By Erica Fischer and Negar Elhami Khorasani
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The wildland-urban interface (WUI) is where structures or other human development interfaces or intermixes with wildland vegetation. Today, many WUI communities are being threatened by fire or consumed by it. To understand how this predicament came to be with respect to wildfire impacts on communities, the policies and mitigation practices of the past must be understood. At the beginning of the 20th century, the U.S. enacted policies that stipulated fires should be put out immediately within the wildland to protect timber crops. These policies were in reaction to the Big Blowup of 1910, which consisted of over 1,700 fires across Idaho, Montana, and Washington, burning more than three million acres of federal and private land and killing at least 85 people. These fires were estimated to have destroyed about 7.5 billion board feet of timber.
However, scientists concluded in the middle of the 20th century that fires were actually beneficial to our landscape and that putting out all fires was detrimental to many plants and animals. It took the U.S. until the end of the 20th century to roll back these policies. Meanwhile, communities were growing and developing within high-risk and fire-prone regions. Today, fires are still suppressed, especially if they threaten people and communities. However, the expansion of communities into the wildland, together with the effects of climate change, has led to destructive wildfires in recent years.
To combat this threat to communities, the U.S. Forest Service and other land management agencies have been working to manage fires within the wildlands. The U.S. Department of Agriculture and Interior developed the National Fire Plan after the devastating destruction of communities due to wildfires in 2000. In 2001, a 10-year Comprehensive Strategy Plan was developed to reduce wildfire risks to communities and the environment. This effort led to the Healthy Forests Restoration Act of 2003, which developed a framework for Community Wildfire Protection Plans. In May 2016, the White House issued an executive order (EO 13728) for enhanced wildfire risk mitigation in the WUI. Despite all existing efforts, due to the proximity of communities to these wildlands and the changing climate, the current approach to managing the fire hazard is neither sufficient nor sustainable. The result is what we see today: fires are increasing in intensity and frequency.
When fire intrudes within a community, and homes ignite, they will release substantially more energy than vegetation and will burn for much longer than vegetation. The danger of this is that in many of the recent major fires we have witnessed (e.g., 2018 Camp Fire, 2021 Marshall Fire, and 2023 Maui Fire), high winds occurred simultaneously with the fire. Therefore, embers or firebrands generated by the burning vegetation and structures can travel several miles (in the 2021 Bootleg Fire in Oregon, embers traveled 5 miles), igniting other parts of the community. During a wildfire, structures are not only an asset to protect but also the fuel for the fire within the community. Structures can intensify the fire demand itself substantially. Destructive fires lead to significant losses, damaging homes and critical infrastructure such as hospitals, schools, water networks, transportation networks, etc. As structural engineers, we understand the resulting societal and economic impacts of damage to such infrastructure systems, given the existing research and established practices for other hazards.
Therefore, a future solution must be a multi-prong approach that engages civil engineers such that land management agencies are not the only ones tasked with reducing risk. To reduce the risk of home loss, local communities and homeowners must work together with land management agencies (Figure 2). While land management agencies (e.g., U.S. Forest Service, Bureau of Land Management) can reduce the probability of home exposure to wildfires through wildland vegetation management, civil engineers, particularly structural engineers, can have an impact through home hardening and creating defensible space to reduce the susceptibility of homes and critical infrastructure to wildfire damage.
State-of-the-Practice for Wildfire Mitigation Within Communities
The state-of-the-practice for wildfire mitigation within communities aims at slowing fire spread by reducing fuels or preventing home ignition. These mitigation practices are based on the methods of fire spread within communities. Fire can spread by three mechanisms: (1) direct flame contact, (2) radiative heat transfer, and (3) embers or firebrands. Fire spread from direct flame contact within a community can occur when vegetation or combustible material touching a house ignites. For example, a bush or bark mulch next to the house ignites, causing the house to ignite. Radiative heat transfer can cause fire to spread in a community when a structure (e.g., another home, a shed) close to the home is burning but not touching the home. The fire can spread from the ignited structure to the home in question through radiative heat transfer. The last method of fire spread within a community is the most common, which is through embers or firebrands. These are ignited pieces of combustible material (vegetation or pieces of a structure) that travel through the air due to wind. Embers can accumulate on porches, decks, roofs, and gutters, igniting the house; enter through vents of the house and ignite the house from the inside; land in bark mulch or vegetation touching the house and cause home ignition through direct flame contact.
