A methodology for quantifying structure fragility to fire in the wildland–urban interface utilizing past event investigations and fire and ember modeling

Background The increase in destructive wildland–urban interface (WUI) fires necessitates improved risk assessment methods for communities. Specifically, there is a need to quantify the impact of fire conditions on structures. Aim We present a framework to develop fragility functions that express the probability of structure destruction considering damageability, given exposure to fire hazard. Methods Lacking field measurements during wildfires, we simulate flame and ember conditions for the 2017 Tubbs Fire case study. Logistic regression is used to relate hazard, exposure and damage, incorporating fire modeling, property characteristics and post-fire damage data. The resulting fragility functions indicate the probability of structure destruction as hazard level increases. Key results Notably, zero simulated exposure does not equate to zero destruction chance, and maximum hazard is not 100% correlated with destruction – owing to the exclusion of structure-to-structure fire spread in simulations. Destruction probability was lower, regardless of hazard level, for structures subject to stricter building codes, greater structure separation and increased vegetation clearance. Conclusions This methodology links structure loss in the WUI to hazard exposure using landscape-scale fire modeling. Implications Though demonstrated using one event, the fragility analysis framework is broadly applicable and scalable, supporting improved understanding and mitigation of WUI fire risk.