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Learning from experience

Fire Investigation in Great Britain

By:
Dougal Drysdale
This article first appeared in the combined November 2000 issue of Fire Engineers and Fire Prevention magazines.
2000 Institution of Fire Engineers
www.ife.org.uk
and
The Fire Protection Association
www.thefpa.co.uk




The study and investigation of fires is an essential component in the development of fire safety engineering as a discipline. But, argues Dougal Drysdale, misleading conclusions can be drawn from fire scenes unless the investigator understands the wide range of factors that can influence the fire process.

ACCIDENT INVESTIGATION is an essential part in the development of any engineering discipline. It provides a means of improving our knowledge and understanding of the subject. For example, we accept that civil engineers are able to design and construct bridges that will be safe. But structural design and the relevant theory of structures is underpinned by the analysis of past failures. The same argument is true in the context of fire. Fire investigation is an essential component in the development of fire safety engineering as a discipline.

Certain fires have provided information that has been invaluable in improving our understanding of the fire process, which in turn should allow a more rational approach to the identification and mitigation of fire hazard. However, the response to significant failures (defined by a large life loss) has been dominated by societal reaction, followed by the imposition of new regulations to prevent recurrence. This is well illustrated by the aftermath of the Bradford City Stadium fire (1985)1 and the King's Cross Underground Station fire (1987)2. While such tragedies continue to provide valuable information for the fire safety scientist, that information is seldom used to its maximum advantage. It is gradually being absorbed into the body of knowledge required by the fire safety engineer but is seldom fed back into the fire investigation community. Although some fires provide the opportunity to expand our knowledge of fire behaviour, most fire investigations are routine and require only a rudimentary knowledge of fire dynamics. Occasionally contemporary knowledge is insufficient to resolve a particular problem, or the characteristics of the fire were such that the damage pattern is ambiguous, leading to possible misinterpretation. In both cases, a thorough investigation will have the dual purpose of identifying fire behaviour that may not previously have been recognised, as well as adding to the knowledge base.

Understanding the past

The study of fire dynamics has developed rapidly since the 1960s, but its adoption by the forensic community has been slow. Fire behaves according to the basic laws of physics, particularly those relating to fluid dynamics and heat transfer. Whether one is trying to predict how a fire might behave following ignition or to locate from the physical evidence at a fire scene where a fire started and how it developed, the same laws of physics must be applied.

From time to time a fire occurs leaving physical evidence and eyewitness accounts that are very difficult to understand. For example, there may be insufficient evidence to identify the locus of origin, or there may be features of the damage pattern which lead to conclusions inconsistent with eyewitness accounts, or even with other physical evidence. Some of the earlier texts on fire investigation were misleading in that they preserved some of the myths of fire behaviour, ignoring research results that clearly showed otherwise. One of these is the adage that all wood chars at the same, constant rate of one fortieth of an inch per minute. Not only does this seem unlikely, but research published in 19713 showed it to be false: the rate of charring depends strongly on the level of radiant (or convective) heating to which the wood has been exposed. Nevertheless, depth of char can be used as a valuable guide when attempting to interpret a fire scene. With care, understanding and the application of common sense, useful corroborative conclusions may be drawn.

Most fire investigators rely on their experience and have a general appreciation of fire behaviour, often allowing some details to be overlooked to no significant disadvantage. However, fires do occur where such details are of vital importance and can be of great assistance in the interpretation of the evidence. This will be illustrated with reference to three fires: the Stardust Club fire (Dublin, 1981), the King's Cross Underground Station fire (London, 1987) and the Garley Building fire (Hong Kong, 1996).

