Quantifying wind‐driven firebrand production from roofing assembly combustion

Abstract

Large outdoor fires present a risk to the built environment. Examples often in the international media reports are wildfires that spread into communities, referred to as Wildland-Urban Interface (WUI) fires. Other examples are large urban fires including those that have occurred after earthquakes. Firebrands are a key mechanism on how rapidly fires spread in urban fires and WUI fires. An experimental protocol has been developed to ignite full-scale roofing assemblies and quantify the degree of firebrand production during the combustion process. As wind is an important factor in firebrand generation, the experiments were conducted under a range of wind speeds at the Building Research Institute’s (BRI) Fire Research Wind Tunnel Facility (FRWTF). A further unique aspect of this work is the experimental results are compared to firebrand size and mass distributions collected from an actual large-scale urban fire in Japan. Results of these experiments demonstrate that when only oriented strand board (OSB) is applied as sheathing, a significant number of firebrands collected from roofing assemblies were less than 1 g and 10 cm2. It was also observed that experiments on individual building component firebrand generation provided useful insights into actual urban fire firebrand generation.

Role of firebrand combustion in large outdoor fire spread

“…While it is interesting to burn an entire structure in order to obtain firebrands, it is hard to control all the parameters which may or may not affect the production. Hence, a systematic series of experiments were conducted with decreasing scale and complexity: a real-scale structure, a simple full-scale structure combustion experiment, full-scale building components combustion experiments, and bench-scale building components [48][49][50][51][52][53] combustion experiments. In all experiments, firebrands were collected in pans with water, and the projected area and the mass of each firebrand were measured in the same manner for all experiments in order to keep consistency to directly compare results.…”

“…As a further simplification, full-scale building component combustion experiments were also performed with wall assemblies as well as roofing assemblies [50][51][52]. A repeatable ignition method was developed.…”

Development of a new approach to characterize firebrand showers during Wildland-Urban Interface (WUI) fires:

“…[12,13] for firebrand surface temperatures) but are out of the scope of the present work and will not be considered here. Firebrands produced during the burning of structural materials have been investigated for experimental configurations ranging from simple building components, i.e., wall assemblies [14][15][16] and roofing elements [17,18], to full scale structures, both indoor [19,20] and outdoor [21][22][23]. Firebrands generated by vegetative fuels were also studied for various configurations, including the burning of single trees in laboratory conditions [24][25][26], as well as prescribed [27][28][29][30] and actual fires [31,32].…”

Garnering understanding into complex firebrand generation processes from large outdoor fires using simplistic laboratory-scale experimental methodologies

“…The roofing angle was maintained constant and selected to match those commonly observed (25°). The sizes of the mock-up assemblies were determined based on extensive investigations by the authors into firebrand generation from structure combustion processes over several years [27,[29][30][31][32][33][34]. The schematics of the mock-up roofing assembly and the full-scale roofing assembly are shown in Figure 1.…”

“…The schematics of the mock-up roofing assembly and the full-scale roofing assembly are shown in Figure 1. The full-scale roofing assembly methodology has been described elsewhere but the data here is new, as the sheathing type is plywood as opposed to oriented strand board (OSB) [33]. The recent OSB experiments were conducted using a large-scale wind tunnel in Tsukuba, Japan operated by the Building Research Institute (BRI).…”