightwells. The generic term for the building type tends to be ‘atrium’ and, by their very nature, atrium buildings often run contrary to the traditional Building Regulations’ approach in terms of horizontal compartmentation and vertical separation. The original atrium was an entrance hall in a Roman house and was one of the most important rooms in the building. The concept of this space has evolved architecturally over the past few hundred years and now applies to structures much larger than the typical Roman house. Atria today are designed as undivided volumes within a structure, intending to create visually and spacially an ideal external environment — indoors6. In Roman times the control of any smoke and hot gases that may have issued from a fire in a room adjacent to the atrium was likely to have been a simple matter. Providing there were no adverse wind conditions (due to local topography of adjacent structures), then the smoke and heat would undoubtedly vent through the open portion of the atrium roof known as the ‘compluvium’ (generally used for lighting purposes). Modern atrium buildings tend to contain large quantities of combustible material and often have open-plan layouts which increase the risk of the spread of fire. The populations within such buildings are also greater; hence there has been a substantial increase in the number of people to be protected and evacuated in an emergency. Modern atrium buildings are usually designed with the atrium as a feature which can be appreciated from within the adjacent rooms. The room/atrium boundary is usually either glazed or completely open. Thus when compared to ‘conventional’ buildings, this architectural/aesthetic requirement imposes additional problems of life safety during a fire, since smoke, hot gases and even flames may travel from one (or more) rooms into the atrium and thence affect areas which would have remained unaffected in the absence of an atrium. In conventional multi-storey structures there is always the possibility of fire-spread up the outside of the building, with flames issuing from one room and affecting the floors above. Recent examples of this mode of fire-spread have been an office block in São Paulo7 and the Villiers building in London. If the escape facilities from the various rooms are of a suitable standard and segregated from other compartments (as required in the UK), there should not (in theory) be any serious hazard to life safety in this fire condition. It is only when the means of escape are inadequate or the parameters dictating their design are violated, that the loss of life may occur.

1

If a building has an atrium then this fire condition can also occur internally, since there is generally a maximisation of the window area and/or open boundary between the rooms and the atrium. Hence there is an increased risk to other levels of the entry of smoke, toxic gases and possibly flames from a fire. Recent experience of fires in atrium buildings in the USA8,9 has shown the problem of flame travel internally through the atrium to be minor in comparison to that of hot and toxic gases accumulating and building down in the atrium — spreading throughout the building and affecting escape routes. Thus there appears to be a need for a properly designed smoke control system in atrium buildings. The ideal option would be to prevent any smoke from a room fire entering the atrium at all. An easily understood way of achieving this is to ensure that the boundary between the room and the atrium is both imperforate and fire-resisting, and that the atrium base has only a very restricted use. This option has frequently been used, but is widely regarded as being architecturally restrictive. Consequently it is not often favoured by designers. The concept has been labelled the ‘Sterile tube’6. Where the boundary between the room and the atrium is open, it is sometimes feasible to provide a smoke ventilation system within the room, to maintain smoky fire gases above the opening to the atrium. Unfortunately it is often very difficult, impracticable, or extremely expensive to fit a separate 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 the atrium (this can occur, for example, when the room layout is of a large area, is predominantly open-plan and open-fronted). There have been several examples of this. Nevertheless it remains generally true that this option is rarely found to be appropriate for most atrium buildings. Another possibility is that the atrium should be pressurised to prevent smoke 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 or a room whose glazing has fallen away in whole or in large part). This is because the air speed needed from the atrium into the room in order to prevent the movement of smoky gases the other way through the same opening, can vary between about 0.5 ms21 and approximately 4 ms21 depending on gas temperature, etc. All of this air must be continuously removed from within the fire room in order to maintain the flow. The quantities of airhandling plant required 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 where the atrium façade has only relatively small leakage paths.

2

Where smoke from a fire in a room can spread into the atrium, with the possibility of rapid further spread affecting other parts of the building, there will be an extreme threat to safe evacuation of occupants from those parts. Similar threats will occur if there is a serious fire in the atrium space itself. In either case, the threat to means of escape which are either within the atrium or in spaces open to the atrium, can develop rapidly unless some form of smoke control is used in the atrium in order to protect those means of escape. In other words a smoke control system in the atrium is essential to make certain that escape is unhindered, by ensuring that any large quantities of thermally buoyant smoky gases can be kept separate from people who may still be using escape routes, or awaiting their turn for evacuation. Therefore the role of a smoke control system is principally one of life safety. It should however also be remembered that firefighting becomes both difficult and dangerous in a smoke-logged building. It follows that to assist the fire services, the smoke control system should be capable of performing its design function for a period of time longer than that required for the public to escape, allowing a speedier attack on the fire to be made after the arrival of the fire service. There has been no readily usable guidance available to designers of atrium smoke control systems within the UK. There have been a number 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). The National Fire Protection Association of the USA has recently been developing a Code13 which sets out a fire engineering approach to the design of smoke control for atria (termed ‘smoke management’ in the USA). While this code is in many ways very comprehensive and broader in purpose than the present Report, some of the approaches used differ from alternatives with which UK designers are more familiar, or are more approximate than methods currently used by the Fire Research Station. This particularly applies to smoke entering the atrium from adjacent rooms. The purpose of this Report is to provide guidance to assist designers of smoke control systems in atrium buildings in line with current knowledge. The guidance is based on results of research where possible, including as yet unpublished results of experiments, but also on the cumulative experience of design features required for regulatory purposes of many individual smoke control proposals. Many of these design features have been evolved over a number of years by consensus between regulatory authorities, developers and fire scientists, rather than by specific research. Such advice has been included in this Report with the intention of giving the fullest picture possible. It is therefore likely that some of this guidance will need to be modified in the future, as the results of continued research become available.

A Code of practice14 for atrium buildings is currently being prepared by the British Standards Institution (BSI). The aim of this present Report is to provide guidance only on design principles of smoke control and it is hoped to support the code rather than to preempt it. The Report cannot cover all the infinite variations of atrium design. Instead it gives general principles for the 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 procedures are of necessity simplified, it also gives their limitations so that, when necessary, a more detailed design by specialists can be carried out.

Figure 1 ‘Sterile tube’ atrium

Such an approach may employ field modelling, which exploits the new techniques of computational fluid dynamics (CFD) to deduce how, and at what rate, smoke would fill an enclosure. It does this by avoiding resort (as far as is currently possible) to experimental correlations, and returning to first principles to solve the basic laws of physics for the fluid flow. As a consequence, with adequate validation, this type of modelling should have a wide applicability. The use of a computer is necessary since the technique involves the solution of tens of thousands of mathematical equations for every step forward the simulation makes. An atrium can be defined as any space penetrating more than one storey of a building where the space is fully or partially covered. Most atria within shopping centres may be considered as part of the shopping mall and treated accordingly. A BSI Code of practice15 specifically for shopping complexes has been published, and also a BRE Report16 giving advice on smoke ventilation of enclosed shopping centres. Where atria have mixed occupancies including shops then reference should be made to these documents, or specialist advice sought.

Figure 2 Closed atrium

In order for a design to be achieved, it is necessary to identify the various ‘types’ of atrium that are built. These can be simply defined as follows: (a) The ‘sterile tube’ atrium The atrium is separated from the remainder of the building by fire-resisting glazing (FRG). The atrium space generally has no functional use other than as a circulation area (Figure 1).

Figure 3 Partially open atrium

(b) The closed atrium The atrium is separated from the remainder of the building by ordinary (non fire-resisting) glass. The atrium space may well be functional (cafeterias, restaurants, recreation, etc) (Figure 2). (c) The partially open atrium Here some lower levels are open to the atrium and the remaining levels closed off by glazing (Figure 3). (d) The fully open atrium Some of the upper levels or all of the building levels are open to the atrium (Figure 4).

Figure 4 Fully open atrium

3

Chapter 1 General principles of smoke production, movement and control Fire growth and smoke production In most instances, a room (compartment) fire may be assumed to burn in either of two ways: (a) Fuel-bed control When the rate of combustion, heat output and fire growth are dependent upon the fuel being burned. (The ‘normal’ fire condition found in most singlestorey buildings whilst the fire is still small enough for successful smoke control.) (b) Ventilation control Where the rate of combustion, etc is dependent upon the quantity of air available to the fire compartment (assuming any mechanical ventilation systems are inactive).

temperature. The smoke spreads out radially underneath the ceiling and forms a layer which deepens as the compartment begins to fill. If the compartment is open to the atrium, then the gases flow out immediately they reach the opening. If the compartment is glazed or the opening is below a deep downstand then the smoke steadily deepens. As the layer gets deeper there is less height for the plume of smoke to rise before it reaches the smoke layer, hence less air is being entrained, with the result that the temperature of the smoke layer increases with layer depth, even for a steady fire. Most fires will continue to grow larger as the layer deepens, reinforcing this effect.

1 The fire starts for whatever reason, its rate of growth depending upon the materials involved. In most practical compartments there is sufficient oxygen to support combustion in the first few minutes, and the fire growth and smoke production are controlled by the fuel, ie, fuel-bed control.

3 6 mm plate glass may shatter when exposed to gases as little as 100 K warmer than ambient. Thus once this temperature is passed, there is an increasing likelihood that the glass will fracture. If the compartment is sprinklered and the water spray hits the glass, the localised heating of the glass by radiation from the fire and by the gas layer, combined with sudden cooling due to the water spray will increase the likelihood of the glass breaking. The smoke and hot gases will then flow externally to atmosphere, or enter the atrium, or both, depending upon the nature of the compartment and its relative position in the building, the size and position of the fire in the compartment, and the strength of differing glazing systems. If the fire can be accidentally or deliberately vented externally then the threat to other levels via the atrium is greatly reduced.

2 Smoke from the fire rises in a plume to the ceiling. As the plume rises, air is entrained into it, increasing the volume of smoke and reducing its

There will, however, be instances when a fire will vent all its effluent gas into the atrium, and this is generally the worst design scenario (Figure 5).

The quantity of smoky gases produced (ie the mass flow rate of gases) in and from the compartment, and the energy (heat flux) contained therein are different for both regimes. It is therefore important to identify the regime which applies. Hence the mass flow and heat flux within the smoky gases may be determined. It is important to understand the basic mechanisms which control the fire condition. A step-by-step history of a growing fire may be as follows:

Figure 5 Smoke entering an atrium from a fuel-bed-controlled fire in an adjacent room

4

There is so much mixing of ambient air into the plume that, except close to the fire itself, the hot smoky gases can be regarded as consisting of warmed air when calculating the quantity (mass flow rate) being produced in the compartment. 4 Initially this mass flow rate of smoke will be controlled by the fuel-bed, as mentioned above. However, the geometry of the opening on to the atrium has a crucial effect. As the fire grows large in comparison to the area of the opening, the air supply to the fire is ‘throttled’, causing it to burn inefficiently. 5 This leads to the situation where the inability of the compartment to vent the gases effectively due to the restricted area available causes the layer to deepen further which, combined with the increasing fire area, causes the layer temperature to rise. Once the layer temperature reaches approximately 600 °C, then in most compartments the downward radiation from the gas layer is sufficient to cause auto-ignition of the remaining combustible materials in the compartment (Figure 6). Where there is sufficient fuel within the compartment for the entire compartment to become involved, the layer temperature will rapidly rise to flame temperature, very approximately 1200 K (930 °C). The rate of burning, heat output and mass flow leaving the compartment are now strongly dependent upon the geometry of the opening, ie ventilation control (Figure 7). 6 The transition from the fuel-bed-controlled fire with a layer at 600 °C to the ventilation-controlled condition is very rapid, and may take only seconds. This condition is often known as ‘flashover’. 7 There may be an intermediate situation where the compartment has flashed over or the fire simply grown to encompass the entire width of the compartment, but where the quantity of air now required to maintain combustion is adequate, even

though the only surface available for air entrainment is the width of the opening (as opposed to the fire perimeter for a fuel-bedcontrolled fire). This condition is known as the ‘fully-involved, large-opening fire’17. 8 There are many factors which determine the prevailing condition, including the type and disposition of the fuel, the dimension of the enclosure and the dimensions of the ventilation opening. They can however be reduced to two principal parameters for most compartments: Aw=H This is the area of the opening into the atrium Aw, multiplied by the square root of its height H. Af

The area of the fire.

Note: Both the fully-involved large-opening fire and ventilation-controlled fire conditions will almost certainly produce flames from the opening into the atrium. 9 The presence of sprinklers will usually serve to prevent fire growth proceeding to full involvement, and will usually maintain the fire in a fuel-bedcontrolled state where extinguishment is not achieved.

Pressurisation Air is introduced into an escape route (usually a stairway) at a rate sufficient to hold back any smoke trying to pass on to that route. The pressure difference across any small opening on to the route must be large enough to offset adverse pressures caused by wind, building stack effect and fire buoyancy. It must also be low enough to allow the escape doors to be opened with relative ease. The air supply must also be large enough to produce a velocity sufficient to hold back smoke at any large opening on to the pressurised space. Experience of pressurisation designs suggests that the technique is well-suited to the protection of

Figure 6 The onset of flashover

5

stairways used as escape routes in tall buildings, though it can also be useful in other circumstances.

Depressurisation This is a special case of pressurisation, where gases are removed from the smoke-affected space in a way that maintains the desired pressure differences and/or air speeds across leakage openings between that space and adjacent spaces. Note that depressurisation does 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 detail in Chapter 5.

Throughflow ventilation Smoke ventilation (throughflow ventilation) is used when the fire is in the same space as the people, contents or escape routes being protected, without it filling that space. The intention is to keep the smoke in the upper reaches of the building, leaving clean air near the floor to allow people to move freely. This stratification or layering of the smoke is made possible by the buoyancy of hot smoky gases produced by the fire, and it follows that to be most successful the highlevel smoke layer must remain warm. Smoke ventilation is therefore only suitable for atria where fires can cause smoke to enter the atrium space. Such fires can either be fuel-bed-controlled fires at the base of the atrium, or fires in adjacent spaces (rooms) which allow smoky gases to enter the atrium. Much of this Report is concerned with the calculation of design parameters for smoke ventilation systems tailored to the circumstances found in various types of atria. First though, it is worth reviewing the underlying principles of smoke ventilation and the general approach needed for successful design.

Figure 7 A fully-involved ventilation-controlled fire

6

Air mixes into the fire plume as it rises, giving a larger volume of smoky gases. These flow outwards below the ceiling until they reach a barrier (eg the walls, or a downstand). The gases then form a deepening layer, whose buoyancy can drive them through natural ventilators (or alternatively smoky gases can be removed using fans). For any given size of fire, an equilibrium can be reached where the quantity of gases being removed equals the quantity entering the layer in the fire plume — no significant mixing of air occurs upwards into the base of the buoyant smoke layer. Sufficient air must enter the space below the layer to replace the gases being removed from the layer, otherwise the smoke ventilation system will not work.

