Bushfire Safety - Society of Fire Safety

What is Bushfire Safety?
Bushfire Safety

Typically protecting new developments from
bush fire as enforced by legislation.
 Planning
 AS3959

for Bushfire Protection
(2009) Building in Bushfire Prone Areas
Review of existing buildings and infrastructure.
 Victorian
School Safer Place
Water Infrastructure Review
 Electrical Infrastructure
 University Campuses
 Individual Homes
Presented by Phil Couch
 Hunter
Newcastle Bushfire Consulting
What is Bushfire Safety?
Bushfire Practitioners

Review of bushfire prone land maps
 Emergency management planning
 Construction advice and design

Bushfire Safety Planning
Discussion Points
Who can do Bushfire Deemed to Satisfy
Solutions for new developments?
 Who can do Bushfire Alternate Solutions
for new developments?
 Who reviews the Bushfire Solutions:

 Councils
 NSW
RFS
Certifiers (Complying Development)
Fire Protection Association of Australia
Bushfire Planning and Design program
FPAA BPAD
 International Fire Engineers Graduate Fire
Engineer
Bushfire Safety Engineering can be complex,
yet anyone can presently do it.
 Qualitative opinions by untrained people are
often accepted.
 Do structural fire engineers propose models
or designs to non-technical people?
 How do you deal with inconsistency?

 Private
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AS3959 (2009) Bushfire
Behaviour


Method 1 Simplified BAL
Assessment - Measures
Bushfire Risk Based
Purely on Radiant Heat
Method 2 Detailed BAL
Assessment - Measures
Bushfire Risk Based on
Radiant Heat or Flame
Contact
Detailed Fire Modelling




Planning for Bushfire Protection (2006)
Allows you to quickly and easily model vary
bushfire scenarios.
Allows you to control more complex modelling.
First released to Fire Protection Association of
Australia Bushfire Planning and Design (FPA
BPAD) members in December 2007.
Also used by SA CFS, SA Water and Planning
SA.
AS3959 Detailed BAL
Assessment


Limitations of AS3959 (2009)
Full crown fire interaction is not measured.
The design fire methodology outlined in AS3959-2009
Detailed BAL Assessment Appendix B, provides the
calculation methodology and accepted inputs that the
AS3959-2009 Simplified BAL Assessment Matrix was
derived from. The design fire model consists of the
below calculation components:
 Rate of Fire Spread;
 Flame Length;
 Fire Intensity;
 Transmissivity; and
 Radiant Heat.
Once out of the flame contact zone the radiant heat
exposure drops significantly.
Limitations - Fuel Arrangement
Photo courtesy of ABC news.
Crumpled Paper/Loose Fuel versus Stacked Compacted Fuel
Overall fuel hazard assessment guide 4th edition July 2010
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Grassfire
Grassland



AS3959 (2009) considers all grassland the same.
All grassland will place a building in the flame zone
where within 6 metres of Grassland on level slope.
In accordance with Cheney and Sullivan (1997) The
Rate of Forward Spread is directly proportional to
flame height. At a ceiling rate of spread of 10 km/h a
ceiling flame height of:


Grassfire image from http://www.rfs.nsw.gov.au/
Grassland
Grass < 25cm
Grass Longer than 5cm
180cm tall sorghum
<2 metres will be achieved based upon mown or grazed
grasslands less than 25 centimetres tall.
<4 metres will be achieved based upon natural ungrazed
pasture between 50 and 80 centimetres tall.
Detailed Fire Modelling
Sawtooth – Effect by dwellings
69 metre APZ
DTS 100 metre APZ
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Scenario – Sawtooth Effect
Radiant Heat Shielding – Multiple
Shields
Section View
Results for Sawtooth Shield
Calculation
Base Calculation
5.51 metre hill shielding Calculation 2.26 kw/m2
0.027
1.14 kw/m2
0.013
12.4 kw/m2
0.145
3.31 metre dwelling shielding Calculation
Final Calculation
Section View of Person Safety
Radiant View Factor
Heat
15.82 kw/m2 0.185
Section View
SHIELDING FROM HILL LANDFORM. MINIMAL RADIANT HEAT EXPOSURE AT RL 38 WHERE A FIRE FIGHTER IS EVACUATING RESIDENTS
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Practical Fire Science
In an evolving fire
Intensity, Flame Temperature
and Emissivity are related.
Conservative figures of all
will overestimate significantly,
as will the flame length.
40m Asset Protection Zone Needed
Fire Growth
Measuring Slopes




Slopes for Rate of Spread assume the
vegetation slope.
The vegetation slope or “effective slope” is that
slope which most significantly influences fire
behaviour
Slope for the View Factor measures the site
slope.
It is the slope along the ground by line of sight
between the predominant vegetation and the
site.
From Bushfire CRC Jim Gould’s project on Fire Growth Transition
Measuring Slopes
Vegetation Slope (Limits)



PBP (2006) “The effective slope is that slope
within the hazard which most significantly affects
fire behaviour of the site having regard to the
vegetation class found.”
AS3959 (2009) “It may be necessary to consider
the slope under the classified vegetation for
distances greater than 100 m in order to
determine the effective slope for that vegetation
classification.”
Vegetative/Effective Slope (Limit 30 degrees
downslope)
 Should
be limited to 30 degrees downslope as
convective heat from bushfire flames is no longer
negligible in rate of spread calculations.
 Note: I cap Rate of Spread at 14 km/h
 Upslopes - attempt to model an adjacent burn if it is
possible (Level/Upslope)
 View factor model becomes inaccurate for steep
upslopes within vegetation.
 AS3959 (2009) says 15 degrees upslope is the
maximum.
 Project Aquarius showed upslope point ignitions can
burn quicker than downslope.
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Transitioning Slopes
When a slope transitions from one slope
how do we measure this?
 At the moment slope measure is
subjective. Take a few measurements and
pick the worst case.
 Why is this even an issue? If slope is
wrong than so is ROS, Flame Length,
Intensity and Radiant Heat Exposure.
 Recent Case -100 m fire run at 15
degrees upslope then 30 metre fire run at
15 degrees downslope.

2.5 hectares of vegetation. 2 hectares flat forested wetland. 40 metre fire run 7
degrees downslope. A 3 km/hr fire will travel 40 metres in 50 seconds.
Acceleration Flame Angle
Transitioning/Accelerating Fire has a reduced View
Factor than a fire in equilibrium state of spread.
Many hectares of vegetation. 40 metre fire run 7 degrees downslope. A
kilometre of steep upslope on the opposite side of the creek bank.
Douglas and Tan Integrating Site Assessment and Performance Planning Outcomes
for Bushfire Prone Areas
Transitioning Slopes
A more objective measure.
Numerical Integration of existing
fire growth models OR
 Newtons Law of Motion


 Constant
acceleration
= F/m
 a = Δv/Δt = (vfinal - vinitial) / (tfinal - tinitial)
 a = (v2-u2)/2s
 a = 2(s - ut)/t2
 Derived velocity – v = v0 + at
a
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Slope Transition Objective
Measure
ROS = RelativeROS x exp-1.05/t
 Fire Run = RelativeROS∫(1510) exp-1.05/t * dt

Select a point on the
acceleration curve to derive
what the accelerating fire
should be
Discussion Points
Bushfire Safety Engineering can be complex,
yet anyone can presently do it.
 Qualitative opinions by untrained people are
often accepted.
 Do structural fire engineers propose models
or designs to non-technical people?
 How do you deal with inconsistency?

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