Large outdoor fires involving building-to-building fire spread are catastrophic, often resulting in significant loss of life and property. Previous research investigating ignition of buildings from exterior fires has focused on exposures from wildland fuels, which typically have shorter residence times compared to structure fires. This paper presents the details of full-scale building-to-building fire spread experiments conducted on residential exterior wall assemblies. Three wall assemblies sided with exterior plywood panels (hereafter referred to as T1-11), exterior insulation finishing system (EIFS), and fiber cement board were exposed to a post-flashover compartment fire at 1.8, 3.0, and 4.3 m separation distances to evaluate reaction-to-fire characteristics. All three wall assemblies tested ignited from the source exposure within this range of separation distances, which are common in residential areas with high structure density. Combustible siding (T1-11 and EIFS) ignited at the 3.0 and 4.3 m separation distances, whereas sheathing behind the noncombustible siding (fiber cement) wall assembly ignited at the 1.8 m separation distance. Compared to results from experiments with an identical protocol that examined the performance of double-pane window assemblies, the maximum heat load (time-integrated heat flux) at the time of ignition at the 3.0 m separation distance was consistently higher than the heat load at the time of cracking of the outer pane of plain (annealed) glass window assemblies but lower than that for cracking of both panes of a double-pane window assembly. Current codes, standards, and test methods should address fire penetration, as well as vertical and lateral flame propagation of exterior wall assemblies.

Each year severe wildfires continue to cause significant destruction resulting in the loss of life, property, critical infrastructure, and the environment. In an effort to increase community preparedness and resilience to wildfire, international jurisdictions have adopted both guiding principles and prescriptive codes that apply to both urban planning and fire engineering design of buildings within the wildland-urban interface. These measures are intended to protect occupants, enhance the survivability of structures from different fire exposure mechanisms, and increase the chances of successful firefighting operations. However, research has identified (i) inconsistent approaches to regulation and governance; (ii) limited research on which urban design and building standards are based; and (iii) misaligned or contradictory urban design and building standards. This not only stifles the use of development proposing suitable performance-based design that could achieve the required outcomes but can increase administrative burdens and development costs without increasing safety. The aim of this current study is to contribute to addressing identified shortfalls by identifying and distilling the last 23 years of research in the field related to (i) the development of evidence-based performance requirements, and (ii) the application of effective governance arrangements in order to enhance urban design and wildfire engineering practices. These aims are achieved through a systematic literature review. Ultimately, however, of the 608 initial articles captured in the identification phase of the SLR, not a single article provided insight into the most effective regulatory or governance mechanism, and only three provided criteria suitable for adoption as a performance requirement. While the aims of this study were only partially achieved, it does provide a foundation for the field by way of identifying and distilling the current state of practice.

Windows are a potential vulnerability of structures during exterior fire exposures, such as those from wildland-urban interface (WUI) fires or a neighboring structure fire in a residential community. Window failure creates a pathway for embers, flames, and hot gases to enter a building and ignite interior combustibles. There is a need to investigate the failure of different types of multi-pane windows during exposures similar to those incident on buildings immediately adjacent to a burning structure. Eight experiments were performed with 16 window pane assemblies mounted in a target facade placed in front of a compartment fire that transitioned through flashover. Double pane window assemblies with both panes plain (annealed) glass, both panes tempered glass, and one plain glass, one tempered glass pane were examined. Window pane assemblies with a fire side plain glass pane and back side tempered glass pane performed notably better than assemblies with the opposite orientation. The heat load (heat flux integrated over time) at the time of complete failure (both panes cracked) was typically between 2.5 MJ/m2 and 10.0 MJ/m2 for window pane assemblies with a plain glass back side pane and between 9.0 MJ/m2 and 17.5 MJ/m2 for those with a tempered glass back side pane. Total heat flux measured behind the pane assemblies exceeded critical values for non-piloted ignition of common household materials, even before complete failure. Results from this study can be used to inform building codes and homeowner guidance related to the installation of tempered pane windows in areas prone to WUI fire hazards.

Embers, also known as firebrands, are the leading cause of building ignition during wildland-urban fires. This is attributed to direct ignitions, materials on, in, or attached to the building, and indirect ignition when they ignite vegetation or combustible material near the building which can result in direct flame contact and radiant heat that ignites the building. Where and when embers accumulate near a building and ignitable fuel is present the potential for indirect ember ignition of the building exists. Factors that influence ember accumulation near a building include building geometry, such as flat wall and re-entrant corner, building wind angle, and wind speed. Experiments conducted at the IBHS Research Center will full-scale homes with the above-mentioned factors provide quantification of ember accumulation, in terms of mass per unit area.