Chapter 2 Design fire size The calculation of the quantity of smoke and heat produced by a fire requires a knowledge of its size in terms of area, perimeter and heat flux developed per unit area or from the fire as a whole. When designing smoke ventilation or depressurisation systems, the mass flow rate and heat flux developed in the room are major parameters in the calculation of the system requirements, changes in which can substantially affect all of the subsequent smoke flow conditions. The preferred choice of design fire would be a timedependent growing fire, to which the means of escape and evacuation time for the particular building occupancy could be related, allowing the increasing threat to occupants to be calculated as time progresses. Unfortunately there is no available research, at the time of preparing this Report, which allows assessment of the probability distribution describing the variation of fire growth curves for areas typically associated with atria. Clearly, one does not want an ‘average’ fire for safety design, since typically half of all fires would grow faster. It is much simpler to assess the maximum size a fire can reasonably be expected to reach during the escape period, and to design the system to cope with that. Such assessments can sometimes be based upon available statistics on fire damaged areas, but may have to depend upon experienced judgement based on the anticipated fire load where a more rigorous approach is not feasible. Work on design guidance for smoke ventilation systems in shopping centres used the principle of selecting a fixed size of fire that would cater for almost all of the fire sizes likely to be found in that class of occupancy, and then deducing a pessimistic heat output from that fire16,18. This procedure has been adopted for occupancies other than retail which are also commonly associated with atrium buildings — offices and hotel bedrooms19,20. The procedure has no time dependency and does not reveal any information regarding the growth characteristics of the fire. It is therefore usual to assume that the fire is at ‘steady-state’. This assumption allows the smoke control system to cater for all fires within the accepted design fire size, and by not considering the growth phase of the fire, introduces a significant margin of safety to the system design. It follows from the foregoing that there is a strongly subjective element in assessing what 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 adopted. In the present work the following, in terms of fire area and convective heat flux, are used to illustrate the calculation procedures adopted: (a) Retail16 (sprinklered shops) 10 m2, 12 m perimeter. (b) Offices19 (sprinklered) 16 m2, 14 m perimeter.

Offices19(unsprinklered) 47 m2, 24 m perimeter.

(c) Hotel bedrooms20 Floor area of the largest bedroom. It should be noted that the design fire size for (a) was originally chosen by the Home Office and Scottish Home and Health Department, and that for those of (b) and (c) there is no official ‘approved’ choice. After considering the heat losses to the structure of the room, and any losses to sprinkler sprays, etc, the commonly used heat outputs are approximately17: Sprinklered office

1 MW

Unsprinklered office

6 MW

Hotel bedroom*

1 MW

Gases flowing into the atrium from a fire deep inside a large-area office with operating sprinklers may be cooler than is assumed in the ‘sprinklered office’ design fire above. The mass flow rate of gases entering the final reservoir will be less than would be calculated using the value given above. Even for this scenario therefore, the above value should err on the side of safety. Designers wishing to take sprinkler cooling in the fire compartment more rigorously into account should adopt a fully fire-engineered approach appropriate to their specific circumstances, for example by using the methods described in the section ‘Effects of sprinkler systems in smoke reservoirs’ (page 14) to assess the effect of sprinkler cooling on the outflowing gases. The hotel bedroom fire represents a fully-involved unsprinklered fire20. Where sprinklers are present this will be clearly unrealistic and a value of 500 kW (for a 6 m perimeter fire) may be more appropriate for designers wishing to adopt a fire-engineering approach to a design. This Report will however assume the fully-involved value for design purposes as this will introduce 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 approximately 20 m2

7

The use of the bedroom floor for the hotel bedroom design fire reflects the situation where there are no sprinklers present. Unpublished research on sprinklered bed fires (P G Smith and J V Murrell, Fire Research Station; private communication, 1986), where the low heat output per unit area was comparable to values for hotel bedrooms, suggests that the much lower fuel load (compared to an office) expected in a hotel bedroom utilising conventional sprinklers should make it possible for the smoky gases to be cooled sufficiently to be retained within the room of origin (assuming the window is not open). The operation of sprinklers is likely to cool any smoke from a fire and suppress that fire to such an extent that the glazing to the bedroom will probably remain intact. This is particularly true for double-glazed windows. The same research indicates that 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 produce hazardous conditions in the atrium, at least above the floor of fire origin. Since there is no statistical data available on fires in sprinklered hotel bedrooms in the UK, any choice of design fire size will be subjective. Should a designer wish to examine the effect of a plume emanating from an open window in a sprinklered hotel bedroom, it would not seem unreasonable to use a value of 6 m perimeter (equivalent to a single bed) with a convective heat output of around 500 kW as the design fire. Research into the use of fast-response sprinklers in a residential environment21,22 has clearly shown that at the time of operation of these sprinklers the conditions inside the rooms were still tenable, ie there was no lifesafety risk from the smoke, even with excessive ceiling level temperatures. This indicates that for any gases flowing into the atrium (eg through an open window) the further entrainment induced by the rising smoke plume will ensure that conditions within the atrium must be tenable, regardless of the smoke temperature or smoke production rate in the room. While it is possible that this may also be true for cellular offices employing fast-response sprinklers, there is no evidence (experimental or empirical) to validate this, and so, to err on the side of safety, this Report will regard sprinklered offices employing fast-response sprinklers in the same way as offices using conventional sprinklers. Further research and statistical data are desirable in this area. As mentioned in the Introduction the retail atrium is considered separately in the Report on covered shopping complexes16, and will not be considered further in this Report. The fire sizes on the previous page (excluding (a)) will be those used throughout. Furthermore, when considering an unsprinklered office occupancy, there exists the potential for flashover to occur and for the entire floor to become involved in fire. Even if the building geometry can 8

accommodate this fire condition, the destructive power of a fully-involved office room fire is such that smoke control systems cannot usually be designed to satisfactorily protect means of escape in this situation, except for fires in small rooms. An accurate assessment of the mass flow rate and heat flux from a room fire will allow the potential for flashover to be estimated, and thence whether additional precautionary measures are required, eg sprinklers. This Report will only provide guidance for the design of smoke 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 equations, figures, etc given here may no longer apply, and advice should be sought from experts.

Chapter 3 Smoke control on the floor of fire origin Within the fire room In any situation involving the potential movement of smoke into escape routes, it is always preferable to control the smoke in the fire room and hence prevent its passage to otherwise unaffected areas. Ventilation of the fire room may be achieved by either a dedicated smoke exhaust system or by adapting and boosting an air-conditioning or ventilating system. If the compartment is open to the atrium, then it must have either a downstand barrier to create a reservoir within the compartment, or a high-powered exhaust slot at the boundary edge to achieve a similar effect (Figure 8). The minimum height of the smoke layer base in the room must be compatible with the openings on to the atrium, with the layer depth being no lower than the soffit of the opening (Figure 9). Where no downstand exists and an exhaust slot is used instead, the exhaust capacity provided will need to be compatible with the layer depth (Figure 10). See the section on exhaust slots (‘Slit extract’) on page 19.

(a) Use of a downstand to create a smoke reservoir

Figure 9 Plume height and layer depth with a downstand

Having established the clear layer height in the room, the mass flow rate of smoke can then be calculated. Recent work by Hansell23 drawing on work by Zukowski et al24 and Quintiere et al25 has shown that the rate of air entrainment into a plume of smoke rising above a fire (Mf) may be obtained by using the equation: Mf = Ce P Y 3/2

kgs21

...(1)

where Ce = 0.188 for large-space rooms such as auditoria, stadia, large open-plan offices, atrium floors, etc where the ceiling is well above the fire. Ce = 0.210 for large-space rooms, such as open-plan offices, where the ceiling is close to the fire.

(b) Use of a ‘slot exhaust’ to prevent smoke entering the atrium

Note: As the two values are approximately 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. Ce = 0.337 for small-space rooms such as unit shops, cellular offices, hotel bedrooms*, etc with ventilation openings predominantly to one side of the fire (eg from an office window in one wall only). Most small rooms will therefore take this value.

Figure 8 Smoke ventilation within a compartment

P=

Perimeter of the fire (m).

Y=

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

* Prior to flashover or full involvement

9

Flow of hot gases out of the room of origin into the atrium The mass flow rate of smoky gases passing through a vertical opening (Mw) may be found from23:

Mw =

Ce P W h 3/2  2/3 1 Ce P  2/3 W + Cd  2  

 3/2  

kgs21 ...(2)

where W = Width of opening (m) h = Height of the opening above the floor (m) Figure 10 Plume height and layer depth with a slot exhaust

Equation 1 has been validated experimentally26 for values of Y up to 10 times =Af, for fires in large spaces, for values of the heat release rate between 200 and 750 kWm22. There is no information available to show how Equation 1 (or any current alternatives) should be modified to allow for the effects of sprinkler spray interactions. Consequently, it is used here unmodified. The quantity of smoke entering a ceiling reservoir or flowing layer given by Equation 1 is shown graphically in Figures 11 and 12 for both cellular and open-plan offices (Ce = 0.337 and Ce = 0.188 respectively) and for sprinklered and unsprinklered offices (P = 14 m and P = 24 m respectively). For further discussion on the criteria for selecting a value of Ce, see the section ‘Flow of hot gases out of the room of origin into the atrium’ that follows.

Cd = Effective coefficient of discharge for the opening Where the smoke flow directly approaches a ‘spill edge’ with no downstand (eg where the ceiling is flush with the top of the opening), Cd = 1.0. For other scenarios the following procedure may be adopted: Where the smoke flows beyond a downstand or lower ceiling level in the form of a plume of height Dd (Figure 13(a) and 13(b)), it has been shown23 that the height of rise of the plume has an effect on the rate of flow of smoke leaving the opening. This effect can be expressed as a modification to the coefficient of discharge as follows:  Dw + Dd  Cd = 0.65    Dw 

1/3

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 enough fire resistance, or have a large enough opening to sustain the replacement air supply needed for such large fires (see the section ‘Inlet air’ on page 17). Figure 12 also has a ‘cut-off’ below which the temperature of the gas layer will exceed 600 °C and flashover of the room will almost certainly have occurred. The mechanism of flashover may well start to occur prior to this critical point, and gas temperatures in excess of 500 °C may be considered a conservative lower limit for flashover potential27. The ‘danger-zone’ is shown shaded on Figure 12. Mass flow rates should be above this shaded zone for the smoke control systems to operate safely.

Figure 11 Rate of production of hot smoky gases — sprinklered offices

10

...(3)

where Dw = Flowing layer depth in the plane of the opening (m). Dd = Depth of downstand or height of rise of plume beyond the opening (m). Where Dd > 1.0 then Dd may be taken as 1.0 for most openings of practical interest. For a plain opening with no downstand obstruction (Figure 14), Dd can be considered as the rise of the plume beyond the balcony edge. The flowing layer depth (Dw) may be found from: 1  Mw  Dw =   Cd  2W 

2/3

...(4)

A simple procedure for calculating the mass flow rate, etc is as follows: (a) Set Cd to 0.65. (b) Calculate mass flow rate from: Ce P W h 3/2 Mw =  1  W 2/3 +  Cd 

 Ce P   2

  

2/3

   

3/2

kgs21

Figure 12 Rate of production of hot smoky gases — unsprinklered offices

(c) Calculate buoyant layer depth from: 1  Mw  Dw =   Cd  2W 

2/3

Figure 13 Flow out of an opening

m

(d) Calculate discharge coefficient from:  Dw + Dd  Cd = 0.65    Dw 

1/3

(e) Use the new value of Cd and repeat from step (b), until the difference between the currently calculated value of Mw(Mw(n)) and the previously calculated value of Mw(Mw(n21)) is less than 0.1%. ie: Mw(n) – Mw(n – 1)    3 100 , 0.1  Mw(n) 

(a) With downstand and projecting balcony

This procedure usually converges after about five iterations and will therefore quickly yield Mw, Cd and Dw. Figures 15 and 16 give the mass flow values in graphical form for various opening heights and widths, using the above procedure. A ceiling and projecting balcony 4 m above the floor have been assumed. It should be noted that the shaded areas on the graphs represent the onset of flashover (calculated using Mw and Qw appropriate to the example illustrated and a layer temperature of approximately 550 °C), and values of mass flow lying within this band should be regarded with caution.

(b) With high balcony

11

Figure 14 Plain opening Cd ≈ 0.8

Figure 15 Mass flow rate through a vertical opening — open-plan offices

Figure 16 Mass flow rate through a vertical opening — cellular offices

12

The demarcation between a cellular room and an open-plan layout is determined by the ability of the incoming air to flow into the rising plume from all sides. The narrower the room becomes, the less easily the air can flow behind the plume. In this Report cellular offices are considered to be those in which the maximum room dimension is less than or equal to five times the diameter of the design fire size, and the incoming air can only enter from one direction (Figure 17). This demarcation dimension was chosen arbitrarily and has no theoretical derivation. Research in this area is highly desirable.

positioned across the balcony to limit the size of this reservoir to ensure that the smoke retains its buoyancy. Each reservoir should be limited to an area not exceeding 1300 m2, with a maximum length of 60 m by analogy with shopping malls16. The screens around the balconies will, in general, be fairly close to potential fire compartments (eg offices). Being close, smoke issuing from such a compartment will deepen locally on meeting a transverse barrier. The depth of these screens should take into account local deepening (see page 17). Smoke removed from these lower level reservoirs should usually be ducted to outside the building but can be ducted into the ceiling reservoir of the atrium (Figure 20). The mass flow rate of smoke to be exhausted from the atrium roof will then be that calculated for the under-balcony condition28. Early experiments with smoke flow in shopping malls29 and unpublished work17 at FRS (also N R Marshall, Fire Research Station; private communication, 1984) have shown that the smoke flowing from a room with a deep downstand and then under a balcony beyond the opening becomes turbulent with increasing mixing of air. This subsequent evidence suggests that for the purpose of engineering design the mass flow rate of smoke entering the balcony reservoir (MB) can be taken to be approximately double the amount given by Equation 2, ie: MB = 2Mw

kgs21

...(5)

Figure 17 Limiting size of a cellular room

Ventilation of the balcony space If the smoke cannot be contained within the room of origin because: ●

the rooms have demountable partitions,

●

insufficient replacement air can be provided, or

●

the engineering implications are too costly or difficult to apply,

then the smoke and hot gases will be able to travel from the room of origin into the space beyond, including the atrium.

Figure 18 Schematic section of an atrium with balconies

Some atria are designed with balconies around the perimeter of the void, serving all the rooms at that level (Figure 18). Figure 19 illustrates in schematic form an atrium with floors (two levels only are shown in both the figures) which have balconies that leave a considerable area for pedestrians. On each level there is a large area situated below each balcony. If screens (activated by smoke detectors or as permanent features) are hung down from the balcony edges, the region below each balcony can be turned into a ceiling reservoir. This is similar to the procedure used in multi-storey shopping complexes16. This balcony reservoir can then be provided with its own extraction system. Other screens can be

Figure 19 An under-balcony smoke reservoir

13

●

alternative escape routes,

●

shorter escape paths along the balcony, and

●

the installation of sprinklers to cool the gases further.

Effects of sprinkler systems in smoke reservoirs Offices, shops, assembly, industrial and storage or other non-residential purpose groups are now expected to have sprinklers if they have a floor more than 30 metres above ground level. Multi-storey buildings in the assembly, shop, industrial or storage purpose groups will also be fitted with sprinklers if individual uncompartmented floors exceed a given size. Sprinklers may also be required in other circumstances for insurance purposes.

Figure 20 Under-balcony smoke reservoir venting into an atrium smoke reservoir

Entrainment into smoke flows from compartments is being studied23. The purpose of this is to determine more accurately the influence of such factors as compartment opening geometry, the presence of a downstand fascia and balcony/downstand combinations. It follows that Equation 5 may be superseded in due course.

