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Firebrand Protection as the Key Design Element for Structure Survival during Catastrophic Wildland Fires

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Firebrand Protection as the Key Design Element for Structure Survival during Catastrophic Wildland Fires. Joseph W. Mitchell & Oren Patashnik. *. *. M-bar Technologies & Consulting [email protected] Firebrands & structures. Historical data & analyses:

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Firebrand Protection as the Key Design Element forStructure Survival during Catastrophic Wildland Fires

Joseph W. Mitchell & Oren Patashnik



M-bar Technologies & [email protected]

Fire and Materials 2007

firebrands structures
Firebrands & structures
  • Historical data & analyses:
    • Firebrands as leading cause of ignition
    • Water spray as an effective countermeasure
  • Cedar Fire, 2003
    • Curved-tile roofs: Some “good” roofs not so good
    • Wind-Enabled Ember Dousing – a wind-resilient spray concept

Fire and Materials 2007

wildland fire losses in california are driven by catastrophic wind driven events
Wildland fire losses in California are driven by catastrophic wind-driven events
  • Intense winds
  • Rapid spread
  • Firefighter intervention improbable
  • High density of brands
  • Extreme fire behavior

- Oct 2003: Cedar + others – 4,443 structures

  • Oct 1991: Oakland (Tunnel) – 2,886 structures
  • June 1990: Paint – 641
  • Sept 1923: Berkeley – 624
  • Nov 1961: Bel Air – 505
  • Sept 1970: Laguna – 382

Fire and Materials 2007

structure ignition by firebrands
Firebrands are the leading cause of structure loss

G.C. Ramsay, 1987 – study of 1148 structures

Ethan Foote, Paint Fire analysis, 1993

Chen & McAneney, 2004 – 50% structure ignition at 45 m or more (satellite analysis)

Cohen analyses of structure ignition potential

Plus others…(but no comprehensive 2003 data published!)

As determined by:

Structures too far from fire front

Observed ignition points (roof, attic, decks, fences)

Civilian protection highly effective

Observed density of brands

Forensic evidence

Structure ignition by firebrands

Fire and Materials 2007

another entry mechanism curved tile roofs
Another entry mechanism:Curved-tile roofs

This is considered a “good roof”

Fire and Materials 2007

data on roof type structure survival from the cedar fire
Data on roof type & structure survival from the Cedar Fire
  • Collected in November 2003, Scripps Ranch, San Diego by Amy & Oren Patashnik
  • Types included wood-shake, curved tile, flat tile, and stone-covered steel
  • Divided into a portion with initial firefighter protection (“front loop”) and without (“back loop”)

Fire and Materials 2007

structure ignition data
Structure ignition data

Fire and Materials 2007

statistical significance
Statistical significance

Hypergeometric distribution (aka “Lotto”)

  • n is the number of houses in a population N
  • t is the number of houses of type T
  • s in the number of houses in a subset S of N (S will be either the surviving or the destroyed houses)
  • p computes the exact probability, in a randomly chosen subset S of N of size s, that S contains at most m houses of type T
  • This is a one-tailed distribution

Curved-tile results: Back loop p = 0.039 Adjacent destroyed home p = 0.011

Fire and Materials 2007


Separate the problem of radiant heat & flame protection(answer: distance from fuel)

from the problem of firebrand protection…

Fire and Materials 2007

firebrand protection
Firebrand protection
  • Protection from firebrand entryStructural characteristics (Australian building codes; Ramsay & Rudolph; CA SFM Interface Fire Building Standards)
    • Only as good as the weakest point / maintenance
    • Not good for existing at-risk structures
  • Protection from secondary ignitionVegetation management / clearance adjacent to structure
    • Accumulation of litter & leaves in gutters, corners
  • Ember extinguishingWater systems / gels
    • Subject to wind disruption
    • Roof protection adds no value to fire-resistive roofs
    • Massive water use / undependable supplies
    • Gels require manual application

Fire and Materials 2007

water spray systems
Water-spray systems
  • Sufficient spray density to affect ignition
  • Sufficient lifetime to protect during/after fire front
  • Resilient against wind disruption

Fire and Materials 2007

re evaluation of paint fire data ethan foote thesis results on water spray
Re-evaluation of Paint Fire dataEthan Foote thesis results on water spray

Multivariate analysis found significance at >90% confidence level (but not 95%)

Fire and Materials 2007

water spray protection
Water-spray protection
  • If spray density is high enough, brands can be directly extinguished.
  • Water accumulates on surfaces and around structures, creating a “moat”.
  • Spray & vapor can hydrate light fuels.

Sensitive to wind disruption

Fire and Materials 2007

wind enabled ember dousing weeds
Wind-Enabled Ember Dousing (WEEDS)


  • The wind blows it back onto the structure
  • Spray accumulates where embers do
  • Low spray densities needed to protect from brands (as opposedto radiant heat)

Fire and Materials 2007

weeds design features
Low flow rate (~120 l/min)

Agricultural spray nozzles

5000 US gl water tank(plus municipal supply)

12 kW generator (propane)

1.5 kW pump

3-4+ hour protection window

Potential improvements: gravity feed, 10k gal tank, automated or remote triggering

WEEDS design features

Fire and Materials 2007

is it sufficient
Crib experiments suggest 1.5 -4.0 gm/m2sec is sufficient to extinguish cribs (reviews: Novozhilov et al., Grant et al.)

Simulation of droplet in wind

Used similar nozzle for droplet size distribution

Achieves extinguishment zone around the structure at nominal design

Is it sufficient?

Fire and Materials 2007

wind resilience of spray
Wind resilience of spray
  • Results conservative – don’t take airflow into account
  • Overlap of spray patterns to 50 km/hr
  • 40% of spray onto roof / eaves at high wind speed

Wind speed = 20 km/hr

Fire and Materials 2007

testing of system october 26 2003
Testing of systemOctober 26, 2003
  • Cedar Fire
  • Nominal operation
  • Apparent success
  • Structures lost on all adjacent properties
  • 60-70% loss rate / no professional fire protection
  • Forensic evidence of brands on property

Not proof, but a case study (Fire Safety Journal, Sept. 06)

Fire and Materials 2007

  • Prevent firebrand ignitions and you prevent the majority of structure ignitions in wildland fire.
  • Approach radiant heat and firebrands as separate problems.
  • Low-volume water spray systems are effective, and can be used to supplement vegetation management.
  • Water spray systems must operate DURING and AFTER fire-front passage.
  • Watch for curved tile as a potential avenue for brand entry.
  • Better data is needed – lack of organized data collection in 2003 was a tragedy.
  • Structures can withstand extreme wildfire conditions without professional intervention.
  • Design for WIND!

Fire and Materials 2007