The Race Against Smoke
Building evacuation is fundamentally a race between the time occupants need to reach safety (RSET) and the time before fire conditions become untenable (ASET). Fire engineers must demonstrate that ASET exceeds RSET with an adequate safety margin. This seemingly simple comparison involves complex human behavior, crowd dynamics, building geometry, and fire development — making egress analysis one of the most challenging aspects of fire safety design.
Components of Egress Time
RSET comprises four sequential phases: detection time (seconds to minutes, depending on detector type and fire growth), alarm time (typically negligible for automatic systems), pre-movement time (often the longest phase, involving recognition, decision-making, and preparation), and movement time (traveling to exits and passing through them). Pre-movement time is the most variable and uncertain component, heavily influenced by human psychology and social dynamics.
Hydraulic Flow Model
The movement phase can be modeled using a hydraulic analogy where people flow like fluid through corridors, doorways, and stairs. Each building element has a capacity determined by its width and the specific flow rate (persons per meter per second). Bottlenecks occur at narrowing points — typically doorways and stair entries — where queues form and density increases. This simulation uses the Fruin/SFPE hydraulic model to calculate flow times and identify capacity constraints.
Beyond the Hydraulic Model
Real evacuations involve complex human behaviors that simple hydraulic models cannot capture: way-finding errors, counter-flows (people returning for belongings), group dynamics (families staying together), elevator usage, and the effects of smoke on walking speed and route choice. Agent-based models like EXODUS, Pathfinder, and STEPS simulate individual decision-making to produce more realistic predictions. However, the hydraulic model remains the standard tool for code compliance calculations.