The investigation of the Stardust Club fire4 revealed significant shortcomings in regulatory control that allowed an extremely hazardous configuration of combustible materials to be used in a place of public assembly. A full-scale test was carried out to demonstrate how and why the fire developed so rapidly but, in fact, enough was known about the particular configuration of combustible materials to have been able to make at least a qualitative assessment of the hazard. On the other hand, the configuration of fuel (the wooden escalator) which led to the extremely rapid growth of the King's Cross fire had not previously been identified as a specific hazard (other than the undesirability of having extended combustible surfaces in a public space). The Court of Inquiry uncovered the so-called 'trench effect'. In the case of the Garley Building5, the fire risk could be said to have been obvious, but the characteristics of the fire were such that the patterns of damage in the area where the fire is alleged to have started were ambiguous and open to misinterpretation.

The Stardust Club

The fire at the Stardust Club in Dublin in February 1981 was responsible for the deaths of 48 people. The investigation and subsequent Inquiry revealed a large number of factors which had contributed to the high toll (including locked escape doors), but the root cause was the rapidity with which the fire developed.

In circumstances where there has been rapid fire development, there is always a suspicion that the fire has been started maliciously with an accelerant. This was found not to be the case in the Stardust fire, but it was appreciated that the combination and configuration of combustible materials - namely carpet tiles which were used to line the walls - in the alcove where the fire is known to have started was hazardous. The Fire Research Station carried out a full-scale demonstration of the fire's development, which confirmed the speed with which the fire grew in its early stages, as the carpet tiles on the rear wall became involved6, 7. Indeed, the ferocity of development demonstrated not only the hazard of combustible wall-lining materials but also how a low ceiling can make matters even worse. The fire scientist knew that this combination was very dangerous but at that time (and perhaps even now) was unable to quantify the problem. This is a major disadvantage. Quantification would have allowed the fire safety engineer to make a substantive case to the authorities that such a combination was totally unacceptable. Similarly, the fire investigator would benefit if after the event he was able to demonstrate by rational argument, backed up by calculation, that this had been the cause of such rapid fire development. In a sense, the Stardust Club fire was analogous to the failure of a bridge that alerted the design engineer to a fault to which his attention had not previously been drawn.

The fire investigator who is aware of the basic principles of fire science and fire dynamics will be able to recognise configurations of fuel that could be hazardous. As our knowledge of fire behaviour increases, reliable modelling techniques will become available which will allow the validity of conclusions to be demonstrated8, 9. For example, ongoing research into flame spread on vertical surfaces is showing how data from small scale tests (specifically, the cone calorimeter) can be used to model the rate of upward fire spread. Further developments will soon enable the rate of fire growth, even under complex circumstances, such as those in the Stardust Club, to be predicted. With such models available, there would be no further need to carry out expensive full-scale tests simply for demonstration purposes.

King's Cross

The King's Cross Underground Station fire in November 1987 revealed a significant gap in our appreciation of fire behaviour. The location of origin was in no doubt. The fire was first seen underneath the steps of escalator No. 4 (one of three escalators which served the Piccadilly Line from the King's Cross Station booking hall), approximately one-third of the way up from the lower level. This was confirmed in the subsequent investigation as the lowest point of damage10. The fire spread above the escalator steps and the damage to the ceiling of the escalator shaft suggested that flames had 'corkscrewed' diagonally upwards from the lowest point to the other side of the escalator shaft, meeting the opposite side about 8m from the booking hall. Eyewitness accounts indicated that the fire had spread extremely rapidly in the two or three minutes before the booking hall became totally engulfed in flames. One witness referred to a 'jet of flame' emerging into the booking hall from the Piccadilly Line escalator shaft11. It was some time before an explanation was found to account for this rapid development.

