A Review of Fire Resistance Expectations for High-Rise UK Apartment Buildings

In the UK, prescriptive fire safety guidance offers a simple means of meeting statutory building regulation requirements, for most common building situations. In the context of prescriptive structural fire resistance, buildings are afforded a fire resistance period based upon their height and use. This fire resistance expectation is either to be achieved inherently by structural elements or via protection to them. The broad aim of such prescriptive guidance is the delivery of a consistent level of risk across all building types. For this to be achieved, as the frequency of fires and consequence of failure increases, the reliability of the fire resistance system must increase. This typically manifests in an increase in a building’s fire resistance expectation as height increases. Despite this common practice, this paper fundamentally reviews the concept of fire resistance as a height dependant metric for residential buildings, identifying the limitations of such an approach, utilising single stair apartment buildings as a basis for demonstration and further investigations. A risk correlation is proposed that seeks to explicitly define the structural fire resistance design goal (reliability of the fire resistance system) as a function of both height and occupancy. The correlation is calibrated against UK statistical data to determine what might constitute a common building. It is found that in single stair buildings, an appropriate benchmark case would constitute 7 apartments per level, ranging in size from 1 to 3 bedrooms (28 m² to 101 m²). This benchmark case is used to determine a risk score which, for the purpose of achieving a consistent level of risk, becomes a constant in the proposed risk correlation. Four example building cases are chosen to demonstrate the application of the correlation. The cases are of the same notional height, but constitute differing occupancy (apartment) numbers. The proposed correlation indicates that the four cases, despite being of the same height, have significantly different fire resistance system reliability demands (96.3% to 98.5%) and, thus, fire resistance demands (154 min to 173 min). In light of the findings, it is concluded that, if conventional structural fire resistance thresholds are not to be exceeded (in the UK typically limited to 120 min), then, in tall residential buildings, the reliability/efficacy of the sprinkler system becomes increasingly important. Finally, a brief discussion is provided regarding the limitations of the approaches presented.