This article is an excerpt from NIST Technical Note 1748.1
of homes and businesses from Wildland Urban Interface (WUI) fires has
been steadily escalating, as has the fire suppression costs associated
with them. Since 2000, more than 3,000 homes per year have been lost to
WUI fires in the United States. The WUI fire problem affects both
existing communities and new construction.
One of the fundamental issues driving the destruction of homes at the interface is the very limited consideration of potential wildland fire and ember exposures in building codes and standards. The limited information currently available does not address the full range of realistic WUI exposures and offers little context for the design of ignition resistant landscapes and buildings. While the principles of ignition and fire spread at the WUI have been known, actual exposure quantification has been very limited. The resulting gap between exposure and structure ignition has therefore resulted in a lack of tested and implementable hazard mitigation solutions.
As an example, there is currently little quantifiable information that links the ember generation from wildland fuels (treated or untreated) to building assemblies testing. Additionally, there has been no consideration of first responder and homeowner safety to ember/ fire exposure.
WUI fires present a unique
challenge to the firefighting and fire protection engineering
communities. The scale of the events can be vast, spanning in many cases
more than 40,000 ha (100,000 acres), and the moving fire perimeter can
be tens of kilometers long with potentially thousands of structures at
The severity of the fire
depends on vegetative (wildland and ornamental) and structural fuels,
topography, and weather. Compared to hurricanes and earthquakes, fire
intensity can vary significantly over relatively short distances
(fractions of a kilometer) requiring complex fire suppression and
exposure scale concept is based on quantifying expected fire and ember
exposure throughout an existing or proposed new WUI community. The
proposed WUI scale can be used to explicitly identify WUI areas that
have a fire and ember exposure problem, as opposed to areas that meet
housing density or wildland vegetation requirements. The scale therefore
can be used to provide the boundaries where specific land use and/or
building construction regulations would apply.
CURRENT WUI BUILDING CODES AND STANDARDS
building construction is influenced by codes and standards developed
from the cumulative expertise and experience of the participating
committee members. This includes the evaluation of structural
performance during past WUI fires, limited laboratory work, and very
limited WUI fire modeling.
post-fire assessments consider structural performance, and if conducted
systematically, should be used as part of a comprehensive approach that
includes laboratory and full-scale experiments as well as computer
modeling to guide and confirm the effectiveness of changes to buildings
codes, standards, and best practices.
date, post-fire WUI field data collections have failed to address three
critical components: impact of defensive actions on structure
survivability, systematic documentation of structure response to WUI
fires, and quantification of fire and ember exposures.
EXISTING HAZARD SEVERITY ASSESSMENT SYSTEMS
example of an existing community-scale hazard severity assessment
program is the one developed by California Department of Forestry and
Fire Protection (CAL FIRE). The CAL FIRE and Resource Assessment Program
(FRAP) Fire Hazard Severity Zone is used to determine fire hazard on a 9
m (30 ft) grid. This information is applied in areas under state
jurisdiction. FRAP is one of the few programs in the United States that
links fire severity (exposure) and building codes (construction
attributes). The FRAP system, with respect to building construction, is
two-tiered: a structure is either in the WUI or it is not.
While FRAP links expected exposure to specific building code requirements, its classification system focuses primarily on proximity to wildland fuels and does not address the likelihood that buildings could be destroyed due to other sources of fire and ember exposures, such as from an adjacent burning structure. Other similar programs with less complex WUI hazard rating systems exist and are implemented across the United States.
The Home Ignition Zone (HIZ)
concept represents another WUI hazard severity assessment framework
designed to be implemented at a parcel or structure level. HIZ includes
the home and surrounding area within 30 to 60 m (100 to 200 feet). The
method has been successfully used to educate homeowners on the different
parameters that affect structure survivability.
primary limitation of the HIZ methodology in the context of this
article is that it does not offer a framework to link the fire and ember
exposure threat to building codes and standards. An additional
limitation of the HIZ system is that it does not account for WUI
scenarios with higher housing densities. A framework similar to the HIZ
is also used by the International Wildland-Urban Interface Code2 as well as many other national and state hazard mitigation programs.
THE PROPOSED WUI SCALE
behavior in the wildlands and the WUI is a function of fuel (vegetative
and structural), topography and local weather during the event. A fire
and ember exposure driven WUI scale, therefore, needs to account for
these local environmental conditions. Using such a rating, an overall
WUI area may receive a range of ratings. The ratings will reflect the
potential severity of a WUI fire event at specific locations.
Additionally, the framework links fire and ember exposure and resident
and firefighter safety.
scale is designed to the range of fire and ember exposure conditions
experienced by structures at the WUI. Fire and ember exposure can be
traced to four primary sources of WUI fuel: wildland fuels, ornamental
vegetation, structures (including homes, auxiliary buildings such as
sheds and garages), and vehicles (Figure 1). The WUI-scale is designed
by considering these sources as well as the local weather. These
combined parameters are referred to as FTLW, which is short for fuels,
topography, and local weather.
