‘Waiting time’ for evacuation in crowded areas
Introduction
The construction industry in the big cities of the Far East is developing rapidly. This refers to both the speed of construction, quantity and quality of new buildings. Some older existing buildings in densely populated districts cannot be demolished. Better living quality can only be provided by upgrading the facilities for environmental control and safety [1].
Consequent to the serious fires occurred since 1996, both from accidents such as the big Garley Building fire in Hong Kong [2] and not from accidents such as the underground railway arson fires in Korea [3] and in Hong Kong [4], people are more aware of the hidden problems of fire safety. The prescriptive codes [5], on both passive construction elements and active fire engineering systems, might not provide adequate protection for those big crowded public areas.
Shopping malls and public access areas are now commonly found in large-scale development projects (e.g. [6]). These public areas are always crowded with people during holidays. The occupancy levels (or occupant loadings) are observed to be much higher than the design values. Because of the high occupant loading, the large space volume and the possibility of storing combustible items in the retail shops in the malls, occupants must have sufficient egress time in case of fire. Evacuation design is considered to be the key item and depends on the occupant loading under crowded conditions. The design figures, both the fire load density and occupant load factors, are specified in the local codes. However, survey studies [6], [7] showed that the number of occupants in some malls can be significantly different from the design values because of the frequent and substantial changes in using the spaces. The malls might be fully crowded with occupants, almost packed together, if the retailing business is good. Using the space as an exhibition area temporally might change the travel distances and even the exit configurations. This will reduce the usable floor area. Designing only a single set of evacuation patterns may not be appropriate.
There are many components [8], [9], [10] proposed in the total evacuation time TET. But at least, three components on the human response time tresp, the travel time ttrav and the waiting time twait can be divided. TET is given by
The values of tresp depend on the living style of the occupants, a factor difficult to study. There might even be no realistic figures in some developing countries. The values of ttrav depend on the travel speed and might have similar magnitudes across different places. The travel speed might be reduced under crowded conditions. Waiting time depends on how crowded the space is. The values might not depend on different races if the occupants can still be kept in order when there is a fire. It is a key element in evacuation and will be studied in this paper.
The geometry, escape routes design and other key parameters of 69 malls and public access areas were surveyed [6]. Evacuation in those spaces with different sizes and geometrical shapes were studied with the aid of the commercial computer-based evacuation package buildingEXODUS ([11], [12], [13], [14]. Different occupant loadings were assigned with the occupants staying at the public access areas in following local codes [15], [16], [17], [18]. These results will be analyzed for studying the waiting time.
Section snippets
Atria surveyed
A field survey was carried out to study the geometry and escape route design of local malls [6]. Sixty-nine samples were selected for detailed evacuation studies in this paper. The surveyed results are shown in Table 1. The floor area varied from 86 to 1802 m2, and the height from 7.4 to 28.7 m. The floor plans of the malls with distribution of exit openings had been reported and will not be repeated in here.
All the malls are in use and so local fire safety requirements were satisfied before
Evacuation software
The software buildingEXODUS [11], [12], [13], [14] is commonly applied to simulate the evacuation in many building types under crowded conditions in the Far East. This evacuation model includes all the four contributory aspects controlling the evacuation process. These are configuration; behavior; environment; and procedures.
The entire space of the enclosure in buildingEXODUS is described by a mesh of nodes representing a portion of space. The size of these spaces is dictated by the length of
Numerical simulations
Evacuation for those 69 malls under two different occupancy loadings were simulated using the software buildingEXODUS ([11], [12], [13], [14]. A bigger sample size was used to give better understanding on the waiting time.
Two occupant loadings were simulated:
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Loading 1: In the simulation for normal use of the malls, the population density was taken to be less than 3 m2/person, following the local codes [15], [17] for shopping mall. The assigned maximum number of occupants Nmax in each atrium
Waiting time
Waiting time is taken as the simulated TET minus the travel time. The maximum travel distance in each mall as shown in Table 1 was taken to calculate the travel time.
As reviewed in SFPE handbook [8], [9], travel speed vt (in m/s) in crowded exits like corridor or staircase is related to the population density D (in number of persons/m2 of floor area) as
In taking metric system with vt in m/s and D in person/m2, k is 1.4 for corridor and a is 0.266.
Under crowded conditions with the two
Conclusion
Waiting time for occupants staying at the floor of 69 atria under two occupant load densities [15], [17] were studied. Two occupant loads specified in the local codes, 3 m2/person for shopping mall and 0.5 m2/person for public accessible area, are considered. Results on numerical evacuation with buildingEXODUS were used.
It is observed that the evacuation time would follow the expected maximum value of 150 s for occupant loading 1. The maximum waiting time is only up to 66 s. However, for occupant
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