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Fire Safety Engineering & Structures in Fire

Fire Safety Engineering & Structures in Fire

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Fire Safety Engineering & Structures in Fire

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

  2. Preliminaries • Martin Gillie • Acknowledge • Prof. Asif Usmani • Many former and current Phd students • The fire group at Edinburgh • Other fire researchers • Aims • Introduce research background • Describe key aspects of analysis and design of heated structures • Presentations • Introduction • Material behaviour at high temperature • Heat transfer in structures • Structural mechanics at high temperature • Modelling of heated structures • Comments on design codes

  3. Fire is a “load” on structures Caracas Los Angeles Taipei

  4. Piper Alpha Mont Blanc Kings Cross Kobe

  5. INTRODUCTION • Traditional fire design is geared towards FIRE PROTECTING steel so that temperatures remain below 550°C (in a standard fire test) • Two well-recognised problems with this approach • Fire scenario is artificial • Whole structure response is very different from single element response • 500C is still a high temperature – no guarantee of conservatism

  6. Traditional fire resistance design of steel structures • Standard fire resistance test • Standard fire • Single elements of construction • axially unrestrained • Applied fire protection to reduce the temperature of steel during a fire

  7. The fire resistance test • History • Procedure formalised ~80 years ago (based on test done ~100 yrs ago) • Standard temperature-time curve -1918(USA), 1932(UK) • Testing single elements or assemblies • Unrestrained (UK) • Restrained/unrestrained (USA) • Furnace • Characteristics differ around the world • No two furnaces provide the same exposure

  8. Column tests

  9. Test criteria • Stability • the ability of a load bearing element of construction to continue to perform its function • Integrity • prevent passage of flames or gases through holes, cracks, fissures or by collapse (cotton pad) • Insulation • should not allow the temperature rise on the unheated side of the element to exceed 140°C above its initial value

  10. Standard fire curves No cooling branch Logarithmic curve T=To+345(log0.113t+1)

  11. Standard and Natural fires

  12. Standard fire test in furnace • Determinate: so stresses governed by equilibrium and depend on applied load • Free expansion of heated beam • Deflections dominated by mechanical strains at given stress level • “Runaway” collapse when strength declines to stress value corresponding to loading

  13. Failure of a simple beam

  14. Need for new methods • Despite the obvious lack of scientific rigour traditional design approach has provided good service, so why change? • The economic burdens of fire in general • Order of 1% of GDP in industrialised countries • Overall total economic burden of fire in USA over $128 billion! Not including losses of productivity, environmental impact • Innovative designs not possible or difficult using the Standard Fire test approach • Need for ability to understand and predict real structural behaviour.

  15. Cost of Fire • Fire costs as % of GDP (Snell, 2001) • USA 0.80 • Japan 0.78 • UK 0.66 • Canada 0.91 • Design methods based on poor science are inefficient • increased burden on economy • less competitive! • Very inflexible – innovative design difficult.. • Solution: Applying better science & technology=>RESEARCH

  16. Research Context • Natural fire vs. standard fire has attracted considerable effort • Heat transfer models are reasonably reliable for design • Steel material behaviour to heating reasonably well know • Concrete behaviour less predictable • Relatively less effort on understanding whole structure response, until recently

  17. UK situation in Composite Steel-Concrete design c1997 • Nearly all design geared towards maintaining steel temperatures below 550°C through fire protection • UK fire protections costs are approximately 15% of total structural steel cost, reduced from over 20% in 1981, but not through any changes in design procedures

  18. Milestone events in UK • A number of severe accidental fires • Broadgate Phase 8 (1990,London,under-construction & unprotected) • Fire temperatures of over 1000 °C, no structural failure • Structural repairs 5% of total repair cost • Churchill Plaza (1991,Basingstoke,built 1988,protected for 90-min) • Fire engulfed -floors (8-10 of 12 storeys), complete burnout in 4 hrs • Total repair cost £17m. No structural damage or repairs

  19. Milestone experiments • Evidence suggesting that the prevalent fire design procedures were grossly over-conservative was getting stronger • Six full scale fire tests were performed at the BRE large building test facility at Cardington (4 by British Steel - Now Corus & 2 by BRE) in 1995-96 • Variety of fire compartments with mostly unprotected beams and protected columns • Again no structural collapse occurred

  20. BRE Large Building Test Facility

  21. Cardington Frame

  22. 3x5 bay frame: 4 tests by British Steel

  23. 2 tests by BRE

  24. Restrained beam test

  25. Restrained beam test

  26. Restrained beam test • 3mx8m compartment testing 305x165 mm unprotected secondary beam spanning 9m between 254x254 mm columns. • Maximum deflection (midspan) 232 mm at 887°C • Beam lower flange buckles at both ends just inside the compartment with tensile cracking in concrete slab • The best test to benchmark computational codes and other analytical models (because its simple and most key events occur)

  27. BS TEST2: “Plane Frame”

  28. Plane frame test

  29. Plane frame test • 3mx21m compartment (whole building width) with all beams unprotected and columns unprotected over the top 800mm • Columns squash at 670°C steel temperature (max. atmosphere temperature was 750°C) • Primary beam to secondary beam connections failed due excessive deflection of primary beam and the squash event • Main lesson “always protect columns to full exposed height”

  30. BS TEST 3: “Corner Test”

  31. Corner Test: Plan

  32. Corner test

  33. Corner test • 10mx7.5m compartment with atmosphere temperatures over 1000°C. All internal beams unprotected (edge beams protected) • Maximum deflection 428 mm at steel temperature of 935°C at midspan of the middle secondary beam, with deflection recovering to 296 mm after cooling • The lower flanges of all unprotected beams buckled • The best test to benchmark computational codes and other analytical models for 3D behaviour

  34. BS TEST4: “Demonstration Test”

  35. Demonstration Test: Plan

  36. Demonstration Test

  37. Demonstration test

  38. Demonstration test • Large compartment simulating and open plan office • Real office equipment and wood cribs equivalent to 46 kg/m2 fire load density. • All steel beams unprotected • Maximum deflections of 650 mm achieved with unprotected steel reaching 1150°C. Concrete temperatures were not recorded • Considering the high temperatures achieved and no steel protection this test (along with BRE large compartment test) provides the greatest confidence in robustness of behaviour • Difficult test to model as no concrete temperatures available

  39. Milestone modelling event • Edinburgh University (with British Steel and Imperial College) began the project for computational modelling of the Cardington fire tests (Sept 1996 to March 2000) with DETR funding • Objective: • ‘To understand and exploit the results of the large scale fire tests • at Cardington so that rational design guidance can be developed • for composite steel frameworks at the fire limit state’

  40. British Steel Test 1 (Restrained beam test)

  41. Grillage model for Restrained beam test

  42. Deflected model

  43. Deflections

  44. Total axial force in the composite beam