BRE186+(design+smoke+ventilation+shopping+centres)

Building Research Establishment Report

Design principles for smoke ventilation in enclosed shopping centres

H P Morgan, BSc, CPhys, MInstP, AIFireE, MSFSE and J P Gardner*, BSc *Colt International Ltd

Fire Research Station Building Research Establishment Garston, Watford WD2 7JR

Prices for all available BRE publications can be obtained from: Construction Research Communications Ltd 151 Rosebery Avenue London, EC1R 4QX Telephone 0171 505 6622 Facsimile 0171 505 6606 E-mail [email protected] BR 186 ISBN 0 85125 462 4 © Crown copyright 1990 First published 1990 Reprinted with corrections 1991 Second reprint 1991 Republished on CD-ROM 1997 with permission of the Controller of HMSO and the Building Research Establishment, by Construction Research Communications Ltd Anyone wishing to use the information given in this publication should satisfy themselves that it is not out of date, for example with reference to the Building Regulations Applications to copy all or any part of this publication should be made to: Construction Research Communications Ltd, PO Box 202, Watford, Herts, WD2 7QG

Contents Page Foreword

v

Introduction

1

Chapter 1

General principles

3

Chapter 2

Design procedure for the mall smoke control system

6

Smoke reservoirs

6

Design fire size

6

Single-storey malls

7

Two-storey malls with small voids

8

Two-storey malls with large voids

10

Multi-storey malls

14

Calculating smoke temperature

15

Mall sprinklers

17

Flowing layer depth

18

Local deepening

18

Inlet air

19

Minimum number of extract points

20

Natural ventilation - area required per reservoir

21

Powered ventilation

22

Chapter 3

Large shop opening onto a mall

23

Chapter 4

Some practical design considerations

25

Factors influencing the design fire

25

The effect of wind on the efficiency of a smoke ventilation system

25

False ceilings in the mall

25

The use of a plenum chamber above a false ceiling in a shop

26

Stores with internal voids

27

Sloping malls

27

Assessment of effective layer depth

27

Smoke flow in low narrow malls

27

Basement service levels

27

Enclosed car parks

29

Smoke transfer ducts

29

Entrances within the smoke layer

30

Other situations

30

Chapter 5

Some operational factors

31

References

32

iii

Foreword This Report has been prepared by the Fire Research Station of the Building Research Establishment (BRE) and results from the latest scientific knowledge and practical experience in smoke movement and control. It is an update of Smoke control methods in enclosed shopping complexes of one or more storeys: a design summary published in 1979, and is based upon preliminary work carried out for BRE by Colt International Limited. The Report takes into account the comments of a Liaison Committee consisting of industrial and government representatives. The Fire Research Station is also grateful to the Society of Fire Safety Engineers, the Institute of Fire Engineers and the UK Chapter of the Society of Fire Protection Engineers for providing detailed comments which have as far as possible been included. The Fire Research Station acknowledges the assistance given by Colt International Limited in the preparation of the final Report.

v

Introduction This Report is intended to assist designers of smoke ventilation systems in enclosed shopping complexes. Most of the methods advocated are the outcome of research into smoke control by smoke ventilation at the Fire Research Station, but also take into account the recommendations1 of the Working Party on fire precautions in town centre redevelopment, as well as experience gained and ideas developed whilst the authors and their colleagues have discussed many proposed schemes with interested parties. The primary purpose of this Report is to summarise the design advice available from the Fire Research Station at the time of its preparation, in a readily usable form. As such, the Report is neither a detailed engineering manual nor is it a scientific review article. Perhaps most important of all, it is not a summary of the totality of approaches possible. New methods such as those based upon computational fluid dynamics, will be developed as time passes and there will always be special cases where existing alternative methods can be adopted. At peak times a shopping centre can be occupied by thousands of people, and some larger centres by more than a hundred thousand. A typical centre may comprise many individual shop units opening onto a common mall. Although the individual units may be separated from each other by a dividing wall of fireresisting construction, usually the shop is either open fronted or only separated from the mall by a glass shop front. This means that the public areas of the entire centre can be effectively undivided. Means of escape from within each shop unit will, in general, be specified (eg BS 5588 Part 2 2) in the same way as for shops which are not part of an enclosed complex. This means that escape from within the shops is specified as if the mall were as much a place of safety as the usual open-sky street. Unfortunately, the mall is a street with a roof, and so cannot be regarded as being as inherently safe as an open-topped street. People escaping from a shop into a mall will still need to travel along the mall before exiting to a true place of safety. It follows that the mall constitutes an additional stage to the escape route, which needs to be protected from the effects of fire and smoke. Ideally, it should approach the same level of safety as a street for as long as people need to escape through it — even if smoke enters the mall from the shop on fire. Details of means of escape provisions for the malls can be found elsewhere1. A shopping complex is a public building and the occupants will be a cross section of the community including the elderly, children and the disabled. They will not necessarily be familiar with the building, or perhaps more importantly, with all the escape routes that might be provided for them. In many types of building it is widely recognised that people will

commonly try to escape by the same route they had used to enter the premises (see for example, Canter3). It follows that escape via the malls must be assumed, even where other exits are provided1. In this situation a long evacuation period can be expected. A detection and alarm system is required to give early warning of a fire, and sprinklers are needed to control its spread. Statistics of fire deaths show that the majority of fatalities are due to the effects of smoke. The ideal option would be to prevent any smoke from a shop fire entering the mall at all. In the majority of cases, it would either be very difficult or extremely expensive to fit a separate smoke extraction system to each and every shop, however small1. Note that large shops are, however, an exception to this rule and the provision of smoke ventilation systems for such shops is discussed in Chapter 3. Occasionally circumstances dictate that smoke ventilation fitted to each shop unit, even small units, is the most viable option for protecting part or all of a mall. (This can apply, for example, when an old complex is being redeveloped, but the mall is too low and/or too narrow to allow the installation of a viable smoke ventilation system in the mall itself). There have been several examples of this. Nevertheless it remains generally true that this option is rarely found to be appropriate for most malls. Occasionally, one still hears the suggestion that the mall should be pressurised to prevent smoke moving from a shop into the mall. This is not usually a viable option by itself where the opening between shop and mall is large (eg an open-fronted shop, or a shop whose glazing has fallen away in whole or part). This is because the airspeed needed from mall into shop in order to prevent the movement of smoky gases the other way through the same opening, can vary between 1/2 and c. 2 ms-1 depending on fire size, gas temperature etc. All of this air must be continuously removed from within the shop unit in order to maintain the flow. The quantities of air-handling plant required will exceed the size of smoke ventilation systems for most typical shop-front openings. Where smoke from a fire in a single shop unit could spread rapidly via the malls through the entire centre, a smoke ventilation system in the malls is essential to ensure that escape is unhindered, by ensuring that any large quantities of thermally buoyant smoky gases can be kept separate from (ie above) people who may still be using escape routes through the malls. Therefore, the role of a smoke ventilation system is principally one of life safety. It should also be remembered, however, that firefighting becomes both difficult and dangerous in a smoke-logged mall. It follows that to assist the fire services, the smoke ventilation system should be capable of performing its design function for a period of time longer than that required for the

1

public to escape: so an immediate attack on the fire can be made after the arrival of the fire brigade. Guidance on design principles for smoke ventilation systems was summarised in a report by Morgan4 published in 1979. This was based on the knowledge available at that time. Since then a great deal of relevant research has been carried out, which for the most part has confirmed the guidance given in the original report, but has in some areas highlighted the need to modify that guidance. A great deal of practical experience has been gained in designing such systems. Also in the intervening years, shopping centres themselves have become larger and more intricate with many open levels, interconnecting voids, sloping floors and atrium features. These can result in a very complex path of smoke flow from the shop in which the fire starts to the eventual point of extraction. The purpose of this Report is to update the guidance available in the earlier design summary4 to reflect these changes, to assist designers of smoke control systems in enclosed shopping centres. As was the case with the previous work, this Report only gives guidance in line with current knowledge and generally accepted practice. The guidance is based on results of research where possible, but also on the cumulative experience of design features, required for regulatory purposes, of many individual smoke ventilation proposals. Many of these design features have been evolved over a number of years by consensus between regulatory authorities, developers and fire scientists,

2

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 practice5 for enclosed shopping centres is currently being prepared by the British Standards Institution (BSI). The aim of this Report is to provide guidance only on design principles for smoke ventilation and it is hoped to support the Code rather than pre-empt it. The Report cannot cover all the infinite variations of shopping centre design. Instead it gives general principles for the design of efficient systems, with simplified design procedures for an ideal model of a shopping mall and then further guidance on frequently encountered practical problems. Because the design procedures are of necessity simplified, the Report also gives their limitations so that, when necessary, a more detailed design by specialists can be carried out. A Code of practice for atrium buildings is also being prepared by the BSI. 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. Where atria have mixed occupancies (eg shops and offices) then reference should be made to the above document, when available, or specialist advice sought.