Smoke layer temperature The mean temperature rise of the smoke layer above ambient (U) can be calculated from: U=

Qw °C

...(6)

Mc where Qw = Heat output at window or exit point (kW) M = The mass flow of smoke (eg Mw or MB) (kgs21) c

= Specific heat capacity of the gases (kJkg21K21)

Tables 1 and 2 give the temperature rise (U) for a 1 MW and 6 MW fire, taking into account the cooling processes mentioned in Chapter 2. In unsprinklered fire situations a high smoke layer temperature will result in intense heat radiation which may cause difficulties for people escaping along a balcony beneath the smoke layer, especially if the balconies form a major escape route. The maximum smoke layer temperature which will allow safe evacuation without undue stress is in the order of 200 °C. If this gas temperature (or lower) cannot be achieved then consideration should be given to: 14

The action of a sprinkler system in an office on the cooling of gases flowing from that office to the atrium is accounted for in the derivation of the 1 MW heat output17. Where the smoke layer is contained wholly within the room of origin by a smoke control system and has a large area, the sprinklers will cool the smoke layer further. Similarly, where smoke is collected within a balcony reservoir adjacent to sprinklered offices, operation of sprinklers under the balconies will lead to increased heat loss reducing the buoyancy of smoke, which in turn can contribute to a progressive loss of visibility under the smoky layer. However, gases sufficiently hot enough to set off sprinklers will remain initially as a thermally buoyant layer under the balcony ceiling, and will not be pulled out of the layer by the sprinkler sprays. When the fire occurs in an office, the operation of sprinklers under the balcony will not assist in controlling it. If too many sprinklers operated under the balcony, sprinklers in the office could become less effective as the available water supply approached its limits. It follows therefore that sprinklers need only be installed in a smoke reservoir if: ●

the smoke layer temperature is likely to exceed 200 °C and thus produce sufficient radiation to impede escape, or

●

if there is the likelihood of sufficient combustibles being present to pose a significant threat of excessive fire-spread.

A powered extract system, to a reasonable approximation, removes a fixed volume of smoke irrespective of temperature. Therefore if the extent of sprinkler cooling is overestimated, the system could be underdesigned. A system using natural ventilators depends on the buoyancy of the hot gases to expel smoke through the

Table 1 Volume flow rate and temperature of gases from a 1 MW fire (including cooling within room of origin) Mass flow rate (Mass rate of extraction) (kgs21)

Temperature of gases above ambient (°C)

Volume rate of extraction (at maximum temperature) (m3s21)

4 6 8 10 12

250 167 125 100 83

6.0 8.0 9.5 11.0 12.5

15 20 25 30 35

67 50 42 33 28

15.0 19.5 22.5 27.5 32.0

40 50 60

25 20 17

36.0 44.5 53.0

temperature beyond the radius of operation of the sprinklers. This radius is generally not known.

ventilators. In this case the system would be underdesigned if the sprinkler cooling were underestimated.

In the absence of better information, it may be reasonable to assume that no more sprinklers will operate than are assumed when calculating the design of sprinkler systems and their water supply (eg 18 heads for Ordinary Hazard Group 3).

The heat loss from smoky gases to sprinklers is currently the subject of research, although data suitable for design application are not yet available. Nevertheless, an approximate estimate can be obtained as follows: If the smoke passing a sprinkler is hotter than the sprinkler operating temperature, that sprinkler will eventually be set off and its spray will cool the smoke. If the smoke is still hot enough the next sprinkler will operate, cooling the smoke further. A stage will be reached when the smoke temperature is insufficient to set off further sprinklers. The smoke layer temperature can thereafter be assumed to be approximately equal to the sprinkler operating

For powered extract systems the cooling effect of sprinklers can be ignored in determining the volume extract rate required. This will err on the side of safety. Alternatively, this further cooling and the consequent contraction of smoky gases can be approximately estimated on the basis of an average value between the sprinkler operating temperature and the calculated initial smoke temperature. Where the fan exhaust openings are sufficiently well separated it can be assumed that one opening may be close to the fire, and

Table 2 Volume flow rate and temperature of gases from a 6 MW fire (including cooling within room of origin) Mass flow rate (Mass rate of extraction) (kgs21)

Temperature of gases above ambient (°C)

Volume rate of extraction (at maximum temperature) (m3s21)

10 12 15 20 25

600 500 400 333 240

25.5 27.0 29.5 32.0 38.0

30 35 40 50 60

200 171 150 120 100

41.5 46.5 50.5 59.0 67.5

75 90 110 130 150

80 67 54 46 40

80.0 92.5 107.0 123.5 140.0

200 300 400

30 20 15

181.0 263.0 345.0

15

will extract gases at the full initial temperature given by Equation 6. The other openings in these circumstances can be assumed to be outside the zone of operating sprinklers, and will extract gases at the sprinklers’ effective operating temperature. The number of potential ‘hot’ and ‘cool’ intakes must be assessed when calculating the average temperature of extracted gases. If the sprinkler operating temperature is above about 140 °C, or above the calculated smoke layer temperature, then sprinkler cooling can be ignored for natural ventilators. Note that the effect of sprinkler cooling is to reduce the heat flux (Qw) without significantly changing the mass flux. It follows that once a new value of U has been estimated, the new heat flux can be found using Equation 6.

Flowing layer depth Smoke entering a ceiling reservoir will flow from the point of entry towards the exhaust points. This flow is driven by the buoyancy of the smoke. Even if there is a very large ventilation area downstream (eg if the ceiling downstream were to be removed), this flowing layer would still have a depth related to the width available under the remaining ceiling (which can now be considered a balcony), the temperature of the smoke and the mass flow rate of smoke. Work by Morgan30 has shown that this depth can be calculated for unidirectional flow as follows: 1  MB  DB =   Cd  2WB 

2/3

m

...(7)

where DB = Flowing smoke layer depth under the balcony (m) MB = Mass flow rate under the balcony (kgs21) WB = Balcony channel width (m) Cd = Coefficient of discharge Note: Values of Cd will vary for differing flow geometries. However, for the purpose of engineering design Cd can be taken to be 0.6 if a deep downstand is present at right angles to the flow, or 1.0 in the absence of a downstand. At the time of writing, values of Cd for intermediate depth downstands cannot be stated with confidence for the wide range of geometries to be found in practice. It is suggested that either of the extreme values should be adopted in seeking a conservative design approach. The resulting values of layer depth for different balcony reservoir widths and mass flow rates of smoke are 16

shown in Table 3 for layer temperatures UB ≥ 65 °C17. This ignores the effects of cooling (See the section ‘Smoke layer temperature’ on page 14). Each depth shown in this table is the minimum possible regardless of the smoke extraction method employed downstream; consequently it represents the minimum depth for that reservoir. The depth must be measured below the lowest transverse downstand obstacle to the flow (eg structural beams or ductwork) rather than the true ceiling. Where such structures exist and are an appreciable fraction of the overall layer depth, the depth below the obstacle should be found using Table 3(b) rather than 3(a).

Table 3 Minimum reservoir depths or minimum channelling screen depths required under balconies for both 1 MW and 6 MW convective heat output (a) Unimpeded flow Mass flow rate entering reservoir (kgs21)

Width of reservoir WB or channelling screen width L (m)* 4

6

8

10

12

15

10 15 20 25 30

1.1 1.4 1.7 2.0 2.3

0.8 1.1 1.3 1.5 1.7

0.7 0.9 1.1 1.2 1.4

0.6 0.8 0.9 1.1 1.2

0.5 0.7 0.8 1.0 1.1

0.5 0.6 0.7 0.8 0.9

40 50 70 90 110

2.8 3.4 4.5 5.6 6.7

2.2 2.6 3.4 4.3 5.1

1.8 2.1 2.8 3.5 4.2

1.5 1.8 2.4 3.1 3.6

1.4 1.6 2.2 2.7 3.3

1.2 1.4 1.9 2.3 2.8

130 150

7.8 9.0

6.0 6.8

4.9 5.6

4.2 4.9

3.8 4.3

3.2 3.7

(b) Impeded flow — deep downstand Mass flow rate entering reservoir (kgs21)

Width of reservoir WB or channelling screen width L (m)* 4

6

8

10

12

15

10 15 20 25 30

1.8 2.3 2.8 3.3 3.8

1.4 1.8 2.2 2.5 2.9

1.2 1.5 1.8 2.1 2.4

1.0 1.3 1.5 1.8 2.1

0.9 1.1 1.4 1.6 1.8

0.8 1.0 1.2 1.4 1.6

40 50 70 90 110

4.7 5.7 7.5 9.4 11.2

3.6 4.3 5.8 7.2 8.6

3.0 3.6 4.8 6.0 7.1

2.6 3.1 4.1 5.1 6.1

2.3 2.7 3.6 4.5 5.4

2.0 2.4 3.1 3.9 4.7

130 150

13.1 15.0

10.0 11.5

8.2 9.5

7.1 8.2

6.3 7.2

5.4 6.2

* For bi-directional flow of smoky gases this should be twice the actual reservoir width.

Local deepening Where a buoyant layer of hot smoke flows along beneath a ceiling and meets a transverse barrier, it deepens locally against that barrier31 and, as the gases are brought to a halt, the kinetic energy of the approaching layer is converted to buoyant potential energy against the barrier. When designing a smoke ventilation system for atria, in which the balconies are acting as reservoirs, it is often necessary to control the path of smoke flow using downstand smoke curtains. These are typically installed around the edge of the voids to prevent smoke flowing up through the voids. If the void edge is close to the room this local deepening could cause smoke to underspill the smoke curtain and flow up through the void, possibly affecting escape from other storeys. Clearly, the void edge screens must be deep enough to contain not only the established layer, but also the additional local deepening outside the room on fire. The extent of local deepening can be found from Figure 21. The depth of the established layer (DB in Figure 21) under the balcony immediately downstream of the local deepening must first be found using the design procedure given in the preceding sections. Usually this means in the channel formed between the void edge screen and the room façade. The additional depth ∆DB can then be found by inspection of Figure 21, allowing the necessary minimum overall depth (DB + ∆DB) of the void edge screen to be found. It can be shown that the following scale-independent formula can be used to approximate Figure 21:

 1 – Loge  ∆DB = 0.4Hc    Loge 

     5WB       Hc    5DB     Hc 

...(8)

where ∆DB = the additional deepening at the transverse barrier (m) Hc = the floor-to-ceiling height (m) DB = the established flowing layer depth (m) WB = distance between the opening and the transverse barrier (ie balcony depth) (m)

Inlet air There must be adequate replacement air for the efficient operation of a smoke ventilation system. When ventilating compartments directly, if the façade is normally sealed then facilities should be provided for the necessary quantity of replacement air to be supplied to the fire room automatically. This requirement often makes the provision of smoke ventilation to the room of origin prohibitive or undesirable. The provision of replacement air to a system employing balcony reservoirs is far easier, provided the balconies are open to the atrium. If the area available for inlet becomes too restricted, incoming airflow through escape doors may be at too high a velocity for easy escape. Such air inflows through doors in public buildings could hinder

Figure 21 Local deepening at a transverse barrier

17

escapees. Recent research32 into the ability of people to move through an exit against an opposing airflow has shown that movement is not impeded for airspeeds below 5 ms21, and is not seriously impeded below 10 ms21 (although some discomfort was reported at these higher airspeeds). This suggests that inflow airspeeds should not usually exceed 5 ms21.

an opening (A J M Heselden, Fire Research Station; private communication, 1976), beyond which air will be drawn through the layer. This critical exhaust rate (MCRIT) may be found from:

Other values may be appropriate for other circumstances. For example, in buildings where the population is largely familiar with the escape routes; where the incoming air is entering the fire room directly, or where (in the instance of the inlet air being supplied via the atrium) the major escape routes are away from the atrium; then a less onerous parameter can be applied. Current advice regarding pressurisation system design33 recommends a maximum pressure drop across a door of 60 Pa. This accords to a face velocity across a rectangular inlet opening of about 6 ms21. The pressure drop criterion may be increased if the population of the building is adult and physically fit, to perhaps 100 Pa (8 ms21).

or

A fan-driven inlet air supply may be employed, but can give problems when mechanical extraction is used (the building will usually be fairly well sealed in such circumstances). This is because the warmed air taken out will have a greater volume than the inlet air. As the fire grows and declines, the mismatch in volume between the inlet air and the extracted fire-warmed air will also change. This can result in significant pressure differences appearing across any doors on the escape routes. For this reason simple ‘push-pull’ systems should be avoided.

Minimum number of extraction points The number of extraction points within the reservoir is important since, for any specified layer depth, there is a maximum rate at which smoky gases can enter any individual extraction point. Any further attempt to increase the rate of extraction through that point merely serves to draw air into the orifice from below the smoke layer. This is sometimes known as ‘plugholing’. 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. Table 4, which is based on experimental work34, subsequently modified (A J M Heselden, Fire Research Station; private communication, 1976), lists the minimum numbers needed for different reservoir conditions and for a variety of mass flow rates being extracted from the ventilators in the reservoir. Table 4 strictly applies to ventilators which are small compared to the layer depth below the ventilators (eg where the diameter is much less than the depth of the layer). Where sprinklers are installed and additional cooling of the smoke layer needs to be accounted for, the number of extraction points required will differ from those shown in Table 4. The number can be determined by calculating the critical exhaust rate for 18

MCRIT = β(gD5 To U/T2)1/2

kgs21

...(9)

where β = 1.3 for a vent near a wall (kgm23) β = 1.8 for a vent distant from a wall (kgm23) g = Acceleration due to gravity (ms22) D = Depth of smoke layer below the extraction point (m) To = Absolute ambient temperature (K) U = Excess temperature of smoke layer (°C) T = To + U (K) The required number of extract vents (N) is then given by: M N≥ ...(10) MCRIT where M = The mass flow rate entering the layer (ie Mf or MB) (kgs21) Table 4 Minimum number of extraction points needed in a smoke reservoir (a) 1 MW heat output Total mass rate of extraction (kgs21) 0.5

Depth of layer below extraction point (m) 0.75

1.0

1.25

1.5

1.75

2.0

9 12 15 18

21–28 29–40 39–54 50–68

8–11 11–15 14–20 18–25

4–5 6–8 7–10 9–13

3–3 3–5 4–6 5–7

2–2 2–3 3–4 4–5

1–2 2–2 2–3 3–3

1–1 1–2 2–2 2–3

20 25 30 40

57–79 77–107 99–137 149–205

21–29 28–39 36–50 54–75

10–14 14–19 18–25 27–37

6–8 8–11 10–14 15–21

4–6 5–7 7–9 10–14

3–4 4–5 5–6 7–9

2–3 3–4 4–5 5–7

(b) 6 MW heat output Total mass rate of extraction (kgs21) 0.5

Depth of layer below extraction point (m) 0.75

1.0

1.25

1.5

1.75

2.0

12 15 18

26–35 31–43 37–51

10–13 12–16 14–19

5–7 6–8 7–9

3–4 4–5 4–6

2–3 2–3 3–4

2–2 2–2 2–3

1–2 1–2 2–2

20 25 30 40

41–56 51–70 62–85 85–118

15–21 19–26 23–31 31–43

8–10 9–13 11–15 15–21

5–6 6–8 7–9 9–12

3–4 4–5 4–6 6–8

2–3 3–4 3–4 4–6

2–2 2–3 2–3 3–4

Note: In both tables the first number of each pair denotes extraction points well away from the walls, and the second is for those close to the walls.

Where very large or physically extensive ventilators are used (eg a long intake 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 with the ‘width of reservoir’ being taken as the total horizontal accessible perimeter of all the ventilators within the reservoir (eg the total length of intake grilles in the example above) and the ‘minimum reservoir depth’ corresponds to the depth of the smoke layer beneath the top edge of the intake orifice. In practice, for a given mass flow rate and layer depth, Table 3(a), 3(b) or Equation 7 can be used to find the minimum value of accessible perimeter.

Such a system is likely to work best with further extraction distributed within the fire room, which for a sprinklered room may possibly be provided by the normal ventilation extraction system (the normal ventilation input and recirculation of air being stopped) or, for an unsprinklered room, by a partial smoke exhaust system. Whilst this system is designed to prevent smoke entering the atrium void, it will not necessarily maintain a clear layer within the room itself, and any balcony may become ‘fogged’. The extraction should be provided very close to the opening from a continuous slit which may be situated in the plane of the false ceiling.

