Fire Severity for Structural Design

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1 Fire Severity for Structural Design A UK Perspective Susan Deeny, PhD

2 2 Broadgate Phase 8

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5 5 Shaping a better world

6 Experience of working in Abu Dhabi 6 UAE

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8 Fire Severity for Structural Analysis Building Fire Behaviour Fire Severity Structural Response 8

9 Building Fire Behaviour Fuel Ventilation Geometry Boundaries 9

10 Small-medium compartments

11 Post flashover fires All fuel within enclosure is burning; Gas temperatures are generally uniform T s z l D M f A o L 11

12 12 Spatial variation compartment temperatures

13 Average compartment temperature Average compartment temperature Average compartment temperature Post flashover fires Considering the impacting factors again - Fuel: Well distributed affects duration. - Ventilation: affects duration and peak temperatures - Geometry affects growth rates and peak temperatures Fire duration Fire duration Fire duration 13

14 Post-flashover fires Tools Parametric design fires Single gas temperature time relationship Heat and cooling phase Available in national design documents Considers Ventilation Fuel load Thermal boundaries Compartment size Validated up in tests up to 100~144m 2 compartments. Likely to be unphysical for large compartments (1000m 2 )

15 15 Post Flashover Flame Projection

16 Open plan compartments

17 Vertical Villages Compartment floor Atrium floors Typical Village

18 Travelling fires Witnessed in real building fires - World Trade Centres, Torres Windsor, Delft Faculty of Architecture Little to no experimental data (Current programmes in Europe) 18

19 Travelling fires Fuel: Distributed Ventilation: Large, fuel controlled fires Geometry: Large (100m 2 +) z l A o T s A o D M f L 19

20 Travelling fires - Tools Arup UoE Methodology (Stern Gottfried & Rein) Near and Far field temperatures Fuel load density and burning area determine travel speed Family of fire curves required Far Field Alpert Near Field 1200 C 20

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25 Very Large Compartments

26 Localised fires Fuel: Low/localised fuel source Ventilation: Large fuel controlled burning Geometry: Large volume low feedback A o A o T s A o z l D M f L 26

27 Localised fires Structural Effects Exposure: - High temperatures/heat flux local to flame - Limited duration due to restricted fuel source 27

28 Localised fires Tools Plume Temperatures: - Heskestad (SFPE Handbook/EC1) - Hasemi (SFEP Handbook/EC1) - Alpert Ceiling Jet Correlations - TM19 Plume Correlations Inputs: - Heat Release Rate (kw) - Fire Area (m 2 ) - Fuel Load (MJ) Heskestad method (left) and Hasemi method (right) 28

29 29 Combustible Construction

30 Temperature (C) Design Fire Severity Long Duration Travelling Fire Medium Duration Travelling Fire Short Duration Travelling Fire 600 Parametric Fire Standard Fire Time (min) 30

31 Temperature (C) ASTM E-119 Time (minutes)

32 Temperature Goals Solution Time

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34 Temperature Temperature Ingberg s Goal Equal areas Time Time

35 Temperature Temperature Goals Constraints Time Solution Time

36 Design Fire Selection - Current method How do we pick a design fire? Design Fire Heat transfer analysis Structural model 36

37 Average compartment temperature Average compartment temperature Average compartment temperature Post flashover fires Fire duration Fire duration Fire duration 37

38 Design Fire Selection - Current method How do we pick a design fire? Design Fire Heat transfer analysis Its not practical to design our building to resist every possible fire scenario BS 9999 acknowledges this: an acceptance criteria for design is defined (based on consequence of failure) Structural model 38

39 Consequence of failure Building type

40 Allowable failure rate Building type

41 Low High Likelihood Consequence likelihood consequence = Risk Risk Building type

42 Design Fire Selection - Current method How do we pick a design fire? Design Fire Heat transfer analysis Structural model Its not practical to design our building to resist every possible fire scenario BS 9999 acknowledges this: an acceptance criteria for design is defined (based on consequence of failure) BS 9999 recognises that the standard fire is inadequate, and adopts parametric curves We now recognise that parametric fires are not always appropriate This approach will allow us to discard the most onerous fires, and select specific fires for structural design 42

43 Risk Based Approach to Design Fire Selection 1) Physical Inputs based on probabilistic distributions Compartment geometry Fuel Load Fire size 2) Maximum protected steel temperature used to characterise fire severity H p/ A analysis 3) Acceptance criteria (allowable failure rate) and selection of key design fires Design Fire Heat transfer analysis Structural model 43

44 Risk Based Approach to Design Fire Selection 1) Physical Inputs based on probabilistic distributions Compartment geometry Fuel Load Fire size 2) Maximum protected steel temperature used to characterise fire severity 3) Acceptance criteria (allowable failure rate) and selection of key design fires Design Fire Heat transfer analysis Structural model 44

45 (1) Physical Inputs Key Variables Key Variables Fuel Load Compartment Area Fire Burn Area Heat Release Rate Flame Temperature Ventilation Controlled for probabilistic distribution and confidence Monte Carlo Analysis to consider potential variation 45

46 Probability distribution Probability distribution Probability distribution (1) Physical Inputs - Possible Distributions More low HRR/UA: More medium HRR/UA: More high HRR/UA: Max HRR/UA Min HRR/UA 250kW/m² 550kW/m² 250kW/m² 550kW/m² 250kW/m² 550kW/m² HRR/UA HRR/UA HRR/UA 46

47 Risk Based Approach to Design Fire Selection 1) Physical Inputs based on probabilistic distributions 2) Maximum protected steel temperature used to characterise fire severity H p/ A analysis 3) Acceptance criteria (allowable failure rate) and selection of key design fires Design Fire Heat transfer analysis Structural model 47

48 48 (2) Fire Severity Maximum Steel Temperature

49 Risk Based Approach to Design Fire Selection 1) Physical Inputs based on probabilistic distributions 2) Maximum protected steel temperature used to characterise fire severity 3) Acceptance criteria (allowable failure rate) and selection of key design fires Design Fire Heat transfer analysis Structural model 49

50 (3) Acceptance Criteria & design fire selection At 18m design fractal is 80% giving 60 minutes FR At 40m design fractal is 96% Fires that the structure will not be designed to resist Most onerous design fires With sprinklers, this is reduced to ~80% selected for input to FE model Fires that the structure must be able to resist 90 minutes of fire protection 50

51 Cumulative Frequency (3) Acceptance Criteria & design fire selection 100% Range of worst case design fires Target reliability Target reliability 0% Limiting temp Limiting temp 51

52 Temperature (C) Fires for Structural Analysis Range of fires: parametric curves, travelling fires and standard Engineering judgement required to determine appropriate range Long Duration Travelling Fire Medium Duration Travelling Fire Short Duration Travelling Fire Parametric Fire Standard Fire Time (min) 52

53 Structural Response Structural Response

54 Office Tower Compartment floor Atrium floors Typical Village

55 55 Multi-storey fire

56 56 Fire spread to multiple floors - Columns

57 57 Long-cool & Short-hot

58 58 Travelling Fires

59 Temperature (C) An envelope of fire behaviour Long Duration Travelling Fire Medium Duration Travelling Fire Short Duration Travelling Fire Parametric Fire Standard Fire Time (min) 59

60 An Envelope of fire behaviour Buildings structures and fire hazard are increasingly complex will require Risk based methods can remove subjectivity in selecting design fires Structural fire behaviour should be tested under an envelope of design fires Combustible construction presents a new challenge to fire severity 60