Chapter 1 General principles Smoke from a fire in a shop rises in a plume to the ceiling. As the plume rises, air is entrained into it, increasing the volume of smoke and reducing its temperature. The smoke spreads out underneath the ceiling and forms a layer which deepens as the shop begins to fill with smoke. As the layer deepens, there is less height for the plume of smoke to rise before it reaches the smoke layer. Less air is being entrained, with the result that the temperature is higher on reaching the smoke layer. As this continues, the increasing smoke layer temperature at some point will operate the sprinklers. The fire may shatter the shop front glazing (if present) unless that glazing is fireresistant. The smoke will then flow out of the shop into the mall (Figure 1). The change of direction as the smoke flows out of the shop front results in increased mixing with the surrounding air. There is so much mixing that, except close to the fire itself, the hot smoky gases can be regarded as consisting entirely of warmed air, when calculating flow. The quantity of smoke particles (and hence the optical density and visibility through the smoky gases) produced in a fire, depends strongly on the nature of the burning material6. The quantity of hot gases carrying these particles is, however, mainly dependent on the size and rate of burning (related to the heat output per second) of the fire. Increasing the visibility through these gases requires their dilution with clean air, but improving visibility (and reducing toxicity) by dilution alone in a mall to safe levels after smoke has entered, is not a practical proposition. One suggested safe level of visibility7 (about 8 metres — rather a short distance for a mall) would require the hot smoky gases to be diluted by a mass ratio possibly larger than 300, even after the gases leave the shop of origin. It follows then that hot smoky gases should always be kept spatially separated from escaping people. One can state as a general principle that air will mix into a rising stream of hot smoky gases in large

quantities, but will not mix appreciably into a horizontally flowing stream of hot smoky gases except under special (and usually local) conditions. Smoke flowing from the shop onto the mall will rise to the mall ceiling (Figures 1 and 2). Air will mix into the smoke as it rises. If no smoke control measures are present, the gases will then flow along the mall as a ceiling layer (Figure 2a) at a speed8 typically between 1 and 2 m/s. This is faster than the probable escape speed of pedestrians in a crowded mall9. When the smoke reaches the end of the closed mall it will dip down to a low level and be drawn back towards the fire10 (Figure 2b). If the end of the mall is open, even light winds blowing into or across the opening will cause severe local disturbance and mixing, and once again smoke at low level will be drawn back towards the fire10 (Figure 2c). Hence a single-storey mall could become smoke logged within minutes. An unsprinklered fire in a single-storey shopping centre in Wolverhampton11, is thought to have caused a 100-metre-long mall to become untenable within one minute. Similarly short times can be expected to apply to the upper floors of a multi-storey mall. Since it is not usually practical to prevent smoke entering the mall, except for larger shops, a mall smoke control system is necessary to control and remove heat and smoke. The Fire Research Station has extended the ideas developed during its earlier work on the fire venting of factories12 to apply to malls8 in order to use the buoyancy inherent in fire gases to keep those gases safely above the heads of people in the malls (Figure 3). There are three essential features of a smoke ventilation system, without any one of which it will not function effectively: 1 There must be some means of forming a smoke reservoir to prevent the lateral spread of smoke,

Figure 1

Smoke spread and main entrainment sites in single and two-storey malls

3

Figure 2a

Creation of a moving smoke layer beneath the ceiling of an unventilated mall, showing movement of the displaced air

Figure 2b

Recirculation of smoke in an unventilated and closed mall

Figure 2c

Mixing of smoke into the air being drawn into an open-ended mall, caused by wind

Figure 3

Principles of system needed to contain smoke in a well-defined layer (section along mall)

which would result in excessive loss of buoyancy. This reservoir must be designed to contain the smoke layer base well above head height. Generally speaking, malls with high ceilings or rooflights will allow a deeper smoke reservoir and hence a more effective smoke ventilation system than in low, narrow malls. 2 There must be extraction of smoke within the reservoir, to prevent the smoke layer building down below the design depth. This can be natural, 4

buoyancy driven ventilation or mechanical extraction, depending on the circumstances. The rate of the exhaust must equal the rate at which the smoke enters the reservoir from below. 3 Since the gases being extracted consist almost entirely of air that has mixed with the original fire gases, fresh air must enter the mall to take its place. It must enter at a rate equal to the rate of extraction of smoke and at a low enough height not to mix prematurely with the smoke.

In a fire most of the important factors are time dependent: the time for a fire to grow from ignition to the design fire size, time for the mall to smokelog and time needed for evacuation. Currently there are no reliable data on fire growth rates in retail premises. Smoke filling times are known to be rapid but are not always easily quantified. The time needed for evacuation is unknown but it could be considerably longer than the 2.5 minutes escape time used to size exit widths to cope with the peak flow rates during this evacuation period. There is considerable anecdotal evidence within the UK fire services supporting this view. These uncertainties lead to many problems in designing a time dependent smoke ventilation system. The design principles given in this Report are based instead on steady state conditions. A design fire is used which has a low probability of being exceeded, and the smoke ventilation system is designed to remove the mass flow rate of smoke necessary to maintain clear conditions in the mall for escape. Sprinklers in shops are an essential part of the smoke ventilation design in order to prevent the fire growing beyond the design fire size.

There are other essential factors that should be borne in mind when designing a smoke control system. When the mall has more than one level, it is also necessary to restrict or channel smoke on the lower level in order to restrict entrainment into the plume rising through the upper level. Once the smoke has entered a ceiling reservoir it will flow towards the extraction points. Since this horizontal flow is driven by the gases’ own buoyancy, there will be a minimum depth for any particular combination of smoke temperature and mass flow rate. This is discussed in greater detail in Chapter 2, but this minimum depth will set a limit to the design for the smoke reservoir, which cannot be shallower. The capacity of the extraction system depends mainly on the height the gases have risen to the layer base, and not at all on the area of the floor or the volume of the space. Therefore a simple percentage rule for natural venting, such as 3% of the floor area, or an air change rate, such as six air changes per hour for powered ventilation, are misleading.

5

Chapter 2 Design procedures for the mall smoke control system The following sections outline a general procedure which can be followed when designing the mall smoke control system. First plan the positions of the smoke reservoirs, calculate the mass flow rate of smoke entering the reservoir, and either the ventilation area of a natural ventilation system or the extract rate of a powered system to remove the same amount of smoke. These procedures have been developed from ‘idealised models’ of shopping malls studied at the Fire Research Station. Commonly encountered variations are discussed further in Chapter 4.

Smoke reservoirs No smoke layer has a perfectly defined interface with the colder, clearer air below; there is always a small amount of cross mixing. This cross mixing has no significant effect on the quantity of smoke in a horizontal ceiling layer but can cause progressive loss of visibility in the air beneath. This loss of visibility occurs more rapidly when:

• a ceiling layer is cool (indeed very cool gases will not persist as a layer)

• the hot smoke and the air beneath have a high relative velocity

• turbulence disturbs the interface Heat is lost from the smoke layer by radiation downwards and by radiation and conduction to the surfaces of the reservoir. To prevent excessive heat loss the size of an individual smoke reservoir within the mall is usually taken to be limited to 1000 m2 (or 1300 m2 if mechanical extraction is used as its efficiency is less susceptible to heat loss). This recommendation evolved during the early 1970s and is perhaps best understood by noting that in many ‘industrial’ simple undivided compartment applications of smoke ventilation, it has long been the practice to limit reservoir areas to between 2000 m2 and 3000 m2. In a mall, smoke from shops of up to 1000 m2 area (if the mall is naturally ventilated) or up to 1300 m2 (if the mall has extract fans) can enter the mall reservoir, giving a total area affected by smoke similar to the long-standing practice stated above. The maximum distance between screens forming the boundary of the reservoir should be 60 m. This distance recommendation follows earlier considerations1 (confirmed in this form by the Liaison Panel consulted in preparation of the present document), and derives from concern over the distance people should be expected to walk below a smoke layer while escaping. For complex geometries of smoke reservoirs specialist advice should be sought. 6

The smoke reservoir can often be formed by the downstand fascia of the shops, combined with screens at intervals along the mall (Figure 4). The fascias are then necessary, not to prevent the smoke flowing into the mall from a fire in the shop, but to prevent smoke contained in the mall reservoir flowing into the shops adjoining the mall. If there are raised rooflights above the malls, these can often be utilised as smoke reservoirs. Although the height through which the smoke may have to rise to a ventilator installed at the top of a high rooflight may be greater than for lower malls, provided that the smoke is allowed to build down to the same level, the mass flow rate of smoke entering the reservoir is no greater, but the deeper reservoir of smoke will produce a greater buoyancy pressure at the ceiling. The screens which form the smoke reservoir should be arranged so that smoke from a fire in any shop can only flow into one reservoir. Even with a smoke reservoir limited to 1000 m2, excessive cooling and/or downward mixing can occur in stagnant regions of the smoke layer. To prevent this the extraction outlets, be they natural or mechanical, should be distributed over the reservoir so as to prevent stagnant regions being formed. Potentially stagnant regions of the smoke reservoir can sometimes be avoided by using smoke transfer ducts (see Chapter 4).

Design fire size Before any smoke ventilation system can be designed it is essential to determine a suitable size of fire, for design purposes. This fire size then forms the basis of a smoke ventilation system design.

