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Design Fires A Structural Engineering Perspective

Design Fires A Structural Engineering Perspective. Dr M Gillie. Content. Existing fire models Standard Fire Swedish curves Parametric fires CFD models Future directions Rein’s fires Structural behaviour in travelling fires. Standard Fire Test.

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Design Fires A Structural Engineering Perspective

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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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