Safety, with respect to evacuation, is measured in time, predominantly the time required for all occupants to reach the outside of a building. The shorter this time, the safer the building is deemed to be. The height of many modern tall buildings, combined with the limited number of vertical escape routes, extend travel times such that the stairwells must act as the outside. They must be designated a safe zone
which should guarantee the safety of occupants once reached and allow safe transit to a place of refuge, within or outside the building. In effect the tall building becomes a collection of single storey buildings. This then allows for different evacuation philosophies (staged, phased, total) to be applied to tall buildings. Furthermore these travel distances extend evacuation times to a magnitude comparable with that of the heating times of structural elements and, by extension, comparable with potential failure times. An increase in vertical escape provisions (stair numbers and widths) and novel technologies (egress lifts, etc.) will not yield sufficient impact to prevent this overlap. The considerable time that occupants spend within the stairwells means that for any fire strategy to be successful, stairwells must remain smoke and heat free and the entire building structurally sound. Without adequate protection the number and width of stairwells is irrelevant, as smoke-logged stairwells are unusable and the Fire Safety Strategy is therefore void. Fig. 2 shows the rates of premature loss of stairwell tenability levels i.e. significant levels of smoke within at least one stairwell whilst still being used for egress by occupants, reported in the surveyed fire investigations and reports. One significantly reoccurring theme reported was that failure occurred early in the evacuation process although exact failure times were not given. Another was that fire fighting activities were associated to approximately a third of the reported failures. A common method to ensure smoke free stairwells is the provision of pressurisation systems. Stair pressurisation was developed in the 1960-70s through experimental work [19]. This work identified criteria that enabled the definition of a pressure range (upper and lower bounds). Systems are then designed such that the pressure difference between the stairwell and its surroundings remains within this range under both everyday and operational conditions in order to maintain the smoke free requirement. The upper bound pressure exists to ensure that occupants never struggle to open doors leading to a stair, thus not hindering evacuation. This pressure is usually defined by the force that an average person can exert. The lower bound pressure is designed to maintain gas flow from the stairwell to its surroundings. It is therefore defined in terms of the pressures and temperatures produced by the fire. If the pressures induced by the effects of the fire are greater than those of the lower bound, smoke will flow through doors and smaller gaps and openings into the stairwell thus rendering it unusable. It is therefore crucial to correctly determine the pressures that a typical fire might produce. While this model has
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been expanded to account for the complexities brought about by modern stairwell geometries [20], the resulting pressurisation systems have been shown to be limited by their narrow ranges of operation [21] and the uncertainties associated to the nature of the fire. The experiments [22] on which this approach is based were conducted in a 10 storey tower, with surrounding compartment floor area of approximately 18 m2 containing a propane burner. The experiments tested a variety of stair pressurization systems to assess the effects of doors opened into the stairwell during evacuation on the ability of the systems to continue to keep smoke out of that stair. Clearly, a fire in a large, open plan environment containing combustible furniture may have considerably different fire dynamics, thus the lower bound pressure definition used for these systems has little relevance for modern open plan scenarios. Reported failure rates for stairwell smoke control systems from the fire report surveys are shown in Fig. 3 and account for 90% of cases where such systems were mentioned. This implies that safe stairwell tenability levels are currently not guaranteed, thus the cornerstone of contemporary tall building fire safety design may not be valid. A fundamental component for the success of this element of the Fire Safety Strategy is the correct definition of the lower bound pressure, and thus also the exact nature of the fire.