1. Primary Fuels Responsible for Fire and Ember Exposure at the WUI –
Wildfire approaching a WUI community (top), with parts of the community
proposed framework, an exposure rating is uncoupled from ignition, so
that the exposure rating is independent of the response to a particular
structural element or landscaping attribute. The figure at the beginning
of this article illustrates community ember exposure zones from a
wildland fire. Figure 2 illustrates the proposed matrix for capturing
fire and ember exposures from widland fuels.
Figure 2. Capturing Exposure from Wildland Fuels
proposed WUI scale is developed with the primary objective of reducing
the ignition risk of buildings in the WUI. This will be accomplished by
linking the ignition resistance required of structures to anticipated
exposures by using the exposure scale. Also, an understanding of
exposure can help improve the effectiveness of wildland fuel treatments.
a WUI fire, a given structure can be exposed to fire and/or embers.
Both threats need to be independently quantified and addressed. A
structure can be hardened for embers, fire, or both. Table 1 is used to
illustrate how three distinct building elements may be vulnerable to
exposure from embers and/or fire.
|Building Elements||Potential Ignition Vulnerability|
|Metal Frame Closed Window||No||Yes*|
|Untreated Wooden Deck||Yes||Yes|
Table 1. Building Element Vulnerability to Ember and Fire Exposure
* Window may break under direct flame exposure.
† Combustible insulation may ignite from embers inside attic, away from exterior attic vents.
issues must be addressed to make the scale quantitative: the critical
lack of quantitative information on the exposure of structures to embers
and fire; and the lack of a well-characterized, systematic effort that
combines pre- and post-fire observations, laboratory, and field
experiments, and fire modeling needed to characterize the ignition
regimes of different WUI fuels.
The following assumptions are used in the development of the WUI scale:
- The fire and ember exposure conditions at a given location can originate from fire in wildland fuels and fuels within the WUI community. The fire and ember exposure each zone experiences is the linearly combined exposures of the external (wildlands) and internally generated exposures. As an example, structures within a zone may experience a significant ember assault from its proximity to wildland fuels, and from any burning fuels within the zone itself.
- During a WUI fire, both the fire exposure and ember assault at a given location will change with time. The fire and ember scales are intended to capture both the peak intensity and maximum duration of the exposure/assault.
distance from the interface and width of each zone will be a function
of fuel, topography and local weather (FTLW). The four zones selected
for each of the fire and ember exposures are described next specifically
as to exposure from the wildlands.
FIRE AND EMBER EXPOSURE FROM FIRE IN WILDLAND FUELS
fire and ember exposures in very high-risk areas can result in
significant structural losses at the perimeter of many communities.
Field observations from first responders have identified burning homes
as large ember generators, posing a significant threat to surrounding
and particularly downwind structures and vegetation. By preventing the
ignition of structures in very hazardous locations, significant
reductions in further fire spread are achievable within WUI communities.
proposed approach will therefore initially focus on fire and ember
exposure from the fire in wildland fuels. Fire and ember exposure from
burning structures, ornamental vegetation, or vehicles will be
considered at a later date following the same framework. This exposure
framework, together with supporting updates to building codes and
standards, will make the WUI scale directly applicable to new
current approach will enable the WUI scale to be used for evaluating
existing communities, highlighting weaknesses and identifying retrofit
solutions. Figure 2 illustrates the fire and ember exposure matrix for
wildland fuels. The proposed exposure matrix is developed using three
categories for terrain: flat, steep slope, and ravine; and three
categories for wind: no wind, low wind, and high wind. Four fuel
categories will be used to provide an initial characterization:
homogeneous surface fuels (such as prairie grasses), inhomogeneous
surface fuels (such as palmetto), inhomogeneous shrubs and low
vegetation (such as chaparral), and canopied forest (such as what is
found in the Intermountain West). The selected topographical, weather,
and fuel attributes, while not all-encompassing, provide realistic input
ranges for the characterization of fire and ember exposures. Modeling
and field data collection from prescribed burns will be used to define
the specifics of the topography, weather, and fuel attributes.
3. Fire Exposure from Burning Structure on Ornamental Vegetation – as a
function of distance from the burning structure (NIST Photo,
Witch/Guejito Fire, CA 2007)
the future, a similar type of matrix will provide the fire and ember
exposure from burning structures, ornamental vegetation, and vehicles in
different local weather and topographical conditions.
This work was made possible through technical collaboration with numerous organizations including but not limited to CAL FIRE, San Diego Building Codes Department, the International Code Council, and the National Fire Protection Association
also acknowledge Dr. Shyam Sunder, director, NIST Engineering
Laboratory. This work is in part funded by the Joint Fire Science
Program Project 11-1-3-29, "Evaluating the Effectiveness of Mitigations
Activities in the Wildland Urban Interface.”
Maranghides is with the National Institute of Standards and Technology.
William Mell, Ph.D., is with the U.S. Forest Service.
- Maranghides, A. and Mell, W. "Framework for Addressing the National Wildland Urban Interface Fire Problem – Determining Fire and Ember Exposure Zones using a WUI Hazard Scale,” NIST Technical Note 1748, National Institute of Standards and Technology, Gaithersburg, MD, 2013.
- International Wildland-Urban Interface Code, International Code Council, Washington, DC, 2012.