Characterising Large Open-Plan Compartment Fires: The ‘Real Fires for the Safe Design

Full Article Name: Characterising Large Open-Plan Compartment Fires: The ‘Real Fires for the Safe Design of Tall Buildings’ Project

By Dr Juan P. Hidalgo, Dr. José L. Torero, Dr. Adam Cowlard, Dr Cristian Maluk, Dr. Cecilia Abecassis-Empis, Dr. Agustín Majdalani

Since the early 20th century, the fire safety engineering community has strived towards establishing “fires” that can be used for design. The purpose of a design fire is to provide a realistic, sufficiently probable fire scenario that allows testing the Fire Safety Strategy. The Fire Safety Strategy is an essential aspect of building design that links the different individual fire safety provisions for the purpose of achieving overall performance. This performance aims to deliver, primarily, adequate life safety to occupants and responders, but also to maximise property protection. The formulation of any Fire Safety Strategy relies on the explicit definition of design targets, such as structural, egress or fire characteristics. The design fire is the starting point for the engineering assessment of a design to ascertain whether these targets are attained.

The Compartment Fire Framework established by pioneers of this discipline — Philip Thomas, Kunio Kawagoe, Margaret Law and Tibor Harmathy among others — remains the basis for understanding fully developed compartment fire dynamics1. This framework places greater emphasis on a subset of fully developed fires that can be characterised as ventilation-controlled fires, also denoted as Regime I fires, which the original researchers deemed to be the more-severe regime.

This fire regime is characteristic of quasi-cubic compartments with limited openings and, thus, is limited in its range of applicability to contemporary infrastructure — in particular, tall buildings. Current architecture emphasises mixed-use tall buildings where small (hotel bedrooms, residential, etc.) and large compartments (offices, lobbies, retail, etc.) coexist. Fires in this type of building can range from ventilation-limited to fuel-controlled.

Fuel-controlled fires, a subset deemed as Regime II by the Compartment Fire Framework, were less-characterised due to the more-complex fire dynamics. Subsequent studies have extended the range of validity of the Compartment Fire Framework to, for instance, the SFPE Engineering Standard on Calculating Fire Exposures to Structures2, the Regime I approach to a design fire originally proposed by Law3 and later embedded in the Eurocode4, i.e., elements such as the parametric curves. Nevertheless, none of these approaches covers the range of geometries used at present, nor are fires in these spaces understood sufficiently to allow for extrapolation beyond the original data and geometries.

In the last two decades, substantial efforts have been made to study the dynamics of fires in large compartments to move further from the conventional conception of Regime I fires. An example of these efforts is the Real Fire for the Safe Design of Tall Buildings (RFSDTB) project5,6, initially instigated at the University of Edinburgh in 2011. The primary objective of the RFSDTB project was to establish a methodology that would generate real fire inputs for the definition of a Fire Safety Strategy for tall buildings in a broad range of conditions that departed from the traditional Regime I data.  

The RFSDTB project encompassed two full-scale experimental programmes designed and commissioned by staff and PhD students at the University of Edinburgh. In 2013, the first programme, called the Edinburgh Tall Building Fire Tests (ETFT)7, was carried out at the BRE Burn Hall in Watford, UK. In 2014, the second programme corresponded to the Large-Scale Demonstrator Malveira Fire Test, carried out in a vacant building in Malveira, Portugal.

The ETFT programme was a milestone in the history of fire testing — the tests had the largest amount of instrumentation reported to date, with nearly 2,200 sensors consisting of thermocouples, thin-skin calorimeters, bi-directional velocity probes, gas analysers, obscuration devices, flow meters, and imaging devices. A 17.8 m by 4.9 m by 2 m compartment with one side fully open with a 0.5 m overhang was built to perform the fire experiments.

Two phases were planned for this programme: 10 tests using gas burners and variable ventilation to reproduce several opening factors and burning modes (a fully developed fire, a moving fire, and a spreading fire), and two tests using wood cribs with two extreme opening factors. The intent was to explore characteristic fire dynamics parametrically that lay outside the experimental database used to characterise Regime I and Regime II fires.

The Malveira Test, while less densely instrumented (in the order of 380 sensors), included a system to quantify the mass loss of the wood cribs during the fire, thus allowing a rough estimation of the heat release rate in the compartment. The Malveira Test consisted of a single fire experiment with similar dimensions and ventilation as the ETFT compartment, however, with different types of linings within the compartment.

These sets of experiments followed the Natural Fire Safety Concept tests(1994), where a large open-plan compartment with different insulating linings was used; the Dalmarnock Fire tests9 (2006), where similar instrumentation density to the RFSDTB project was applied to a small compartment, consistent with the experiments used to define the Compartment Fire Framework; and a large set of small-scale tests performed at the University of Edinburgh10 (2012), where geometrical variations were explored in a systematic way.

The detailed level of instrumentation was intended to provide sufficient data resolution to validate current fire models and generate correlations to characterise fires in these types of enclosures. The design of experimental programmes of this nature, with such a level of instrumentation, is a challenging endeavour, given the size, complexity, and destructive nature of the experiments. From the construction of a robust enclosure to the design and installation of the instrumentation, these experiments required the input of experienced fire engineers and academics.