Intermediate size intakes (ie where the ventilator size is comparable to layer depth) cannot be treated so simply and it is recommended that Table 4 be used since it errs on the side of safety.

It has been shown that powered exhaust from a slit at right angles to a layer flow could completely prevent smoke passing that slit, provided that the extraction rate at the slit was at least 5/3 times the flow in the horizontal layer flowing towards the slit (H G H Wraight, Fire Research Station; private communication, 1984). This allows a useful general method for sizing such an extract:

Required ventilation rate (powered exhaust) A powered smoke exhaust system consists of fans and associated ductwork designed to remove the mass flow rate of smoke entering the smoke reservoir, and to be capable of withstanding the anticipated smoke temperatures.

(a) First calculate the flow rate of gases approaching the opening (or gap in the balcony edge screens) using as appropriate the following sections:

The controls and wiring should of course be protected, to maintain the electrical supply to the fans during a fire. The mass flow rate of smoke determined from the previous sections can be converted to the corresponding volumetric flow rate and temperature, using Tables 1 or 2 or the following equation for selection of the appropriate fans: MT Vl =

m3s21

ρο Το

...(11)

where Vl = Volumetric exhaust rate required in the reservoir (m3s-1) M = Mf or MB determined from the previous section (kgs21) T = U + To (K) ρo = Density of ambient air (kgm23)

Slit extract When removing the smoke from a common balcony reservoir, and there is no possibility of using downstand screens to prevent the passage of smoke into the atrium, a slit extract system may be employed over the length of the flow path to supplement the underbalcony exhaust system and replace the screens (Figure 22). Similarly, a slit extract system can be used across a room’s openings to prevent any outflow of smoke.

Figure 22 Slit extraction

19

‘Flow of hot gases out of the room of origin into the atrium’ (page 10) ‘Ventilation of the balcony space’ (page 13), and ‘Smoke layer temperature’ (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 appropriate to ‘Effects of sprinkler systems in smoke reservoirs’ on page 14), calculate the volumetric exhaust rate required from the slit, using Equation 6 to calculate the mean gas temperature drawn through the fan, and Equation 11 to calculate the required fan capacity.

False ceilings Where there is an unbroken false ceiling in the fire room or balcony it must be treated as the top of the smoke layer. If the false ceiling is porous to smoke, ie if it has an appreciable free area, any smoke screens forming the smoke reservoir must be continued above the ceiling. If the proportion of free area is large enough, the reservoir and its screens may even be totally above the false ceiling. The permeable ceiling ought not to interfere appreciably with the flow of smoke from the fire to the smoke ventilation openings above the false ceiling. It has been shown experimentally35 that a minimum free area of 25% can be used as a rule of thumb value for allowing safe escape. For balcony reservoirs cool smoke can be expected to affect some nearby rooms under some circumstances, but would not significantly hinder safe escape. Free areas of less than 25% are possible in some circumstances, but expert advice should be sought where this possibility is felt desirable.

The use of a plenum chamber above a false ceiling Some designs have been seen in which the space above the mainly solid false ceiling in a roof or above a balcony is used for the extraction of air for normal ventilation purposes. A fan extracting air from this space (effectively a plenum chamber) reduces its pressure and so draws air from the space below through a number 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 chamber in a similar way. A potentially valuable bonus of such a system in a sprinklered building is that the sprinklers which are normally required in the space above the false ceiling will cool the smoky gases before they reach the fan. Furthermore, it can be desirable to leave the false ceiling below the extraction points ‘solid’ (ie not able to pass smoke) to prevent air being drawn up through the smoke layer. A sufficiently extensive area of solid 20

false ceiling will ensure that the smoke passes through at least one sprinkler spray en route to the extract. The plenum chamber should not be larger in area than its associated smoke reservoir. Larger chambers should be subdivided by smoke screens extending the full height of the chamber and below the false ceiling to form a complete smoke reservoir below. The minimum number of openings through the false ceiling required within a single subdivision can be found from Table 4. The total area of such openings per reservoir should be decided by consideration of the design pressure differences between chamber and smoke layer, and of the flow impedance of the openings concerned. A system of reasonably wide (perhaps oneor two-metre) slots surrounding a region of false ceiling could perhaps be used instead of screens below the false ceiling.

Chapter 4 Smoke ventilation within the atrium Smoke movement in the atrium When the smoke and heat cannot, 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 smoke control is that most readily understood by most as ‘smoke ventilation’ and is based upon a defined buoyant smoke layer being established at some point within the structure, with a ‘clean’ layer of air beneath. The mass flow of gases entering this layer is equivalent to that flowing out through the exhaust system (Figure 23). The base of such a layer is usually at a height chosen for safety reasons or to avoid breaching the practical ‘cut-off’ limits outlined in the section ‘Limitations to the use of throughflow ventilation’ on page 35. Once the height of this layer base is chosen for a lowest-level fire, the height above the top of the opening (or above the edge of the projecting canopy or balcony over the opening where relevant) must be established where the fire is in an adjacent room. Note that when the fire is on the floor of the atrium and is directly below the smoke layer that forms under the atrium ceiling, entrainment into the rising plume is different to entrainment into spill plumes. This special case of fires on the atrium floor is discussed later on page 33. In general however, the worst condition to be catered for is a fire in an adjacent room on the lowest level, as

this results in the most entrainment in the rising smoke plume and hence the largest quantity of smoky gas entering the buoyant layer. The fire condition in the compartment (the design fire) should be specified, and the mass flux leaving through the compartment opening and any entrainment under the projecting balcony or canopy can be calculated as described in the first three sections of Chapter 3. As the smoke flows through the room opening into the atrium space it will either: ●

rotate upwards around the top edge of the opening and pass directly into the atrium space as a plume, or

●

flow under a horizontal projection such as a balcony beyond the opening, pass to the edge of the projection and rise upwards into the atrium space as a plume.

Such plumes are often referred to as ‘spill’ plumes, or as ‘thermal’ line plumes. The term ‘line’ denotes that the base of the plume immediately following rotation is long and relatively narrow. Line plumes may take one of two forms: 1 Adhered plumes, where the smoky gases project directly from a compartment opening, and the plume attaches to the vertical surface above the opening whilst rising upwards. This will also occur when there is a vertical surface immediately above any rotation point into the void. The surface of the plume in contact with the ambient atmosphere in the atrium will cause additional air to be entrained

Figure 23 Throughflow ventilation of the atrium

21

into it (Figure 24(a)). This type of plume is also known variously as a single-sided, attached or wall 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 entrainment on both sides of the plume along its spill width (Figure 24(b)), for which reason they are also known as double-sided plumes. The degree of entrainment into the rising plume, and hence the total quantity of gases entering the smoke layer forming under the ceiling of the atrium space, is governed basically by four initial parameters36: (a) The mass flow rate or temperature of the gases at the edge of the rotation point into the atrium. (b) The heat flux of the gases. (c) The length of the line plume entering the atrium, measured along the edge past which the smoke spills. (d) The height through which the plume must rise. Reductions in the mass flow rate of smoke entering the smoke layer can usually be effected by changes to (c) and (d). In practice the height of rise of the plume is usually chosen to permit safe evacuation, leaving only a dependency on the length of the line plume.

Channelling screens When the atrium has a plane façade with no horizontal projections, the length of the plume is determined by the width of the opening through which the smoke is passing. When, however, smoke is able to flow unrestricted under a horizontal projection, eg a balcony, it will flow forwards towards the balcony edge, and laterally sideways. It will continue to flow sideways until it meets an obstruction or loses

(a) Adhered plume

Figure 24 Line plumes within the atrium

22

(b)

Free plume

sufficient energy to become stagnant, and will then 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 rate of smoke entering the layer in the atrium roof. This excessive entrainment can be reduced by restricting the sideways travel of the smoke under the balcony and hence reducing the length of the line plume. The devices used to achieve this are commonly known as channelling screens, and literally ‘channel’ the smoke from the exit of the room to the balcony edge (Figure 25(b)). This concept is used in smoke control systems in multi-storey shopping centres16. The minimum depth required for a pair of these screens to channel all the smoke is dependent on their separation at the void edge (L). Some values for 1 MW and 6 MW fires in offices are given in Table 3(a) for a balcony with no downstand obstruction at the void edge (unimpeded flow), and Table 3(b) for a balcony with a downstand at the void edge, which is deep relative to the approaching layer (impeded flow). Alternatively the minimum channelling screen depth may be calculated using Equation 7, where WB will be the channelling screen separation width L. Whichever procedure is chosen, the resulting depth is the maximum flowing smoke layer depth and hence minimum screen depth. Good practice suggests that a safety margin should be considered. An additional depth of 0.25 m would not seem unreasonable. Screens may be fixed or may descend upon smoke detection. As described above, the final mass flow entering the layer is a function of four initial parameters, one of which is the plume width at the balcony edge. The narrower the plume at its base, the less the mass flow entering the layer. Thus the closer the screens may be installed to each other, the more the smoke base may be allowed to rise for the same heat flux.

Figure 25(a) Smoke spreading sideways beneath a projecting canopy or balcony

Figure 25(b) Smoke confined to a compact spill plume by channelling screens

These screens must, of course, meet the wall of a compartment where it meets the balcony. Any screen fixed midway across a compartment opening will serve no purpose, since smoke will flow on both sides simultaneously.

Entrainment into spill plumes rising through the atrium

Recent research37 suggests that channelling screens may be unnecessary if the balcony projects no more than 1.5 m beyond the fire room. This research has also shown that balconies which are shallow (<2 m) will cause the rising plume to curl inwards towards the structure (Figures 26(a) and 26(b)). If there are higher balconies a vortex will be created between them, smoke-logging the balcony levels above the fire floor.

Calculations of entrainment into the smoke flows rotating around the opening/balcony edge and into the subsequent rising plume follow the procedures of Morgan and Marshall36,38 for free plumes, using the modifications introduced by Morgan and Hansell17. Entrainment into smoke flows rotating into a rising adhered plume can be calculated using the similar method of Morgan and Hansell17, although it should be noted that the entrainment constant appropriate to an adhered line plume is about half that for a free plume37,39. A value of 0.077 has been used for the present calculations37. This calculation method is outlined in detail in Appendix B. The algorithm described can be used either directly, or as the basis for a computer program.

(a) Deep balcony projection

(b) Shallow balcony projection

Figure 26 The effect of balcony depth on plume trajectory

23

Once the desired height of the layer base (hb) has been chosen, the opening width established, or the 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 the screens contact the walls separating the rooms, the mass flow rate of smoke entering the layer forming the ceiling space of the atrium can be found. Results of calculations of smoke production due to entrainment into the rising plume are shown graphically in Figures 27–58. Heat outputs of 1 MW and 6 MW are considered. Downstand fascia depths of 0 m, 0.5 m, 1.0 m and 1.5 m are used with an overall compartment height of around 4 m. Opening widths or channelling screen separations of 5 m, 10 m, 20 m and 40 m are shown for both cellular and open-plan offices. A choice is provided for either adhered or free plumes. The results given in Figures 27–58 are representative of typical room/atrium geometries, with a nominal slabto-slab height of 4 m. In practice the room opening geometry, the presence and absence of a deep downstand fascia and a balcony beyond, and different floor-to-floor heights will affect the mass flow rate of smoke. For example, some atria may have upper levels set back above the 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 the room opening W is identical with the line plume length L. Where such rooms have a downstand fascia, the plume rise (hb) should be measured from the bottom of the downstand. Figures 27–58 can again be used to estimate the entrainment into the plume, but a more precise calculation for this case is feasible, using the procedures of Appendix B. This fire engineering approach is of necessity more complicated and needs individual consideration. An alternative method of calculating the entrainment into the line plume is due to Thomas40. This treats the plume in a ‘far plume’ approximation apparently rising from a line source of zero thickness some distance below the void edge. The relevant formula is: 1/3

2/3

 gQw L2   0.22(hb + 2∆) Ml = 0.58ρ   (hb + ∆) 1 +  ...(12)  ρ c To   L  where Ml = Mass flow of smoky gases entering the smoke layer at height hb (kgs21) ρ

= Density of warm gases at height hb (kgm23)

Qw = Convective heat flux in gases (kW)

24

L = Length of void edge past which gases spill (m) c

= Specific heat of air (kJkg21K21)

To = Absolute ambient temperature (K) ∆ = Empirical height of virtual source below void edge (m) hb = Height of rise of thermal plume above void edge (m) It should be realised that the derivation of Equation 12 limits its application to scenarios where smoky gases issue directly from the compartment on fire, with a balcony projecting beyond. With appropriate changes to the value of ∆ to cater for changes in room/opening geometry, and hence the mass flow under the balcony (see the second and third sections of Chapter 3), the Equation 12 and Figures 43–58 should give broadly similar results, since although both methods use different empirical approaches, these constants are obtained by fitting to the same data36. It should also be realised that Equation 12 only describes a free or double-sided plume and cannot be adopted for an adhered or single-sided plume. For a free plume, the appropriate changes to ∆ may be inferred by inspection of the graphs and comparison with the calculated results, eg for a free plume from an openplan office with a 1.0 m downstand, ∆ = 0.83H. An analysis by Morgan41 for a shopping centre geometry suggests that for practical purposes it can be taken that ∆ = 0.3 times the height of the compartment opening H, ie ∆ = 0.3H, but note that this result strictly applies for the specific experimental geometry and for the entrainment above the electric convector heaters in the ‘fire compartment’ used to simulate the fire. Recent research37 has suggested that the entrainment into a high-temperature spill plume might be lower than into a thermal plume. The effect is not sufficiently well studied to allow quantitative advice to be given, beyond the statement that the effect becomes apparent for values of UB (or of Uw where the compartment opening has no projecting canopy) greater than approximately 300 °C. Where the entrained mass is the critical design parameter (eg for estimating the capacity of powered smoke exhaust ventilators) it is recommended that the same calculation procedures be followed as for lower temperature thermal plumes, since this will result in an overestimate of the exhaust capacity and an underestimate of the smokereservoir’s layer temperature — giving an extra margin of safety. It is expected that all the calculation procedures for spill plumes described in this Report will give sufficiently erroneous results for flame plumes (eg typically for UB (or Uw) greater than about 550 °C) that they should not be employed. At the time of writing it is not clear what calculation procedures can be adopted for flame spill plumes.