Figure 4

Smoke reservoir in mall with a ceiling height greater than that in shops (section across mall)

Ideally the design fire should show the physical size and heat output of the fire increasing with time, allowing the growing threat to occupants to be calculated as time increases. 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 retail areas. 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. Note that even here, the statistical evidence is not strong (see for example Morgan and Chandler13) for shopping malls. Further research is currently in hand to improve this statistical basis. It follows from the foregoing that there is a strongly subjective element in assessing what fire size is acceptably infrequent for safety design purposes. A 12 m perimeter (3 m × 3 m) 5 MW sprinkler controlled fire has become the accepted basis in the UK for a smoke ventilation system in a sprinklered shopping centre1,4,8. Some factors affecting the design fire size are discussed in Chapter 4. The design principles outlined in this Report are based upon a 12 m perimeter 5 MW fire. Should a different design fire be considered for whatever reasons, the equations, figures etc given in this Report may no longer apply and advice should be sought from experts. Other fire sizes have occasionally been specified by designers, for both sprinklered and unsprinklered shops. The problem of unsprinklered shops has been discussed in more detail by Gardner14, who has shown the importance of considering ‘flashover’ in such units and the consequent need to consider potentially very large fires.

Single-storey malls The minimum height of the smoke layer base must be 2.5 m from the mall floor to ensure safety, and preferably at least 3 m in a single-storey mall (Figure 5).

Figure 5

Smoke ventilation in a single-storey mall

It may be necessary to raise the smoke layer base above this for practical reasons. This will result in an increased mass flow rate of smoke due to the additional entrainment, but as the smoke layer base is higher, a greater degree of safety will have been achieved. Shop fascias should extend below this height otherwise the layer base would be low enough to enter unaffected shops. Indeed, fascias serve no other useful smoke controlling function, except as part of a separate smoke extraction system within a shop, or to contain smoke from a very small fire. Having established the clear layer height in the mall, the mass flow rate of smoke can then be calculated. Recent work by Hinkley15 has confirmed the rate of entrainment of air into a plume of smoke rising above a fire as: M = 0.19 P Y 3/2

(1)

where M = mass flow rate of smoke entering the smoke layer within the shop (kg/s) P = the perimeter of the fire (m) Y = the height from the base of the fire to the smoke layer (m) 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. Experiments have shown16 that the smoke flowing from the shop onto the mall becomes turbulent with increasing mixing of air. The mass flow rate of smoke entering the reservoir is approximately double the amount given by Equation 1, where Y is now the height from the fire to the base of the mall smoke layer (Figure 5). Figure 6 shows the mass flow rate of smoky gases entering the layer at different layer heights in a single-storey mall. This can be calculated from: M = 0.38 P Y 3/2

(2)

The Fire Research Station is currently studying entrainment into smoke flow from compartments. The purpose of this work 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 2 may be superseded in due course. If the clear layer in the mall is much higher than the shopfront opening, then the plume of smoke rising to the smoke layer will be similar to a two-storey shopping centre (Figure 7). Where this height difference (hr) between the shopfront opening and the smoke layer base is more than 2 m (Figure 7) the mass flow rate should be obtained using the procedure for two-storey shopping centres given later in this chapter on page 10.

7

Two-storey malls with small voids When a fire occurs on the lower level of a two-storey mall, a plume of smoke has further to rise before entering the smoke reservoir. This results in greater entrainment of air, hence a larger quantity of smoke. The smoke layer on the upper level will then be cooler and less well defined. In this case the smoke layer should be not less than 3 m above the upper floor level and preferably more than 3.5 m. Again, shop fascias should extend below this height otherwise the layer base would be low enough to enter unaffected stores. In a shopping centre which has small voids connecting levels, smoke from a fire in a lower level shop will flow out of the shop and spread in a complicated horizontal circulation pattern beneath the ceiling (ie beneath the upper deck) (Figure 8). Where smoke reaches the edge of a void linking the two levels, some will flow over the edge producing an extensive plume above each void, rising through the upper level. Air mixes into these plumes, resulting in extremely large quantities of very cool gas collecting in the upper level ceiling reservoir. This in turn reduces the efficiency of buoyancy-driven venting as well as increasing downward mixing from the ceiling layer. To minimise this mixing of air into the plume, smoke screens of at least 1.5 m depth17 (actuated by smoke detectors or as permanent features) should be hung below the lower level ceiling (ie below the upper deck) in order to restrict the lateral spread of smoke and ensure that all the smoke from a fire passes through only one void (Figures 9 and 10). The approach outlined in this section will only apply when the void is small enough in relation to the mall, and the screens are arranged so that smoke can flow freely into the void around its perimeter in a ‘swirling’ pattern (Figure 9). Figure 6

Rate of production of hot smoky gases in a single-storey mall from a 5MW fire

The perimeter of the rising plume, which strongly affects the rate of the mixing of air into the plume, then depends only on the size of the void. Experiments with scale models17 suggest that the largest void consistent with this type of smoke control system would have a perimeter of between 35 and 45m. There are no adequate theories to relate the quantities of smoke entering the reservoirs, to the height from the upper floor to the smoke base. Figure 11 shows the values of the mass flow rate (M) entering the reservoir for different heights of ceiling reservoir smoke base above the upper floor, and for three different sizes of void (indicated by their respective perimeters), based on the voids used in the scale-model experiments. Other void perimeters must be inferred by interpolation.

Figure 7

8

Additional entrainment with the smoke base well above the shop front opening

Figure 11 applies to a 5 MW fire occurring in a shop on the lower levels, and is derived from experiments on a one-tenth scale model17 representing a mall of 5 m floor-to-floor height. Note that extrapolating too far

Figure 8

Plan view of smoke circulation pattern below upper deck. Without smoke-restraining screens

Figure 9

Plan view of smoke circulation pattern below upper deck. With smoke-restraining screens

Figure 10

Smoke and air movements in two-storey mall with small connecting voids

9

Figure 11

Mass of smoky gas entering ceiling reservoir per second from a 5 MW fire — small voids

will risk an increasing (but unknown) error, since these curves are purely empirical.

entrainment of air that takes place may result in very large quantities of cool smoke.

The fire on the upper level can be treated for design purposes as a fire in a single-storey mall. If the smoke ventilation system will cope with the smoke from a fire on the lower level, it should usually be able to cope with fires on the upper level. Note, however, that upper level fires can result in higher temperature smoke in the reservoir.

Screens installed beneath the balcony to channel the smoke from the shopfront to the edge of the balcony will restrict lateral spread of smoke (Figure 13), thereby producing a more compact line plume. This results in less entrainment of air and therefore a more manageable quantity of hotter smoke. These screens can be permanent structures or, alternatively, screens activated automatically on smoke detection. Recent, as yet unpublished, research18 suggests that channelling screens may be unnecessary if the balcony projects no more than 1.5 m beyond the shop front. 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 a 5 MW fire in a typical mall are given in Table 1a for a balcony with a downstand at the void edge, which is deep in relation to the approaching layer, and Table 1b for a balcony without a downstand at the void edge (based on Morgan19). If sideways spillage occurs in quantity, the visibility on the upper level can be significantly worse.

Two-storey malls with large voids When a shopping centre has large voids through the mall connecting levels, the smoke flow is somewhat different from that given earlier for small voids. An idealised model of a two-storey mall with large voids is shown in Figure 12, where the upper pedestrian walkways take the form of a balcony on either side of a large void. Again, the ‘worst case’ is when a fire occurs in a shop on the lower level. Smoke will flow from the shopfront to the underside of the balcony. It will then flow forward to the balcony edge, as well as sideways beneath the balcony (Figure 12). Because of this sideways flow beneath the balcony, there may be an extensive line plume flowing upwards from the balcony edge like an ‘inverted waterfall’. Unless the length of this line plume flowing over the edge of the balcony is limited, the extensive 10

The mass flow rate entering the reservoir can, in principle, be related to the height from the balcony to the reservoir layer base and to the plume length (ie to the screen separation) by the theory of Morgan and Marshall20. Unfortunately, this theory applies to a plume rising through free space, where the air outside the plume is uniformly at ambient temperature.

Figure 12

The smoke layer in a mall ceiling reservoir does not have a well defined base (especially in a two-storey mall where there might be a deep layer of cool smoke). Even below the nominal layer base (ie d1 below the ceiling, Figure 13, which corresponds closely to the visible layer base17), there is a temperature excess relative to the temperature of the incoming air which increases with height (Figure 14). To apply the theory20 to a calculation of the mass flow rates of smoke entering the reservoir, one must introduce a correction factor for the smoke layer depth in a reservoir. Experiments with flat roofed models21 have shown that for calculating plume entrainment, the effective layer depth (d2) is 1.26 times the nominal and visible layer depth (d1) which has been chosen for reasons of visibility and safety. In practice many shopping malls have roofs which are not flat, and it is necessary to assess the result of this on the effective layer depth. This is examined more closely in Chapter 4.

