Fire Spread in Car Parks09 June 2009Guidance for fire safety in car parks is based on data on vehicles which are now decades old. Here we report on the findings of a three year government-sponsored project carried out by BRE to investigate the fire characteristics of modern vehicles and the potential fire spread hazard in car parks. [This is an abridged version of the authors' article. You can download the original version here]
Fires in car parks are rare, and, although there have been few deaths or injuries recorded to date in the UK, there are concerns regarding new and emerging risks from modern cars and alternative fuels. The existing guidance for fire safety in car parks in England and Wales is in Approved Document B (Fire safety) to the Building Regulations in England and Wales (AD B)1. The basis for this guidance for fire safety strategies in car parks relates to fire initiation and fire growth involving cars whose designs are decades old. There has been increasing concern about the consequences of fires in car parks associated with modern car design (e.g. plastic fuel tanks) and how these fires may spread to other vehicles parked adjacently and nearby. This concern has been heightened by the entry into the market place of cars powered by alternative fuels such as LPG. So there was a need to gather up to date information on fires involving cars in car parks, in order that the current fire safety guidance could be reviewed and, if necessary, updated. The overall aim of the project has therefore been to gather information on the nature of fires involving the current design of cars so that, in due course, guidance on fire safety strategies for car parks can be reviewed and brought up to date, as appropriate. This project was concerned with fire spread in car parks that fall under the Building Regulations for England and Wales2 and included open sided car parks, fully enclosed car parks, and basement car parks. The work started off with desktop studies and involved collecting and analysing information and data on the identified topics. Over 200 items were reviewed, mostly from the UK but many from overseas. Topics reviewed included:
A review of the UK fire statistics was carried out, for the 12 years from 1994 to 2005 (the most recent year for which detailed data was available to BRE3), the main findings of which are summarised below. CFD modelling was used at various stages throughout the project to guide the research, assess findings and validate current methodologies. Fire behaviour of materials Fire spread between cars The objectives of this task were to benchmark car fire sizes for a range of vehicle types, and to determine the spread of fire between cars, the severity (heat release) of car fires, and the associated conditions (heat, smoke, toxic gas) to which car park occupants might be exposed, under typical conditions. Three large scale fire tests were carried out in a car park test rig under the large calorimeter hood in the BRE Burn Hall, each involving three cars. The test rig was 12m long and 6m wide. The (underside) ceiling height was 2.9m from the floor. The rig comprised a steel frame with breeze block infill and the roof was of hollow-core concrete slabs. One end of the rig was open, but with a 0.5m deep down stand. Ventilation openings were provided along one side and the back wall. Instrumentation was essentially identical (with some minor variations) for all the full scale tests. It primarily comprised three thermocouple columns within the test rig, thermocouples on the roof slabs (above and below), thermocouples within and on the surface of each car (although not Test 1). Heat flux meters were located in five positions and gas sampling (CO, CO2 and O2) in three locations. Fire gases were collected in the 9m hood and analysed for heat release rate. For all tests, three cars were located in the test rig, Cars 1 and 2 next to each other in adjacent bays, Car 3 separated by a space (equivalent to an un-used parking bay). For all of these tests, the fire initiation simulated an arson attack. The driver's window and offside passenger window of Car 1 were open (i.e. on the side furthest from Cars 2 and 3). All other car windows were closed. The fire was started using a No. 7 crib6 on the driver's seat of Car 1. Hertfordshire Fire and Rescue Service were on hand for all these tests.