Firewise USA developed methods of mitigation based on decades of research on fire spread within communities and based on post-fire investigations. The mitigation approach is based on different zones surrounding the house and is divided into vegetation management or defensible space and home hardening (Figure 3). The goal of defensible space is to reduce the combustible material surrounding the house such that if embers do land on the property, there is not enough fuel on the property to develop a fire that will spread to the house itself and ignite the home. The other goal of defensible space is to provide space for firefighters to perform defensible actions on the house. Civil engineers and structural engineers specifically can engage with Firewise USA through the second part of the mitigation strategy of home hardening. Home hardening addresses the siding and roofing materials, types of windows, and detailing of the house, such as vent coverings, soffit details, roofing details (e.g., eaves), etc. Post-fire investigations have shown that implementation of these strategies on properties can reduce the probability of home destruction during a wildfire.
Regions of the country that adopted the Firewise planning program or were within 30 miles of a Firewise community have been shown to experience fewer burned structures than regions that did not adopt the Firewise program. Syphard et al. concluded that the most effective methods of mitigation were to reduce woody cover by up to 40% in the immediate region adjacent to a home and ensure vegetation does not overhang or touch the home.
Many of the above-mentioned details were adopted within the 2021 International Wildland Urban Interface Code (IWUIC), which in turn was adopted in the 2022 California Building Code (CBC). The IWUIC is the nationwide code of reference for wildfire-resistant design provisions. The use of code is intended for both public and private sectors. The IWUIC provides the minimum requirements for access to the site, water supply for the use of fire protection services, and architectural features considering the construction materials and details, such as roofing, eaves, gutters, underfloor enclosures, appendages and projections, exterior glazing, openings, and requirements for defensible space. Guidelines for the preparation of a fire protection plan, when required by the code official, are also provided. Considerations of location, topography, flammable vegetation, climatic conditions, and fire history are recommended aspects to be included in the site-specific risk assessment and the fire protection plan. California adopted these provisions into the 2008 version of the CBC within Chapter 7A. Investigations after the 2018 Camp Fire showed that about 80% of homes built before 2008 were destroyed within the Camp Fire; however, about 40% of homes built after 2008 were destroyed (Figure 4).
The Colorado WUI Hazard Assessment Methodology and the Wildfire Hazard Assessment Guide for Florida Homeowners are other examples of existing guidance in the US. Other documents, such as those by the National Fire Protection Association (e.g., NFPA 1141 and NFPA 1142), are also available. A study by Intini et al. completed a review of guidelines on the design and construction of the built environment against WUI fire hazards in North America, Europe, and Oceanic countries. Based on the review, provisions, and recommendations from the U.S., Australia, as well as IWUIC, include the most detailed information related to building construction in WUI.
Developing Fire-adaptive Communities
Disaster resilience of WUI communities depends on spatial dependencies and unique community characteristics, such as biophysical drivers of wildfire hazard and spatial relationships between structures and vegetation, as well as among structures themselves. A structure’s physical exposure to a hazard depends upon its environmental setting, community design, and location, quantifying the likelihood of a wildfire intrusion into the community and the transmission through the community in vegetation or from structure-to-structure ignition. The data from Paradise shown in Figure 4 demonstrates the need for widespread mitigation. Because of the way fire spreads through a community, effective wildfire mitigation cannot be implemented on a property-by-property scale. Rather, mitigation on neighboring properties will influence the amount of vulnerability on an individual property and vice versa. When there is a large-scale lack of mitigation within a community, the risk for the community increases. Also, the distance between structures, the width of streets, and the incorporation of fire breaks influence the probability of fire spread.