Figure 27 Adhered plume from open-plan sprinklered office Heat output: 1 MW Downstand depth at opening: 0 m

Figure 29 Adhered plume from open-plan sprinklered office Heat output: 1 MW Downstand depth at opening: 1.0 m

Figure 28 Adhered plume from open-plan sprinklered office Heat output: 1 MW Downstand depth at opening: 0.5 m

Figure 30 Adhered plume from open-plan sprinklered office Heat output: 1 MW Downstand depth at opening: 1.5 m

25

Figure 31 Adhered plume from open-plan unsprinklered office Heat output: 6 MW Downstand depth at opening: 0 m

Figure 33 Adhered plume from open-plan unsprinklered office Heat output: 6 MW Downstand depth at opening: 1.0 m

Figure 32 Adhered plume from open-plan unsprinklered office Heat output: 6 MW Downstand depth at opening: 0.5 m

Figure 34 Adhered plume from open-plan unsprinklered office Heat output: 6 MW Downstand depth at opening: 1.5 m

26

Figure 35 Adhered plume from cellular sprinklered office Heat output: 1 MW Downstand depth at opening: 0 m

Figure 37 Adhered plume from cellular sprinklered office Heat output: 1 MW Downstand depth at opening: 1.0 m

Figure 36 Adhered plume from cellular sprinklered office Heat output: 1 MW Downstand depth at opening: 0.5 m

Figure 38 Adhered plume from cellular sprinklered office Heat output: 1 MW Downstand depth at opening: 1.5 m

27

Figure 39 Adhered plume from cellular unsprinklered office Heat output: 6 MW Downstand depth at opening: 0 m

Figure 41 Adhered plume from cellular unsprinklered office Heat output: 6 MW Downstand depth at opening: 1.0 m

Figure 40 Adhered plume from cellular unsprinklered office Heat output: 6 MW Downstand depth at opening: 0.5 m

Figure 42 Adhered plume from cellular unsprinklered office Heat output: 6 MW Downstand depth at opening: 1.5 m

28

Figure 43 Free plume from open-plan sprinklered office Heat output: 1 MW Downstand depth at opening: 0 m

Figure 45 Free plume from open-plan sprinklered office Heat output: 1 MW Downstand depth at opening: 1.0 m

Figure 44 Free plume from open-plan sprinklered office Heat output: 1 MW Downstand depth at opening: 0.5 m

Figure 46 Free plume from open-plan sprinklered office Heat output: 1 MW Downstand depth at opening: 1.5 m

29

Figure 47 Free plume from open-plan unsprinklered office Heat output: 6 MW Downstand depth at opening: 0 m

Figure 49 Free plume from open-plan unsprinklered office Heat output: 6 MW Downstand depth at opening: 1.0 m

Figure 48 Free plume from open-plan unsprinklered office Heat output: 6 MW Downstand depth at opening: 0.5 m

Figure 50 Free plume from open-plan unsprinklered office Heat output: 6 MW Downstand depth at opening: 1.5 m

30

Figure 51 Free plume from cellular sprinklered office Heat output: 1 MW Downstand depth at opening: 0 m

Figure 53 Free plume from cellular sprinklered office Heat output: 1 MW Downstand depth at opening: 1.0 m

Figure 52 Free plume from cellular sprinklered office Heat output: 1 MW Downstand depth at opening: 0.5 m

Figure 54 Free plume from cellular sprinklered office Heat output: 1 MW Downstand depth at opening: 1.5 m

31

Figure 55 Free plume from cellular unsprinklered office Heat output: 6 MW Downstand depth at opening: 0 m

Figure 57 Free plume from cellular unsprinklered office Heat output: 6 MW Downstand depth at opening: 1.0 m

Figure 56 Free plume from cellular unsprinklered office Heat output: 6 MW Downstand depth at opening: 0.5 m

Figure 58 Free plume from cellular unsprinklered office Heat output: 6 MW Downstand depth at opening: 1.5 m

32

Fires on the atrium floor This relatively simple case can be treated in the same way as a fire in a single-storey space, where the plume can rise unhindered from the fire directly into the base of the layer. The design fire can be specified in terms of area (Af) and perimeter (P), based on expert assessment of the fire load at the atrium floor (which can vary from trees to cars, from furniture to exhibitions). If known, the calorific value of the likely fuel can be used to estimate the heat flux in the rising gases (Qf). Examples of known heat fluxes may be: (a) A group of four easy chairs clustered together, forming a perimeter of around 6 m, with a heat flux of 2 MW. (b) A sprinklered office environment (providing the sprinklers can operate over the fire area) with a total convective heat flux (qf) of about 115 kWm22 of fire. Note: if the atrium ceiling is high, special provisions may have to be made to ensure effective sprinkler operation. (c) An unsprinklered office environment with a total convective heat flux (qf) of about 185 kWm22 of fire. (d) A vehicle (car) with a fire perimeter of 12 m and a total convective heat flux (qf) of 2.5 MW.

2

 AvCv    1/2 2 Ml Tl 1  AiCi  Τl Το  AvCv =   ρo  2g DB UlTo 

...(13)

where Av = Measured throat area of ventilators (m2) Ai = Total area of all inlets (m2) Cv = Coefficient of discharge (usually between 0.5 and 0.7) Ci = Entry coefficient for inlets (typically about 0.6) Ml = Mass flow rate of smoke to be extracted (kgs21) ρo = Ambient air density (kgm23) g

= Acceleration due to gravity (ms22)

DB = Depth of smoke beneath ventilator (m) Ul = Temperature rise of smoke layer above ambient (°C) Tl = Absolute temperature of smoke layer (K) To = Absolute temperature of ambient air (K)

If the heat flux is not known for the predicted fuel load, a convective heat flux (qf) of 0.5 MW per m2 of fire area is a usefully pessimistic rule of thumb covering many cases. The mass flow rate in the plume as it enters the smoke layer may be established from Figure 59 or Equation 1. This procedure can be used for Y < 10.0 =Af. For larger values of Y it would be better to seek specialist advice on the use of ‘small fire’ plume theories.

Throughflow ventilation — area of natural ventilation required A natural ventilation system uses the buoyancy of the smoke to provide the driving force for extraction. The rate of extraction is largely dependent upon the depth and temperature of the smoke. The advantage of a natural ventilation system is that it is very simple and reliable, and can cope with a wide range of fire conditions. Should for any reason the fire grow larger than the design fire size, a greater depth and temperature of smoke leads to an increased extraction rate, so to an extent a natural ventilation system has a self-compensating mechanism. The precise relationship between the mass flow rate extracted, the ventilator area, the inlet area and the smoke layer is42: Figure 59 Rate of production of hot smoky gases from a fire on the atrium floor

33

Table 5 gives the minimum free area of ventilation required, ignoring the effect of any inlet restriction (ie assuming an infinite area of inlet ventilation).

overpressure and suction have been identified for all possible wind directions, design of ventilators or fans can proceed as before.

The effect of limited fresh air inlets can be allowed for, using the following approximations:

A powered extract system should be used where positive wind pressures are likely to be a problem, or where it is necessary to extract smoke via an extensive ductwork system.

●

If the inlet area to the atrium is twice the exhaust ventilation area given by Table 5, the indicated ventilation area and the inlet area should both be increased by approximately 10%.

●

If the inlet area is equal to the exhaust ventilation area, the indicated ventilation area and the inlet area should both be increased by approximately 35%.

●

If the inlet area is half the exhaust ventilation area, the indicated ventilation area and the inlet area should both be increased by approximately 125%.

When natural ventilators are used for smoke extraction, it is important that they are positioned where they will not be adversely affected by external wind conditions. A positive wind pressure can be much greater than the pressure head developed by a smoke layer. Should this occur the ventilator may act as an inlet rather than as an extract. However, if sited in an area of negative wind pressure, the resultant suction force on a natural ventilator would assist smoke extraction. Tall buildings or taller areas of the same building (such as rooftop plant rooms, etc) can create a positive wind pressure on nearby lower roofs. Steeply pitched roofs, ie roofs over 30° pitch, may also have a positive wind pressure on the windward slope.

Table 5 Minimum total ventilation area (m2) needed for a smoke reservoir (from Equation 13 with Cv = 0.6) (a) Q = 1 MW Mass flow rate (exhaust rate) (kgs21) 1.5

Smoke depth beneath ventilators (m) 2

3

4

5

7

10

4 6 8 10 12

2.1 3.2 4.5 5.9 7.4

1.8 2.8 3.9 5.1 6.4

1.5 2.3 3.2 4.1 5.2

1.3 2.0 2.7 3.6 4.5

1.1 1.7 2.4 3.2 4.0

1.0 1.5 2.1 2.7 3.4

0.8 1.2 1.7 2.3 2.9

15 20 25 30 35

9.9 14.5 19.6 25.3 31.4

8.5 12.5 17.0 21.9 27.2

7.0 10.2 13.9 17.9 22.2

6.0 8.9 12.0 15.5 19.2

5.4 7.9 10.8 13.9 17.2

4.6 6.7 9.1 11.7 14.5

3.8 5.6 7.6 9.8 12.2

40 50 60

37.9 52.2 67.9

32.9 45.2 58.8

26.8 36.9 48.0

23.2 32.0 41.5

20.8 28.6 37.2

17.6 24.2 31.4

14.7 20.2 26.3

(b) Q = 6 MW A suggestion sometimes advanced for offsetting wind overpressure is to increase the total area of natural ventilation per reservoir. Since the overpressure is, by definition, force per unit area, this will usually not work and indeed could exacerbate the problem by allowing even greater quantities of air to be driven through the ventilator to mix into the smoke. In some cases it may be possible to retain natural ventilation openings in a vertical plane by arranging them to face inwards to either a region sheltered from wind action, or where the wind will always produce a suction. In other cases the erection of suitably designed screens or wind baffles (outside the vertical wall or window holding the ventilators) can overcome wind interference and may even be able to convert an overpressure into a suction. There is also the possiblity of selectively opening ventilators in response to signals from a wind direction sensor. Expert advice should be sought for such designs. Due to the complexity of wind-induced air flow over some atrium buildings and the surrounding buildings, it may sometimes be desirable to carry out boundary layer wind tunnel studies to establish the wind pressure over the building’s envelope. Once areas of 34

Mass flow rate (exhaust rate) (kgs21) 1.5

Smoke depth beneath ventilators (m) 2

3

4

5

7

10

10 12 15 20 25

5.5 6.4 7.8 10.2 12.9

4.7 5.5 6.7 8.9 11.1

3.9 4.5 5.5 7.2 9.2

3.3 3.9 4.8 6.3 7.9

3.0 3.5 4.3 5.6 7.0

2.5 2.9 3.6 4.7 6.0

2.1 2.5 3.0 4.0 5.0

30 35 40 50 60

15.6 18.6 21.6 28.2 35.2

13.5 16.1 18.7 24.4 30.5

11.1 13.1 15.3 19.9 24.9

9.6 11.4 13.2 17.2 21.6

8.6 10.2 11.8 15.4 19.3

7.2 8.6 10.0 13.0 16.3

6.1 7.2 8.4 10.9 13.6

75 90 110 130 150

46.7 59.2 77.2 96.8 117.8

40.4 51.2 66.9 83.8 102.0

33.0 41.8 54.6 68.5 83.3

28.6 36.2 47.3 59.3 72.1

25.6 32.4 42.3 53.0 64.5

21.6 27.4 35.8 44.8 54.5

18.1 22.9 29.9 37.5 45.6

200 300 400

175.9 313.1 474.2

152.3 271.1 410.7

124.4 221.4 335.3

107.7 191.7 290.4

96.3 171.5 259.7

81.4 144.9 219.5

68.1 121.2 183.7

Note: To account for the restriction imposed by the inlet area, add the following to both inlet and exhaust ventilation areas: 10% if the inlet area is twice the ventilation area, 35% if the inlet area is equal to the ventilation area, and 125% if the inlet area is half the ventilation area.

Throughflow ventilation — remaining design procedures Other design calculations are essentially the same as described in the previous chapter, eg mean smoke layer temperature (page 14), flowing layer depth (page 16), inlet air (page 17), minimum number of extraction points (page 18) and required ventilation rate of powered exhaust ventilators (page 19).

Limitations to the use of throughflow ventilation As may be seen from the graphs in Figures 27–58, the mass flow rate generated by the entrainment into the rising plume is very large, and hence the plume cools quickly with height. This large increase in mass flow with increases in height tends to suggest there may be some cut-off point in the rise of the plume, above which it might become economically impracticable in terms of a smoke control system. Experience suggests that this is often true for flows larger than 150–200 kgs21. Another effective limit may occur if the temperature of the smoky gas layer forming in the roof void is too low. If internal day-to-day heat gains (solar, plant, etc) are allowed to accumulate within the atrium roofspace (eg passive solar atria) then high-level air temperatures within the atrium may be very high. Roofspace temperatures have been recorded at or above 50 °C. Smoke spreading into an atrium during the incipient stages of a fire will naturally be very cool, and the entrainment processes will draw in the surrounding ambient air as the plume rises. In most instances this ambient air will be at or near 20 °C ( due either to ventilation or air-conditioning), producing a plume temperature which may be considerably lower than the air within the roofspace. Unless the hot air can be removed sufficiently quickly, it will result in the initial smoke layer forming at a point lower down in the building than may be desirable. This process is known as early (or premature) stratification (Figure 60). As the fire is probably growing, the plume temperature will progressively rise with time. This may result in hotter smoke ‘punching’ its way through the cooler smoke layer and forming another warmer layer above. The process may continue until the smoke ‘strata’ have become sufficiently mixed to rise up as a single bulk of

smoke. This problem of early stratification can to some extent be overcome by providing smoke detectors at many heights within the atrium or located to ensure detection of smoke close to the fire. Once a forming smoke stratum is detected and the smoke ventilation system caused to operate, the hottest (and therefore highest) gases will be removed first, allowing any cooler strata to rise to take their place. Hence smoky gases will reach the ventilators and the smoke ventilation system should settle into its ‘design’ state. The timescale for this process is uncertain and hence early detection of smoke in these circumstances is essential. A further problem which could be encountered may be more problematical during cooler weather. Atria with large areas of external glazing will present a large surface area to the smoke layer, which can lead to considerable heat losses from it. Most throughflow smoke control systems are designed with an arbitrary limitation to the ceiling reservoir of between 1000–3000 m2 (see for example References 16, 36 and 42), one reason being to prevent excessive energy loss from the buoyant smoke layer. Many atria cannot physically or architecturally adopt such reservoir formations and, if larger than the areas mentioned above, will cause additional energy to be lost from the layer. This energy loss will increase with distance, the further the smoke 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 deepen beyond the desired design depth, perhaps considerably so. Cool smoke will also be sensitive to airflow movements, such as air currents (draughts) due to ventilation, air conditioning or weather conditions. Furthermore, experimental evidence37 has shown that excessive air movement (such as that which may occur due to an arbitrary air change rate) into a cool but otherwise stable smoke layer can cause it to become unstable, spreading further throughout the building. The formation of a smoke layer depends upon buoyancy for the maintenance of stability. Smoke layers which have temperatures (and hence densities) approaching that of the incoming replacement air supply will have a tendency to ‘mix’ with this air, rather than ‘float’ above it. This process is known as dilution ventilation and is frequently used in industry to reduce contamination levels in buildings (eg welding shops).

Figure 60 Early (or premature) stratification

35

The mechanisms involved in dilution ventilation can easily induce downward mixing of a smoke layer to the extent that, with sufficient air movement, complete smoke-logging of an atrium can occur. It follows therefore that the atrium smoke layer should be at a temperature compatible with stable stratification. There is little information available on the destabilisation of cool buoyant layers, so a precise limiting temperature beyond which the above effects will lessen cannot be given. Further research is desirable in this area. Experience and experimental observation however indicate that these effects may be severe in terms of smoke control, perhaps leading to smoke spreading to otherwise unaffected escape routes. In the absence of the necessary experimental data, as a result of practical experience this Report will adopt a temperature band of between 15–20 °C above ambient as the critical layer temperature below which undesirable effects may occur. This temperature rise should be regarded as that which the layer will have after suffering heat losses to the structure containing it (see Chapter 7). The practical limitations to the use of throughflow ventilation are therefore a maximum mass flow rate of 150–200 kgs21 and/or a minimum smoke layer temperature of 15–20 °C above ambient. Which limit is reached first will depend upon the situation being considered, ie on the type of fire, the construction of the compartment, the geometry of the atrium, etc. Experience (and Figures 27–58) suggests that one or other limit is usually reached when the height of rise above the fire room opening exceeds 8–12 m. It follows that it does not usually appear to be practicable to design a throughflow ventilation system requiring more than three to four storeys (sometimes less) to be kept free of smoke, regardless of whether it is powered or natural smoke ventilation. This limitation would appear to pose a serious threat to the design of interesting atria, and indeed building codes and fire safety standards in the USA are apparently reflecting this43. The large, open atrium designs for which American architects have become renowned are no longer widely regarded as acceptable, and new atrium designs are sometimes restricted to a maximum of three open (non fire-separated) floor levels, one of which must be the ground floor of the atrium. However, various methods exist whereby this limitation may be overcome to a certain extent.

