Editorial for special issue on Interflam 2019: Mass timber and fire resistance

Since 1979, the Interflam conference series has been one of the leading conferences, which focus on the most contemporary fire safety research topics. An inspection of previous proceedings exemplifies this as the conference evolved to include many subjects that matured in the field and represent the contributions in a platform where results and analysis could be timely shared.1 Fire resistance as a specific topic has always had a presence in Interflam. However, it was not until the early 1990s that the topic came to fruition. For example, in the first decade of the conference, only six articles were presented on the topic of fire resistance. Since the inclusion of a critical analysis of the then conducted steel framed Cardington test series by 1996, nearly 100 full articles on the topic of fire resistance have appeared in Interflam proceedings. That test series was a fundamental turning point for how fire resistance of structures is considered and for the creation of computational tools used for structural fire design. By 1999, the Interflam conference series featured full sessions on related fire resistance topics. Today, this special issue recognizes the diversity of the topics that differentiate from steel-only focuses and feature a range of contemporary issues. Mass timber, for example, began to emerge as a separate topic branching distinctly from fire resistance alone. This issue of Fire and Materials features 10 articles prepared and reviewed in greater detail than their counterparts received at Interflam in 2019. The first five articles in this special issue focus on various aspects of fire performance of timber construction. The first article reports the results of an experimental study in which the effect on compartment fire severity of a cross-laminated timber (CLT) ceiling vs a reinforced concrete ceiling was investigated.2 The study confirms that the use of a combustible timber ceiling results in higher temperatures within the enclosure, regardless of whether the fire is fuel or ventilation controlled. The article further demonstrates that methods to estimate the fully developed fire temperatures in a compartment of noncombustible construction require refinement to obtain accurate temperature predictions when a combustible ceiling is present. The work described in the second article was motivated by the large scatter in the tensile strength data at elevated temperatures for wood reported in the literature.3 Steady-state tensile strength experiments below charring temperatures were performed. The results were then used to assess the tensile strength of a wooden beam subjected to a fire using Fire Dynamics Simulator. The third article focuses on the thermo-mechanical behavior of CLT slabs under standard and natural fire exposure conditions.4 Experimental data are presented that provide new insights into the behavior of unprotected CLT slabs exposed to fire. A detailed analysis of the thermo-mechanical performance of the slabs leads to a better understanding of mechanical failure during and after fire exposure. The last two articles on timber construction deal with the use of gypsum board as a protective membrane. Chorlton et al describe the results of a two-stage experimental program with the purpose of understanding multilayered encapsulation performance of timber columns exposed in nonstandard fires that include a natural cooling phase.5 The first stage involved a timber column protected with fire-rated gypsum board exposed to a fire within a large, open farm structure from ignition to burnout and cool down. The test showed that three layers of 15.9 mm Type X gypsum board were insufficient to protect the column. The second stage of the research involved four laboratory experiments in which one face of an encapsulated timber column was exposed to a methanol pool fire. These tests imply that timber elements can be adequately protected from fire exposure, provided the screw spacing is strictly adhered to and redundant layers of gypsum are installed. The final article on timber construction presents the results of seven standard fire resistance tests on wood frame wall assemblies.6 The study examined the effect of insulation (cellulosic, glass, and rock fiber), wood stud spacing, and mid-height blocking on the fire resistance of protected interior wall assemblies with a single or double row of wood studs. This special issue also includes three articles related to fire resistance of concrete. The first article illustrates the use of probabilistic methods for demonstrating adequate safety in the framework of British Standard PD 7974-7:2019.7 Three different approaches to determine an absolute safety target for a concrete column in an office building are explained. The two subsequent articles concern the fire spalling of concrete. An innovative material screening test method, with concrete molded in steel pipes to create a restraint during testing, is illustrated using a comparative study.8 This seems to be a promising test setup, although it appears that the size of the specimen is critical. In the second article on fire spalling of concrete, it is shown that addition of even relatively small amounts of polypropylene fiber to the fresh concrete mix can reduce fire spalling.9 This study also presents findings indicating that small unloaded concrete cubes exposed to the same heating as large loaded slabs exhibit no spalling, while some equivalent large slabs spalled beyond the layer of reinforcement. This demonstrates once again that tests based on small, unloaded specimens are not relevant when assessing fire spalling of larger cross-sections loaded in compression. Novel experimental studies that extend the study of materials beyond standardized test specifications are also considered in the issue. This includes an experimental study using the novel heat-transfer rate inducing system test method, which considered the influence of substrate thermal conditions on the behavior of thin intumescent coatings.10 The substrate thermal conditions were found to govern the swelling of intumescent coatings and, therefore, their effectiveness in protecting load-bearing structural elements. A separate study also examined partitions and scale effects.11 In this study, the influence of height of glazed partitions with fire resistance tests that exceed the dimensions of the sample walls was considered. The study concluded that the height of the partition does not have a significant impact on its insulation and integrity subjected to limitations on the size of the glass panes, symmetry, and stiffening of mullions.