Tuesday, 5 February 2013

Like gravity, wind and quakes: Fire science leads to better infrastructure

While high-rise designers make sure that gravity, winds, quakes and fires do not take their ever complex structures to catastrophic collapse, researchers study structural mechanics, aerodynamics, seismology and fire dynamics so that engineering calculations continuously improve and contribute to safer and safer infrastructure. I had the professional pleasure of being involved in this context first hand, and see some of my fire research work be embodied into real buildings [1].

UPDATE (7/2013): Our research on traveling fires has been highlighted in 'Engineering News-Record' in the article titled "9/11 Blazes Debunk Code Assumptions About FireBehavior in Open-Plan Offices

High rises in Madrid. Photo by Oscar Villarejo.

Cast in the PhD theses of Dr Jamie Stern-Gottfried ([2], now at Arup Berlin) and Dr Angus Law ([3], now at Arup Leeds), I led the research team that pioneered the thermodynamics concept of travelling fires for structural engineering. This concept has already impacted on the way industry designs modern infrastructure. Funded by Arup, the work has been applied to a building in the City of London in 2012 even before publication of the latest journal papers [1, 4]. More buildings in London, Cardiff and Manchester have followed. This represents one of the fastest knowledge transfer from research to industry seen in the field.

The idea started when we realized that the current structural design for fire protection is not well suited for 21st Century architecture. Traditional methods for specifying the fire load to the structure assume uniform burning and homogeneous temperature conditions throughout a compartment, regardless of its size. This is in contrast to the observation that accidental fires in large, open-plan compartments tend to travel across floor plates, burning over a limited area at any one time and do not burn simultaneously throughout the whole enclosure. These fires have been labelled travelling fires [1, 4]. Despite these observations, traditional structural fire design methods do not account for this type of fire. Traditional methods are only valid for small enclosures, like those typical of older architecture (eg, apartment blocks vs. modern office space or modern airport lounges).

We used travelling fires to produce more realistic fire scenarios in large, open-plan compartments than the conventional methods. This has been published widely [1-6]. The methodology that we developed is purposely simple but based on actual fire physics. It is also posed in a manner that is compatible to the way structural engineers prefer to think about fire loads and design. It considers a family of fires that includes the full range of physically possible fire sizes, from very small to very large. Traditional methods consider only one fire, two at most, and always of the largest size possible. Small fires spread slowly, large fires spread fast, and fires that occupy the whole compartment area do not spread, they simply burn in place. With this framework in mind, we then split the thermal environment into two regions: the near field (the flames) and the far field (smoke away from the flames). Both fields move along the compartment as the fire spreads. See Figure 2.

Fig. 2. (a) Illustration of a travelling fire and (b) Near field and far field exposure durations at an arbitrary point within the fire compartment. From [1].
Small fires travel across a floor plate for long periods of time (slow spread) with relatively cool far field temperatures, while large fires have hotter far field temperatures but burn for shorter durations (faster spread).

Heat transfer calculations show how much the concrete and the steel members heat up due to different fires. As structural members heat up, they lose strength and induce deformations thus posing a collapse hazard to the building. The higher the temperature the larger the hazard. We found that travelling fires lead to the highest temperatures and have a larger impact on the performance of both concrete and steel structures. They are the most onerous fire scenario to the stability of the building. Thus, in the course of this research, we learnt that conventional design approaches cannot be assumed to be conservative. The results indicate that the worst case scenario would be a medium sized travelling fire between 10% and 25% of the floor area. See Figure 3.

Fig. 3. (a) Gas phase and concrete temperatures for rebar depths of 20, 30, 42 and 50 mm and (b) Peak bay temperature vs. fire area and rebar depth. From [1].

The work [1 to 6] represents the foundation for using this concept for structural analysis and design. The results show that the impact of travelling fires is critical for understanding true structural response to fire in modern, open-plan buildings. See Figure 4. We recommend that travelling fires be considered widely for structural design and the structural mechanics. The four recent buildings mentioned above are the very first structures designed purposely to withstand the thermal load of a travelling fire.

Fig. 4. Comparison of concrete temperatures calculated using the travelling fires (base case) and three traditional methods (standard fire curve, and two Eurocode curves).

The work is continued as part of the EPSRC project "Real Fires for the Safe Design of Tall Buildings" [7] led by Prof Torero and which counts with substantial support from industry (AXA, Arup, BRE, BuroHappold, FM Global, SOM). This project aims to produce data on large-scale fire behavior and remove the main barrier to progress in travelling fires; (as noted in [1]) "incorporating travelling fires into design is challenged by the lack of large scale test data".

Note: One of the first journal papers we published on the topic [5] received the 2011 Lloyd’s Science of Risk Prize in Technology. You can read this past article in the blog here.

  1. J Stern-Gottfried, G Rein, 2011, Travelling Fires for Structural Design. Part II: Design Methodology, Fire Safety Journal 54, pp. 96–112, 2012.
  2. J Stern-Gottfried, 2012, Travelling fires in building design, PhD thesis, University of Edinburgh. 
  3. A Law, 2010, The Assessment and Response of Concrete Structures Subject to Fire, PhD thesis, University of Edinburgh.
  4. J Stern-Gottfried, G Rein, 2012, Travelling Fires for Structural Design. Part I: Literature Review, Fire Safety Journal 54, pp. 74–85, 2012.
  5. A Law, M Gillie, J Stern-Gottfried, G Rein,2011,The Influence of Travelling Fires on a Concrete Frame, Engineering Structures 33, pp. 1635–1642 (open access).  (Winner of 2011 Lloyd's Science of Risk Prize in Technology).
  6. G Rein, 2012, Introduction to Fire Dynamics for Structural Engineers, Training School for Young Researchers COST TU0904, Malta.
  7. EPSRC-funded project, Real Fires for the Safe Design of Tall Buildings.