The hazard posed by wildland–urban-interface (WUI) fires is recognized by the international fire research community and features as one of nine research need priority threads in the Society of Fire Protection Engineers (SFPE) Research Roadmap. We posit that the first step in the journey to enhancing fire safety engineering at the WUI is to develop a common understanding between developers, engineers, planners, and regulators of the development scope, wildfire problem, technical design solutions, and verification methods to be used. In order to define a fire safety engineering consultation process appropriate for the wildfire context, this paper aims to translate well-established and evidence-based performance-based design (PBD) consultation frameworks and approaches from traditional fire safety engineering to the wildfire context. First, we review international English-language fire safety engineering frameworks that have been developed for the urban context. Next, we distil the results into a streamlined framework, which we call the “CAED Framework”. Finally, we apply and discuss the contextualization of the CAED Framework to the WUI context through a comparative case study of urban and WUI development. In doing so we seek to provide a structure for the development of standardized PBD within the WUI context across jurisdictions internationally, as well as to embed best practices into the emerging field of performance-based wildfire engineering.

In the last twenty years, wildland-urban interface (WUI) fires have been growing in severity and size. The structures destroyed by WUI fires have devastated entire communities and have cost billions of dollars while significantly impacting the social fabric and economic well-being of entire regions. Structural losses have been attributed to exposures from embers (firebrands) and fire (radiation and/or convection). As structural losses continue to increase, there is need for a comprehensive hazard assessment and mitigation methodology to harden appropriate structures and parcels effectively and efficiently against ember and fire exposures. The Hazard Mitigation Methodology (HMM) described in this report was developed explicitly to meet these needs. The science-based methodology uses the knowledge collected from post-fire field observations spanning over a dozen years and tens of thousands of hours of field data integration and analysis. Additionally, the HMM utilizes the latest technical knowledge gained from laboratory and large-scale research in fire propagation and hazard mitigation in the WUI. The HMM is the result of a sixteen-month collaborative effort between NIST, the California Department of Forestry and Fire Protection (CAL FIRE), and the Insurance Institute for Business and Home Safety (IBHS). The HMM is a spatial hazard mitigation methodology that provides an implementable path forward by considering the spatial relationships of fuels and exposures and hardening at the structure and parcel levels. The HMM demonstrates how complex structure hardening is, and how and why hazards associated with both fire and ember exposures need to be mitigated. The methodology further demonstrates where structure hardening in the absence of parcel hardening does not work by highlighting the relationships between exposure and hardening. Housing density, structure separation distance, and parcel layouts are also addressed. The methodology was explicitly designed to address the current building stock, i.e., to solve the retrofit challenges, and efforts were made to limit retrofit expenses. While the methodology was developed primarily for retrofits, the presented strategy can also be applied to new construction. This report documents the methodology and addresses the critical issues of mitigation effectiveness at the parcel and community levels. The impacts of partial mitigation at the parcel and community level were addressed for different types of WUI communities.

The primary objective of this project is to assess structure-to-structure fire spread for structures located in the Wildland Urban Interface (WUI). Full-scale fire experiments will be conducted in which various types of structures (sources of fire) will be used to generate typical radiative and convective heat exposures on target structures (residential dwellings). The spacing between the source and target structures will be varied to identify safe structure separation distance (SSD). Most experiments will be conducted with construction materials currently listed in State of California building codes and using code compliant structural assemblies. A limited number of experiments will be conducted with enhanced, ignition resistant, materials that may not currently be listed by the State of California. The project is divided into three phases. Phase 1 will test sheds as fire sources, Phase 2 will test in-law buildings (small accommodation structures built close to primary residential structure) and Phase 3 will test single-family homes. Source terms from 1.39 m to 24.8 m (15 ft2 to 267 ft2) will be tested in Phase 1. Phase 2 will examine exposures from 40.87 m (440 ft2) in-law buildings, and Phase 3 will characterize exposures from approximately 92.90 m (1000 ft2) residences. Dimensions of Phase 3 single residences (source term) will be finalized after numerical modeling of the results from Phase 1 and Phase 2. This test plan focuses on Phase 1 research. Phase 1 aims to quantify the effects of shed sizes, construction types, fuel loading, and separation distance on the ignition of primary structures (residential dwellings). Experiments will be conducted at the National Institute of Standards and Technology (NIST) and the Insurance Institute for Business and Home Safety (IBHS). The NIST experiments will be conducted at the National Fire Research Laboratory (NFRL), and the IBHS experiments will be conducted just outside of the wind loading test cell. The opening of the test cell (13.7 m (45 ft) wide) will dictate the largest source structure that can be evaluated at IBHS.