During the early stages of the investigation, considerable attention was paid to the flame spread properties of the multiple layers of paint on the ceiling of the escalator shaft as it was known that multiple paint layers could be a serious fire hazard. Other suggestions included unusual cross radiation effects within the escalator trench, but ultimately it was shown that a very different mechanism was responsible. This came to be known as the 'trench effect'. The first indication of this phenomenon was revealed by some early computational fluid dynamic (CFD) modelling12 of the flows of hot gases in the escalator trench (inclined at an angle of 30). Instead of the hot buoyant gases rising vertically, out of the confines of the trench, they became 'attached' to the trench floor and flowed upwards within the trench, hinting for the first time that the flames from the burning wooden escalator would flow in a similar fashion and preheat unburnt fuel (wooden balustrades, steps and risers), thus causing very rapid flame spread up the trench towards the booking hall. This mechanism was subsequently confirmed in laboratory tests on a small scale model13, and ultimately in a one-third scale mock up which included the escalators, the escalator shaft and the booking hall14. Subsequent CFD modelling of the behaviour of flames on inclined surfaces (with and without 'sidewalls') has confirmed the experimental findings and has provided valuable insight into the general problem15,16. The technical aspects of the report of the Inquiry into the King's Cross fire should be required reading for any fire investigator. Some of the evidence was confusing and capable of being misinterpreted if only the basic rules of thumb were applied. A combination of an understanding of fire dynamics and the liberal application of common sense will help avoid misinterpretation of ambiguous clues that may be found at the fire scene.

The Garley Building

An interesting example of an ambiguous clue was encountered during the investigation of the fire in the Garley Building in which 40 people lost their lives. This occurred on 20 November 1996 in Tsim Sha Tsui, Hong Kong.

The building, which was 16 storeys high, was operating as a multiple occupancy commercial building. The first three floors were occupied by a department store (CAC) and the remaining floors housed a variety of businesses. One stairway (A) had connected the lift lobbies from the ground to floor 3 (3/F) but had been sealed off at ground and 3/F to form a series of storerooms. These were separated from the lift lobbies on 1/F and 2/F by plywood partitions. Furthermore the fire-resisting walls between the lift lobbies and CAC on 1/F and 2/F had been replaced by roller shutter doors. These closed automatically on activation of smoke detectors but were not fire rated. Two of the four lifts serving the building were under refurbishment at the time of the fire. Lift shafts 1 and 3 had been completely stripped and were open at every level, only protected by wooden hoardings. Bamboo scaffolding had been erected within the shafts to allow access for the installation of fixed hardware.

Fire was first observed in the 2/F lift lobby by a workman who climbed up the bamboo scaffolding in lift shaft 3. He described flames behind some wooden planks, which were leaning against the hoarding, about 2m from the edge of lift shaft. Within a short period of time flashover appears to have occurred within this hoarding and shafts 1 and 3 were soon involved. Unusually, the fire spread downwards to involve the 1/F lift lobby within a matter of minutes. While unexpected, this may be a consequence of the burning characteristics of bamboo, which when involved in fire explodes as the air trapped between the internal nodes expands, scattering burning fragments over a wide area17.

From this stage onwards, serious fires were burning in the lift lobbies on the first and second floors, drawing air from the two open lift shafts. There was serious fire damage on the ground, first, second, 13th, 14th and 15th floors. All but one of the fatalities were trapped on the upper floors. Levels 4 to 12 were scarcely touched by the fire.

It was concluded by the Commissioner of the Public Inquiry that the fire had started in the second floor lift lobby as a result of welding debris falling from the 13th floor, where work was being carried out at the rear wall of the lift shaft. However, significant aspects of the evidence of the witness who first observed the fire conflicts with some of the conclusions that were accepted by the Inquiry.

Figure 1: The lift lobby on the second floor of the Garley Building as it was prior to the fire. Lift shafts 1 and 3 we re completely open, protected by wooden hoardings (WH). The fire wall separating the lift lobby from the department store (CAC) had been re m oved and replaced by ordinary roller shutters (RS). Stair A closed off at G/F and 3/F had been conve rted into store rooms, with no fire separation between the stairwell and the lift lobbies at 1/F and 2/F.
Figure 2: Plan of the 2/F lift lobby, showing the location of the low leve l damage outside Lift Shaft 3 at A. The wooden hoardings (WH) were almost totally consumed. The fire damage to wooden boards and planks at B was consistent with an intense fire burning in the vicinity of A, as we re the remains of a pile of corrugated card board at C. The roller shutter doors (RS) were in place at the end of the fire. A wooden Chinese Altar at D was totally consumed.
Figure 3: The early and late stages of the fire. (a) The flows associated with fires in both lift lobbies on 1/F and 2/F before the involvement of the store rooms in Stair A. (b) The flow that would have developed after the wooden partitions in Stair A had burned through, allowing the fire in the 1/F lift lobby to vent through the 2/F lift lobby into the lift shafts.