Smoke spread beneath a balcony producing a long line plume

Results from such calculations for a number of values of channelling screen separation, are shown graphically in Figure 15. These results include an allowance for entrainment of air into the ends of the plume. This correction method follows that of Morgan and Marshall21, which supersedes their earlier approach20. Once the height of the nominal layer base (h-d1) has been chosen on safety grounds, and the channelling screens separation L (and hence also channelling screen depth using Tables 1a or 1b) has been chosen on practicability grounds, (eg such that the screens contact the walls separating the shop) then Figure 15 can be used to find the mass flow rate of smoke entering the reservoir. It should be noted that once the height of the layer base (h-d1) has been selected, d1 can be used instead of d2 for greater simplicity in interpreting Figure 15 and in designing the consequent smoke extraction. This results in an ‘overdesign’ of the extract capacity, which 11

Figure 13

thus errs on the side of safety. In experiments values of (h-d2) as low as a full-scale equivalent 0.75 m were obtained. For a very deep layer, one sometimes finds that (h-d2) can sometimes be negative. This corresponds to a plume moving downwards which is impossible in this context and shows that the method breaks down under these conditions. It follows that wherever (h-d2) is less than 0.75 m, it would be safe to use (h-d1) instead for estimating entrainment. The results given in Figure 15 are representative of typical shop/balcony geometries. In practice the shopfront geometry, presence or absence of a deep downstand fascia and a balcony will affect the mass flow rate of smoke. For example, many malls will have the upper walkway set back above the shop units on the storey below with no balcony projecting beyond the lower shop front. In such designs the shop walls themselves act as channelling screens. Where such a shop has a downstand fascia, the plume’s rise (h-d2) 12

The use of smoke-channelling screens to produce a compact rising plume

should be measured from the bottom of the downstand. Figure 15 can again be used to estimate the entrainment into the plume, but a more precise calculation for this case is feasible22. Similarly, results given in Figure 15 are for a line plume rising through an upper level where air is entrained on both sides. If the plume is rising against a vertical surface (such as a wall or shopfront on the level above), then air will only be entrained into one side. Recent research work22,23 has enabled a more detailed analysis of the fire compartment conditions and subsequent plume entrainment to be carried out taking into account these factors. 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 Thomas24. This treats the plume in a 'far plume' approximation apparently rising

Table 1a Minimum depth of channelling screens — downstand at void edge Screen separation at edge L (m)

Minimum screen depth (m)

4 6 8 10 14

2.7 2.4 2.2 2.1 1.8

Note: The minimum depth is that below the lowest transverse obstacle

Table 1 b Minimum depth of channelling screens — no downstand at void edge Screen separation at edge L (m)

Minimum screen depth (m)

4 6 8 10 14

1.6 1.4 1.3 1.2 1.1

Note: The minimum depth is that below the lowest transverse obstacle

from a line source of zero thickness some distance below the void edge. The relevant formula is:

M = 0.58ρ

×

 gQL2



 ρCpT1 

1 +



 1/3  (z + ∆) 

(3)

0.22 (z + 2∆)  2/3  L 

where M = mass flow of smoky gases passing height z (kg s-1) ρ = density of warm gases at height z (kg m-3) Q = heat flux in gases (kW) L = length of void edge past which gases spill (m) Cp = specific heat of air (kJ kg-1 K-1) T1 = absolute ambient temperature (K) ∆ = empirical height of virtual source below void edge (m) z = height above void edge (m)

Morgan’s re-analysis25 of Law’s earlier paper26, concluding that the effective depth d2 of the reservoir layer should be used when the plume rises within a closed space such as a mall, should apply equally to Thomas’s work. This means that z should be taken to be (h-d2), as earlier in this section. Again from Morgan25, one can take ∆ = 0.3 times the height of the compartment (ie shop unit) opening, although comparisons between Equation 3 and the modified form22 of Morgan and Marshall’s method20 suggest that the value of ∆ may be sensitive to the parameters of the horizontal flow approaching the void. In this context it is noteworthy that Law26 derived a value of ∆ = 0.67 times the height of the compartment based on different experimental data. It should be realised that the derivation of Equation 3 limits its application to scenarios where smoky gases issue directly from the compartment on fire, with a balcony projecting beyond. For two-storey malls Equation 3 and Figure 15 should give broadly similar 13

Figure 14

A typical temperature profile for a reservoir layer

results, but under some circumstances significant discrepancies can occur because of the apparent variability of ∆. Nevertheless, Equation 3 can be a very useful way of estimating entrainment for geometries departing significantly from Figure 15.

Multi-storey malls From the previous section it can be seen that when a fire occurs in a ground floor shop and rises through an upper level, a very large quantity of smoke is produced. If this is extended to a three-storey mall the result is an impracticably large quantity of very cool smoke. Therefore it is not usually possible to design a practical smoke ventilation system which allows smoke from more than the top two levels of a multi-storey mall to rise up through the mall and maintain a clear layer for escape on the upper level. The limiting factor here is not the height to the top of the smoke reservoir, but the height of rise to the smoke layer base. A multi-storey mall can instead be treated as a stack of single-storey malls, with each level having a separate smoke ventilation system. Clearly this technique can also be used in a two-storey mall if so desired. Figures 16 to 18 illustrate in schematic form a mall whose upper floor (two levels only are shown in the figures) is penetrated by voids which leave a considerable area for pedestrians. On the lower level there is a large area situated below this upper deck. If screens (activated by smoke detectors or as permanent features) are hung 14

down from the void edges, the region below the upper deck can be turned into a ceiling reservoir similar to that of a single-storey mall, albeit a more complicated geometry. This reservoir can then be provided with its own smoke extraction system. Other screens can be positioned across the mall to limit the size of this reservoir, as for a single-storey mall. The screens around the voids will, in general, be fairly close to potential fire compartments (ie shops). 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 deepening27 — see page 18. 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 top floor ( Figure 18). The mass flow rate of smoke exhaust for the top floor can be calculated as if it were a single-storey mall. There will often be some small smoke spillages under the void screens, which will contribute to a ‘fogging’ of the upper levels. This can be controlled by permitting some ventilation of these upper levels to operate, to flush out this stray smoke. To minimise such spillages and limit the amount of smoke below the ceiling layer on the affected level, it is desirable to simplify the geometry of the ceiling reservoir where possible.The

Figure 15

Mass of smoke entering ceiling reservoir per second from a 5MW fire — large voids

void screens tend to split the smoke flow into separate streams within the reservoir (Figure 16). These streams can meet further along the reservoir, as shown. Such opposed smoke flows produce turbulence and downward mixing of smoke into the air below. It follows that it is an advantage to keep a simple reservoir geometry. It is also important to provide the full ‘flushing’ clean air inflow below the ceiling reservoir at the affected level. A lesser amount of ‘flushing air flow’ is desirable on the top level when lower reservoirs are vented to a common top level reservoir (Figure 18). This can easily be provided by increasing the smoke extraction from the top reservoir by 10% above that needed to remove all the smoke arriving from below.

Calculating smoke temperature The temperature rise of the smoke layer, above ambient, is given in Table 2, for a 5 MW fire (ignoring any further cooling) and can be calculated from: θ =

Q

(4)

MCp where Q = heat flow rate (kW) θ = temperature rise of the smoke layer above ambient (°C) Cp = specific heat capacity of the gases (kJ/kgK) M = mass flow rate of smoky gases (kg/s) High smoke layer temperature will result in intense heat radiation causing difficulties for people escaping 15

16

Figure 16

Schematic plan of multi-storey mall with a smoke reservoir on each level

Figure 17

Schematic section of a two-storey mall with a smoke reservoir on the lower level

Figure 18

Schematic section of a two-storey mall with lower smoke reservoir venting into the upper reservoir

Table 2 Volume flow rate and temperature of gases from a 5 MW fire (ignoring cooling) Mass flow rate (Mass rate of extraction) kg/s 9 12 15 18 24 30 36 50 65 80 95 110 130 150

Temperature of gases above ambient °C 556 417 333 278 208 167 139 100 77 63 53 45 38 33

beneath the smoke layer in the mall. To reduce the intensity of heat radiation the smoke layer temperature in the malls should be less than 200°C. In general a higher clear layer beneath the smoke layer will lead to more air being entrained into the rising smoke plume and therefore lower smoke temperatures. If the height of the mall is restricted, then it may not be practical to increase the clear layer height in order to reduce the smoke layer temperature, in which case consideration may be given to installing sprinklers into the malls specifically to reduce the smoke layer temperature by sprinkler cooling. This is discussed further in the following section.

Mall sprinklers Sprinklers in shops are an essential part of the smoke ventilation design in order to prevent a fire growing beyond the design fire size. Sprinkler operation in the malls 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 ceiling.

Volume rate of extraction (at maximum temperature) m3/s 21.5 24 26 29 34 39 43 55 67 79 91.5 103.5 120 136

Sprinkler cooling can be used in the malls to reduce the smoke layer temperature to below 200°C, above which heat radiation from the layer is likely to impede escape beneath. A natural ventilation system relies on the buoyancy of the smoke for extraction, therefore if sprinkler cooling is underestimated, the use of unrealistically high smoke temperatures could lead to the system being underdesigned. Conversely 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. The heat lost from smoky gases to sprinklers in the mall is currently the subject of research although data suitable for design application is not yet available. An approximate calculation approach can be used, as follows:

When the fire occurs in a shop, operation of sprinklers in the mall will not assist in controlling it. If too many sprinklers operated in the malls sprinklers in the shops could become less effective as the available water supply approaches its limits.

