Analysis of building vulnerability and firebrand exposure to mitigate wildfire damage

Abstract

Wildfire intensity and resulting damage to communities and infrastructure is increasing worldwide; this thesis investigates the vulnerability of wildland-urban interface (WUI) buildings to wildfires, focusing on firebrand exposure. The research objective is to improve understanding of wildfire damage mechanisms to aid effective risk reduction practices. A comprehensive literature review establishes the need for quantitative building vulnerability assessments and advanced understanding of firebrand deposition and accumulation around solid obstacles. To address these needs, a statistical analysis on two large wildfire damage databases from Portugal and California is conducted to investigate the relationship between building design and damage extent. The results contribute to the development of a preliminary building resistance index (WRI) to assess building vulnerability to wildfire damage. The thesis progresses to characterise the feasibility of using the Fire Dynamics Simulator (FDS) to simulate firebrand creep movement and accumulation around solid obstacles. A sensitivity analysis quantifies the impact of input parameters on final particle position, revealing a relationship between sensitivity and the Tachikawa number, which describes the aerodynamic properties of simulated particles. A comparison between two FDS Lagrangian Particle model modalities assesses their efficacy in simulating firebrand accumulation based on previously published experimental results. The analysis explain the connection between particulate transport physical mechanisms and the effective operational FDS use for simulating wildfire firebrand exposure. FDS simulations are subsequently conducted to identify regions of firebrand contact exposure, connected to firebrand ignition hazard, around three different obstacles mimicking common building components. Results characterize the combined influence of wind speed and obstacle geometry on firebrand contact exposure. Finally, the applicability of established sand protection measures to protect infrastructure from firebrands is explored by reviewing literature and conducting exploratory FDS simulations. The effectiveness of a trench protection measure to inhibit firebrand accumulation on an infrastructure component is investigated, indicating its potential when combined with contextual information on infrastructure design and ambient conditions.