36

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. Pauls44 has shown that escape periods in multi-storey buildings can vary from 10 minutes for a 15-storey building, where escapees were ‘caught’ on 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. It 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.

Compartment 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. In 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 façade should be well sealed. If the gases in the atrium become hot, as they often may locally to the fire, the façade 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 on 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 façade need not be sealed, but can be allowed to be leaky, even if the upper atrium is filled with smoke. Examples of such ‘leaky façade’ 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 on 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

Figure 61 Smoke-logging in a ‘leaky’ closed atrium

much quicker if all accommodation were kept clear of smoke. Hence smoke must be prevented from passing in appreciable quantities through these small leakage openings. One way of achieving this may be by depressurising the atrium45,46. Natural depressurisation In any structure with natural ventilation openings at high and low level, and with a quantity of heat trapped inside, a ventilation rate will be created due to the ‘stack effect’. In order for air to move out through the high-level opening, the pressure at high level inside must be greater than the external pressure otherwise there would be no air movement. Similarly, for air to flow inwards at low level the pressure at low level inside must be less than that outside. Thus there must be a position within the structure where the pressure inside is equal to that outside. This is known as the ‘neutral pressure plane’ (NPP). Any openings situated at the neutral pressure plane will have no airflow through them, as there will be no pressure differential at that point. In buildings where a throughflow ventilation system is installed, where the inlet area is equal to the exhaust vent area, then the neutral pressure plane will exist approximately midway within the smoke layer (Figure 62). If the inlet vent area is smaller than the exhaust vent area, then the neutral pressure plane will move upwards (Figure 63). Any openings above the neutral pressure plane will be

under a positive pressure (defined positive outwards from the atrium). Thus there will be a flow of smoke from the atrium into rooms above the neutral pressure plane through any leakage path which may exist. However, careful manipulation of the neutral pressure plane can raise it to a safe height above sensitive levels, where there is little or no threat from the positive pressure above (Figure 64). The pressure in the atrium below the neutral pressure plane will be at a pressure lower than ambient, thus any airflow will be from the room into the atrium. Hence the levels below the neutral pressure plane are protected from heat and smoke contamination. Appendix A gives a description of a fire that occurred in the IMF Building in Washington. It started on the tenth floor of a 13-storey atrium, and by the time the fire service arrived (16 minutes later) the smoke level had descended below the tenth floor. An 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 apparent failure of the venting system was attributed to the use of natural ventilation in a ‘tall’ building, where the smoke had insufficient buoyancy to reach the vents. However, the fire occurred on the tenth floor, and for all practical purposes, when the fire broke out it was effectively in a three-storey building with a deep basement. Natural ventilation works extremely well in ‘shallow’ buildings, and therefore there must have been some other mechanism in action affecting the operation of the ventilation system.

Figure 62 Neutral pressure plane — throughflow ventilation

38

Figure 63 Neutral pressure plane — vent larger than inlet

The atrium had no apparent inlet facility and accordingly, instead of the ventilators providing a throughflow ventilation effect, the atrium became depressurised in the manner described above. This in turn prevented smoke from spreading beyond the atrium, despite being smoke-logged to ground-floor level at one stage. The neutral pressure plane will lie somewhere within the depth of the smoke layer in the atrium depending upon factors such as inlet/vent area ratio, gas temperatures, wind pressures, etc. It is not, and should not be confused with, the actual base of the smoke layer. The equation describing the above relationship, in the absence of wind effects is45,46: (AvCv)2 (AiCi)2

where

=

Tl Dmax  To  –1  X 

...(14)

X = The height from the base of the smoke layer to the desired position of the NPP (m) Dmax = Maximum depth of smoke layer from the centre line of the exhaust ventilator (m)

This equation is represented graphically in Figure 65.

Equation 14 represents the condition where the atrium has a single, dominant inlet leakage path from the exterior (eg access doors) but smaller leakage paths between the atrium and accommodation and the exterior (Figure 66). With the technique as described above it is quite possible for the atrium to be entirely filled with smoke (see Appendix A — IMF Building), in which case Dmax will approach the height of the atrium (Ha), eg Dmax → Ha. It is a straightforward task to calculate the ventilation requirements for a ‘pure’ depressurisation system using Equation 14 or Figure 65 where the smoke layer temperature is known or can be determined as shown in Chapter 6. If the neutral pressure plane were to descend below the desired design depth then some of the higher storeys may become endangered. This can arise from an increase in the actual inlet leakage area available, for example, where the fire brigade have opened access doors to the atrium to investigate the severity of the fire. A successful depressurisation design should be able to prevent smoke infiltration into adjacent spaces on the higher floors even in this condition. In addition it is possible that the fire may cause windows to break on both the external façade and the atrium façade of the fire room. In this case the broken areas can act as a ‘dominant’ leakage path from the exterior.

Figure 64 Neutral pressure plane above highest ‘leaky’ storey

39

storeys above the layer’s base the same depressurisation principle can be employed, but a more complicated ‘flow network’ calculation must be used. This is best left to specialists in the field. It is difficult to give a simple general rule to identify when a building can be regarded as having a single dominant inlet. Nevertheless, it may be sufficient to adopt a guideline from the related field of ‘air infiltration’, so that one can assume a dominant inlet if the total area of all openings below the layer base is more than twice the total area of all openings above the layer base (excluding the area of the ventilators themselves)47. Natural depressurisation and wind effects The neutral pressure plane is sensitive to the effects of wind, and ‘adverse’ wind pressures might cause the NPP to fall to a lower position on the leeward side of the building, possibly contaminating the topmost leeward storeys. It follows that the depressurisation design procedure must take wind force into account. To assess the efficiency of operation of a depressurisation system a knowledge of the wind pressure coefficients acting upon a building will be necessary. These are a well established way of relating the wind pressure anywhere on a building to the wind velocity at roof level.

Figure 65 Solution to neutral pressure plane equation (14)

Thus all potential inlet leakage paths must be assessed when using Equation 14 or Figure 65. It should be noted that the simple approach set out here will be invalid where the leakage paths across the atrium boundary have appreciable areas on several storeys (although all leakage areas below the smoke layer’s base can be aggregated and regarded as being at the layer’s base for calculation purposes when using Equation 14 or Figure 65). Where there are appreciable significant leakage paths on several

Wind pressure coefficients have often been measured so that structural wind-loading can be calculated. There is a considerable body of data in existence. Where complete certainty is required for a novel or complicated building, wind-tunnel observations using scale models will yield usable results. In general however it should often be possible to obtain reasonable values for the wind pressure coefficients needed for smoke control calculations from available literature (see Reference 47 for example). Figure 67(a) shows the typical three-dimensional complicated pattern of wind pressure coefficients over a tall tower block48. In practice it would be necessary to identify the most pessimistic values for each storey, in which case the problem can be simplified to twodimensional as shown in Figure 67(b).

Figure 66 Neutral pressure plane — dominant inlet

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Providing the requirements of Equation 15 are satisfied then natural ventilation will work at all wind speeds. This implies that the roof ventilation system should be subjected to suction wind pressures at all times. However, if it is impossible to employ a natural ventilator on a particular building, fans can be used instead.

(a) Three-dimensional distribution for typical tower block

Powered depressurisation The necessary capacity is a little harder to calculate, and the best fan is one which is not affected by wind pressures on its exhaust. With a fan however, a maximum wind speed must always be assumed for design purposes. The required volumetric flow rate may be calculated from45: 2 Tl AiCi  (Cpi – CpL) v wind + 2g U l X  Vl =      Το   Tl 

where Vl (b) Two-dimensional distribution for typical tower block

1/2

...(17)

= Fan capacity required (m3s21)

vwind = Design wind velocity (ms21) A natural smoke control system will be affected by the wind pressures operating against all the openings in the structure; thus pressure differentials vary with wind direction and opening position, and the throughflow of air will vary with wind velocity. However when the hole in the roof is replaced by a fan, the pressure differentials within the building now have to be changed by mechanically altering the throughflow of air. Therefore the system must be designed with a maximum design wind velocity to cater for all conditions.

Figure 67 Wind pressure coefficients around buildings

With these data established for any specific building, the design procedure for checking on the performance of a natural depressurisation system is fairly simple where there is a single dominant opening. To prevent smoke leakage into the top leeward storeys for all wind speeds45: 

( A – 1 ) Cpv – A CpL + Cpi 

  

≤ 0

Further sophistication may be achieved by the use of an anemometer and by having ‘groups’ of fans, each group operating at a different wind velocity. So if the wind was light, one group would operate and, if the wind speed increased, further groups might be activated as necessary.

...(15)

where Cpv = Wind pressure coefficient at the vent CpL = Wind pressure coefficient at the topmost leeward storey of the building Cpi = Wind pressure coefficient at the inlet 2

and

A=

To AvCv    +1 Τl AiCi 

...(16)

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Chapter 6 Depressurisation/smoke ventilation hybrid designs Principles

Hybrid designs usually follow one of two approaches:

In Chapters 2 to 4 we have indicated how smoke ventilation can only keep a limited number of lower storeys clear of smoke below the buoyant smoke layer formed in the atrium. The technique does in principle, however, allow those lower storeys to have adjacent spaces — and their escape routes — open to the atrium. The 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 façade immersed in the smoke layer.

1 Mass flow based, where the atrium is designed with a number of open levels above the atrium floor, requiring a plume of a specific height. The maximum number of levels will be determined by either the magnitude of the mass flow rate entering the layer, or the smoke layer temperature falling below the minimum value of 15–20 °C (see Chapter 4).

Depressurisation does not however protect any large leakage openings on any storey above the layer base in the atrium, nor will it protect any escape routes on that storey open to the atrium. In this context a large opening is one where the opening in the atrium façade is larger than the sum of openings further along the same leakage path away from the atrium (eg if the atrium façade opening is larger than openings in the external wall). But it will often be the case that architects will want to maximise use of the atrium space, and an obvious way is to combine the smoke ventilation approach of Chapters 2 to 4, allowing greater freedom of design on the lowest storeys, with the lesser freedom of ‘leaky façades’ allowed by the depressurisation technique set out in Chapter 5. In this ‘hybrid’ design the ratio of vent area to fresh-air inlet area will be determined by Equation 15, whereas the actual values of these areas must be consistent with the necessary smoke extraction requirement as defined in the two sections on throughflow ventilation design in Chapter 4 (pages 33 and 35). It should be appreciated that in such a hybrid design the smoke layer temperature in the atrium required for the depressurisation calculations is a natural outcome of the plume entrainment calculations needed for the smoke extract calculation. Note that hybrid designs are similarly possible where powered ventilators are used for atrium smoke exhaust.

2 Temperature based, in order to cool a potentially hot smoke layer by the deliberate entrainment of ambient air into the rising plume. This may enable the use of façade materials that cannot withstand high temperatures (eg float glass).

Design procedures for hybrid systems Mass flow based systems (see Figure 68) (a) Determine the height of rise of the smoke plume required to clear the open levels (hb), with the design fire (from Chapter 2) chosen on the lowest open level. This will also yield the smoke layer depth (D), measured from the centre line of the ventilator. (b) From Figures 27–58 or by detailed calculation and with the desired channelling screen separation (L) or opening width (W), determine the mass flow rate (Ml) entering the base of the layer. If the fire is on the atrium floor, determine Ml using the section ‘Fires on the atrium floor’ on page 33. (c) Calculate the total surface area of the smoke layer (the atrium surface area in contact with the smoke layer plus the area of the layer base), and determine the likely smoke layer temperature, using Chapter 7. If the smoke layer temperature is below 15–20 °C above ambient then the number of open levels may need to be reconsidered, or some (or all) of the lower levels vented independently from the atrium, using the procedures set out in Chapter 3.

Figure 68 Principles of hybrid smoke ventilation system — mass flow based

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(d) Set the neutral pressure plane height (X) to that required above the base of the smoke layer, and determine the value of (AvCv /AiCi)2 from Equation 14 or Figure 65. (e) With these values of (AvCv /AiCi)2, DB, Ml and Ul calculate the ventilation area required from Equation 13. (f) With the values of (AvCv /AiCi)2 and AvCv calculate the quantity of inlet ventilation required. In the event that the actual inlet area available is greater than that required by calculation, then the ventilation area should be increased to maintain the ratio of (AvCv /AiCi). (g) Using Equations 15 and 16 and the appropriate wind pressure coefficients, check the system operation with regard to wind effects.

(c) From Figures 27–58 or by detailed calculation and with the channelling screen separation (L) or opening width (W), determine the height of rise (hb) to the base of the layer, necessary to give the required mass flow rate. (d) With the design fire at the lowest level and taking into account the necessary height of rise (hb) for cooling purposes, determine the maximum smoke layer depth (Dmax). Set the neutral pressure plane height (X) to that required above the base of this smoke layer depth, and determine the value of (AvCv /AiCi)2 from Equation 14 or Figure 65. (e) With the required value of hb, determine the shallowest smoke layer depth (DB), compatible with the depressurisation concept (this is often the second level beneath the NPP).

(h) In the event that the wind effects may adversely affect the operation of a natural ventilation system, calculate the fan capacity required using Equation 17, with an appropriate value of design wind velocity.

(f) With this value of (AvCv /AiCi)2, DB, Ml and Ul calculate the ventilation area required from Equation 13. In the event that the actual inlet area available is greater than that required by calculation, then the ventilation area should be increased to maintain the ratio of (AvCv /AiCi).

(i) Check that the anticipated suction pressure and/or air inflow velocities do not in themselves endanger the safe use of any escape routes away from the atrium (see the section ‘Inlet air’ on page 17).

(g) Using Equations 15 and 16 and the appropriate wind pressure coefficients, check the system operation with regard to wind effects.

Temperature based systems (see Figure 69) (a) Decide upon a smoke layer temperature rise (Ul) compatible with the façade material employed. For float glass a temperature rise of 70 °C above ambient will give a reasonable safety margin to the system design. Toughened glass may be capable of withstanding higher temperature rises (eg 200 °C). (b) Calculate the total surface area of the smoke layer (the atrium surface area in contact with the smoke layer plus the area of the layer base), and determine the mass flow rate required to give the desired temperature rise, using Chapter 7. As a simplification incorporating a margin of safety, this step can be omitted and the mass flow rate calculated using Equation 6.

(h) In the event that the wind effects may adversely affect the operation of a natural ventilation system, calculate the fan capacity required using Equation 17, with the appropriate value of design wind velocity. (i) Check that the anticipated suction pressure and/or air inflow velocities do not in themselves endanger the safe use of any escape routes away from the atrium (see the section ‘Inlet air’ on page 17).

Figure 69 Principles of hybrid smoke ventilation system — temperature based

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Chapter 7 Atrium smoke layer temperature In most smoke control system designs no account is taken of the heat losses to the structure. It is assumed there is conservation of heat and that all of the heat flux entering a smoke reservoir is contained in, and remains in, the smoke. Experimental work in the past has shown that for relatively small smoke reservoirs with medium to high thermal resistance, or for high mass flow rates of smoke, this assumption holds good. When considering atria however, the assumption can no longer be considered entirely valid. An atrium generally has a large surface area, which is predominantly glazed in most cases, thus providing a good heat sink. There will be a passage of heat energy from the smoke layer into the structure, and accordingly the smoke layer will suffer a reduction in temperature. Figure 70 shows the heat balance in an atrium. This model was used to determine the loss of energy from the smoke layer, based upon ‘worst case’ assumptions for the façade46. The façade fabric is assumed to be thin glazing, with no apparent delay in the transfer of the energy from the layer. The results of using this model are shown graphically in Figures 71, 72 and 73 for 1 MW, 5 MW and 6 MW fires respectively. As can be seen from the graphs — despite the fact that many differing atrium geometries were considered, with different values of external exposure — the resultant calculation points may be plotted comfortably as single curves for each value of mass flow rate. At high values of mass flow rate there is little change in the atrium smoke layer temperature for wide variations in smoke layer surface area. This is due to the gas flow being the prime mover of energy, and tends to justify the assumption that loss of heat to the structure of a building may be ignored for relatively small contact areas.