If the smoke passing a sprinkler is hotter than the sprinkler operating temperature that sprinkler will eventually be set off. The sprinkler spray will then 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 simply be assumed to be approximately equal to the sprinkler operating temperature if natural vents are used.

Malls should be sprinklered if they contain sufficient combustibles to support a fire larger than the design fire size of 5 MW, 12 m perimeter, during their operational lifetime. Note however that sprinklers installed at high level in a multi-storey mall are unlikely to operate unless the fire size reached is much larger than this.

The cooling effect of sprinklers in the malls can be ignored in determining the volume extract rate required for fans. This will err on the side of safety. Alternatively this cooling and the consequent contraction of the smoky gases can be approximately estimated on the basis of an average value between the sprinkler operating temperature and the calculated 17

initial smoke temperature, since a fire may occur close to at least one of the extract openings, whereas the other openings may be well outside any probable zone of operating sprinklers. The number of potential ‘hot’ and ‘cool’ intakes must be assessed separately for each shopping complex when calculating the average temperature of extracted gases. If the sprinkler operating temperature is set high enough and is above the calculated smoke layer temperature, then sprinkler cooling in the malls can be ignored. Note that the effect of sprinkler cooling is to reduce the heat flux Q without significantly changing the mass flux. It follows that once a new value of θ has been estimated, the new heat flux can be found using Equation 4. Note that sprinklers must not be installed close to ventilator extract positions.

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 3b rather than 3a. It is also necessary in some malls to determine the flowing layer depth between channelling screens beneath balconies. This flowing layer depth will be altered by the presence or absence of a downstand which is deep in comparison to the approaching layer. If there is a deep downstand at the balcony edge use Table 1a.

Local deepening When a buoyant layer of hot smoke flows along beneath the ceiling and meets a transverse barrier it deepens locally against that barrier27; the kinetic energy of the approaching layer is converted to buoyant potential energy against the barrier as the gases are brought to a halt.

Flowing layer depth Smoke entering the ceiling reservoir will flow from the point of entry towards the vent or fans. This flow is driven by the buoyancy of the smoke. Even if there is a very large ventilation area downstream (eg if the downstream roof were to be removed) this flowing layer would still have a depth related to the width of the mall, the temperature of the smoke and the mass flow rate of smoke. Work by Morgan19 has shown that this depth can be calculated for unidirectional flow under a flat ceiling, as follows:  2/3 d1 =  1/2   γθ W   MTc

where

(5)

d1 = flowing smoke layer depth (m) Tc = absolute temperature of the smoke layer (K) θ = temperature rise of the smoke layer above ambient (°C) W = channel width (m) γ = downstand factor 36 if deep downstand is present at right angles to the flow; downstand factor 78 if no downstand is present at right angles to the flow M = mass flow rate of smoky gases (kg/s)

The resulting values of layer depth for different reservoir widths and mass flow rates of smoke are shown in Table 3a. This ignores the effects of cooling in the layer, therefore if sprinklers are installed in the mall Equation 5 should be used after estimating the effects of such cooling (see previous section). 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 18

Table 3a Minimum reservoir depth (m) Mass flow rate entering reservoir (kg/s)

Width of reservoir (m)* 4

6

8

10

12

15

10 15 20 25 30 40 50 70 90 110 130 150

1.1 1.4 1.7 2.0 2.3 2.8 3.4 4.5 5.6 6.7 7.8 9.0

0.8 1.1 1.3 1.5 1.7 2.2 2.6 3.4 4.3 5.1 6.0 6.8

0.7 0.9 1.1 1.2 1.4 1.8 2.1 2.8 3.5 4.2 4.9 5.6

0.6 0.8 0.9 1.1 1.2 1.5 1.8 2.4 3.1 3.6 4.2 4.9

0.5 0.7 0.8 1.0 1.1 1.4 1.6 2.2 2.7 3.3 3.8 4.3

0.5 0.6 0.7 0.8 0.9 1.2 1.4 1.9 2.3 2.8 3.2 3.7

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

Table 3b Minimum reservoir depth — deep downstand across the flow Mass flow rate entering reservoir (kg/s)

Width of reservoir (m)* 4

6

8

10

12

15

10 15 20 25 30 40 50 70 90 110 130 150

1.8 2.3 2.8 3.3 3.8 4.7 5.7 7.5 9.4 11.2 13.1 15

1.4 1.8 2.2 2.5 2.9 3.6 4.3 5.8 7.2 8.6 10.0 11.5

1.2 1.5 1.8 2.1 2.4 3.0 3.6 4.8 6.0 7.1 8.2 9.5

1.0 1.3 1.5 1.8 2.1 2.6 3.1 4.1 5.1 6.1 7.1 8.2

0.9 1.1 1.4 1.6 1.8 2.3 2.7 3.6 4.5 5.4 6.3 7.2

0.8 1.0 1.2 1.4 1.6 2.0 2.4 3.1 3.9 4.7 5.4 6.2

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

When designing a smoke ventilation system for shopping centres of more than one level, it is often necessary to control the path of smoke flow using downstand smoke curtains. These are typically installed around the edge of voids to prevent smoke flowing up through the voids. If the void edge is close to the shop this local deepening could cause smoke to under-spill 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 shop unit on fire. The extent of local deepening can be found from Figure 19. The depth of the established layer (dm in Figure 19) in the mall 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 shop front. The additional depth ∆ dw can then be found by inspection of Figure 19, allowing the necessary minimum overall depth (dm + ∆ dw) of the void edge screen to be found. Alternatively it has been shown28 that the formula given below can be used

Figure 19

to determine approximate values for local deepening: ∆dw

=

2(1 – logedm)

(6)

logew where ∆ dw = additional deepening below the established smoke layer at the transverse barrier (m) dm = established layer depth (m) w = distance between the shop front and the transverse barrier (m) Note that extrapolating too far will risk an increasing (but unknown) error, since equation 6 is a purely empirical fit to the data and has no theoretical derivation. Note also that Equation 6 only applies strictly to a 5 m high ceiling. Figure 19, on the other hand, can readily be scaled.

Inlet air There must be adequate replacement air for the efficient operation of a smoke ventilation system. The ratio of inlet area to extract area is used in the calculation of natural ventilation area required to account for the effect of inlet restrictions on the

Local deepening at a transverse barrier

19

efficiency of the system (see page 21). If doors are required to open for replacement air, an appropriate entry coefficient should be used. Natural vents in unaffected smoke zones can often be opened automatically, and simultaneously with the main smoke ventilation system extracting smoke, contribute to the total air inlet required. If a powered smoke extract system is used, smaller areas for inlet air may be sufficient. If the area available for inlet becomes too restricted, incoming air flow through escape doors may be at too high a velocity for easy escape. Studies at the Building Research Establishment29 have shown that winds above 5 m/s can cause discomfort to pedestrians. Such air inflows through doors would hinder escapees and could be dangerous since they might already be predisposed to panic. It would perhaps be wise to design a smoke ventilation system such that air velocities through doors are less than 3 m/s. A high relative velocity between the smoke layer and incoming air occurs when air is drawn in through an inlet of limited area and the resulting air stream (or jet) passes below the smoke layer immediately. Smoke will be drawn down into such a jet by the venturi effect17, causing a significant loss of visibility in the lower cold air regions; this can occur when doors are used for inlet air (Figure 20). It can be minimised by placing screens defining the end of the reservoir at least 3 m back from the air inlet, giving the inflow an increased cross-section and a drop in velocity. This measure also permits turbulence in the entering air stream (caused by external winds) to damp out before

Figure 20a

Smoke from a buoyant ceiling layer mixing into a high velocity air inflow

it can disturb the smoke/air interface and cause excessive loss of visibility. If the layer base is designed at least 2 m above the top of the doors (or air inlets) there is no need to set back the reservoir screen17. Any stagnant region in the cold clearer air beneath the smoke layer would suffer from a steady accumulation of smoke. The fire and other major entrainment sites will act as air pumps and cause air to flow from the inlets towards themselves. The air inlets should therefore be chosen to ensure that these flows of cold air will flush through all areas of the malls below the ceiling reservoir. Any smoke wisps that enter the lower clearer air will thus be swept back into the main body of the hot smoke. A fan-driven inlet air supply 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 extract points The beginning of this chapter outlined the importance of distributing extraction points about a smoke reservoir to prevent the formation of stagnant cooling regions. The number of extraction points within the reservoir is also important since, for any specified layer depth, there is a maximum rate at which smoky gases can enter any individual extract vent (be it natural, chimney, or mechanical). Any further attempt to increase the extraction through that vent merely serves to draw air into the orifice from below the smoke layer. This is sometimes known as ‘plug-holing’. It follows that, for efficient extraction, the number of extract points must be chosen to ensure that no air is drawn up in this way. Table 4, which is based on experimental work30 subsequently modified by Heselden31, lists the minimum numbers needed for different reservoir conditions and for a variety of mass flow rates being extracted from the vents in the reservoir. Table 4 strictly applies to vents which are small compared to the layer depth below the vents. If calculation is preferred to using Table 4, the following apply at the critical point where air is about to be drawn into the openings31: At an opening, m = α (gd5 T1 θ/Tc2)1/2

Figure 20b

20

Mixing is reduced by allowing the incoming air flow to slow before contacting the smoke layer