Three tests were carried out: Test 1 was a freely burning test with no sprinklers fitted. Test 2 was similar to Test 1 but included a sprinkler system. Test 3 was a repeat of Test 1 but with larger vehicles. Cars fuelled by LPG
Seven tests in all were carried out on single cars. Stacker test The objective of this test was to examine the processes of fire spread from the lower car to the upper car on a stacker, and the time to full development of the fire, starting in the passenger compartment of the lower car with the driver's window open. For this, a steel and block work test rig able to support one car above another in a typical stacker configuration, was constructed under the BRE 9m hood. Analysis
Car park fire statistics Most car park fires (68%) occurred in buildings reported as ‘car park buildings'. 6% of fires in a car park occur in ‘flats'. Most injuries from fires in car parks occur in car park buildings (45%), followed by flats (26%); hence 6% of fires in car parks are causing 26% of the injuries (car parks in flats) while 68% of the fires have caused only 45% of the injuries (car park buildings). It follows that fires in car parks within flats are the more dangerous. So in summary, the desktop research findings show that:
Findings
Three of the full scale tests, Tests 1, 3 and 4, all demonstrated the ease with which a car fire in a car park might spread to nearby cars. Once a very severe fire has developed, ignition will occur on cars separated by an unoccupied parking bay. Test 2 showed the effectiveness of a sprinkler system at containing a fire within the car of origin and slowing its rate of growth. Some minor heat damage was incurred by the second car in this test but actual fire spread did not occur. In this situation, where a number of cars are burning simultaneously, the fire is exacerbated by heat feedback and heat release rates in excess of 16 MW might be achieved from two or three cars. In Test 1, Car 1 burned at around 2 MW for about 20 minutes and it was only then that Car 2 became involved (although Car 3 then ignited very soon after). However in Test 3, all three cars were burning after around 10 minutes. In Tests 1 and 3, the fire was terminated once Car 3 was involved to avoid overloading the calorimeter. Heat release rate measurements for all the relevant tests are shown in Figure 2 and the heat release rate results are summarised in Table 1.
In Test 4 (LPG in Car 2), Car 2 was alight after 21 minutes and all four cars were burning after around 23 minutes. In Test 8 – an engine fire test with a nearby car ‘nose to nose' – the fire spread to the second car within five minutes. In Test 11, the stacker test, the fire spread up to involve the second car within six minutes and fully involve it within nine minutes. The ventilation limitations on such a fire in an enclosed car park result in a very hot ceiling jet, which spreads the fire to
nearby cars, with the dominant mechanism of heat transfer being radiation from the flames and hot gas layer. There were only a few cars in each of these tests (a maximum of four); however escalation to many cars within a specific proximity in an actual car park must be expected under these conditions. Computer modelling has been employed to examine these larger scenarios. Gas temperatures in the enclosed rig (beneath parts of the ceiling) reached 1100OC in all
tests, exceeding 1200OC briefly in Test 4. The LPG tank did not explode in Test 4 since the pressure relief valve provided with the LPG tank appeared to work effectively – even in quite a severe multi-car fire. The single car tests demonstrated that a fire in a sealed (i.e. windows closed) passenger compartment is unlikely to develop; that modern cars are sufficiently well sealed that a fire starting within the passenger compartment is likely to go out through lack of air. It may be presumed that most cars left in public car parks will have their windows closed. These tests demonstrated that a fire starting in the engine compartment of a modern car can grow within the engine compartment and can break through the bulkhead into the passenger compartment, where a substantial fire can develop and involve the whole vehicle.
Computer modelling Electric cars
Conclusions The ease with which a car fire in a car park might spread to nearby cars has been demonstrated. Once a very severe fire has developed, ignition will occur even to cars separated by an unoccupied parking bay. The potential use of computer modelling in predicting the spread of fire and smoke in car parks has been demonstrated and validated. The findings from this work should be available to feed into standards referred to by ADB, specifically BS 7346-7: 2006 Components for Smoke and Heat Control Systems – Part 7: Code of practice on functional recommendations and calculation methods for smoke and heat control systems for covered parking areas for cars 8. Similarly, the findings from this research, in particular the ‘raw' experimental results, will provide a data resource for the fire safety engineering of car parks. It is intended that the full report from this research programme will be made available on the CLG website in due course. References
The research and paper on which this article is based was commissioned by the Communities and Local Government department (CLG) and carried out by Martin Shipp, Jeremy Fraser-Mitchell, Richard Chitty, Róisín Cullinan, David Crowder, and Phil Clark of BRE. Read the full article here. Any views expressed are not necessarily those of CLG, with whose permission the article is published. The authors wish to acknowledge the assistance we were given by many individuals and organisations and in particular the Advisory Group, formed from representatives of stakeholders, who advised the project from its inception.
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