Post-fire evidence

The post-fire evidence showed that there was serious low level damage in front of the opening to lift shaft 3, which was interpreted as identifying the locus of ignition. There was also evidence on the metal architrave around the opening to the shaft that suggested that flames had emerged into the shaft at a very low level. There were further directional markers which were taken to confirm that the fire had started on the floor close to the lift opening and spread from there outwards. However, when the damage pattern is considered in the context of the way in which the fire subsequently developed, it must be concluded that this evidence is at least ambiguous.

The fire burned for many hours. The storerooms in the enclosed stairwell between the ground and 3/F eventually became involved. At some stage during the fire, a flow path would have been created from the first floor lift landing to the second floor lift landing, through the storerooms, which contained large quantities of combustible material. As the contents became increasingly involved, the partitions would have burnt away and allowed a vigorous fire flow to develop between the two lift lobbies. Prior to this event, the fires in the first and second floor lift lobbies would have entrained air from the lift shaft (Figure 3(a)). Each level would have behaved as a conventional compartment fire, in which fresh air is entrained through the lower part of the opening and flames and hot combustion products emerge from the upper part. In the unusual conditions of the Garley fire, the 2/F lift lobby fire would have been entraining the products arising from the fire in the 1/F lift lobby. Once the partitions between the lift lobbies and the storerooms had burned through, the flames and gases from the fire on the 1/F lift lobby would have vented through the stairwell into the 2/F lift lobby.

The two lift lobbies and the stairwell would then become one compartment, fresh air being entrained at the lower level (1/F) and the hot combustion products flowing into the lift shaft from the 2/F lift lobby (Figure 3(b)). The damage to the floor outside lift shaft 3 on the second floor lift lobby is entirely consistent with a flow of this nature, as flames would have completely filled the opening as they emerged into the lift shaft. (It has been observed that when a fully developed fire vents through a single opening, the emerging flames fill the upper half, to two-thirds of the opening18, 19). This flow would have produced intense heat transfer to the floor of the lift lobby outside lift 3 and provided high levels of radiant heating, which would have enhanced the burning rate of any combustible materials in the vicinity. The low-level damage to the floor outside lift shaft 1 was much less severe, but it may have had some degree of protection from the fire when it was at its most intense. The fuel load in that corner of the lift lobby was relatively low and the single hoarding which surrounded the opening may not have been consumed as quickly as the hoarding around lift 3.

The interpretation of the evidence in the 2/F lift lobby is consistent with this mechanism. The directional markers that were used to help pinpoint the seat of the fire as being immediately in front of lift shaft 3 would have been caused by intense low level burning in this location at a later stage in the fire. In fact, there is a strong argument that the damage observed in the 2/F lift lobby would have been caused by any long-burning fire which came to involve the contents of the storerooms in Stair A - regardless of where ignition first occurred. Consequently, the low level damage to the floor and surrounding materials outside lift shaft 3 cannot be assumed to be evidence of the locus of origin.

Using all available evidence

The three fires discussed were all major incidents involving multiple loss of life. Such tragedies are fortunately rare, but they do provide unusual opportunities for in-depth studies of fire behaviour. Many, if not most, fires that are investigated are quite clear-cut in much of the detail. But it is a duty of the fire investigator to draw on as much available evidence as possible to confirm any conclusion about the way in which the fire started and spread.
In many cases, it is entirely sufficient to identify the most likely cause, but this should be combined with identification of the evidence that confirms where the fire started. Certain clues that may be found at the fire scene can be misleading. For example, deep charring at specific locations may be interpreted as evidence for the use of flammable liquids, particularly if this damage is at low level in an enclosure. However, serious burning can be a result of the way in which the fire has behaved within the compartment, either as a consequence of ventilation being provided close to that region, or from some unusual fire flow condition that gave rise to high rates of heat transfer in that locality, such as in the Garley Building fire.