(7)

where m = critical extract rate for efficient venting at one vent (kg s-1) α = 1.3 for a vent near a wall (kg m-3)

Table 4 Minimum number of extraction points needed in a smoke reservoir Total mass rate of extraction (kg/s) 9 12 15 18 20 25 30 40 50 70 90 110 130 150

Depth of layer below extraction point (m) 1

1.5

2

3

4

5

7

10

4–5 5–6 6–8 7–9 8–10 9–13 11–16

2–2 2–3 2–3 3–4 3–4 4–5 4–6 6–8 8–11

1–1 1–1 1–2 2–2 2–2 2–3 2–3 3–4 4–5 6–8

1–1 1–1 1–1 1–1 1–1 1–1 1–1 1–2 2–2 2–3 3–4 4–5

1–1 1–1 1–1 1–1 1–1 1–1 1–1 1–1 1–1 1–2 2–2 2–3 3–3 3–4

1–1 1–1 1–1 1–1 1–1 1–1 1–1 1–1 1–1 1–1 1–1 1–2 2–2 2–3

1–1 1–1 1–1 1–1 1–1 1–1 1–1 1–1 1–1 1–1 1–1 1–1 1–1 1–1

1–1 1–1 1–1 1–1 1–1 1–1 1–1 1–1 1–1 1–1 1–1 1–1 1–1 1–1

Note: In reading the above table, the first number is for extraction points well away from the walls, the second is for extraction points close to the walls

or

α = g = d = T1 = θ = Tc =

1.8 for a vent distant from a wall (kg m-3) acceleration due to gravity (ms-2) depth of smoke layer below the vent (m) ambient absolute temperature (K) excess temperature of smoke layer (°C) T1 + θ.

The required number of extract vents (N) is then given by: N ≥ M m

(8)

where M is the total extract rate required from the reservoir Where very large or physically extensive vents are used (eg a long intake grill in the side of a horizontal duct) an alternative method is possible. For this case, Table 3a can be used with the ‘width of reservoir’ being taken as the total horizontal accessible perimeter of all the vents 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 one can use Table 3a or Equation 5 to find the minimum value of accessible perimeter.

rate of extraction is largely dependent upon the depth and temperature of 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 selfcompensating mechanism. Care must be taken to ensure that natural ventilators are not sited in a position subject to positive wind pressures (see Chapter 4). If smoke ventilation is required in such positions powered extraction must be used instead of natural vents. Note that natural vents and powered extracts should never be used together in the same reservoir. Table 5 gives the minimum aerodynamic free area of ventilation required, ignoring the effect of any inlet restriction. To allow for the effect of limited fresh air inlets the following guide can be used: If the inlet area for the whole mall is the same as the vent area for the reservoir given by Table 5, this indicated vent area should be increased by approximately 35%. If the total inlet area is twice the reservoir vent area, the indicated vent area should be increased by 10%.

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

The precise relationship between the mass flow rate extracted, the vent area, the inlet area and the smoke layer is12:

Natural ventilation — area required per reservoir

AvCv =

A natural ventilation system uses the buoyancy of the smoke to provide the driving force for extraction. The

M  Tc2 + (Av Cv/AiCi)2ToTc 1/2   ρo   2g d θ T b

c

(9)

o

21

where AvCv = aerodynamic free area of natural ventilation (m2) Av = measured throat area of ventilators for the reservoir being considered (m2) Ai = total area of all inlets (m2) Cv = coefficient of discharge (usually between 0.5 and 0.7) Ci = entry coefficients for inlets (typically about 0.6) M = mass flow rate of smoke to be extracted (kg/s) ρ = ambient air density (kg/m3) ° g = acceleration due to gravity (m/s2) db = depth of smoke beneath ventilator (m) θc = temperature rise of smoke layer above ambient (°C) Tc = absolute temperature of smoke layer (K) To = absolute temperature of ambient air (K) Natural ventilation can sometimes be enhanced by using chimneys to increase the buoyant head of hot smoke. However, a system of chimney vents can be difficult to design. The flow resistance must be taken into account. Typical values may be found in the CIBSE Guide32, but it should be noted that unpublished experiments at FRS have shown the entry coefficient to take on a value of approximately 0.7 (as compared with the more usual value of 0.5 used in HEVAC calculations) for a sharp-edged opening in drawing gases from a relatively shallow buoyant layer. 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.

Powered ventilation A powered smoke extract system consists of fans and associated ductwork designed to remove the mass flow rate of smoke entering the smoke reservoir, and to be

22

capable of withstanding the anticipated smoke temperatures. 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 section can be converted to the corresponding volume flow rate and temperature, using Table 2 (or the following equation) for selection of the appropriate fans: V =

M

Tc

(10)

ρ oT o

where V = volume flow rate of gases to be extracted from the smoke layer (m3/s) Table 5 The minimum total vent area (m2) needed in one ceiling reservoir (from Equation 9 with Cv = 0.6) Mass rate of extraction (kg/s) 9 12 15 18 24 30 36 50 60 75 90 110 130 150

Smoke depth beneath vents (m) 1.5

2

3

4

5

7

0

4.8 6.1 7.5 9.0 12.2 15.6 19.2

4.1 5.3 6.5 7.8 10.5 13.5 16.6 25

3.4 4.3 5.3 6.4 8.6 11.0 13.6 20 25 34 43 57 71 87

2.9 3.7 4.6 5.5 7.5 9.5 11.8 17.5 22.0 29 37 49 62 75

2.6 3.3 4.1 4.9 6.7 8.5 10.5 15.7 19.7 26 34 44 55 67

2.2 2.8 3.5 4.2 5.6 7.2 8.7 13.2 16.7 22 28 37 47 57

1.8 2.4 2.9 3.5 4.7 6.0 7.4 11.1 13.9 18.6 24 31 39 48

Note: Add on 10% if the total inlet area in the mall is twice the vent area. Add on 35% if the total air inlet area in the mall is equal to the vent area

Chapter 3 Large shop opening onto a mall As outlined in Chapter 2, the preferred option for the majority of shops is to provide a common smoke ventilation system in the malls unless the shops are greater than 1000 m2 (if the mall is naturally vented) or 1300 m2 (if the mall has powered extraction). Shops larger than these sizes need additional measures to protect the mall. A smoke layer within a large shop will lose heat as smoke spreads throughout the store. This heat loss is caused by the cooling action of the sprinkler spray and heat loss by conduction and radiation to the building structure. Thus the cooling is related to the size of the shop. If there is too much heat loss, the effect in the mall will be as if the mall reservoir itself were too large, since there will again be excessive downward mixing and loss of visibility in the mall. To ensure safe conditions in the mall therefore, smoke from large stores must be prevented from flowing onto it. To achieve this the store must be either isolated from the mall or have its own smoke ventilation system. Fire shutters should only be used to isolate the store from the mall if there is no other practicable choice — and if the implications for means of escape and the possible psychological effect on escapees have been fully taken into account. This excessive cooling of smoke is thought only to be serious in stores over 1000 m2 in area (or 1300 m2 if powered extraction is used in the mall). If they are not isolated from the mall, such large stores should have ceiling reservoirs formed by similar methods to those for malls. Since no smoke is allowed to enter the mall, each reservoir can be limited to 2000 m2 (or 2600 m2 if powered extraction is used) to prevent excessive cooling, ie to the same maximum area as the combined maxima for the mall plus a small shop unit. Note that any area less than 1000 m2 ‘left over’ from this internal sub-division, and adjacent to the mall, can then be ventilated via the mall in the same way that a small shop would be. The quantity of smoke entering the ceiling reservoir within the shop is given by Equation 1, and results for a 5 MW fire are shown graphically in Figure 21. Having determined the mass flow rate of smoke, the design procedure given for the mall ventilation system in Chapter 2 can be used to determine the extract capacity required within the store. It should be noted that, provided smoke cannot enter the mall, the height of the smoke base in the shop need not be the same as it would be in the mall. When calculating the overall smoke layer temperature the procedure for mall sprinklers given on page 17 should be used (but note that any individual intake or duct might be located immediately above any fire and this should be taken into account when specifying the

temperature requirements for fans and ducts). An alternative, particularly appropriate where the total width of openings between the store and the mall are restricted, is to provide large capacity ‘slit’ extraction33 in the ceiling over the whole width of such openings, including doors, but not including fixed glass windows (Figure 22). Such a system is likely to work best with further extraction distributed within the store, which may possibly be provided by the normal ventilation extraction system, with the normal ventilation input and recirculation of air being stopped. Whilst this system is designed to prevent cool smoke entering the mall, it will not necessarily maintain a clear layer within the store itself. The extraction should be provided very close to the opening from a continuous slit which may be situated in the plane of

Figure 21

Rate of production of hot smoky gases in a store from a 5MW fire

23

the false ceiling. The extraction rate (V) can be found from:

V=

where

3.45  3.24 W 

W = Wm = H = Vs =

Figure 22

24

–

0.74Vs   Wm HW2/3 

m3/s–1

(11)

total width of stores opening (m) width of stores opening onto the mall (m) height of store opening (m) volume extract rate from store ventilation system (m3/s)

