N A Townsend and A Nicholson
The Millennium Dome - A Fire Engineered Approach
Introduction
When the concept of the Millennium Dome at Greenwich was first born some people drew parallels with the Summerland complex Isle of Man. After all the proposal was to build a leisure complex for a large number of people contained within a structure made of synthetic material . Should a fire occur would the consequences not be the same resulting in total loss of the building and large loss of life?
Whilst this view might be termed as alarmist we should study what we actually learnt from this disaster and what steps we have taken to prevent a re-occurrence.
The similarities between the Summerland building and the Millennium Dome are readily apparent; both are purpose built leisure sites accommodating a large number of members of the public. These visitors will be unfamiliar with the building and concentrating on enjoying themselves and the exhibitions rather than the means of escape. Both structures being enclosed in a synthetic material. So, there appears sufficient evidence to suggest that there may be some credibility to the alarmist view.
How then do we prevent a repeat of the Summerland disaster at the Millennium Dome and how sure are we that we have got it right this time?
Adopting a fire engineered approach in order to meet the fire safety requirements in the Dome both fire consultants and enforcing authorities should be able to quantify the likelihood and consequences of a fire.
Fire Engineering Strategy
Means of Escape
The means of escape for the Dome cannot obviously follow the prescriptive codes due to its size, the diameter of the Dome is 320m giving a maximum travel distance of 160m which is far in excess of any code based recommendation. It therefore becomes obvious that designing an evacuation strategy based on travel distances is not a feasible option and the fire engineers would need to demonstrate that the time for escape is less than the time to untenable conditions. This concept is not a ground breaking one and in fact features in the proposed British Standard series BS 9999
and DD240 where it is considered more important how long it takes occupants to escape rather than how far they have to travel. Also because a phased or staged evacuation strategy is being adopted , where the Dome is divided into segments like a clock face, some occupants may have to remain inside for some time. Adopting this approach however does mean that a number of other factors now interact and impact on the means of escape , for example the method of smoke control.
In the calculation of evacuation time the elements that need to be considered are;
There are problems specifically associated with places of entertainment , like the dome, where people generally become involved interactively with the exhibitions and it may be difficult, in the early stages, to convince occupants that the unfolding emergency is not part of the show. This becomes even more difficult when a phased or staged evacuation is proposed where most of the occupants will be left inside the dome and are able to view the fire. Will their movements be away from the fire or towards the fire? If they feel relatively safe then it could be suggested that the natural reaction of the occupants will be one of curiosity and try to move towards the fire.
The issue of how the operation of the Dome is structured could have a significant bearing on the means of escape in terms of available exit widths. If visitors are allowed to wander through the dome and visit any exhibition at any time then the population distribution may not be that of normal distribution. Certain exhibits may be more popular than others and result in local crowding .This may not be a problem if the escape strategy is fluid in that parts of the outside walls can be opened up to increase the exit widths available locally. If however the exits are fixed then problems may arise in the event that local crowding occurs.
Fire Growth
The basis of any fire engineered solution is the ‘design fire’. The size (convective output) of this design fire will determine the fire engineering strategy in such matters as smoke vent size, escape time and type of automatic detection.
A design fire can be determined by a number of factors and within the dome these have been identified as,
In the case of ‘normal’ buildings, the design fire concept is relatively straight forward as the layout and contents of the structure are known and usually consistent with other buildings being put to the same use.
With the Dome however, in order to achieve a maximum design fire strict managerial control will be required to limit the type and amount of combustible load being introduced.
There are three main components within the Dome, in terms of structure, these being:
The hospitality blocks, of which there are at present six, can be treated as ‘normal’ buildings for fire safety arrangements in terms of compartmentation, detection and suppression. Prescriptive codes are adequate and suitable for these buildings and fire is managed within these structures by a combination of sprinkler controlled and containment within a fire rated structure.
The central area of the Dome is the show area where it is planned to have daily shows with the audience around the circular auditorium. In essence the middle of the Dome is a theater and prescriptive rules for places of entertainment should apply, with restrictions on the number of seats, widths of aisles etc. In terms of travel distance the central show area represents the worst case, in cases about 150m to the nearest exit.
The fire growth in this area is influenced by:
The main perceived problem within the Dome is that of the exhibitions themselves. Each exhibition has an allocated area to which services have been provide e.g. water for suppression systems. A guide to exhibitors has been produced that outlines the type of materials that may be used in the construction of the exhibits.
These exhibitions represent the greatest potential for fire growth for the following reasons:
Smoke Control
The ‘clear layer’ concept of smoke management would not be possible within the Dome due to its size. Smoke control will be achieved by a combination of extract and dilution methods. Fans are fitted , 12 in all, within the supporting ‘legs’ of the Dome for environmental reasons and each have a rating of 50m3/s and are designed to withstand smoke at 150 / 200 oC for two hours. CFD modelling has demonstrated that for a fire size of 20Mw these extract fans are adequate in terms of volumetric capacity and temperature rating, although it appears that some ‘plugholing’ will occur. An additional feature to assist in smoke clearance is the provision of openable roof vents that are capable of 400m2 with 500m2 of inlet air at lower level.
Any residue smoke left, either because of lower temperature or because of lack of capacity of the extract fans, will be diluted to such a level that it should not pose a threat to occupants. Having an internal volume of over 2 million m3 in order to reduce visibility limits to a region of 10m it would take a sustained fire of some magnitude although some local smoke logging make occur however this smoke is likely to be more of a nuisance than life threatening.
Fire Alarm
The fire alarm for the Dome will be an analogue addressable alarm system, made up of a number of networked control panels, with a voice alarm. The alarm system will be ‘zoned’ in segments to co ordinate with the proposed phased evacuation strategy so whilst the affected segment can be evacuated adjacent segments will be put on alert.
Each segment control panel will be networked together and a central control computer will be located within the back of house control centre. Each zone or segment will include the installation of automatic fire detection devices and, according to the type of risk being covered, comprise one or more of the following:
The fire alarm system will interfaced with magnetic door holders, lifts, escalators, smoke control systems and voice alarm system and will also alert neighbouring risks i.e. the Blackwall tunnel and North Greenwich underground station.
Any ‘stand alone’ structure e.g. exhibition or catering unit deemed large enough shall be provided with its own fire alarm system, interfaced with the main Dome system.
Conclusion
From the size and complexity of the Dome it would appear that the only way of achieving a satisfactory fire safety solution is by means of a fire engineered solution. As with many buildings where this appears to be the case about 75% of this solution is achieved by use of prescriptive codes. If there was no roof fitted over the Dome then it would be a collection of buildings that for over 90% would fall within the scope of one fire code or another and the need for any involvement for fire engineering would not be required. Encapsulating this exhibition under a synthetic roof has created a situation similar to that of putting a roof on a high street and calling it a shopping mall. Large shopping complexes are now an everyday part of fire safety and we are satisfied that in the event of fire in these premises it is unlikely to result in a large loss of life. The Dome, as with other large structures, the main component for fire safety will the management of such issues and the continuing programme of training and maintenance of the systems installed.