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

Abstract

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.

The performance of wood and tile roofing assemblies exposed to continuous firebrand assault

“…Firebrands generated in this study were commensurate to those measured from burning structure components. For reference, the average projected area of firebrands generated in this study tailored to burning structure components was 1.56 cm 2 ± 0.85 cm 2 , as compared to 0.77 cm 2 ± 0.2 cm 2 , in prior studies of continuous firebrand generation tailored to burning trees and data from an actual WUI fire, where Douglas‐fir wood pieces were used to generate firebrands .…”

“…The feeding rate was varied to determine the optimal conditions for continuous firebrand showers. Based on prior work, a wood mass loading 150 g to 200 g every 15 s resulted in adequate firebrand generation . As a result, in these experiments, 170 g ± 30 g, fed into the Dragon every 15 s provided an adequate firebrand generation rate to ignite building materials.…”

“…When the air pressure was applied, the sliding rod of the pneumatic cylinder moved forward and separated the wood chips from the storage area to the first gate, where they were then dropped into the second gate that led to the Dragon where they were ignited (see Figure 1). Care was taken to select the pneumatic cylinder (15-cm bore with 17.8-cm stroke; maximum pressure of 1.7 MPa and maximum load of 12 kN; this is about 4 times more force than our previous device of 3.1 kN; see [15,16]); smaller sized pneumatic cylinders were observed to be unable to cut through the wood and would jam. The twogate system was required to contain the fire from spreading from the Dragon to the feed system, and each gate was driven by pneumatic cylinders as well.…”

“…As a result, in these experiments, 170 g ± 30 g, fed into the Dragon every 15 s provided an adequate firebrand generation rate to ignite building materials. A supply of 170 g corresponded to approximately 560 wood pieces deposited every 15 s. It is important to note that finer control, and therefore less uncertainty per each feed, was obtained when feeding with machined Douglasfir pieces, as compared to the wood chips used here [15,16]. The reason for this is that the Douglas-fir pieces are all of uniform size, while chips vary to some extent.…”

Understanding structure ignition vulnerabilities using mock‐up sections of attached wood fencing assemblies

“…Examples of attached building components include wood fencing assemblies. The authors demonstrated that full-scale fencing assemblies may be ignited by firebrand showers, but the capability of the ignited fencing to transfer the fire to an adjacent structure, such as wall assembly, was not considered [4]. …”