An investigation of the proportionality of safety margins with increased ceiling heights in relation to BS9999:2017

Commonly, in both the United Kingdom (UK) and international building regulatory guidance, a key factor used in escape route design is that of prescribed travel distances. These are maximum distances permitted for a building occupant to travel until they reach a place of safety, or a place of relative safety.

When fire occurs, due to the temperature difference between the products of combustion and the surrounding ambient air, the smoke is buoyant and rises, tending to fill a volume from the ceiling down. Although the mass flow of a plume above a fire increases exponentially with height, an increase in ceiling height will nevertheless increase the time until escaping occupants are affected by the descending smoke layer. To the authors knowledge the only design document which explicitly takes account of the advantage of increased ceiling height is BS 9999:2017 [1]. Designing in accordance with this document allows a trade-off between increased ceiling height and an increase in travel distance.

This paper examined the variation in the safety margin achieved for occupants escaping a simple one room office with a single means of escape, as the ceiling height is varied. The analysis comprised a comparison of the Required Safe Egress Time (RSET), with the Available Safe Egress Time (ASET) with varying ceiling heights and travel distances proportioned according to BS9999:2017. It was concluded that the margin of safety afforded between ASET and RSET, increased with ceiling height and that, for the example analysed, the RSET remained almost constant. This was attributed to queuing at the exit door dominating travel; the marginal increases in travel distance were irrelevant as the furthest away occupants were travelling to join a queue to exit.