Slit extraction

Chapter 4 Some practical design considerations Factors influencing the design fire Unsprinklered shops If sprinklers are not installed in the shop, it is likely that the fire will grow larger than the 5 MW, 12 m perimeter, design fire14. An analysis of fires in public areas of retail premises13 has suggested that fewer than 4% of fires exceed the design fire area. (Note that the ‘sprinklered’ curve in Figure 23 is drawn to conform to Morgan and Chandler’s13 interpretation of the statistical data and to have approximately the same form as the 'unsprinklered' curve; it can be seen that the latter is far more reliable). The same statistical probability of being exceeded when sprinklers are not present gives a fire size in excess of 100 m2 (Figure 23) — potentially 10 times larger. This could lead to ‘flashover’, when the fire will develop very rapidly to involve the entire shop. Evidence from actual fires suggests that flashover can happen quickly34. A fully involved shop fire in a ‘typical’ shop unit may not have a large enough opening onto the mall for sufficient oxygen to be available to support complete combustion within the shop. This will result in unburnt gases flowing from the shop, mixing with air and further combustion taking place in the mall itself, resulting in considerable downward heat radiation. Unless the mall roof is high it is unlikely to be practical to design a smoke ventilation system that will ensure safe escape, for people in normal clothing, below the mall ceiling smoke reservoir. Clearly if sprinklers are not installed in the shops a 5 MW design fire size is inappropriate, and the formula, equations and much of the guidance given in this Report are invalid. This is discussed in more detail in a paper by Gardner14. Multi-occupancy malls As shopping centres are becoming increasingly multiuse developments, it is important to consider the potential fire risk in occupancies other than shops, if they adjoin the mall without any fire separation. Information on fire sizes in some other classes of occupancies has been developed during research into smoke control in atrium buildings35,36. It is important to assess the likely fire size in any of these occupancies, and a check should be made to test whether a system designed on the basis of a 12 m perimeter 5 MW fire will be adequate to deal with a fire from any alternative occupancy opening onto that section of the mall. The mall smoke ventilation system should be designed to cope with the most severe consequences of any of the selected fire sizes in any of these occupancies.

The effect of wind on the efficiency of a smoke ventilation system 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 extractions. Tall buildings or taller areas of the same building (such as roof top plant rooms etc) can create a positive wind pressure on lower nearby roofs. Steeply pitched roofs, ie those with over 30° pitch, may also have a positive wind pressure on the windward slope. A suggestion sometimes advanced for offsetting wind over-pressures is to increase the total area of natural ventilation per reservoir. Since the over-pressure 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 vent to mix into the smoke. A powered smoke extraction system should be designed to overcome the anticipated wind pressures. 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 vents) can overcome wind interference and may even be able to convert an overpressure into a suction. There is also the possibility of selectively opening vents 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 a shopping centre and 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 over-pressure and suction have been identified for all possible wind directions, design of vents or fans can proceed as before.

False ceilings in the mall Where there is an unbroken false ceiling in the mall 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 25

Figure 23 Retail premises — horizontal fire damaged area

smoke reservoir must be continued above the ceiling. If the proportion of free area is large enough the reservoir and its screens may 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 experimentally37 that a minimum free area of 25% can be used as a ‘rule of thumb’ value for allowing safe escape. Cool smoke can sometimes be expected to affect nearby shops but would not significantly hinder safe escape. Free areas of less than 25% are possible in some circumstances; expert advice should be sought where this possibility is felt desirable.

The use of a plenum chamber above a false ceiling in a shop Some designs have been seen in which the space above a mainly solid false ceiling in a large store 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 store through a number of openings in the false 26

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. This system can of course also be used for malls. A potentially valuable bonus of such a system 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. The plenum chamber should not be larger in area than its associated reservoir. Larger chambers should be subdivided by smoke screens that are the full height of the chamber and which extend downwards to form a complete smoke reservoir below the false ceiling. 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 one or two metres) slots surrounding a region of false ceiling could perhaps be used instead of screens below

the false ceiling. 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’ false ceiling will ensure that the smoke passes through at least one sprinkler spray en route to the extract.

for any given reservoir. Until more reliable information becomes available, it seems reasonable to assume that the equivalent flat roof is one where the layer base is at the same height and the layer has the same cross-sectional area. The effective layer depth can then be determined with d1, now the depth of smoke beneath the equivalent flat roof position.

Smoke flow in low narrow malls The principles described in this section could be used with extraction through a shaft vent (or chimney) from the space above the false ceiling provided that sprinklers were not installed in this space, since they would rob the gases of buoyancy.

Stores with internal voids When a store of more than one level, with internal voids, is open to a multi-level mall, smoke flow onto the mall at more than one level simultaneously should be avoided. In some circumstances this may mean isolating the store from the mall on one or more levels. This can be achieved in a number of ways, including the use of fire shutters on detection of smoke to isolate the store. Whenever these are used a number of factors need to be assessed, such as the implications for means of escape and the psychological effect on people of fire shutters operating.

The problems of smoke flow in low narrow malls are more often encountered when dealing with the refurbishment of older existing shopping centres. Table 3 gives the depth of flowing layer in malls of different widths and varying mass flow rates. Narrow malls less than 5 metres floor-to-floor or with deep beams across the mall beneath which the smoke must flow, could have insufficient clear height for escape and correspondingly high smoke temperatures. These high smoke layer temperatures can be reduced by installing sprinklers in the malls, specifically to cool down the smoke layer. The smoke layer base cannot be raised by increasing the rate of extraction unless there is sufficient floor-toceiling height to achieve this. From Table 3 it can be seen that if the smoke flows in two directions in the smoke reservoir, the flowing layer depth is smaller than for a single direction flow.

Sloping malls If the mall floor slopes significantly, a 3 m clear layer at one end of the smoke reservoir may be considerably less at the other end. The smoke ventilation system should be designed to maintain an acceptably clear layer at the high end for a fire in a shop at the low end, resulting in a greater height of rise to the smoke layer base and mass flow rate of smoke. There may be a difference in floor level between the mall and the shops either side. If so, the height of rise to the smoke layer base should be measured from the lowest shop level and the clear layer in the malls should be measured from the highest mall level.

Assessment of effective layer depth As explained in Chapter 2 the effective layer depth introduces a correction in the procedure for determining entrainment, to account for the layer of warmed air beneath the visible smoke layer base. The experimental work was carried out on a scale model with a flat roof as shown in Figure 13. The depth of smoke beneath both the ventilator and the ceiling was found to be the same. In practice pitched roofs, pyramids, domes or rooflights are commonly used as smoke reservoirs. No experimental data exist on these other reservoir forms and using the depth beneath the apex of a pitched roof, for example, will increase the effective layer depth, possibly resulting in an underestimate of the mass flow rate of smoke (Figures 24a and 24b). Therefore it is desirable to determine an equivalent flat roof position

This can be achieved by ensuring that 50% of the extract capacity is installed at either end of the reservoir such that bi-directional flow will occur. Again from Table 3 it can be seen that if the mall is made wider the flowing layer depth is reduced. One method of achieving this without moving the shop fronts back, which has been used in practice, is shown in Figure 25. The mall is widened at high level without any change at lower levels by ‘stepping back’ the shop fronts. For aesthetic reasons this could be above a permeable false ceiling, allowing the flowing layer to exist wholly above the false ceiling line.

Basement service levels Where regulatory authorities feel that a smoke ventilation system is required for the basement service level of a shopping centre, there has previously been no published guidance. A fire size of 7 MW, 15 m perimeter corresponding to a fully burning tractor (of an articulated vehicle) or a partly burning trailer or a van, has become widely used as a maximum practical fire size for smoke ventilation design. Note however that larger fires are possible for large delivery vehicles or trailers. The design procedure is similar to that for a large store given in Chapter 3. Figure 26 gives mass flow rates for varying clear layer heights, for the basement service level design fire size. The temperatures and volume extract rates for fans can then be determined from Table 6. If natural 27

Figure 24a

Assessment of effective layer depth from apex of pitched roof

Figure 24b

Assessment of effective layer depth from the equivalent flat roof position

Figure 25

28

Reducing the flowing smoke layer depth by widening the mall at high level

building design, it can be noted that a fire in one car is unlikely to spread to a neighbouring parked car, even one with a ‘plastic’ body shell, when sprinklers have been fitted in the car park. This comment should not be taken as a general recommendation that sprinklers are regarded as essential in all enclosed car parks. The design procedure is similar to that for large stores given in Chapter 3 since the perimeter is the same as for a shop fire. Figure 21 can be used to obtain the mass flow rate for varying clear layer heights. However, since the heat flux is 2.5 MW the temperature and volume extract rates must be determined from Table 7. If natural ventilation is to be used the aerodynamic free area can be determined from Equation 9.

Smoke transfer ducts Stagnant regions of a smoke reservoir will suffer from continued heat loss resulting in downward mixing into the air below as discussed in Chapter 2. Good distribution of ventilator extract positions can prevent this being significant. Where this solution is impracticable smoke transfer ducts can be installed to move smoke from the stagnant region to another part

Table 6 Volume flow rate and temperature of smoke from a 7 MW 15 m perimeter fire (ignoring cooling)

Figure 26

Mass flow rate of smoke entering the ceiling reservoir from a 7MW fire, 15m perimeter

ventilation is used the aerodynamic free area required can then be found from Equation 9.

