|Defining Exposure at the Wildland Urban Interface - Challenges and Pitfalls|
Issue 32: Defining Exposure at the Wildland Urban Interface - Challenges and Pitfalls
By Alexander Maranghides
Even amongst fire protection engineers, the term "exposure" can mean
different things. When investigating human tenability, "exposure"
typically refers to toxic gases, heat fluxes and temperature. When
conducting a design calculation on the response of a structural member
to a specific design fire, "exposure" typically refers to the heat
fluxes and temperatures that will impact a structural element during a
design fire. In both cases, in order to correctly characterize exposure,
one must clearly define the fire scenario as well as the progression of
the fire event. Complications arise when the selected design fire does
not adequately represent the real life fire scenario. This typically
occurs because either the fire scenario is not clearly understood or
from an inappropriate determination of equivalent fire exposure.
The Wildland Urban Interface (WUI) has many definitions; however, for
the purposes of this article, WUI refers to locations where
topographical features, vegetation types, local weather conditions and
prevailing winds result in potential for ignition of structures from
flames and embers of a wildland fire.1
Between October 2003 and October 2007, seven California WUI fires destroyed a total of 8,877 structures2
- on average over 2,200 structures per year. These seven fires resulted
in 29 deaths, and over 317,000 hectares (783,000 acres) burned. The
2003 Cedar fire and the 2007 California Firestorm are among the top four
fire incidents for the number of structures destroyed and acres burned.
The Witch fire, the largest of the fires that occurred during the 2007
California firestorm, burned 80,124 hectares (197,990 acres) and
destroyed 1,125 residential structures, 509 outbuildings and 239
vehicles. Additionally, 77 residential structures and 25 outbuildings
These fires typically start in the wildlands and spread into communities. Frequently, large WUI conflagrations occur under severe weather conditions with high winds and low humidity, making these fires difficult to control. Under such severe conditions, the ignition and destruction of buildings at the WUI pose a significant challenge for the fire protection engineering community.
Traditionally, passive and active fire protection measures in
structures have been aimed at limiting the spread and damage from a fire
initiating inside the home. A paradigm shift is needed for structures
that resist ignitions from both the inside and outside. However, very
little quantitative information is available on the actual exposure
experienced by structures in WUI fire settings.
Even though there are other fuel sources, structures at the WUI
typically ignite from other structures or from residential or wildland
vegetation. Traditionally, it had been believed that the ignition
process was driven by direct flame contact or radiative heating. Embers
have been known to play a role in the ignition process during WUI fires.3,4
To predict the ignition of structures from WUI fires, the production of embers from different types of vegetation and from burning structures needs to be characterized. However, removing embers as a source of structure ignition will not solve the problem of structure to structure fire spread in its entirety.
Structure to structure fire spread can become a dominant mode of structure destruction in high density subdivisions. In a recent full-scale laboratory experiment at the National Institute of Standards and Technology (NIST), it took less than 80 s for flames from a simulated house with combustible exterior walls to ignite a similar house 1.8 m (6 ft) away.5 In another experiment, involving the same type of structures, the flames from one simulated house again reached the second, but a gypsum barrier protected the simulated home from sustained ignition.
The experiments showed that an adjacent structure can be ignited if
flames from a fire inside a house exit through window openings. The
experiments illustrated how a fire resistant barrier can, in the
scenario tested, slow down flame spread between two structures separated
by 1.8 m (6 ft). The scenarios tested were not the worst case. Flame
spread between structures is a complex process primarily affected by
structure construction type, structure separation distance, placement
and size of windows and weather conditions. The experiments illustrated
the impacts of high density single family construction on fire spread.
This limited data on the actual contribution of embers to structure ignitions has limited the development of ember specific test methods for building materials and systems. As an example, the Standard for Tests for Fire Resistance of Roof Covering Materials6 is designed to evaluate a roof assembly's ability to resist fire exposure from the outside. The maximum wind used in the test is only set to 19 km/h (12 mph), less than the severe weather conditions seen during many WUI fires. Additionally, the test is not designed to challenge the roof assembly against dynamic ember assault present during severe WUI fires. As new data on the significance and impact of embers become available, the testing community is quickly responding. Recently, the ASTM International Committee E05 formed subcommittee E05.14 specifically for External Fire Exposure Tests.
Engineers have tools available for addressing parts of the problem.
Fire dynamics provides the foundation, while computer models such as the
Fire Dynamics Simulator (FDS)7 and other models offer
methods to simulate fire behavior. The tools, however, will only work as
well as the engineer can correctly understand the evolution of the fire
scenario. Understanding the exposure ultimately requires understanding
the fire environment.
Alexander Maranghides is with the National Institute of Standards and Technology.
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