Ignition of Wood Fencing Assemblies Exposed to Continuous Wind-Driven Firebrand Showers

Post-fire studies conducted by NIST on the Waldo Canyon Fire in Colorado (2012) determined that wood fencing assemblies are believed to be vulnerable to ignition from firebrand showers in Wildland–Urban Interface fires, but again there has never been any experimental verification of this ignition mechanism. As a result, a series of experiments were conducted to examine ignition of wood fencing assemblies subjected to continuous, wind-driven firebrand showers. Western Red Cedar and Redwood fencing assemblies were exposed to continuous, wind-driven firebrands generated by the NIST full-scale Continuous Feed Firebrand Generator installed in the Fire Research Wind Tunnel Facility at the Building Research Institute in Japan. To simulate fine fuels that may be present near fencing assemblies, dried shredded hardwood mulch beds were placed adjacent to the fencing assemblies. The fencing assemblies were varied in length and in orientation to the applied wind field to simulate a range of configurations that may be encountered in realistic situations. Both flat and corner sections of fencing assemblies were used in these experiments. The dimensions of the flat wood fencing assemblies sections were varied from 0.91 m wide, 1.83 m in height to 1.83 m wide, 1.83 m in height. With respect to the corner sections, the dimensions used were 0.91 m by 0.91 m by 1.83 m in height. All configurations considered resulted in flaming ignition (FI) of the mulch beds, and subsequent FI of the wood fencing assemblies. Finally, experiments were also completed to determine if wind-driven firebrand showers could produce FI of fencing assemblies without the presence of fine fuels adjacent to the fence sections. Firebrands produced smoldering ignition (SI) of the fencing assemblies without fine fuels present, and SI transitioned to FI under the applied wind field. These experiments have demonstrated that wood fencing assemblies are vulnerable to ignition by wind-driven firebrand showers.

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