Fire Protection Analysis: Fire Dynamics, Hazard, and Risk
The fire scenario is the unit of analysis
Nearly every analysis question reduces to two things: the heat release rate (HRR) as a function of time, and where the products of combustion go. The fire tetrahedron (fuel, oxygen, heat, uninhibited chain reaction) explains ignition and extinguishment; the flaming-versus-smoldering distinction explains why smoke detection usually precedes heat detection and why smoldering fires produce more toxic smoke per unit of mass burned.
A compartment fire passes through incipient, growth, flashover, fully developed, and decay stages. Growth is conventionally modeled as t-squared, Q = αt², with α values of 0.00293 (slow), 0.01172 (medium), 0.0469 (fast), and 0.1876 kW/s² (ultrafast). Flashover - the transition from a growing fire to a fully developed one - is commonly correlated with an upper-layer temperature near 600 °C or a radiant flux at floor level near 20 kW/m². After flashover, the fire is usually ventilation-limited, so opening geometry rather than fuel load controls the HRR.
Hazard versus risk
A hazard is a condition with potential for harm (a storage pile, an ignition source); risk combines likelihood and consequence. NFPA 551 frames fire risk assessment, and SFPE engineering guides frame design-fire selection. Performance-based design compares available safe egress time (ASET, from the tenability model: visibility, temperature, toxic species) against required safe egress time (RSET = detection + pre-movement + travel).
Worked example
Medium-growth fire: Q = 0.01172 × (200 s)² = 0.01172 × 40,000 ≈ 469 kW. At 400 s, Q ≈ 1,875 kW. Quadrupling the time quadruples nothing - it multiplies HRR by sixteen.
Traps
Confusing hazard with risk; using peak HRR without a growth curve; assuming flashover always occurs (a large volume or a suppressed fire may never reach it); and forgetting that a sprinkler-controlled design fire is capped, often on the order of 1.5 MW.