Figure 70 Heat balance in an atrium

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Figure 71 The atrium layer temperature (Q = 1 MW)

Figure 72 The atrium layer temperature (Q = 5 MW)

Figure 73 The atrium layer temperature (Q = 6 MW)

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Chapter 8 Additional design factors Atrium roof-mounted sprinkler systems Conventional sprinklers mounted in the roof of an atrium will only be of potential benefit if there is a fire in the atrium floor itself. However, due to the height that these sprinklers will generally be mounted above the floor, the fire will be of considerable proportions before the sprinklers activate and they will therefore be of limited benefit. More importantly perhaps, if a smoke layer is just above the operating temperature of the sprinklers, it will be reasonably stable (Ul ≥ 50 °C). The action of all the roof sprinklers operating almost simultaneously (as can be inferred from work by Hinkley49) may rapidly cool the layer and cause it to become unstable. This can occur if a fire is not at the atrium floor, and is therefore highly undesirable. If there is a likelihood that a fire load will be present on the atrium floor, the smoke control system designed for a fire in an adjacent room will generally be able to cope with the products of a much larger fire directly under the roof17. Thus with good housekeeping and proper management to restrict the use of fuel assemblies, such a fire could feasibly exist and burn itself out before becoming a problem to other areas outside the atrium.

Smoke detection systems in the atrium The operation of any smoke control system in an atrium is generally dependent upon a smoke detector operating. If the atrium is particularly tall, stratification of the smoke layer before reaching the ceiling is probable — especially in atria which are airconditioned in the lower portion only, or have a high proportion of roof glazing (see the section ‘Limitations to the use of throughflow ventilation’ on page 35). Thus if the only detection system present is roofmounted it may not operate, or at least its operation may be considerably delayed. This problem can be overcome by the installation of smoke detection to the various rooms or by intermediate detection zones at different heights in the atrium, possibly using beam detectors.

Pressurisation of stairwells and lobbies In some atrium buildings there may be a requirement or desire to pressurise the escape stairs and associated lobbies. If the atrium employs a depressurisation or hybrid smoke control system and the glazing between the fire room and the atrium has cracked or shattered, the pressure within the fire room will, of necessity, be lower than the outside ambient pressure. This reduction in pressure will act as though an extract fan were fitted to the fire room, increasing the pressure differential developed across the escape lobby doors. 46

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 smoke into the escape route.

Air-conditioned atria In some hot countries it is common practice to totally air-condition an atrium, so that its internal ambient temperature is lower than the external ambient temperature. In this case the use of a natural ventilation system would cause a reverse stack effect (ie vents acting as inlets) and could cause problems of cool smoke spreading downwards towards the escape doors. Once the cool atrium air has been flushed out by the warmer ambient air entering through the vents, the system will reverse its direction of flow. This problem of an initial downward movement of smoke may be alleviated by the use of smoke detectors in the rooms, rather than the atrium, causing the ventilation system to operate and creating a balance between the internal and external temperatures prior to the smoke entering the atrium in quantity, or by using powered ventilators.

Channelling screens and hybrid systems It has been shown in Chapter 4 (‘Channelling screens’, page 22) that when smoke passes under a balcony to rise into the roof void above, the quantity of smoke entering the smoke layer in the rising smoke plume can be reduced by restricting the width of the plume as it passes the balcony edge, by the use of channelling screens. The need for plume width restriction is necessary for any smoke control design where a clear layer of air is required above a balcony projection beyond the fire room (eg for escape purposes), and so will apply to a hybrid smoke control system when the height of rise of the smoke plume is fixed by escape requirements (mass flow based systems). As described in the sub-section ‘Temperature based systems’ on page 43, an alternative use of a hybrid system is to cool the smoke layer for some purpose (eg to prevent glazing from cracking) by deliberate entrainment of air into the smoke plume (temperature based systems). When designing a natural ventilation system for this purpose a knowledge of the depth of the smoke layer in the atrium is necessary to calculate the vent area required. This in turn implies a knowledge of the height of rise of the smoke plume. Therefore an estimate of the plume width leaving the room is desirable to determine the height of rise required for cooling purposes, and this estimate should

be reasonably narrow (usually not more than 10–20 m). However, variations between the mass flow entering, and that being vented from, the atrium smoke layer with this type of hybrid system will be immediately compensated for by a change in the depth of the smoke layer. Thus the actual width of plume achieved is irrelevant to the satisfactory operation of the system. Hence there is no practical advantage in physically reducing the width of the rising plume for this form of hybrid system. Therefore in the design of temperature based hybrid systems, channelling screens are unnecessary.

Wind-sensing devices and natural depressurisation The sub-section ‘Natural depressurisation and wind effects’ on page 40 detailed the effects of wind pressures around 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 experience an adverse pressure effect (eg vertically mounted vents), in which case the ventilation system should be controlled by a wind direction indicator, and the required amount of ventilation should operate in a leeward zone only.

47

Appendix A Case history (based on Reference 9) International Monetary Fund Building, Washington DC Building

13-storey square-shaped reinforced concrete office building with penthouse, basement and 4-storey underground garage.

Atrium

A centrally situated enclosed courtyard created the atrium. The windows of the offices facing the atrium were of 6.35 mm plate glass.

Date of fire 13 May 1977.

Fire protection Two ventilation systems recirculated air at the top of the atrium, and at its base there was an air handling unit. Smoke detectors were provided at the fans of the air handling unit and were arranged to shut down the fans when the detectors activated. The units could be manually restarted and put on exhaust. The general office area was fed by penthouse 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 time of the fire. The roof of the atrium was made of clear plastic panels. Six custom-made smoke ventilators were provided in the atrium’s roof and comprised clear plastic panels on hinges equipped with springs and release mechanisms. The release device was operated by one smoke detector located in the atrium roof. Fusible links on individual ventilators were also fitted. Sprinklers were provided at roof level in the atrium, and the building was equipped with manual fire alarm points and hydrant valves on each floor.

The fire At 18.45 h a worker discovered a fire in a small office (3 m 3 4.6 m) on the tenth floor; a plan of this floor is shown in Figure A1. The fire brigade received the alarm at 19.01 h. On arrival firemen found fire venting from the office window into the atrium. The fire floor was hot and smoky and this, coupled with the fact that the fire involved an inner office, made locating of the

fire difficult. Thick black smoke issuing from the office had built down from the roof of the atrium to below the tenth floor. Although the smoke detector had operated, only two of the six smoke ventilators had opened. The other four had released but the springs had lost sufficient strength to open them fully. These units had to be manually opened from outside. Smoke however did not vent effectively and at one stage the atrium was completely smoke-logged. Smoke extractors could not be connected to the smoke ventilators and so firemen used large extractors pointed upward from the atrium ground floor to pull fresh air from the front doors and push smoke upward and out through the ventilators. No building engineering staff were available to advise firemen on the HVAC smoke purge capability until much later. It took 2–3 hours to finally remove the smoke from the atrium.

Conclusions 1 The fire was confined to the room of origin by the closed office door and the wall construction. 2 Windows facing the atrium above the fire floor were cracked by heat but fire and smoke had not penetrated other floors. 3 The temperature of the gas layer in the atrium was insufficient to activate the sprinklers in the atrium roof. 4 Due to an insufficiency of replacement air the existing ventilation system design was inappropriate for clearance of smoke from the atrium, and the ‘dilution’ ventilation approach used by the fire brigade took many hours to clear the smoke. 5 If this had been an atrium with balconies providing access to escape-ways, the smoke may well have caused serious escape problems from upper floors.

Figure A1 International Monetary Fund Building. Plan of the tenth floor, showing location of the office involved

48

6 Despite the fact there were unprotected openings on to the atrium, and that at one point the atrium was totally smoke-logged, smoke did not migrate to other parts of the building. This indicates that the existing ventilation arrangements apparently ‘depressurised’ the atrium.

Appendix B Users guide to BRE spill plume calculations Introduction The Fire Research Station has carried out a number of studies into the movement of smoke in buildings. Part of this work has resulted in the development of a theory by Morgan and Marshall36 to estimate the amount of air entrained into free (or double-sided) thermal spill plumes (see Figure 24(b)). This calculation method is important for smoke control design in that it enables the designer to calculate the required fan capacity or vent area for a smoke ventilation system for large undivided volume buildings (eg multilevel shopping malls and atria). A number of studies have since been carried out which have resulted in the modification of the original theory to include more recent work on thermallybuoyant horizontal flows17 and adhered (or attached, or wall, or single-sided) plumes17,37 (see Figure 24(a)). This appendix presents the modified theory in a form which the designer can follow more easily than in the earlier research papers. The calculations can be done using an electronic calculator having full scientific functions. This however may be time-consuming, particularly where the designer wishes to look at a number of geometries or conditions. The calculations can be incorporated in a computer program where frequent calculations are required. An alternative method to Figure B1 is given later in this appendix (page 53) in order to facilitate such programming.

Many of the variables used in equations in this appendix do not appear in the main body of the Report. To avoid unnecessary complications for the reader who does not wish to use this calculation procedure, the appendix is provided with a separate list of nomenclature (page 54).

Scenarios and assumptions The calculation method strictly only applies to fire scenarios where a horizontally-flowing thermallybuoyant layer of smoky gases approaches a void, through which those gases then rise. More specifically, the following assumptions are made: ●

This approach flow is assumed to be beneath a flat ceiling (or a downstand) at the edge of the void.

●

It is channelled by downstands (which may be either walls or channelling screens).

●

The flow has flow-lines which are everywhere parallel and which approach the edge of the void at a right angle.

●

The approach flow is assumed to be fully developed.

●

There is no immersed ceiling jet.

●

It is assumed that the velocity of the clear air below the smoke layer has a velocity much smaller than the velocity of the layer itself.

Figure B1 Graphical representation (from Reference 50) of the theoretical solution for a plume issuing from a restrained source (F < 1)

49

Fortunately these assumptions correspond to many practical scenarios of interest to designers. It should further be noted that experimental evidence37 suggests that the calculation procedure which is the subject of this guide should not be used for approach flow layer temperatures higher than about 350 °C. Accurate methods for higher temperatures do not yet exist. The present method significantly overpredicts the mixing of air into the rising hot gases for higher temperatures. In practice the designer will have arrived at the key parameters of the approach flow by some calculation procedure independent of the present guide. For example, Equations 6 and 7 of Reference 17 could be used to calculate the flow of smoky gases passing from a room into an atrium void. Another example is where a single-storey mall allows smoke to rise through the void of a two-storey mall: here the flow in the single-storey mall can be calculated in the usual way using, for example, Reference 16, which will give the designer values for both the mass flow rate and the heat flux.

Outline of procedure The calculation proceeds in discrete stages: 1 The designer must know: (a) the internal geometry of his building, including relevant channel widths, and (b) at least two of the key parameters of the approach flow. Useful pairs are: mass flow/heat flux mass flow/mean layer temperature mass flow/ceiling temperature heat flux/mean layer temperature heat flux/ceiling temperature heat flux/layer depth layer depth/mean layer temperature layer depth/ceiling temperature 2 Using the known parameters for the approach flow, calculate the remaining parameters of the flow. 3 Using the results from the preceding stage, calculate the entrainment into the flow as it rotates around the void edge, ie as the smoky gases change from a horizontally-moving flow to a verticallymoving flow. By the end of this stage the key parameters of the vertically-moving gases will be known at the horizontal plane passing through the ceiling/void edge. These parameters are the heat flux, the vertically-moving mass flux, and the kinetic energy of the gases (this last is based only on the vertical component of velocity). 4 The plume at greater heights behaves as if it rises from an infinitely wide source located in the horizontal plane passing through the ceiling/void edge, where that source has horizontal profiles of 50

both buoyancy and (the vertical component of) velocity which can be described by Gaussian functions. This source is, of course, virtual. We have followed Lee and Emmons50 in using this source, and indeed in the method of calculating the plume above the source. We follow Lee and Emmons in calling this source an ‘Equivalent Gaussian source’. Calculate the key parameters of the Equivalent Gaussian source by ensuring that the three key parameters from Stage 3 above keep the same values. 5 Knowing the height above the ceiling/void edge (for example, this is likely to be chosen to be equal to the smoke layer base in the reservoir above the void), calculate the entrainment into the spill plume. This calculation treats the plume as a perfect two-dimensional plume having a length equal to the width of the channel of the approach flow. 6 Calculate the additional entrainment into the free ends of the plume. This assumes that the bulk of the plume is relatively unaffected by these end effects — reasonable for plume heights typically smaller than or comparable to the plume length36.

Detailed procedure Stage 1 Complete all necessary pre-calculations to derive the key parameters of the approach flow described in 1b on this page. Stage 2 Select from the following equations17 to determine the remaining parameters for the approach flow from the initial known parameters: – Calculate the mean layer temperature (Uw) Qw – Uw = Mw c

...(B1)

Calculate the mass flow rate (Mw) at the opening given by30:

Μw =

2 3

Cd3/2 (2g Ucw To)1/2

Wρο

Τcw

dw3/2 κΜ

...(B2)

where ρo = 1.22 kgm23 for an ambient temperature To of 288 K Cd = 0.6 for opening with a deep downstand or 1.0 for no downstand g

= 9.81 ms22

κM = 1.3 for most typical flowing layers

The depth of the layer (dw) at the opening is then given by30 : 2/3 3Μw Tcw   dw =   1/2 2Cd3/2 κΜ Wρο (2g Ucw To) 

...(B3)

The mass-weighted average temperatureUw of the gas layer is30 :

κQ κΜ

– Uw = where

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: First convert Q and My into the corresponding parameters per unit length of plume (ie divide by the channel width (W) to give Qo and A). Then solve the following equations36:

Ucw

...(B4) Qo    ξ = Α +  Το c 

κQ = 0.95 for most typical flowing layers.

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. Greater accuracy can be achieved by calculating the values of the profile correction factor κM and κQ 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 Cd κΜ g Qw Tcw  v = 0.96   κQ1/3 c ρo WTo2 

U    =  T G

ζ =

by17:

1/3

...(B5)

B= 2

g v dw

...(B6)

λ 1  ...(B12) U  2   =π ρo   =3 T  G = 1 + 3λ2  ζ

...(B13)

√ ξ

and:

bG =

1/3

...(B7)

ξ ...(B14)

U     T  G , uG and bG are parameters of the Equivalent Gaussian Source. where

2

...(B8)

Tcw

 Uc  ρo W α′ 2g  3  To  2

...(B11)

uG

Stage 3 Calculate the mass flux (My) rising past the void edge17 :

My =

Qo   To c λ Α +   To c 

1/3

Calculate the horizontal flux (B) of vertical buoyant potential energy17,36 (relative to the void edge): ρo Ucw

Qo = 1 + λ2

2B

uG =

With no downstand at the opening, Cd = 1.0, and:  g Qw Tcw  v = 1.27   c ρo WTo2 

...(B10)

= π ρο

where the empirical thermal constant50 (λ) = 0.9

For a deep downstand, where Cd = 0.6, this becomes:  g Qw Tcw  v = 0.76   c ρo WTo2 

1

1/2

Stage 5 Calculate the entrainment into the rising plume. The Source Froude number (F) for the line plume is36 :

2  F=   π 

1/4

  λ 

α

  U       Τ G 

1/2

uG (g bG)1/2

...(B15)

dw3/2 + Mw ...(B9)

where the entrainment constant (α′) = 1.1

where α = 0.16 for double-sided50 and 0.077 for single-sided line plumes37. Calculate the transformed parameter (υG) for the Equivalent Gaussian Source:

51

υG =

1 ...(B16)

(1 2 F 2 )1/3

Calculate the mass flow per unit plume length (mr) passing the chosen height36 x:

Determine the value of I1(υG) by using the following procedure or an alternative method set out in later in this appendix on page 53.

  mr = =π ρo u b 1 2 p′  

υG represents a value on the vertical axis of Figure B1. Look across to the middle solid curve and find the corresponding value of I1(υG) on the other axis.

