Experimental investigation of wood decking assemblies exposed to firebrand showers

Abstract

Wildland-Urban Interface (WUI) fires have become a problem of great concern across multiple continents. An important mechanism of structure ignition in WUI fires and urban fires is the production of firebrands. During WUI fires, decking assemblies have been observed to be an ignition vulnerability based on post-fire damage surveys conducted by NIST and elsewhere. The authors have conducted scoping experiments and demonstrated the dangers of the dynamic process of continual, wind-driven firebrand showers landing on decking assemblies for wind speeds of 6 m/s. In this study, eight full-scale experiments were conducted with wood decking assemblies under a wind speed of 8 m/s. The basis for these new investigations was twofold: observe possible vulnerabilities of wood decking assemblies to continuous, wind-driven firebrands at higher wind speed as firebrand accumulation patterns were expected to be influenced by wind speed, and examine if wall ignition occurred due to the burning decking assembly. To this end, sections of wood decking assemblies (1.2 m by 1.2 m) were constructed and attached to a reentrant corner assembly. The deck/reentrant corner assembly was then exposed to continuous, wind-driven firebrand bombardment generated by a full-scale Continuous Feed Firebrand Generator installed in the Fire Research Wind Tunnel Facility (FRWTF) at the Building Research Institute (BRI) in Japan. The mass of firebrands required for flaming ignitions under a wind speed of 8 m/s was considerably less compared with those under a wind speed of 6 m/s. This result is postulated to be due to higher firebrand surface temperatures as the wind speed was increased. For the decking assembly to wall ignition studies, the interface between the decking assembly and the wall appeared to be a weak point; this is not addressed in the current test methods.

Critical Ignition Conditions of Wood by Cylindrical Firebrands

“…Additionally, vegetative fuels ignite differently than dense materials, such as wooden structural elements used on residential homes, necessitating the study of solid fuels. Larger-scale studies have mostly focused on the generation of firebrand "showers" and large wind-driven depositions of and ignition by firebrands in a wind tunnel setting (Manzello et al, 2012;Manzello and Suzuki, 2017). Investigations of accumulated firebrand piles under wind are more realistic; however, they are difficult to reproduce on a small-scale in the laboratory and require extensive characterization.…”

Wildland–Urban Interface Fires in Spain: Summary of the Policy Framework and Recommendations for Improvement

“…In this context, it has to be highlighted a recently started novel research effort lead by CERTEC (Universitat Politècnica de Catalunya -BarcelonaTech) and funded by the European civil protection agency aimed at providing scientific answers to residential fuel hazard management focused on both real-scale experiments and computational modelling research [33]. [34,35]. Local turbulence and flame entrainment from residential fuels might be induced in angled and edgy façades, and semi-confined spaces [36].…”

Ignition Vulnerabilities of Combustibles around Houses to Firebrand Showers: Further Comparison of Experiments

“…The time to SI or FI for 10 mm gap decking assembly is similar to that for ‘wet’ hardwood mulch (FMC 9% to 40%). Time to ignition under 6 and 8 m/s wind conditions for 5 mm gap decking assembly is similar ( Figure 6 ), while past studies [ 27 , 28 ] showed the actual mass required to ignite a 5 mm gap decking assembly under 8 m/s were much smaller than the mass required under 6 m/s. Figure 7 shows the time to ignition would be almost the same if the difference of firebrand flux under two wind speeds is taken into consideration.…”

“…The board spacing used in all of the experiments was 10 mm. Even though 5 mm is a recommended spacing in Australia, 10 mm to 12 mm has also been proposed for areas where homeowners are unlikely to wear high heel shoes and ease of litter cleaning is required, larger board spacing could be a worse-case scenario, and due to ignition source used (methenamine tablets as opposed to actual firebrands), ignition could not be achieved using 5 mm board spacing in this simple setup [ 26 , 27 ]. Two deck board thicknesses were used by Macindoe [ 35 ], 2 cm and 5 cm.…”