Two virtual workshops of the permanent working group, sponsored by the International Association for Fire Safety Science (IAFSS), entitled Large Outdoor Fires and the Built Environment (LOF&BE), were held this past August (2020).The first session was held on August 4, 2020 with times selected to suit those in Africa, Europe, and Asia/Oceania.The second session was held on August 6, 2020 with times to suit those in North and South America.The Ignition Resistant Communities (IRC) subgroup is focused on developing the scientific basis for new standard testing methodologies indicative of large outdoor fire exposures, including the development of necessary testing methodologies to characterize wildland fuel treatments adjacent to communities.IRC subgroup progress was presented by Alex Filkov (U Melbourne) and Daniel Gorham (IBHS).The Emergency Management and Evacuation (EME) subgroup is focused on developing the scientific basis for effective emergency management strategies for communities exposed to large outdoor fires.EME subgroup progress was presented by Maria Theodori (Reax Engineering Inc.) and Sayaka Suzuki (NRIFD).The Large Outdoor Fire Fighting (LOFF) subgroup is providing a review of various tactics that are used, as well as the various personal protective equipment (PPE), and suggests pathways for research community engagement, including environmental issues in suppressing these fires.LOFF subgroup progress was presented by Xinyan Huang (Hong Kong Poly U) and Brian Lattimer (Va Tech).

Generation of firebrands from various fuels has been well-studied in the past decade. Limited details have been released about the methodology for characterizing firebrands such as the proper sample size and the measurement process. This study focuses on (1) finding the minimum required sample size to represents the characteristics of the population, and (2) proposes a framework to facilitate the tedious measurement process. To achieve these goals, several firebrand generation tests were conducted at a boundary layer wind tunnel with realistic gusty wind traces. Firebrands were generated from burning structural fuels and collected in 46 strategically located water pans. The statistical analysis showed that the minimum required sample size based on the chosen statistical parameters (standard deviation, confidence interval, and margin of error) is 1,400 for each test. To facilitate characterizing such a large sample of firebrands, an automated image processing algorithm to measure the projected area of the firebrands was developed, which can automatically detect the edges of the background sheet, rotate the photo if its tilted before cropping, detect edges of firebrands, remove erroneous particles (e.g., ash) and finally measures the projected area. To facilitate the weighing process, a Gaussian process regression was performed to predict the mass based on projected area, traveling distance and wind speed. The model can predict the firebrand mass within 5% error compared to the measurement. This framework and model can provide a probabilistic range of firebrand characteristics over the continuous range of the collection region.

Large outdoor fires present a risk to the built environment. Wildfires that spread into communities, referred to as Wildland-Urban Interface (WUI) fires, have destroyed communities throughout the world, and are an emerging problem in fire safety science. Other examples are large urban fires including those that have occurred after earthquakes. Research into large outdoor fires, and how to potentially mitigate the loss of structures in such fires, lags other areas of fire safety science research. At the same time, common characteristics between fire spread in WUI fires and urban fires have not been fully exploited. In this paper, an overview of the large outdoor fire risk to the built environment from each region is presented. Critical research needs for this problem in the context of fire safety science are provided. The present paper seeks to develop the foundation for an international research needs roadmap to reduce the risk of large outdoor fires to the built environment.

Structure loss in wildland fires has significantly increased over the past few decades, affected by increased development in rural areas, changing fuel management policies, and climate change, all of which are projected to increase in the future. This paper is Part II of a two-part review, which presents a summary of fundamental and applied research on pathways to fire spread in the wildland urban interface. Part I discussed the fundamentals of wildland fire spread via radiative heat transfer, direct flame contact, and firebrand exposure. Here in Part II, we cover the response of building components and systems, as well as mitigation strategies used to prevent fire spread into and within communities in the United States. Post-fire investigations, full-scale structural testing, individual component testing, and combined systems or assembly testing have been used to identify building component and system vulnerabilities such as roofs, vents, siding, decks, fences, and mulch. Using results from these tests and investigations at different scales, some knowledge has been gained on specific vulnerabilities and the effectiveness of mitigation strategies, but a quantitative framework has not yet been established. On a community level, the layout of structures and the space between them has been shown to be incredibly important in mitigating wildfire risk. More locally, defensible space around homes has been effective in mitigating exposure from both radiation and direct flame contact. Firebrands still remain a challenge; however, many design recommendations have been proposed to harden structures against firebrand exposures. Recommendations for future research and development are also presented.