Wildfires have a widespread impact on communities in similar ways to other hazards. The 2018 Camp Fire destroyed two schools and damaged multiple other school buildings. The fire also destroyed the emergency room facility and many other resources at the Adventist Health Hospital in Paradise. Damage to homes and depressurization of the water system due to large firefighting efforts caused widespread damage to the water distribution systems of the impacted regions after the 2017 Tubbs Fire, 2018 Camp Fire, 2020 Labor Day Fires in Oregon, 2021 Marshall Fire, and most recently in the 2023 Maui Fire. This damage left communities without access to potable water for up to a year in Santa Rosa and Paradise and for several months in other regions.
Typical vulnerable components of a building to wildfire include roofing, dormers, gutters, eaves, vents, sidings, windows, glazing, decks, patios, fences, and mulch and debris. Tests of specific building components (e.g., roof shingles) to study their response to radiation, flame contact, and firebrands have also been completed. Roofing types are fire-rated based on standard tests; a Class A roofing assembly is recommended for construction in WUI areas. Double-pane windows are less prone to breaking when exposed to extreme heat. Openings in vents can provide a path for firebrands to enter the structure and cause ignition; thus, all vents should be screened with 1/8-inch metal screening. Keeping gutters clean is important as debris within gutters can lead to ignition and the spread of fire to the rest of the structure. Using non-combustible or ignition-resistant material for the exterior walls of a structure reduces the likelihood of ignition by direct flame contact or radiation. Decks, wooden fences, and detached structures can also ignite firebrands or direct flame contact and spread the fire to the next structure. It is recommended to use ignition-resistant material for decks and attached fences.
To develop fire-adaptive communities, comprehensive community resilience plans must be developed. Structural engineers are uniquely qualified to be a partner, alongside experts in sustainable forest management, in developing these plans due to their involvement and leadership in comprehensive resilience plans such as the SPUR Resilience Plan, Resilient Washington State, and Oregon Resilience Plan. Structural engineers can contribute to developing methods and solutions for fire resistance design and construction of critical facilities, reducing the recovery time after a wildfire event, quantifying wildfire risk for communities, and adaptation and mitigation to create resilience against extreme wildfire events. Structural engineers also have a key role in improving building codes and regulations as well as education and outreach to create fire-adaptive communities.
ASCE Fire Protection Committee
The ASCE/SEI Fire Protection Committee has formed a working group with the goal of engaging in research and educational programs on the design of civil infrastructure for wildfires within communities. To make progress towards this goal, the committee hosted a panel at the 2023 Structures Congress that engaged wildfire scientists and structural engineers in a meaningful conversation about wildfires and their impacts on structures. In addition, the committee met in New Orleans to connect ASCE/SEI member experts on this topic with individuals from the US Forest Service, the Insurance Institute for Business & Home Safety (IBHS), utility companies, and academia. During this discussion, the working group was officially formed. Throughout the next year, the working group will develop a white paper that lists sources and reports on WUI fires that are already published and identify gaps in knowledge. Through this process, the working group will examine the potential for collaboration with other professional organizations such as the International Association of Fire Safety Science (IAFSS), the Society of Fire Protection Engineers (SFPE), and the National Fire Protection Association (NFPA). Lastly, the working group aims to continue to write articles, such as this one, to help educate structural engineers about wildfires and wildfire impacts on communities with the goal of engaging more structural engineers in the conversation about fire-adaptive communities. ■
About the Author
Erica Fischer is an Associate Professor at the Oregon State University in the School of Civil and Construction Engineering. Dr. Fischer’s research interests revolve around the resilience and robustness of structural systems affected by natural and man-made hazards such as building fire, wildfires, and earthquakes. (erica.fischer@oregonstate.edu)
Negar Elhami-Khorasani is an Associate Professor at the University at Buffalo. Dr. Elhami-Khorasani’s main area of research is investigating the performance of built environment under extreme loading and multi-hazard scenarios, especially structure fires, wildfires, and fires following earthquakes. (negarkho@buffalo.edu).
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