Fundamentals

Fundamentals

The published laws that fire-safety engineering uses to judge whether occupants can escape a fire: the egress timeline, the design fire, smoke obscuration, walking speed in smoke, toxic and thermal dose, and occupant decisions. These pages are written for PhD students and pedestrian-dynamics engineers who are new to fire toxicology and human behaviour in fire.

Smoke from a fire rises, forms a hot layer under the ceiling and spreads through the building. Its hazards to occupants are heat and toxic gases carried in the smoke, and the obscuration the smoke causes (Engineers Australia Society of Fire Safety 2014, §3.1).

Hazards of a building fireSide view of a room. A fire burns on the floor at the left; its plume rises and spreads as a hot smoke layer under the ceiling, whose lower edge reaches the head of one occupant walking towards an exit at the right. Four hazards are marked, each by an icon and a label: convected heat, drawn as curved arrows of hot gas flowing in the layer towards the occupant; radiant heat, drawn as straight dashed rays from the flame and from the layer to the occupant; toxic and irritant gases, drawn as molecules in the layer around the head; and smoke obscuration, drawn as a dashed sight line from the occupant's eye to the exit sign, which hangs in the smoke.Hot smoke layerEXITConvected heatRadiant heatToxic and irritant gasesSmokeobscuration

Hazards of a building fire, as a sketch, not to scale. The sketch shows mechanisms only; it states no acceptance criteria and no software defaults. Figure inspired by Fig. 1 of the Engineers Australia practice note for tenability criteria (2014).

Each page covers one quantity: the published laws for it, in the source’s own notation and constants, the data it rests on, its known limits, and the primary citations. How pyFDS-Evac implements a law, with its defaults and where it departs from the source, is on the Models pages. The intuition behind the models is on the Concepts page.

For the full treatment, see Part VIII, Human Behavior (Ch. 65–72), of the SFPE Handbook of Fire Protection Engineering, 6th ed. (SFPE 2026).

ASET, RSET and the egress timeline
Detection, warning, pre-travel activity and travel, against the time to untenable conditions.
Design fires
The t² growth law and its four classes, how growth is capped, and what a design fire represents.
Extinction coefficient
Beer–Lambert attenuation, optical density and the mass-specific extinction coefficient.
Visibility through smoke
Jin’s V = C/K, where C comes from, and how it changes in dim light.
Seeing and using exit signs
From a sign barely seen to a sign followed: the waypoint method and the detection studies.
Walking speed in smoke
Jin, Frantzich and Nilsson, Fridolf et al., and the fractional versus absolute readings.
Asphyxiant fractional effective dose
CO, HCN, CO₂ hyperventilation and low-oxygen hypoxia after Purser and ISO 13571.
Irritant gases
FEC as a separate endpoint in ISO 13571; FIC and FLD in Purser.
Heat
Convective and radiant heat: separate equations, separate endpoints.
Incapacitation thresholds
FED 1 against 0.3, the log-normal assumption, and what its population basis is.
Pre-movement time
Why the longest part of escape is often spent before anyone moves.
Exit choice and familiarity
Movement to the familiar, social influence and discrete-choice models.

Sources

  • 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. §3.1 and Fig. 1, p. 7.
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