In principle, the fire investigator should be able to identify any unusual features of the building geometry that could account for the evidence at the fire scene. Evidence for high rate of heat transfer or high temperatures in unusual locations need to be explained. He or she should also be able to identify circumstances that might lead to high rates of heat transfer and, if this is likely to compromise interpretation of evidence, it must be taken into account when coming to final conclusions q

Professor Dougal Drysdale is based at the School of Civil and Environmental Engineering, University of Edinburgh

References

1 Popplewell, O., Committee of Inquiry into Crowd Safety at Sports Grounds: Final Report, HMSO, London, 1986.

2 Fennell, D., Investigation into the King's Cross Underground Station Fire, HMSO, London, 1988.

3 Butler, C. P., 'Notes on the charring rates in wood', Fire Research Note 896, 1971.

4 Report of the Tribunal of Inquiry on the Fire at the Stardust, Artane, Dublin, Stationary Office, Dublin, 1982.

5 Woo, K. H., Final Report of the Inquiry into the Garley Building Fire on 20th November 1996, Hong Kong: Government Printer, 1997

6 Morris, W. A., 'Stardust Disco Investigation - some observations on the full-scale fire tests', Fire Safety Journal 7, 255-65, 1984.

7 Video: 'Anatomy of a Fire', Building Research Establishment, Garston, Watford, 1982.

8 Karlsson, B, 'A mathematical model for calculating heat release rate in the room corner test', Fire Safety Journal 20, 93-113, 1993

9 Grant, G. B. and Drysdale D. D., 'Numerical modelling of early fire spread in warehouse fires', Fire Safety Journal 24, 247-278, 1995.

10 Moodie, K., 'The King's Cross Fire: Damage assessment and overview of the technical investigation', Fire Safety Journal 18, 13-33, 1992.

11 Roberts, A. F., 'The King's Cross Fire: a correlation of eyewitness accounts and results of the scientific investigation', Fire Safety Journal 18, 105-121, 1992.

12 Simcox, S., Wilkes, N. S., and Jones, I. P., 'Computer simulation of the flows of hot gases from the fire at the King's Cross Underground Station', Fire Safety Journal 18, 49-73, 1992.

13 Drysdale, D. D., Macmillan, A. J. R., and Shilitto, D., 'The King's Cross Fire: experimental verification of the 'trench effect', Fire Safety Journal 18, 75-82, 1992.

14 Moodie, K., and Jagger, S. F., 'The King's Cross Fire: results and analysis from the scale model tests', Fire Safety Journal 18, 83-103, 1992.

15 Woodburn, P., and Drysdale, D. D., 'Fire in Inclined Trenches: the dependence of the critical angle on the trench and burner geometry', Fire Safety Journal 31, 143-164, 1998.

16 Woodburn, P., and Drysdale, D. D., 'Fire in Inclined Trenches: time-varying features of the attached plume', Fire Safety Journal 31, 165-1172, 1998.

17 Janssen, J. J. A., Building with Bamboo: a Handbook, pp.21 Intermediate Technology Publications, London, 1988.

18 Babrauskas, V., and Williamson, R. B., 'Post flashover compartment fires: basis of a theoretical model', Fire and Materials 2, 39-53, 1978.

19 Drysdale, D. D., Introduction to Fire Dynamics, Second Edition, JohnWiley and Sons, Chichester, 1998.

The author wishes to thank Dr Sheilah Hamilton, Forensic Focus, Hong Kong, for valuable discussions.

This paper was presented at Interflam '99, the 8th Fire Science and Engineering Conference organised by Interscience Communications Ltd at Heriot Watt University, Edinburgh, in July 1999.

 

 
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