A Dynamic Approach to ASET/RSET Assessment in Performance based Design

The basic concept in the assessment of occupant safety in a building under fire conditions is the determination of the time when occupants are able to safely escape before hazardous conditions sets in. The Available Safe Egress Time/Required Safe Egress Time (ASET/RSET) concept of fire safety assessment in performance based fire safety engineering design has become widely used amongst fire safety engineering practitioners, since its inception more than thirty years ago. However, the adequacy of this approach has only been occasionally deliberated on and not well addressed in detail. Discussions were usually focused on the weaknesses and inadequacies in the assumptions and methodologies which impact upon the outcome of the ASET and RSET parameters, rather than on the ASET/RSET concept itself. The original ASET/RSET approach was derived from a simple two-zone model for a single compartment by Cooper in 1980, but is still being applied to larger and more complex buildings today. This is despite the advancement of sophisticated three- dimensional simulation models producing highly detailed results, whereby the smoke layer is no longer uniform and the extent of the area of untenable conditions is a transient state and may develop only over a portion of the compartment area. The continued application of the ASET/RSET approach in these circumstances may lead to varied interpretation of the egress terms, leading to departures from the intended purpose and scope of providing the basic measure of assessing egress safety that was originally devised by Cooper. There is also little published literature available on viable alternatives to this simplified means of assessing safe egress from fire that was originally derived from a zone model concept. This paper provides a brief review of the ASET RSET methodology, and introduces an alternative means of fire safety assessment based on the utility of a given space over time. The alternative scheme enables a dynamic approach to assessing the level of safety that is more appropriate for use with advanced simulation models providing transient three-dimensional environments in more complex building layouts.