A study on installing water curtain system at refuge floor of supertall buildings in Hong Kong

Abstract

Local code of practice for fire safety in buildings issued by Buildings Department (BD) requires supertall buildings over forty storeys high to provide a refuge floor for every 25 storeys in residential/commercial buildings. However, the protection to external wall openings on refuge floor by a drencher/water curtain system falls within the jurisdiction of the Fire Services Department (FSD). There is a concern on the trend for flatted units in supertall buildings to have large window/glass panes design so as to have more sunlight and a better view. The result is that this design increases the impact of the ventilation factor, which in turn steps up the heat release rate for flashover and fire development in compartment fires. Another concern is the external wall opening sizes on the refuge floor vary greatly in different types of buildings. The differeence in the ratio of the area of an external wall opening to the area of the external wall of the refuge floor greatly affects the effectiveness of cross-ventilation on that floor. For a better understanding, a survey of building plans including 44 building lots (51 blocks) of residential and non-residential buildings was conducted. As statistics from FSD reveal that about 80% of structural fires occurred in government housing estates and private residential buildings, my study focuses on these types of buildings. Detailed inspections of building plans of 29 building lots (31 blocks) were conducted to obtain and analyze, for each of the building, the ratio of the area of an external wall opening to the area of the external wall on the refuge floor, the ratio of the window opening area to the external wall area as well as the ratio of the window opening area to the floor area of the flatted unit immediately below the refuge floor. The trend of refuge floor designs and the cross-ventilation requirement stipulated in the relevant local building code were also analysed. To determine the minimum amount of heat released for flashover in a compartment with large window panes, 31 supertall residential buildings were selected and the Babrauskas method for flashover was adopted. The results showed that the average minimum heat release rate for flashover was 15.3 MW for an unit having an average window opening area of 15.1 m2. To evaluate the effectiveness of a water curtain system in achieving heat reduction, radiation attenuation and smoke reduction from entering the refuge floor, Computational Fluid Dynamics (CFD) model Fire Dynamics Simulator (FDS) was adopted. Before working on the CFD model, a pre-study in four main subject areas was conducted: (1) comparing the simulation results of the FDS model with another CFD model called “SIMTEC and also with the results of full-scale burning test from literature; (2) selecting appropriate fuel types as an input parameter in FDS simulation; (3) comparing the performance of the sprinkler system and the water curtain system; and (4) making FDS trial runs covering different locations of fire seat, domain model geometry and fire sizes, etc. Simulation results from my pre-study showed that the results generated by the FDS model were comparatively more conservative in subject area 1 (i.e. lower temperture reduction and lower radiation attenuation) and agreed well with burning test results from literature (at least in the veocity of outflow gases) and with temeprature profile of the fire plume. In the selection of the appropriate fuel type for FDS simulation study, my prime objective was to look for the adverse effects in the combustion of propane, wood, polyurethane (PU) and polyvinyl chloride (PVC) . Simulation results including outputs of temperature, visibilities, carbon monoxide concentrations and radiation heat fluxes from different scenarios were evaluated. The highly hazardous properties of burning PVC and PU in producing large amounts of carbon monoxide and very low visibilities within a short period of time were carefully considered. The sprinkler system is commonly used in fire protection but this is not the case in the protection of external wall openings on the refuge floor. To compare the performance of the sprinkler system with that of the water curtain system in defined scenarios, the FDS model was adopted. Simulation results showed that the water curtain system performed better in of temeprature reduction and radiation atttenuation. To simulate the performance of the water curtain system on refuge floor external wall openings, a two-storey building model consisting of a fire room measuring 11m (length) x 9 m (width) x 3m (height) on the ground floor with a refuge floor immediately above it was designed. A fire was assumed to be spreading from the fire room after a flashover fire with flames ejecting through broken windows and spreading upwards along the external wall and wall openings of the refuge floor. An in-depth simulation study using FDS in different scenarios and design parameters including different fire sizes, opening widths of the fire room, wind speeds and scenarios with and without the water curtain system were conducted. The results showed that the water curtain sytem performed well in temeperature reduction, radiation attenuation and visibility enhancement on the refuge floor. Attenuation of thermal radiation by the drencher/water curtain system and relevant parameters such as water flow rates, water pressures, spacing of drencher nozzles, water droplet sizes and water density were studied using FDS simulation. Theories of Beer-Lambert's law and its analogy were adopted and simulation data analysed. The relationship between the parameters and the ratios of radiation intensity for scenarios with and without water curtain protection was established. A generalized equation to calculate the maximum attenuation of thermal radiation by a water curtain system was formulated using multiple regression analysis. The introduction of a building fire safety management system to monitor building fire safety based on the total fire safety concept together with an ultimate building fire safety audit system to ensure overall building fire safety has been developed. Methods and procedures in the evaluation of building fire safety levels are also presented. The thesis concludes that in the scenarios studied, the water curtain system performs well in temperature reduction, radiation attenuation and visibility enhancement on the refuge floor; and offers recommendations on how to improve the design of the water curtain system. Limitations of the study and suggestions on further work on the adequacy of ventilation requirement on the refuge floor are also given.