Improved travelling fires for structural design

The collapse of 1WTC, New York City, 10:28am Sept 11, 2001.
Photo by 9/11 Photos CC BY.

Our latest paper on travelling fires for structural design has been published in Structures (journal of IStrutE) with the title Improved formulation of travelling fires and application to concrete and steel structures.

Note: It is open access so you can read and share it without need for a subscription. We have posted in open access also our Matlab code to calculate the fire temperatures in zenodo.


Accidental fire can be disastrous, especially in buildings. The effect of fire on structural stability is critical in regard to safe evacuation and safe access for fire fighters, financial losses, and lost business. This is particularly the case in tall buildings where extended evacuation times are required due to phased evacuation practices. The World Trade Centre Tower fires in 2001 have highlighted the need of a more realistic design tools to represent fires in large compartments. 

Innovative architectural designs of modern buildings already provide a challenge to structural engineers. This is above all the case in structural fire engineering. However, most of the understanding and current design codes are based on the assumption of uniform fires in a compartment. In previous work, we have shown that fires in large, open-plan compartments, typical of modern architecture, travel from one part of it to another with non-uniform temperature distribution. These fires are referred to as travelling fires. And Travelling Fires Methodology (TFM) has been developed to account for the travelling nature of fires.

Illustration of a travelling fire and distribution of gas temperatures.

TFM was born in 2010 and offers a paradigm shift in the structural engineering of modern buildings. The concept has already been applied by engineering firms like Arup, BuroHappold or AECOM in the design of a dozen of iconic buildings in the UK (including the renovation of Battersea Power Station in London). TFM accounts for one of the fastest knowledge transfers from research to industry seen in fire protection engineering. TMF is now being studying in detailed in the USA for possible adoption as well.

The focus of this latest paper is on the improvement of the calculations of traveling fire (iTFM) to account for better fire dynamics, and the analysis of the effect on structural members. The proposed changes represent a simple yet powerful fire model. In particular, our paper shows that:

• Using data from experiments and real fires, we limit the range of possible fire sizes thus reducing the time required for conduct TFM studies.
• Analytical expressions are presented for generating time–temperature curves which are independent of grid size (previous versions of TFM) and can be easily calculated with any mathematical tool. 
• Introduction of flapping term leads to reduced near-field temperatures for smaller fire sizes which cover a range between 800 and 1200 °C, as observed in real building fires. 
• The location of the peak temperature in the compartment is found to occur at the end of the fire path (i.e. far half of the compartment from the ignition source).

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.

References:

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.

Travelling fires paper wins Lloyd's Science of Risk Prize

Feb 2013 update: Read more about this work and how it ended up in a real building in this post.

We have won the 2011 Lloyd’s Science of Risk Prize in the Technology category for the paper "The Influence of Travelling Fires on a Concrete Frame" (published in Engineering Structures 33).

Winners of the 2011 Lloyd’s Science of Risk Prize. Dr Law is is second from the right.

The work argues that the trend towards open plan offices has changed the types of fire likely to occur in modern buildings. His paper uses science to look at ways to improve engineering guidelines and building design, reduce the risk of travelling fires, and help insurers better quantify and model fire risk. The work was founded by BRE Trust and Arup.

 Progression of the 2.5% and the 25% travelling fires across the floor plate (Fig 4 in the paper)

Temperature profiles for the average rebar in the final bay (Fig. 8. in the paper)

The Science of Risk Prize was launched by Lloyd’s in 2010 to stimulate cutting edge research into the latest emerging risks facing businesses.

For more details on the work, see here the paper (open access), a poster and related presentation.


NOTE: My team also won the 2010 Lloyd’s Science of Risk Prize in the same category with a paper on the modelling of tunnel fires. Two in a row :)

Modelling of transient flows in tunnel fires

Our most recent paper on tunnel fires has just been published in the journal Computers and Fluids. The title is:

Multiscale Modelling of the Transient Flows from Fire and Ventilation in Long Tunnels

The paper applies a transient multiscale approach to model ventilation flows and fires in a long tunnel. It couples dynamically a Computational Fluid Dynamics solver with a simple 1D model, allowing for a more rational use of the computational resources without loss of accuracy.

Schematic of the multiscale model of the1.2 km tunnel from portal to portal and including 10 jet fans pairs. The CFD domain of the fire region contains temperature contours showing the fire plume.

After all the fundamentals of the coupling are discussed, the methodology is applied to study the unsteady flow interaction between a growing fire and a ramping-up ventilation system in a modern tunnel (7 m diameter, 1.2 km long). To the best of our knowledge, this is the first time than a growing fire and a growing ventilation are studied together. The results allow for simultaneous optimization of the ventilation and detection systems, and allows engineering answers to questions that could not be posed before by tunnel designers.

Longitudinal velocity field computed 180 s after fire ignition (60 s after ventilation activation) for three ventilation scenarios (3, 5 or 10 jet fan pairs respectively). Velocity values are expressed in m/s.

The work is a continuation of the collaboration between Politecnico di Torino and University of Edinburgh led by Dr Francesco Colella (the work is based on this 2010 thesis "Multiscale modelling of tunnel ventilation flows and fires").

NOTE: An earlier paper related to this received the 2010 Lloyd's Science of Risk Prize.

Call for papers: Fire Technology special issue on WTC Collapse

Fire Technology, the journal of the National Fire Protection Association published by Springer, is preparing an issue on the 2001 fire and collapse of World Trade Center.

The purpose is to collect research, forensic and engineering output of the highest scholarly standards synthesized in the 10 years passed since the event.

Multidisciplinary and international contributions are especially encouraged. Topics of interests include: WTC 1, 2, 5 and 7, the crash, fires, structural response, collapse, forensic conclusions, experiments, modelling, Fire and Rescue intervention, human behaviour, building design, post-collapse fires and recovery, previous attacks on WTC and related subjects.

Submissions will be accepted until 11th Nov 2011 at: http://fire.edmgr.com (choose article type "World Trace Center") .

The call for papers flyer can do downloaded here. Please spread the word, we are looking for a wide range of high quality submissions.

For further information, contact the Associate Editor of this special issue: Dr Guillermo Rein, The University of Edinburgh.

A New York City fireman calls for 10 more rescue workers to make their way into the rubble of the World Trade Center. Photo form Wikipedia, United States Navy ID 010914-N-3995K-01

Invited Talk in Madrid: Travelling Fires in Building Structural Design

Last Thursday 24th Feb, I gave this invited talk on Travelling Fires at the 6th International Congress on Performance-Based Design for Fire, Madrid:

G Rein, Travelling Fires in Building Structural Design, Madrid 2011

A related paper has just been accepted for publication:
A Law, M Gillie, J Stern-Gottfried, G Rein, JL Torero, The Influence of Travelling Fires on a Concrete Frame, Engineering Structures, (in press) 2011. doi:10.1016/j.engstruct.2011.01.034