Wind-induced natural ventilation of the refuge floor of a high-rise building in Hong Kong

An important element in the building fire safety of high-rise buildings in Hong Kong since 1996 has been the use of refuge floors in the building's evacuation system. To prevent smoke collecting and remaining in the refuge floors, the Building Code of Hong Kong requires these floors to have openings on opposite sides to provide adequate wind-induced ventilation. Other researchers using CFD simulations without wind tunnel verification have indicated that under certain conditions smoke could still remain on these floors and thereby reducing the fire safety of the refuge floors. This thesis explores these situations and presents a detailed scientific investigation of the wind movement in and around a refuge floor at mid-height of a high-rise building using wind tunnel testing together with CFD simulations (using CFD CFX-5.6 package). Besides identifying problem areas for smoke logging, this thesis also identifies how the design of a refuge floor can be modified to improve its fire safety. A significant factor on the fire safety of a refuge floor is the blocking effect of the building's central core and its effect on the wind-induced ventilation. Under Hong Kong Building Code, the central core can occupy up to 50% of the refuge floor. Previous investigators did not take into consideration the effect of the maximum core size on natural ventilation of the refuge floor. This thesis investigates the worst case scenario for a refuge floor that has a core occupying 50% of the floor and has two solid walls on opposite side of the floor to identify the problem areas where smoke could collect and remain. In exploring the worst case scenario with two parallel solid walls, the investigations revealed that the ceiling height and the wind direction have a significant effect on the wind ventilation of the refuge floor. These factors were not identified by previous investigators. In the case of the ceiling height, it was found that the head height of the refuge floor should be greater than 0.02 times the building height to achieve the desirable wind environment on the refuge floor. Regarding wind directions, the wind from most angles escapes the floor via the channel-like corridors next to the central core of the building. The main problem area occurred when the wind was perpendicular to the solid side walls. This resulted in noticeable stagnant areas where smoke could remain. To validate the CFD method used in the thesis, wind tunnel experiments were performed to provide the scientific field velocity data of wind flowing over the building and in the refuge floor located at mid-height of the building. Earlier researchers of the refuge floor did not have access to wind tunnel data of a refuge floor. In comparing the wind tunnel experiments with the CFD simulations used in the thesis, acceptable agreement was achieved. These results make it possible for a significant reduction in the CFD computational effort that previous studies required. Based on the findings of the investigations undertaken, design recommendations are proposed to improve the fire safety of the refuge floor in multistorey buildings in Hong Kong.

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