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FULL-SCALE BURNING TESTS METHOD FOR RETAIL SHOP FIRES
Because of the high land price, many retail shops in Hong Kong are small and packed with combustibles [e.g. 1,2], such as shops selling personal computers, video compact disks (VCD),toys and cartoons in shopping malls. The total amount of combustibles stored in a shop can be quantified by the fire load density FLD (in MJm-2),i.e. the total heat generated for burning all items per floor area. The upper limit of FLD under local codes [3] is 1135 MJm-2. Fire safety in these small retail shops should be considered carefully, especially for those in terminals and public transport interchanges where the passenger loading is extremely high during rush hours [4]. For hazard assessment, the heat release rate (HRR) (in MW) has to be known [e.g. 5,6] and this quantity is very different from FLD which would give the maximum amount of heat released (in J) upon burning up all the combustibles. But this would happen only when there is adequate ventilation and high enough temperature.
The ‘design fire scenario’ [7] is one of the primary uncertainties in fire safety engineering. A design fire depends on the use of the building and the materials used and stored, therefore it cannot be decided without understanding the combustibles present. The HRR [e.g. 5,6] has to be known and results can be used as input parameters for fire models in studying fire environment. Values used for different local application [e.g. 1] were up to 7 MW for terminal halls; 5 MW for shopping malls;
and up to 7 MW for atriums. It is difficult to decide the value as there is no database for local combustibles. It appears that the number of fires not due to accident is increasing in the past few years as observed in karaokes in Hong Kong [8], discos in USA [9], World Trade Centre in USA [10], underground railway fire in Korea [11] and the recent underground train fire in Hong Kong[12]. Whether the fire safety provisions are for protection against accidental fire, arson fire, terrorist attack fire or mass fire due to big disasters such as earthquakes or explosion of big gas tanks have to be clarified [10]. Anyway, rigs similar to an ‘industry calorimeter’ in Sweden [e.g. 13] should be developed to burn an actual retail shop for studying how much heat would be released.This is expensive but necessary, and the concept is pointed out with preliminary tests reported in this paper.
There were some data on total HRR for burning combustibles in retail shops, libraries and stores. For example, design fires deduced from large-scale fire tests in a sprinklered calorimeter were reported[5]. But these tests were started from a small fire such as an ‘igniter’ due to a short-circuited electrical appliance, a litter bin, or a gas burner as used in some standard fire tests. There is no radiation heat flux applied to test the samples as in a cone calorimeter. Results are useful for understanding how a fire grows, develops to flashover and then spreads to adjacent areas. But this will not give the contribution of materials, nor their assemblies, to a fire under flashover condition. The HRR measured would not be too high as only a small amount of the combustibles were ignited.
And for most cases, fire suppression systems might be operated to reduce the resultant HRR. The tests then become a demonstration on how the system would act at the tested fire, but not for understanding the actual HRR and the possibility of igniting the combustibles under flashover condition. The situation should be reviewed as more fires other than due to accidents were reported.
Effect of high thermal radiation heat flux (such as 20 kWm-2 for flashover) on combustibles in a shop should be included in studying the possible HRR.Missing this point might be quite serious for retail shops. Combustible items stored there such as plastic dolls, especially those without quality control through standard fire tests, might be ignited easily by the incident heat flux if there is an accidental fire. Note that flashover would occur easily in such small retail shops as raised before [2].
Upon ignition of those combustibles, much larger quantity of heat and smoke would be liberated and spread to the hall space outside the shop. What will happen if the big hall is overcrowded? Better understanding on the probable HRR under flashover is strongly recommended [14].
Preliminary burning tests of several fire scenarios in small retail shops under flashover condition were performed in a new full-scale burning facility, the Chinese Assembly Calorimeter [15]. That facility was just developed as a collaboration project between the Harbin Engineering University (HEU).Results on HRR in flashover shop fires will be reported in this paper.
Flame spreading of building materials used would be controlled. Fibreglass composites and flammable aerosols are usually not allowed. The likelihood of flashover in the shop had been pointed out with points of concern identified:y Consequences of flashover [e.g. 16] in a shop should be watched. The HRR of combustibles stored in the shop are key factors [17]. Storing products of HRR higher than the value giving flashover under a certain ventilation condition should be watched carefully.
y Effectiveness of operating sprinkler system in controlling a fire should be watched and water mist system [e.g. 18] might be used if necessary.
y Fire safety management schemes [e.g. 19] including training of the storekeepers and the building management staff are to be worked out carefully. For example, keeping a fire to be smaller than the design fires for sizing the sprinkler and smoke extraction systems should be ensured.
All points should be analyzed carefully to provide total fire safety. Performing full-scale burning tests on some selected shops is necessary to observe key fire issues. Performance of fire safety provisions such as water mist fire suppression system on those shops in big halls [14,20] can also be demonstrated. Consequences of flashover fires for some dangerous arrangements of concern must be studied. It is difficult to work out fire safety regulations and recommend good fire safety management without in-depth studies on fire dynamics.