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This workshop at the Indian Institute of Science in 2010 aimed to introduce research on high-temperature material behavior, heat transfer, and structural mechanics. It discussed the shortcomings of traditional fire protection for steel structures and highlighted the need for innovative design approaches using better science and technology. Key topics included fire resistance tests, column stability, insulation, and the economic impact of fires on GDP. Participants learned about new methods to predict real structural behavior and the importance of advancing research in fire safety engineering.
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Fire Safety Engineering & Structures in Fire Structural Fire Engineering – Introduction Workshop at Indian Institute of Science 9-13 August, 2010 Bangalore India
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
Fire is a “load” on structures Caracas Los Angeles Taipei
Piper Alpha Mont Blanc Kings Cross Kobe
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
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
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
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
Standard fire curves No cooling branch Logarithmic curve T=To+345(log0.113t+1)
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
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.
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
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
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
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
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
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)
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”
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
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
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’