The analysis of the outcomes from the RFSDTB project continues today at the University of Queensland11 and the University of Maryland. This analysis focuses on the development of a robust methodology to address the energy distribution in the compartment using the experimental data measurements. The objectives of this analysis are:

  1. To analyse, organise, and structure the data in a manner that can be effectively used by others. These experiments provide an invaluable set of data that can be used by modellers and designers. In 2018, the data will be made publicly available via the University of Edinburgh DataShare repository12.
  2.  Similar to the approach used following the CIB tests13, current analyses on the RFSDTB data focus on determining simple engineering correlations for different burning modes and ventilation conditions, using an energy distribution analysis. These correlations are intended to provide adequate inputs for engineering design. Early outcomes of these analyses14 indicate different trends under the combination of different burning and ventilation modes. Additionally, the role of compartment linings has been found to significantly determine the establishment of different burning modes.

The high resolution of instrumentation enables energy distribution approach proposed by the University of Queensland and the University of Maryland. Nonetheless, despite the magnitude of the data collected, various experimental uncertainties have been encountered. Whereas the characterisation of the thermal fields using low-cost thermocouples and thin-skin calorimeters proved to be a success, quantification of the flow fields remains as the main limitation of these experimental sets. This is a constraint often faced in full-scale fire experimentation, but it can potentially be tackled by the use of image velocimetry systems such as PIV (particle image velocimetry). In the case of full-scale fire testing, this approach has proven to be impractical to date.

Despite these challenges, the significant challenge of data interpretation and analysis of the large amount of data remain ongoing and productive now that these milestone experimental programmes have been completed.

Global efforts in structural fire engineering have highlighted the importance of the thermal boundary condition when assessing the performance of structures during and after fire6,15. Recent developments in experimental fire testing of load-bearing materials and structures has experienced a shift in testing philosophy (e.g., H-TRIS16) — away from standard furnace testing and into testing techniques capable of controlling the time history of incident radiant heat flux at the exposed surface of test samples. Among other benefits, techniques like this have enabled experimentalists to replicate a wide range of heating conditions, aligning with the core intent of the RFSDTB project.

Through this work, it is expected that an adequate representation of the thermal boundary conditions for describing the response of structures during a fire event will be identified, independent of the geometry of the compartment17.

Dr. Juan P. Hidalgo with the University of Queensland, Dr. José L. Torero, with the University of Maryland, Dr. Adam Cowlard with TAEC Engineering, Dr. Cristian Maluk with the University of Queensland, Dr. Cecilia Abecassis-Empis, with TAEC Engineering, Dr. Agustín Majdalani with University of Edinburgh 

Acknowledgements
The authors would like to acknowledge the support of EPSRC towards funding the ‘Real Fires for the Safe Design of Tall Buildings’ project (Grant No. EP/J001937/1) and the numerous industry partners that made these experimental programmes possible. The outcomes of this project would not have been possible without the substantial efforts of numerous staff and students at the University of Edinburgh, University of Queensland, and University of Maryland.


References

[1]  Torero, et al. (2014), Revisiting the Compartment Fire, Fire Safety Science 11, 28-45. doi:10.3801/IAFSS.FSS.11-28.

[2]  SFPE (2011), SFPE Engineering Standard on Calculating Fire Exposures to Structures, SFPE (S.01 2011).

[3]  Margaret Law (2002), Some selected papers by Margaret Law: engineering fire safety, Arup, London.

[4]  EN 1991-1-2 (2002), Eurocode 1: Actions on structures, Part 1-2: General actions – Actions on structures exposed to fire.

[5]  http://gow.epsrc.ac.uk/NGBOViewGrant.aspx?GrantRef=EP/J001937/1.

[6]  Cowlard, et al. (2013), Some considerations for the fire safety design of tall buildings, International Journal of HighRise Buildings 2(1), 63–77.

[7]  Hidalgo, et al. (2017), An experimental study of full-scale open floor plan enclosure fires, Fire Safety Journal 89, 22-40, doi:10.1016/j.firesaf.2017.02.002.

[8]  Kirby, et al. (1994) Natural Fires in Large Scale Compartments – A British Steel Fire Research Station Technical Collaborative Project. BSC.

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[10]  Majdalani et al. (2016) Experimental characterisation of two fully-developed enclosure fire regimes, Fire Safety Journal 79, 10–19. doi:10.1016/j.firesaf.2015.11.001.

[11] http://www.civil.uq.edu.au/fire-research.

[12] http://datashare.is.ed.ac.uk/.

[13] Thomas, et al. (1972), Fully developed fires in single compartments, CIB Report No 20. Fire Research Note 923, Fire Research Station, Borehamwood, England, UK

[14]  Maluk, et al. (2017), Energy distribution analysis in full-scale open floor plan enclosure fires, Fire Safety Journal 91, 422–431. doi:10.1016/j.firesaf.2017.04.004.

[15]  Bisby, et al. (2013) A contemporary review of large-scale non-standard structural fire testing, Fire Science Reviews 2, 1. doi:10.1186/2193-0414-2-1.

[16]  Maluk, et al. (2016), A Heat-Transfer Rate Inducing System (H-TRIS) Test Method, Fire Safety Journal, doi:10.1016/j.firesaf.2016.05.001.

[17]  Torero, et al. (2017), Defining the thermal boundary condition for protective structures in fire, Engineering Structures 149, 104-112. doi:10.1016/j.engstruct.2016.11.015.

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