Laboratory Studies on the Generation of Firebrands from Cylindrical Wooden Dowels

Abstract

Wildland urban interface (WUI) fires are increasing in size and severity in the United States. These fires are a major concern because they spread into communities and ignite thousands of homes every year. The main source of home ignitions is not through direct flame contact or radiation; rather, firebrands are the primary pathway for fire to spread into WUI communities. Firebrands are pieces of a burning material that break off and then can be lofted and carried large distances to ignite subsequent fires. The purpose of this thesis is to study the breakage process to better understand how firebrands are produced. Cylindrical wooden dowels are used to represent vegetative fuels in WUI fires. The dowels were exposed to various heating conditions and then three-point bending tests were performed on each dowel to study the effect of combustion on strength properties. It has been found that there are two distinct regimes that describe the breakage. The size of the dowel and the final density of the dowel both control the transition between regimes. A scaling analysis was performed to show that the two regimes and transition point are the same for all species. Predictions of wind velocities needed to produce the measured critical stresses were calculated, because a connection needs to be drawn between the combustion and the wind to fully understand firebrand generation.