Taller Structures Generate More Wildfire-Spreading Embers

Research indicates that taller buildings produce more wind-carried embers, a primary cause of structure ignition in wildfires. These findings, published by Oregon State University College of Engineering researchers, could help predict wildfire spread and inform building design in wildland-urban interface areas. Embers, also known as firebrands, are responsible for up to 90 percent of structural losses in community wildfires.

The study involved burning 21 wooden shed-like structures, ranging from one to 3.6 meters in height. Researchers analyzed the firebrands generated, considering structure scale, roofing types, and siding materials. The test burns occurred outdoors in light breezes, with winds between 2.25 and 4.5 miles per hour.

Deepak Sharma, a research associate, stated that structure scale and exterior building materials influence firebrand production. This is due to how scale affects fire intensity and wind-plume dynamics. Exterior materials also impact the fragmentation and thermal state of firebrands during transport.

Researchers quantified the total firebrand yield and the mass-specific yield. Total yield ranged from 2,000 to 24,000 firebrands. Mass-specific yield, which measures embers per kilogram of combusted material, ranged from 50 to 135. This study is the first to measure firebrand yield from single structures and determine yield relative to burned mass.

David Blunck, a professor of mechanical engineering, noted that embers are the most challenging mode of wildfire spread. Understanding their formation and travel can improve predictions of fire movement. Structures with more flammable exterior materials, such as cedar siding, produced more embers than those with less combustible materials.

These findings will contribute to future empirical models and physics-based fire spread simulations. Researchers plan to investigate firebrand mass distributions, ignition potential, and a broader range of building assemblies and wind conditions.