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Background. Buildings in communities near wildlands, in the wildland-urban interface (WUI), can experience wildfire damage. Aims. To quantitatively assess the relationship between building features and damage, a building wildfire resistance index is developed and validated with the 2013-2017 CAL FIRE (DINS) database from California, USA, and the 2017 Pedrógão Grande Fire Complex post-fire investigation from Portugal. Methods. Three statistical dependence tests are compared to evaluate the relationship between selected building features and damage. The Wildfire Resistance Index (WRI), range: [-1, 1], is proposed and validated as a rating for building wildfire susceptibility. Key results. The most correlated features to wildfire damage are the presence of vent screens and deck materials in California, and exterior walls material and deck materials in Portugal. For Portugal, as WRI increases by 50%, linear regression estimates a 48% decrease in proportion of highly damaged buildings, and a 42% increase in proportion of low damage buildings (R 2 of 0.93 and 0.90, respectively). A total of 65% of California buildings with WRI = 1 were destroyed, compared to average 85% for WRI ≥−0.33. Conclusions. The WRI quantifies the wildfire damage experienced by buildings in two diverse WUI regions. Implications. The WRI could be used as an estimator of wildfire damage but it needs further development.

Windblown embers, known as firebrands, are generated in large numbers during wildfires and can ignite buildings kilometers away from their origin. Firebrands are often the leading cause of building ignition in the wildland-urban interface. Firebrands can either ignite buildings directly by landing on or inside the structure, or indirectly by igniting adjacent vegetation. Studies have investigated firebrand generation from various fuels, wind-driven transport, and ignition mechanisms on different materials and building features. Less research has been conducted on how firebrands deposit and accumulate around buildings; this knowledge is critical in designing buildings safer to firebrand ignition. Here, we address this less understood part of firebrand processes: landing and accumulation. A computation fluid dynamic Fire Dynamics Simulator (FDS) model simulating published experimental data by Suzuki and Manzello measuring firebrand accumulation region in front of a vertical wall under firebrand exposure created with the NIST Dragon and varying wind speeds (4 m/s - 10 m/s) (Suzuki and Manzello, 2017). The simulated accumulation areas in front of the wall are compared to the experimental observations. The height, width, and thickness of the wall and wind speeds are varied, and the accumulation patterns and sizes are studied to learn the critical design parameters affecting firebrand accumulation. Here we present the set up and vision for this project, and preliminary results from the simulations. FDS measurements do not yet agree with experimental results; authors are working on improving the model planning its potential for investigations once validated.

<p>Inhabited areas adjacent to wildland, known as the wildland-urban interface (WUI), often experience wildfire damage. Although knowledge on external fire protection of buildings has greatly advanced through post-fire inspections and experimental studies, the intercomparison between studies in different regions is lacking. Here we quantitatively compare two large post-fire building damage inspection databases: the 2013-2017 California Department of Forestry and Fire Protection damage inspection in the USA, and the 2017 Pedrógão Grande Fire Complex post-fire investigation in Portugal. We compare the relationship between different building features and wildfire damage, and propose the Wildfire Resistance Index (WRI), a preliminary wildfire risk index applied to rural buildings. Results indicate that exterior walls, windows, and vent screens have the strongest correlation to damage level in California, and exterior walls and preservation level in Portugal. The correlation strength indicates each feature’s relative importance in protecting the building from wildfire damage. The WRI value corresponds to the building’s net number of fire-resistant features and has an inversely proportional relationship to the percent of destroyed buildings. In California 93% of buildings with a WRI of -0.4 were destroyed, compared to 73% of buildings with WRI of 1; in Portugal 75% of buildings with WRI of 0.5 were highly damaged or destroyed, decreasing to 44% of buildings with a WRI of 1. Results indicate that the amount of fire-resistant building features directly relates to the building’s damage probability, and that the WRI can be used to quantify building wildfire resistance.</p>