Mass flow kg/s

Temperature of smoke above ambient °C

Volume extract rate m3/s

9 12 15 18 24 30 40 50 70

778 583 467 389 292 233 175 140 100

28 30 33 35 40 45 53 62 79

Enclosed car parks Should regulatory authorities require a smoke ventilation system for an enclosed car park, there has previously been no published guidance. This section outlines an approach that has been used for some time by the Fire Research Station. A design fire size of 2.5 MW, 12 m perimeter corresponding to a single burning car has become widely used as the design fire size based on a recalculation of experimental results38. However, concern has been expressed about the increasing use of plastics in car bodies, and of plastic petrol tanks and the implications that these would have on the design fire size. Further research is needed to evaluate the practical significance of such concern. Where the approving authorities feel that such aspects might significantly influence their opinion of any individual

Table 7 Volume flow rate and temperature of smoke from a 2.5 MW 12 m perimeter fire (ignoring cooling) Mass flow kg/s

Temperature of smoke above ambient °C

Volume extract rate m3/s

9 12 15 18 24 30 40 50 70

278 208 167 139 104 83 63 50 36

15 17 20 22 27 32 41 49 66

29

Figure 27

Use of smoke transfer ducts in otherwise stagnant regions

of the smoke reservoir to rise with an existing flow towards a vent or extract fan (Figure 27). As a ‘rule of thumb’, if the reservoir continues more than 3 times as deep beyond an extract opening as the reservoir is wide, then a smoke transfer duct may be necessary. A currently recommended value of the minimum extract rate is 4% of the smoke layer’s net flow or 1 m3/s, whichever is the greater.

Entrances within the smoke layer There have been shopping centres designed with entrances within the smoke layer (eg from car parks above the shopping levels). Any pedestrian area inside these entrances and open to the mall needs to be small enough to be evacuated quickly through smoke sealed doors.

Other situations The possibilities for special features are by no means exhausted by the above. Whenever another special case occurs that is not explicitly covered by this Report, advice should be sought from experts.

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Chapter 5 Some operational factors All reservoir screens, smoke channelling screens and natural vents that are not permanent features should operate automatically, actuated by smoke detectors, as should mechanical extract systems. Other types of detector are less useful since, at best, they have longer response times and many (eg fusible links) may not operate at all. It is much easier to prevent smoke spreading to escape routes than it is to clear such routes subsequently. If doors are to be relied on as air inlets, devices to open them automatically are needed. When non-permanent automatic drop curtains are used as reservoir screens the gaps between curtains should be minimised. If a powered smoke extract system is installed, care should be taken to ensure that smoke leakage through gaps between curtains does not inadvertently activate adjacent powered extract zones. It can be an advantage to open all the natural vents installed in a shopping centre, since those in zones unaffected by smoke can contribute to the fresh air inflow. Wherever it is feared that wind pressures on vents to unaffected reservoirs may be much more negative than on vents in a smoke-filled reservoir, it is recommended that wind-tunnel studies and/or a detailed pressure analysis be carried out to evaluate the effect on venting efficiency. When mechanical extraction is used for a number of separate smoke reservoirs, it would be better to operate the smoke control system individually by zones. Facilities for manual override should be installed to allow greater flexibility in firefighting, testing and maintenance.

a high level, frequently where smoke would be flowing. This air would simply increase the quantity of smoke that has to be dealt with. Hence the normal ventilation inlet system should be automatically shut down when smoke is detected. If the smoke to be removed from a reservoir is too hot for the ducting or for the fans, sprinklers or water jets could be installed in a suitable part of the extract ducting to cool the smoke. A temperature rating quoted for many (but not all) smoke extract fans is to survive a temperature of 300°C for half an hour. Many authorities regard this time as sufficient for escape from most malls — but note that this should not be regarded as being universally true. All such criteria should be agreed with the approving authorities. All electrical apparatus and power supply cables used in these smoke ventilation systems should, of course, be protected to ensure sustained operation in a fire. Cold smoke tests are sometimes used for the acceptance testing of smoke ventilation systems. Whilst this cold smoke can be used to operate the smoke detection system and therefore activate all the components of the smoke ventilation system, it should be noted that since the smoke is cold it would not have the buoyancy that smoke in a true fire condition would have, and cannot therefore adequately test the ventilation efficiency of the system. Smoke ventilation systems should be regularly tested and adequately maintained.

The normal ventilation systems fitted to many, if not most, malls and shops blow air into the mall or shop at

31

References 1

Home Office and Scottish Home and Health Department. Fire precautions in town centre redevelopment. Fire Prevention Guide 1. London, HMSO, 1972.

2

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3

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Canter D. Studies of human behaviour in fire: empirical results and their implications for education and design. Building Research Establishment Report. Garston, BRE, 1985. Morgan H P. Smoke control methods in enclosed shopping complexes of one or more storeys: a design summary. Building Research Establishment Report. London, HMSO, 1979. British Standards Institution. Fire precautions in the design and construction of buildings. Part 10: Code of practice for enclosed shopping complexes. British Standard BS 5588: Part 10. London, BSI. To be published. Heselden A J M. Fire problems of pedestrian precincts. Part 1. The smoke production of various materials. Fire Research Station Fire Research Note 856, Borehamwood, BRE, 1971.

13 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. 14 Gardner J P. Unsprinklered shopping centres. Design fire sizes for smoke ventilation. Fire Surveyor, 1988, 17 (6) 41-47. 15 Hinkley P L. Rates of production of hot gases in roof venting experiments. Fire Safety Journal, 1986, 10 57–65. 16 Heselden A J M, Wraight H G H and Watts P R. Fire problems of pedestrian precincts. Part 2. Large-scale experiments with a shaft vent. Fire Research Station Fire Research Note 954, Borehamwood, BRE, 1971. 17 Morgan H P, Marshall N R and Goldstone B M. Smoke hazards in covered multi-level shopping malls: some studies using a model two-storey mall. Building Research Establishment Current Paper CP45/76, Borehamwood, BRE, 1976. 18 Hansell G O, Marshall N R and Morgan H P. Private communication. Fire Research Station 1988.

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19 Morgan H P. The horizontal flow of buoyant gases toward an opening. Fire Safety Journal, 1986, 11, 193–200.

Hinkley P L. Work by the Fire Research Station on the control of smoke in covered shopping centres. Building Research Establishment Current Paper CP83/75, Borehamwood, BRE, 1975.

20 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, Borehamwood, BRE, 1975.

9 London Transport Board. Second Report of the Operational Research Team on the capacity of footways. Research Report 95. 10 Heselden A J M and Hinkley P L. Smoke travel in shopping malls. Experiments in co-operation with Glasgow Fire Brigade — Part 1. Fire Research Station Fire Research Note 832, Borehamwood, BRE, 1970. 11 Silcock A and Hinkley P L. Fire at Wulfrun shopping centre, Wolverhampton. Fire Research Station Fire Research Note 878, Borehamwood, BRE, 1971.

32

12 Thomas P H et al. Investigations into the flow of hot gases in roof venting. Fire Research Technical Paper No 7, London, HMSO, 1963.

21 Morgan H P and Marshall N R. Smoke control measures in covered two-storey shopping mall having balconies as pedestrian walkways. Building Research Establishment Current Paper CP11/79, Borehamwood, BRE, 1979. 22 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. 23 Hansell G O, Morgan H P and Marshall N. Smoke flow experiments in a model atrium: Part 2 plume entrainment in the atrium. To be published.

24 Thomas P H. On the upward movement of smoke and related shopping mall problems. Fire Safety Journal, 1987, 12 (3) 191–203. 25 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.

32 Chartered Institution of Building Services Engineers. CIBSE Guide, Volume C, 1986. 33 Marshall N R and Heselden A J M. Smoke control in large stores opening onto enclosed shopping malls. Fire Surveyor, 1986, 15 (1) 18–22.

26 Law M. A note on smoke plumes from fires in multi-level shopping malls. Fire Safety Journal, 1986, 10 197–202.

34 Morgan H P and Savage N P. A study of a large fire in a covered shopping complex: St John’s Centre 1977. Building Research Establishment Current Paper CP10/80, Borehamwood, BRE 1980.

27 Morgan H P and Marshall N R. The depth of voidedge screens in shopping malls. Fire Engineers Journal, 1989, 48 (152) 7–9.

35 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.

28 Hansell G O. Fire Engineers Journal, 1989, 48 (152) 9.

36 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.

29 Penwarden A D. Acceptable wind speeds in town. Building Research Establishment Current Paper CP1/74, Garston, BRE, 1974. 30 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, Borehamwood, BRE, 1974. 31 Heselden A J M. Private communication. Fire Research Station, 1976.

37 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. 38 Butcher E G, Langdon-Thomas G J and Bedford G K. Fire and car park buildings. Fire Research Station Fire Note 10, Borehamwood, BRE, 1968.

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Liaison Panel The preparation of the final report was undertaken with the assistance of a Liaison Panel which consisted of the following members, representing both industry and Government interests: Department of the Environment, Construction Directorate Home Office, Fire Inspectorate Scottish Development Department Colt International Limited Gradwood Limited Nuaire Limited

34

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