Convert to the total mass flow in line plume (ignoring end effects) by multiplying Equation B25 by the channel width (ie mrW).

Calculate the transformed height parameter of x′ corresponding to the desired plume height (x):

Stage 6 Calculate38 the entrainment δMr into the free ends of the line plume. The width of the line plume (and also its axial velocity) can be taken as being approximately constant for most of its height as a first order approximation, and equal to the mean of the values at the Equivalent Gaussian Source and at the chosen height (x).

x′ =

2 =π

α

x ...(B17) bG

Next calculate ∆ I1(υ):

∆ I1(υ) =

and

x′ F2 (1 2 F2)   

I1(υ) = I1(υG) + ∆ I1(υ)

1/3

...(B18)

...(B19)

where:

p′ =

...(B20) 1 ...(B21)

(1 2 F 2)1/3 p′′

b′ = b′′ F2 (1 2 F2) 

1/3

...(B23)

Then calculate the axial vertical velocity component (u) of the gases at height x: u′ uG u=

...(B24) F

52

−− δMr = 4 b u α x ρο

...(B26)

− b = (bG + b) / 2

...(B27)

− u = (uG + u) / 2

...(B28)

Add this to the plume entrainment result from Stage 5 to obtain the total mass flow Mr of smoky gases rising past the specified height (x). Mr = mrW + δΜr

...(B29)

It should be noted that where both ends of a plume are bounded by side walls (eg, as in a shaft) then δMr = 0. Stage 7 Modifications to the above procedure for singlesided17,37,51 (or adhered) line plumes.

...(B22)

Next determine the characteristic half-width (b) of the line plume36 at height x: b = b′ bG

...(B25)

where:

ie: u′ = u′′ F 1/3

  λ    2 1/2  G (1 + λ ) 

The entrainment δMr into both ends of the line plume is then38 :

Determine values of b′, p′ and u′ corresponding to the calculated value of I1(υ) using the following procedure or the alternative method on page 53. I1(υ) 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′′. Then use the following equations to determine u′, p′ and b′,

 U  Τ

Convert both the Equivalent Gaussian Source and the plume into a composite of a real and an imaginary half, so that the centre line of the composite lies along the vertical wall to which the plume is adhering. This is done by doubling values for B, My (and hence A), and Q from Stage 3 before returning to Stages 4 to 6 above. Note that experiments37 show that the value of α needed in Stages 4 to 6 should change from 0.16 (valid for a free or double-sided plume) to 0.077 for the adhered plume. On completing Stage 6, halve the final value of mass flow Mr rising past the desired plume height (x).

Alternative method for determination of I1 (υG) If υG is greater than 1.549 then I1(υG) = (υG 2 0.75)/0.9607 If υG is less than or equal to 1.549 and υG is greater than 1.242 then I1(υG) = (υG 2 0.843)/0.8594 If υG is less than or equal to 1.242 and υG is greater than 1.059 then I1(υG) = (υG 2 0.9429)/0.6243 If υG is less than 1.069 then I1(υG) = (υG 2 1.0)/0.3714

Alternative method for calculating value of b′, p′ and u′ (a) Determination of u′′ If I1(υ) is greater than 1.896 then u′′ = 1.0 If I1(υ) is greater than 0.786 and I1 (υ) is less than or equal to 1.896 then u′′ = 0.0908 I1(υ) + 0.821. If I1(υ) is less than or equal to 0.786 then u′′ = I1 (υ)0.35

(b) Determination of p′′ If I1(υ) is greater than 0.832 then p′′ = 0.9607 I1(υ) + 0.75 If I1(υ) is greater than 0.464 and less than or equal to 0.832 then p′′ = 0.8594 I1(υ) + 0.8429 If I1(υ) is greater than 0.186 and I1(υ) is less than or equal to 0.464 then p′′ = 0.6243 I1(υ) + 0.9429 If I1(υ) is less than or equal to 0.186 then p′′ = 0.3714 I1(υ) + 1.0

(c) Determination of b′′ If I1(υ) is greater than 2.161 then b′′ = 0.938 I1(υ) + 0.82 If I1(υ) is less than or equal to 2.161 and I1(υ) is greater than 1.296 then b′′ = 0.89 I1(υ) + 0.95. If I1(υ) is less than or equal to 1.296 and I1(υ) is greater than 0.896 then b′′ = 0.81 I1(υ) + 1.071. If I1(υ) is less than or equal to 0.896 and I1(υ) is greater than 0.65 then b′′ = 0.619 I1(υ) + 1.214. If I1(υ) is less than or equal to 0.65 and I1(υ) is greater than 0.543 then b′′ = 0.331 I1(υ) + 1.414. If I1(υ) is less than or equal to 0.543 and I1(υ) is greater than 0.421 then b′′ = 0.0627 I1(υ) + 1.55. If I1(υ) is less than or equal to 0.421 and I1(υ) is greater than 0.348 then b′′ = 1.821 2 0.6 I1(υ) If I1(υ) is less than or equal to 0.348 then b′′ = I1(υ)20.4 Now calculate u′, p′ and b′ from Equations B20, B21 and B22 in Stage 5.

53

Nomenclature used in Appendix 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 I1 m δm M δM p′ p′′ Q Qo T u u′ u′′ v W x x′ α α′ ρ U

κM κQ

λ υ ζ ξ

Upward mass flow rate per metre across the horizontal plane through the balcony (kgs21m21) 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 (Wm21) Coefficient of discharge Specific heat at constant pressure of gas (kJkg21°C21) Depth of gas stream beneath ceiling (m) Source Froude number (for line plume) Acceleration due to gravity (ms22) Transformed height (dimensionless) Mass flow rate per unit width of gas stream (kgs21m21) Mass per second per metre of air entrained into hot gas stream at corridor ceiling edge (kgm21s21) Mass flow rate of gases (kgs21) Mass per second of air entrained into free ends of plume (kgs21) Dimensionless buoyancy on plume axis Transformed dimensionless buoyancy on plume axis Heat flux in the gas (kW) Heat flux per second per unit width of gas flow (kWm21) Absolute gas temperature (K) Vertical gas velocity at height x (ms21) Dimensionless vertical gas velocity Transformed dimensionless vertical gas velocity Horizontal velocity component of gas (ms21) 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 (kgm23) 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 (λ = 0.9) Transformed reciprocal of buoyancy (dimensionless) Function defined in text (Equation B12) Function defined in text (Equation B10)

Subscripts o An ambient property c Variable evaluated at highest point in a flow (but outside any boundary layer) G A property of the equivalent Gaussian source r Base of ceiling smoke reservoir w Variable evaluated in the horizontal flow at opening y Variable evaluated in vertical flow past top of opening 54

Acknowledgements

References

We would like to thank Mr N R Marshall of the Fire Research Station for his help in preparing Appendix B.

1

Butcher E G and Parnell A C. Smoke control in fire safety design. London, E & F N Spon Limited, 1979.

2

Department of the Environment and the Welsh Office. The Building Regulations 1991. Approved Document B. Fire safety (1992 edition). London, HMSO, 1991.

3

House of Commons. Public Health Acts 1936 and 1961. London, HMSO.

4

House of Commons. Factories Act 1961. London, HMSO.

5

House of Commons. Offices, Shops and Railway Premises Act 1963. London, HMSO.

6

Saxon R. Atrium buildings. Development and design. London, The Architectural Press, 1983.

7

The Andraeus Building fire in São Paulo, Brazil. Fire Prevention, 1973, 97 (January) 37.

8

Sharry J A. An atrium fire. Fire Journal, 1973, 67 (6) 39–41.

9

Lathrop J K. Atrium fire proves difficult to ventilate. Fire Journal, 1979, 73 (1) 30–31.

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 on a draft of of this Report. These have as far as possible been taken into account.

10 Robinson P. Atrium buildings: a fire service view. Fire Surveyor, 1982, 11 (4) 42–47. 11 Degenkolb J G. Atriums. The Building Official and Code Administrator, 1983, XVII (6) 18–22. 12 Parnell A C and Butcher E G. Smoke movement in atria. Fire Protection (South Africa), 1984, 11 (3) 4–6. 13 National Fire Protection Association. Smoke management systems 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 Standards 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 for smoke ventilation in enclosed shopping centres. Building Research Establishment Report (BRE Bookshop ref BR186). Garston, BRE, 1990. 55

17 Morgan H P and Hansell G O. Atrium buildings: calculating smoke flows in atria for smoke control design. Fire Safety Journal, 1987, 12 (1) 9–35.

31 Morgan H P and Marshall N R. The depth of voidedge screens in shopping malls. Fire Engineers Journal, 1989, 49 (152) 7–9.

18 Morgan H P and Chandler S E. Fire sizes and sprinkler effectiveness in shopping complexes and retail premises. Fire Surveyor, 1981, 10 (5) 23–28.

32 Bosley K. The effects of wind speed on escape behaviour through emergency exits. Summary Report. FRDG Research Report Number 53. London, Home Office, 1992.

19 Morgan H P and Hansell G O. Fire sizes and sprinkler effectiveness in offices — implications for smoke control design. Fire Safety Journal, 1985, 8 (3) 187–198. 20 Hansell G O and Morgan H P. Fire sizes in hotel bedrooms — implications for smoke control design. Fire Safety Journal, 1985, 8 (3) 177–186. 21 Vincent B G, Kung H C and Hill E E. Residential side wall sprinkler fire tests with limited water supply. Fire Science and Technology, 1988, 8 (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 O. Heat and mass transfer process affecting smoke control in atrium buildings. PhD Thesis. London, South Bank University, 1993. 24 Zukowski E E, Kubota T and Cetegen B. Entrainment in fire plumes. Fire Safety Journal, 1981, 3 (2/3) 107. 25 Quintiere J G, Rinkinen W J and Jones W W. The effects of room openings on fire plumes entrainment. Combustion Science and Technology, 1981, 26 (5/6) 193–201.

33 British Standards Institution. Fire precautions in the design of buildings. Part 4. Smoke control in protected escape routes using pressurization. British Standard BS 5588:Part 4:1978. London, BSI, 1978. 34 Spratt D and Heselden A J M. Efficient extraction of smoke from a thin layer under a ceiling. Fire Research Station Fire Research Note 1001. FRS, Borehamwood, 1974. 35 Marshall N R, Feng S Q and Morgan H P. The influence of a perforated false ceiling on the performance of smoke ventilation systems. Fire Safety Journal, 1985, 8 (3) 227–237. 36 Morgan H P and Marshall N R. Smoke hazards in covered multi-level shopping malls: an experimentally-based theory for smoke production. Building Research Establishment Current Paper CP48/75. Garston, BRE, 1975. 37 Hansell G O, Marshall N R and Morgan H P. Smoke flow experiments in a model atrium. Building Research Establishment Occasional Paper OP55. Garston, BRE, 1993.

26 Hinkley P L. Rates of production of hot gases in roof venting experiments. Fire Safety Journal, 1986, 10 (1) 57–65.

38 Morgan H P and Marshall N R. Smoke control measures in a covered two-storey shopping mall having balconies as pedestrian walkways. Building Research Establishment Current Paper CP11/79. Garston, BRE, 1979.

27 McCaffrey B J, Quintiere J G and Harkleroad M F. Estimating room temperatures and the likelihood of flashover using fire test data correlations. Fire Technology, 1981, 17 (2) 98–119.

39 Grella J J and Faeth G M. Measurements in a twodimensional thermal plume along a vertical adiabatic wall. Journal of Fluid Mechanics, 1975, 71 (4) 701–710.

28 Morgan H P and Marshall N R. Smoke hazards in covered multi-level shopping malls: a method of extracting smoke from each level separately. Building Research Establishment Current Paper CP19/78. Garston, BRE, 1978.

40 Thomas P H. On the upward movement of smoke and related shopping mall problems. Fire Safety Journal, 1987, 12 (3) 191–203.

29 Heselden A J M. Fire problems of pedestrian precincts. Part 1. The smoke production of various materials. Fire Research Station Fire Research Note 856. FRS, Borehamwood, 1971. 30 Morgan H P. The horizontal flow of buoyant gases toward an opening. Fire Safety Journal, 1986, 11 (3) 193–200.

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41 Morgan H P. Comments on ‘A note on smoke plumes from fires in multi-level shopping malls’. Fire Safety Journal, 1987, 12 (1) 83–84. 42 Thomas P H, Hinkley P L, Theobald C R and Simms D L. Investigations into the flow of hot gases in roof venting. Fire Research Technical Paper No 7. London, HMSO, 1963.

43 Boehmer D J. Atrium fire engineering — North American experience. Proceedings of seminar ‘Atrium Engineering’ held by Environmental Energy Group of the Institution of Mechanical Engineers, 26 June 1990. London, I Mech E, 1990. 44 Pauls J. Calculating evacuation times for tall buildings. Proceedings of symposium ‘Quantitative Methods for Life Safety Analysis’ held by the Society of Fire Protection Engineers, University of Maryland, March 1986. Boston MA, SFPE, 1986. 45 Hansell G O and Morgan H P. Smoke control in atrium buildings using depressurisation. Part 1: Design principles. Fire Science and Technology, 1990, 10 (1 & 2) 11–26. 46 Hansell G O and Morgan H P. Smoke control in atrium buildings using depressurisation. Part 2: Considerations affecting practical design. Fire Science and Technology, 1990, 10 (1 & 2) 27–41. 47 Building Research Establishment. The assessment of wind loads. Part 8: Internal pressures. BRE Digest 346. Garston, BRE, 1990. 48 Douglas-Baines W. Effects of velocity distribution on wind loads and flow pattern on buildings. Proceedings of Symposium No 16 ‘Wind Effects on Buildings and Structures’ held by the National Physical Laboratory, Teddington, 26–28 June 1963. London, HMSO, 1965. 49 Hinkley P L. The effect of smoke venting on the operation of sprinklers subsequent to the first. Fire Safety Journal, 1989, 14 (4) 221–240. 50 Shao-Lin Lee and Emmons H W. A study of natural convection above a line fire. Journal of Fluid Mechanics, 1961, 11 (3) 353–368. 51 Marshall N R. Air entrainment into smoke and hot gases in an open shaft. Fire Safety Journal, 1986, 10 (1) 37–46.

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Design approaches for smoke control in atrium buildings G O Hansell and H P Morgan

Related titles from IHS BRE Press Design principles for smoke ventilation in enclosed shopping centres BR186, 1990 Smoke control in large stores: an extended calculation method for silt extraction design OP51 A simplified approach to smoke-ventilation calculations IP19/85, 1985 Experiments at the Multifunctioneel Trainingcentrum, Ghent, on the interaction between sprinklers and smoke venting BR224, 1992 Sprinkler operation and the effect of venting: studies using a zone model BR213, 1992

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