While the wildland–urban interface (WUI) is not a new concept, fires in WUI communities have rapidly expanded in frequency and severity over the past few decades. The number of structures lost per year has increased significantly, due in part to increased development in rural areas, fuel management policies, and climate change, all of which are projected to increase in the future. This two-part review presents an overview of research on the pathways for fire spread in the WUI. Recent involvement of the fire science community in WUI fire research has led to some great advances in knowledge; however, much work is left to be done. While the general pathways for fire spread in the WUI (radiative, flame, and ember exposure) are known, the exposure conditions generated by surrounding wildland fuels, nearby structures or other system-wide factors, and the subsequent response of WUI structures and communities are not well known or well understood. This first part of the review covers the current state of the WUI and existing knowledge on exposure conditions. Recommendations for future research and development are also presented for each part of the review.

Experiments were conducted to study the intermittent extension of flames from wind-driven line fires using stationary burners. These fires are thought to share similar features with propagating wildland fires, where forward pulsations of flame have been observed to quickly ignite material far ahead of the mean flame front. However, stationary burners offer the ability to study the movement of the flame and its heating processes in greater detail than a spreading fire. In these stationary experiments, propane gas was used as a fuel with different burner sizes, 25–30cm wide and 5–25cm long in the direction of the flow. A specially-built wind tunnel was used to provide a well-characterized laminar flow for the experimental area. The free-stream flow velocity, measured by a hot-wire anemometer, ranged in the experiments from 0.2 to 2.7m/s. The shape of the flame was measured using a high-speed video camera mounted perpendicular to the apparatus. A method was developed to track the extension of the flame close to the surface, simulating flame contact with unburnt fuel downstream of the fire. This extension length was then measured frame by frame and frequencies of flame presence/absence determined as a function of downstream distance. The location of maximum pulsation frequency, xmax, for each burner/wind configuration, was obtained using a level-crossing approach (essentially the variable-interval time-average (VITA) method). Further study indicates that xmax can be well estimated using mean flame properties. Probability distributions describing the location of the flame over time also showed that, the probability the flame extends far beyond the mean flame front is sensitive to increasing ambient winds and fire size.

Large wildfires of increasing frequency and severity threaten local populations and natural resources and contribute carbon emissions into the earth-climate system. Although wildfires have been researched and modeled for decades, no verifiable physical theory of spread is available to form the basis for the precise predictions needed to manage fires more effectively and reduce their environmental, economic, ecological, and climate impacts. Here, we report new experiments conducted at multiple scales that appear to reveal how wildfire spread derives from the tight coupling between flame dynamics induced by buoyancy and fine-particle response to convection. Convective cooling of the fine-sized fuel particles in wildland vegetation is observed to efficiently offset heating by thermal radiation until convective heating by contact with flames and hot gasses occurs. The structure and intermittency of flames that ignite fuel particles were found to correlate with instabilities induced by the strong buoyancy of the flame zone itself. Discovery that ignition in wildfires is critically dependent on nonsteady flame convection governed by buoyant and inertial interaction advances both theory and the physical basis for practical modeling.

Time-dependent movements described as pulsing, puffing or swaying are among the most visible characteristics of open flames. In the fire safety field, “puffing” of pool fires has been well studied, with scaling of the frequency of flame pulsations with the size of the burner being well correlated with the diameter of the fire size [1]. For smaller fires, a “flickering” instability at a higher frequency than “puffing” is also formed at the top of small flames [2]. Despite many years of study, the dynamics of wind-driven fires, especially those resembling a line fire configuration have not been well documented, instead focusing on their steady or averaged characteristics [3]. This configuration has recently been found to be particularly relevant when attempting to understand propagating wildland fires, prompting this more detailed study of their time-dependent behavior.

In practical applications, flammable materials are often arranged in arrays of discrete objects whose combustion properties may vary compared to that of a homogeneous material. In this study, the influence of spacing between arrays of wooden dowels on the rate of upward flame spread through arrays has been studied. This configuration adds to previous work on single columns of matchsticks (Gollner et al., 2012), in some ways modeling physics that appears in flame spread through wildland fuels and suspended cable trays. A single dowel was ignited at the base of an array of birch dowels with fixed spacings of 0.75, 1.0 and 1.5 cm and allowed to spread upwards. In the wider-spaced cases (1.0 and 1.5 cm), the flame spread upward mostly along the center column, igniting few dowels to its side. In the 0.5 cm spacing, however, flame spread exhibited a two-dimensional nature, spreading throughout the array in a V-shaped pattern. Comparing results to an existing theory for upward flame spread and burning of single vertical columns of matchsticks, the farspaced arrays follow the previous theory (Gollner et al., 2012); however rates of upward spread are decreased through the closer-spaced arrays. This may be due both to impedance of the flow through the array, but perhaps more importantly to a lack of available oxygen for burning. In the denser spacing of this configuration, significant unburnt fuel vapors are seen released above the array of this configuration. Results for mass-loss rates and spread horizontally through the array are also presented.