Fire research for timber structures

This special issue of Fire and Materials presents global research into timber structures and fire safety. This follows the renewed momentum in sustainable timber design in the 21st century. Contemporary practitioners and researchers are tasked with a need for information into the fire safety of these structures and from a sustainability point of view, important combination of timber and other structures (Figure 1).

The special issue was conceived in late 2020. A call for papers was issued in 2021 and advertised on many different social media-based platforms. Practitioners and researchers were invited to submit contributions to gather the state of the art on the subject intended to help guide critical research gaps. Specifically, the following areas were identified for interest in this issue: compartment fire evolution, structural resilience, adhesives and other components of construction, code and design method development, underlying mechanisms of timber degradation (charring, pyrolysis, moisture transport and loss, etc.), useful applications of research in practice, and so as not to restrict papers significantly the issue was open to relevant topics proposed by the submitting authors. Eleven papers were ultimately accepted.

It must be recognized the substantial efforts of all authors herein who faced extraordinary times in completing these studies that are presented in this special issue. Specific acknowledgement being those to our graduate student authors who experienced significant challenges in their studies and academic development during the COVID-19 pandemic.

The resulting collection of papers does not just capture the badly needed research for the subject, but the issue recognizes the perseverance of these researchers in addressing this critical need for our society to produce and maintain safe and sustainable timber structures.

The editorial team on this special issue also recognizes the valuable contributions made by the reviewers for this special issue as per anonymity are not named. These reviewer's feedback and acceptance to review articles made this issue the success it has been and allowed a timely production by 2023.

These papers within the special issue are described below with specific reference to their novelty and practical use.

The first article includes a review of 63 compartment fire tests including timber structures regarding temperature development and charring behaviour.¹ In the reviewed material, timber ceilings had on average a 16% lower charring rate than timber walls and the peak temperatures in most experiments were higher than non-combustible compartments.

The second article includes a comparison of the thermal exposure from external fire plumes in compartment fire tests with façade test methods used in Europe including the European test methodology under development.² In the compartment tests, between 43% and 78% of the surfaces were exposed mass timber. The main conclusion was that the thermal exposure from the external fire plumes corresponded best with the British BS8414 façade fire testing method and the European test method under development.

Timber columns may fail in the cooling phase of a fire scenario. This was explored by using the duration of heating phase methodology, DHP.³ This research showed experimentally that the columns tested failed during the cooling phase after exposure to fire for which the heating phase lasted about 25% of the standard fire resistance of the columns. This was shown to be in line with a previous numerical study.

Modelling of timber structural members exposed to realistic fire impact was developed and explored using the Open Seas platform.⁴ The model enables modelling the heat transfer of timber sections in non-standard fire scenarios and thermo-mechanical analysis for various timber structural members.

This paper⁵ presents experiments on identical timber beam–column subassemblies exposed to the same heating duration but with two different cooling phases. It addresses how thermal fields develop in timber connections during the cooling phase of fires and what influence different cooling rates may have. The results therein showed that exposed steel components conduct heat into the main connection system, which then propagated as a thermal wave through the timber elements.

Researchers also investigated the thermal mechanisms behind the appearance of the second heat release rate peak of spruce materials when a cone apparatus is utilized for testing.⁶ They discuss the governing tests conditions behind this second peak.

In a paper on smouldering ignition of wood, three different regimes were identified: (i) no ignition, (ii) un-sustained smouldering and (iii) self-sustained smouldering.⁷ In the experimental campaign on beech woods, minimum radiant heat flux for smouldering ignition was about 5.5 kW/m² after long time exposure. A criterion for self-sustained smouldering, when smouldering continuous without irradiation, was set to (i) a minimum surface temperature of 350 ± 20°C, (ii) a minimum smouldering front thickness of 30 ± 5 mm and (iii) a minimum mass flux of 3.8 ± 0.4 g/m² s.

A framework is also presented for using building information modelling (BIM) with fire dynamics simulation (FDS) to simulate the fire behaviour of a CLT compartment that would be under construction.⁸ An IFC model language is used for the data transfer between BIM viewing software and PyroSim. This simulation is validated against a real experiment of a medium-sized CLT compartment with various exposed surfaces and the framework is used to decide which walls or ceiling should be protected while under construction. The authors call for more benchmarking with various sized compartments.

The last set of papers is a three-part series^9-11 that addresses that current experimental fire research for timber is generally limited to small compartments (<100 m²). These three papers feature a series of experiments, known as CodeRed, in a facility of 352 m² that was built in France to examine the influence of timber on fire dynamics of large open-plan compartments. In CodeRed #01,⁹ the impact of a timber ceiling on the fire dynamics was investigated. In CodeRed #02,¹⁰ the impact of ventilation was examined by halving the available ventilation. In CodeRed #04,¹¹ the impact of exposed surface of the timber ceiling was investigated by encapsulating 50% of the ceiling. CodeRed #03 was related to the impact of water mist, not a fire dynamics experiment, and hence not part of this special issue and published elsewhere.