A probabilistic system dynamics model for dynamic assessment of occupant risk during fire growth and evacuation

Fires in buildings pose a significant threat to occupant life safety, particularly when fire growth, smoke-layer descent, evacuation delay, and exit-capacity limitations evolve on comparable time scales. This study develops a probabilistic System Dynamics framework for screening-level assessment of occupant exposure during fire growth and evacuation. The model integrates a smoke-layer-based Available Safe Egress Time calculation with an aggregated evacuation model including detection, recognition and response, travel time, queue formation, and total effective exit capacity. Occupants are represented through three stocks: occupants remaining inside the evacuation zone, occupants safely evacuated before untenability, and occupants exposed to untenable conditions. Uncertainty in fire growth, critical smoke-free height, smoke-layer modelling, response time, movement velocity, specific flow capacity, and evacuation timing is propagated through stochastic simulation. The framework is applied to a near-critical retail evacuation-zone scenario in which ASET and RSET are of similar magnitude. Scenario analysis shows that alarm delay, enclosure height, and fire-growth severity are dominant drivers of exposure. Increasing additional alarm delay from 0 to 720 s increases the probability of evacuation failure from 0.0117 to 0.9282, while increasing enclosure height from 2.2 to 3.6 m reduces the expected number of exposed occupants from 160.68 to 14.30. The results demonstrate that the proposed framework can identify near-critical conditions, quantify probabilistic ASET/RSET competition, and support early-stage comparison of fire safety design and operational strategies. The model is intended as a screening and decision-support tool, complementary to detailed fire and evacuation simulations.