Fundamentals
ASET, RSET and the egress timeline

ASET, RSET and the egress timeline

Performance-based fire-safety design compares two times. The available safe escape time (ASET) is the time until conditions in a space, or on the route out of it, become untenable. The required safe escape time (RSET) is the time occupants need to reach a place of safety. A design is acceptable when ASET exceeds RSET by an adequate margin (ISO/TR 16738:2009, §5.6).

The equations as published

ISO/TR 16738:2009 writes the margin of safety and the escape time as

$$ t_{\mathrm{marg}} = t_{\mathrm{ASET}} - t_{\mathrm{RSET}} \qquad \text{(ISO/TR 16738, Eq. 1)} $$$$ t_{\mathrm{RSET}} = t_{\mathrm{det}} + t_{\mathrm{warn}} + \left(t_{\mathrm{pre}} + t_{\mathrm{trav}}\right) \qquad \text{(Eq. 2)} $$

All times are in seconds or minutes. \(t_{\mathrm{det}}\) is the time from ignition to detection, by a system or by the first occupant to notice fire cues. \(t_{\mathrm{warn}}\) runs from detection to a general alarm or warning. \(t_{\mathrm{pre}}\) is the pre-travel activity time, also called pre-movement or pre-evacuation time, and \(t_{\mathrm{trav}}\) is the travel time. The standard notes that the evacuation time \(t_{\mathrm{evac}}\) consists of the last two terms only, and it splits \(t_{\mathrm{pre}}\) into a recognition time and a response time. The Society of Fire Protection Engineers (SFPE) Handbook uses the same terms (Gwynne and Boyce 2016, Ch. 64).

What the timeline rests on

ASET comes from the fire: the time-concentration curves of heat, toxic gases and smoke at the occupants’ positions, compared with tenability limits. The fractional effective dose (FED), irritant, heat and visibility pages give those limits. ISO 13571:2012 (§5.1) states that its time to compromised tenability may reasonably be equated to ASET when escape to a place of refuge is the outcome considered.

Tenability is judged in one of two ways. With fixed limits, conditions at one height are compared with one value per hazard, and a space is untenable as soon as any value is exceeded: for exposures of up to 10 minutes, the Engineers Australia practice note gives 100 °C, 2.5 kW/m², 2,700 ppm CO, 140 ppm HCN and a visibility of 10 m, evaluated 2.0 m above the floor (2014, §5.2 and Fig. 8, p. 15). With a dose, the exposure of each occupant is summed over time along the path they take, and incapacitation is expected at a fractional effective dose of 1 for the median occupant (the same note, §5 and §5.3, after the SFPE Handbook and ISO 13571). The note computes heat exposure as one FED and toxic-gas exposure as another (§5.3). For more susceptible occupants a lower threshold may suit; see Incapacitation thresholds.

Fixed tenability limitsOne occupant standing below a dashed line at 2.0 m above the floor, where the limits are evaluated. Short exposure, up to 10 minutes. Air temperature at most 100 °C; radiant heat at most 2.5 kW/m²; CO at most 2,700 ppm and HCN at most 140 ppm; visibility at least 10 m. Conditions are untenable as soon as any one limit is exceeded.Fixed limits at one heightshort exposure, up to 10 minair ≤ 100 °Cradiation ≤ 2.5 kW/m²CO ≤ 2,700 ppmHCN ≤ 140 ppmvisibility ≥ 10 mlimits evaluated at 2.0 mUntenable as soon asany one is exceeded
Dose along each occupant's pathA graph of fractional effective dose (FED) against time for two occupants, with a dashed threshold line at FED 1. Occupant A takes a short route; the dose rises slowly and A reaches the exit, marked by a circle, at a dose of about 0.35. Occupant B takes a route through denser smoke; the dose rises faster and reaches 1, marked by a cross, where B is incapacitated. The dose is the sum of the dose rate times the time step along each path. The curves are the toxic-gas dose; heat exposure is judged as a second, separate FED.Dose along each pathsummed over time, per occupantFED = Σ dose rate × Δt01gas FEDtime along the path →A reachesthe exitB stops at 1A: short routeB: route through denser smokeHeat: a second, separate FED

Fixed limits: the short-exposure criteria of the practice note (Fig. 8, “Tenability Criteria – Short Exposure”, p. 15); for small enclosures, of the order of 10 m, the note allows a visibility of 5 m. Dose along each path: the dose approach, schematic, for two occupants; the same fire gives each one a different dose. The curves are the toxic-gas FED; heat is judged as a second, separate FED (§5.3). Figure inspired by Fig. 8 of the Engineers Australia practice note for tenability criteria (2014).

RSET comes from people. ISO/TR 16738 (§5.4) reports that the pre-travel activity phase can often be the longest part of the total escape time. Each occupant has their own \(t_{\mathrm{pre}}\) and \(t_{\mathrm{trav}}\), so the standard treats them as distributions, and warns that within an enclosure the two terms cannot be added directly because the distributions interact (§7). See Pre-movement time.

Known limits

The definitions differ in where the clock starts. ISO/TR 16738 Eq. 2 and SFPE Ch. 64 count RSET from ignition, since they include detection. SFPE Ch. 56 (Bukowski and Tubbs 2016) defines RSET from the notification of occupants and ASET from notification to the onset of untenable conditions, while also stating that evacuation times consist of detection, notification, pre-movement and movement times. When two analyses are compared, check which origin each uses. The terms also vary: “escape” in ISO/TR 16738, “egress” or “evacuation” in the SFPE Handbook, for the same quantities.

ASET is not one number. It varies with position and with the tenability criterion chosen, and the criterion itself is set for a fraction of the population (see Incapacitation thresholds).

Sources

  • ISO (2009). ISO/TR 16738:2009 Fire-safety engineering — Technical information on methods for evaluating behaviour and movement of people. ISO, Geneva. iso.org/standard/42887. Read from the public preview (§1–7).
  • ISO (2012). ISO 13571:2012 Life-threatening components of fire — Guidelines for the estimation of time to compromised tenability in fires. ISO, Geneva. iso.org/standard/56172.
  • Bukowski, R. W., & Tubbs, J. S. (2016). Egress concepts and design approaches. SFPE Handbook of Fire Protection Engineering, 5th ed., Ch. 56, 2012–2046. doi:10.1007/978-1-4939-2565-0_56
  • Engineers Australia Society of Fire Safety (2014). Practice note for tenability criteria in building fires, version 2.0, 3 April 2014. Society of Fire Safety, NSW Chapter, Engineers Australia. engineersaustralia.org.au. §5 (p. 13), §5.2 and Fig. 8 (p. 15), §5.3 (p. 17).
  • Gwynne, S. M. V., & Boyce, K. E. (2016). Engineering data. SFPE Handbook of Fire Protection Engineering, 5th ed., Ch. 64, 2429–2551. doi:10.1007/978-1-4939-2565-0_64

How pyFDS-Evac uses this: see the RSET ensemble how-to, and A crowd in a real fire › ASET and RSET for a per-agent comparison on a real FDS fire, and the Schröder room for ASET, RSET and their difference as maps. Each agent carries its own dose along its path; see Fractional effective dose.

RSET from a run without the fire against a coupled run: Evacuation with and without the fire.

How it is verified: the Verification index; ASET and RSET themselves are outputs of a study, not of a test.

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