Tenability in Smoke: Defining Safe Evacuation Conditions with CFD

Tenability in Smoke: Defining Safe Evacuation Conditions with CFD

When it comes to building evacuation, we often hear the term tenability threshold. In simple words, tenability refers to the conditions inside a smoke-filled space that determine whether people can survive and safely escape during a fire. It measures how survivable the atmosphere is in terms of temperature, visibility, toxic gases, oxygen levels, and radiant heat exposure.

Key Tenability Conditions in Smoke

1. Temperature

   • Threshold: ≤ 60 °C at head height for standing adults

   • Higher temperatures cause severe pain, burns, or unconsciousness.

2. Visibility

   • Threshold: ≥ 5–10 meters

   • Below this, occupants struggle to see exit signs or doors, causing delays or disorientation.

3. Toxic Gases

   • Carbon Monoxide (CO): ~500 ppm tolerable for 10 minutes

   • Hydrogen Cyanide (HCN): ~100 ppm can be fatal within minutes

   • High gas concentrations reduce consciousness and can cause death.

4. Oxygen Concentration

   • Minimum: 19.5% (normal air ~21%)

   • < 17% impairs coordination, < 10% is life-threatening.

5. Radiant Heat Flux

   • Threshold: ≤ 2.5 kW/m²

   • Above this, skin burns occur in a few seconds.

Consolidated Minimum Tenable Conditions

• Temperature: ≤ 60 °C

• Visibility: ≥ 5 m

• CO Concentration: ≤ 500 ppm

• O₂ Concentration: ≥ 19.5%

• Radiant Heat Flux: ≤ 2.5 kW/m²

Where Do These Values Come From?

These thresholds are based on decades of research and codified in international standards:

• NFPA (National Fire Protection Association):

  • NFPA 92, NFPA 101, NFPA 130 – cover smoke management and evacuation safety.

• SFPE (Society of Fire Protection Engineers) Handbook:

  • Scientific reference for tenability criteria in performance-based design.

• ISO Standards:

  • ISO 13571 – defines life-threatening components of fire and evacuation time.

  • ISO 16733 – provides guidance on fire scenario selection.

• NIST (National Institute of Standards and Technology):

  • Research on tenability, toxic gas effects, and fire dynamics.

• UK Standards:

  • BS 7974 and Approved Document B – reference tenability in engineered fire strategies.

Selecting Fire Scenarios for CFD Simulation

Computational Fluid Dynamics (CFD) tools like FDS or PyroSim are widely used for fire and smoke modeling. But the accuracy of results depends heavily on how fire scenarios are defined.

Step 1: Identify Study Objectives

• Life safety (evacuation modeling)

• Smoke management design

• Performance-based compliance

Step 2: Define Possible Fire Scenarios

• Fire load & occupancy type: e.g., plastics vs. wood in warehouses, train cars, or offices

• Ignition location: electrical rooms, kitchens, trash bins, vehicles

• Ventilation: natural vs. mechanical, open/closed doors and windows

• Occupancy condition: night (sleeping) vs. peak daytime load

• Fire growth rate (t² fires): slow (600s), medium (300s), fast (150s), ultra-fast (75s)

Step 3: Select Critical Scenarios

• Most severe smoke spread

• Longest egress time

• Highest occupant load

• Failure cases (power loss, vent malfunction, blocked exits)

Step 4: Align with Standards

• NFPA 92, NFPA 101

• ISO 16733 (scenario selection)

• SFPE Performance-Based Guides

• BS 7974 / PD 7974-1

Step 5: Number of Scenarios

• Typically 3–6 justified cases

• Include base case, worst-case, and sensitivity cases

Why Tenability Matters

Performance-based fire safety design depends on comparing Available Safe Egress Time (ASET) with Required Safe Egress Time (RSET):

• ASET: How long conditions remain tenable before thresholds are exceeded.

• RSET: How long occupants need to evacuate.

For a safe design, ASET must always exceed RSET.

Final Thoughts

Tenability thresholds—temperature, visibility, toxic gases, oxygen concentration, and radiant heat—are the foundation of fire safety engineering. By aligning CFD fire scenarios with NFPA, SFPE, ISO, NIST, and BS 7974 standards, fire engineers can design safer buildings and evacuation strategies.

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