Fire Safety Engineering & Structures in Fire

1. Fire Safety Engineering & Structures in Fire Structural Fire Engineering – Design Approaches Workshop at Indian Institute of Science 9-13 August, 2010 Bangalore India

2. Structural design for fire • PRESCRIPTIVE APPROACH • Structural elements protected to remain under a certain temperature • Fire scenario so that they retain adequate strength and stiffness to continue to carry loads. This has been the traditional approach. • PERFORMANCE BASED DESIGN APPROACH • Engineer must show structure meets certain criteria • Requires understanding of behaviour

3. Schematic of design methods Standard fire Natural fires Conventional design Single member behaviour E.g. Eurocode Parametric fire Whole structure behaviour SCI Level-1 Design guide Advanced methods

4. Eurocodes • Replacing British (and other) Standards (for all structural design) • 9 Codes • Eurocode 1 - actions • Eurocode 2 - concrete • Eurocode 3 - steel • Eurocode 4 - composite • Etc • All available online via the library website

5. Eurocode 1 • Covers “actions” for all design • Various parts and sections • Part 1-2 covers fire actions • Allows for • Standard fire curve • Natural fires • Computer analysis of fires • Provides background information on • Parametric fires • Fire load densities • etc

6. Procedure for Fire Engineering Design (1) • Obtain compartment size • From geometry/use of structure • Ventilation openings also needed • Estimate loads on structure • Fire load based on use of structure • Mechanical load with low safety factors • Estimate gas temperatures • Swedish curves • Parametric curves • Zone models • CFD

7. Procedure for Fire Engineering Design (1) • Estimate structural temperatures • Simple calculation • Computer analysis • Check resistance of structure to fire • Calculation on single elements • SCI style design guides • Computer analysis

8. Loads for Fire Design

9. Mechanical Loads for Fire Design • Dead loads always present • γ (safety factor) normally taken as 1 • Imposed loads taken as less than design load • γ typically taken as 0.4-0.9 • Eurocode 1 has • 1 x dead load +0.9 x permanent live load +0.5 x temporary live load

10. Fire Loads • Measure of the combustible material in a fire compartment • Normally measured in MJ/m2 floor area • REMEMBER temperature calculations often use total surface area of compartment • In design adjusted for • Compartment area • “Fire activation risk” • Etc • Extract from Eurocode available

11. Estimating Compartment Temperatures

12. Temperatures in compartment fires • Need to know atmosphere temperatures in order to estimate structural temperatures • Simple approach uses energy balance in a compartment • Swedish method the most common (from physics) • Similar curves in parametric form in the Eurocode 1 Part 2 (curve fire to Swedish method)

13. Compartment Fires

14. Energy balance for a compartment QW QL QW Qc QR

15. 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

16. Evaluation of terms - Qc • Heat release rate given by Kawagoe equation as • Assumes all fuel is wood • Set Qc=0 when all fuel consumed at time

17. Evaluation of Terms - QR • Radiation through opening governed by Stefan-Boltzmann equation • Epsilon value uncertain. Drysdale suggests With K=1.1m-1 and xf the flame thickness

18. Evaluation of terms - QL • The rate of energy loss due to exchange of gases is • The mass flow rate is determined semi-empirically by Prahl and Emmons using Bernoulli’s equation

19. Evaluation of terms - QW • Convection into wall • Conduction through wall • Convection out of wall • Not steady state

20. Temperatures in a compartment boundary

21. Evaluation of terms - QW Conduction out of layer Convection into layer Rate of change in stored energy Heat transfer into first layer of wall

22. Substitute in Heat Balance Equation Intrinsic Needs to be solved numerically

23. Swedish Curves

24. Swedish Curves

25. Swedish Curves • NOTE: Fire load calculated based on TOTAL surface area of the compartment • Implicit nature awkward

26. Parametric T-t Curves • Used in Eurocodes • Avoid implicit nature of Swedish Curves • Growth curve based on opening factor • Peak temperature (time) based on fuel load • Linear decay curve

27. Peak temp depends on fire load or ventilation Linear decay phase Eurocode parametric temperature-time Curve q=400MJ/m2 A=400m2

28. STEEL STRUCTURES

29. General Actions on structure in fire < Strength of structure in fire

30. Spray protection: £6/m2 1 hour, wet trade, poor application in winter conditions

31. Board protection: £8/m2 1 hour, Higher labour – fixing etc. Not good for external, Slower than spray

32. Blanket: same price as boarding, poor appearance, dry trade

33. Paints React to heat, swelling to form a protective coating Mastics Epoxy intumescent Intumescents: Most expensive material, off site option

34. Off site intumescent • Plates and bolts need to be cleaned, primed and painted on site • Bolt caps recently introduced to the market • Aiming for a completely offsite product

35. Why contractor may propose off-site application: • Faster construction • Cost savings • Reduction in site disruption • Improved safety • Better QA • Environmental • Where site access is limited

36. Disadvantages of off site application • Careful handling required • Mechanical damage • Coat connections once erected • Damaged by through deck stud welding • Water-based systems are not sufficiently durable for off-site application

37. Automated application process at ENOB Ltd

38. Elimination of topcoat? • Top coat function • improved durability, cleanability, appearance • protects tfi from mechanical damage, uv degradation • Why remove: Cost! • Do consider • in internal, heated, air-conditioned environments • where tfi is protected from damage • Don’t consider • where high RH, condensation are anticipated • NB • ponding more likely to cause water damage • programme delays could lead to damage

39. Appearance on site (without top coat)

40. Elimination of primer? • Primer function: • prevent corrosion of steel • provide good substrate to which tfi adheres • Why remove: Cost • Only consider in C1 environment • NB: • mechanical damage to tfi will allow rusting • making good - abrading, spot priming, tfi • uncoated areas may be primed or left as black steel • delays - loss of tfi & rusted steel

41. Pros and Cons of Protection materials

42. Pros and Cons of Protection materials

43. Market shares

44. Costs

45. Partially protected steel • Web in-filled columns • blockwork • concrete unreinforced • concrete reinforced • Concrete filled hollow sections • Typically 30-90 minutes FR

46. Partially protected steel –Composite floor systems • Shelf Angle Floor beams • Slim floor beams • SLIMFLOR • SLIMDEK • Up to 60 minutes possible with bare steel flange. • 90 and 120mins fire resistance possible with protected flange

47. Hidden costs • The quality of finish – decorative finishes are more expensive • Difficulty or ease of access, manpower, time on site • Size of project • Location of project e.g. tall congested city centre • Type, size and weight of steel section

48. CONCRETE STRUCTURES

49. Heat penetration in concrete beams

50. Heat penetration in concrete slabs (mm)