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Understanding Fires in Structural Engineering: Analysis, Models & Future Directions

Explore existing fire models like Standard Fire and parametric fires from a structural engineering perspective. Learn about Swedish curves, Rein’s fires, and CFD models, with a focus on compartment fires. Discover the pros and cons of critical temperature testing and explore the implications for structural behavior in traveling fires. Delve into fire loading, gas temperatures, and the influence of temperatures on structural elements. Consider the potential impact on design forces and fire protection strategies.

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Understanding Fires in Structural Engineering: Analysis, Models & Future Directions

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  1. Design FiresA Structural Engineering Perspective Dr M Gillie

  2. Content • Existing fire models • Standard Fire • Swedish curves • Parametric fires • CFD models • Future directions • Rein’s fires • Structural behaviour in travelling fires

  3. Standard Fire Test • Defined using a furnace Test (ASTM-E-119, ISO-834, BS-476 Part 8) Critical Temperature

  4. Pros and Cons • Has many limitations • Not based on real fire data • Test repeatability difficult • No cooling phase • Uniform heating • Uses gas temperature “not fair” • But • Widely used • Can be useful for crudely comparing products

  5. Compartment Fires

  6. Energy balance for a compartment – Swedish Method QW QL QW Qc QR

  7. Assumptions in Swedish method • No heat built-up in pre-flashover phase of fire • Temperature uniform in the compartment • Uniform heat transfer coefficient in compartment boundaries • All combustion takes place in the compartment

  8. Pros and Cons • Limitations • Crude • Rather severe • Implicit expressions (Eurocode parametric curves solve this) • ***Uniform fire…*** • ***…hence maximum size of compartment*** • But • “Not bad” • Can be used in performance-based design

  9. Zone Models • Extension of parametric models • Assume uniform temperatures in each zone • Normally computer based • Several commercial codes available eg. Ozone, CFast • Similar drawbacks and benefits to parametric curves

  10. Fires Travel • Large compartment test at BRE showed travelling fire behaviour • Travel times of the order of 45 minutes within compartment 5m deep Ventilation Crib Crib Crib Crib 5.5m

  11. …other Evidence

  12. Defining Temperature Loading • Need simple model that can capture effects of • Travelling fire • Ventilation conditions • Cooling • Preliminary work undertaken on simplified travelling fire loading by Rein et. al. • NOTE: CFD calculations too complex for design work and… • …can not predict burning behaviour anyway

  13. Rein’s Fire Model • Rein proposes a near field and far field model of temperature loading • Fire field moves around compartment

  14. Proposed Fire Model Structure Cool Hot Cool 200-600C Need size of fire, gas temperatures and nature of travel! 1200C Compartment

  15. Proposed Gas Temperatures T 1200C Good ventilation Size of fire Limited ventilation Size of fire Distance from seat of fire

  16. Typical office fire load =570MJ/m2 Rate of burning = 500 kW/m2 Therefore 19 minutes in every location Total fire time= 19Ac/Af Local fire T=0 Local fire T>0 Local fire Later on Fire compartment Travelling Behaviour

  17. Travelling Behaviour • What “path” does the fire take? • We can’t know • Depends on details of compartment and chance

  18. y x Influence on Structural Behaviour Composite frame “Cardiington like” Travelling fire

  19. Influence of Temperatures

  20. Influence of Temperatures

  21. Conclusions • Current assumptions probably conservative • Travelling fires in large compartments may lead to significantly lower design forces • Current assumptions very severe • Possibilities for significant savings in fire protection • Work to date of conceptual nature

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