building with timber in bushfire-prone areas - lunch & learn

Building with Timber in Bushfire-prone Areas

Building with Timber in Bushfire-prone Areas ©FWPA 2011

Learn more about wood at UTAS

Centre for Sustainable Architecture with WoodGraduate Certificate in Timber (Processing & Building)• 4 units, part time, onlineAreas covered include:• Wood science• Design for durability and service for life• Timber as a renewable resource• Sustainable design and construction• Engineered wood products• International technologies and developments• Plus, selected topics of individual interestMore information: Associate Professor Greg Nolan (03) 6324 4478 or [email protected]


Learning Objectives

After this presentation you should be able to:– Understand the design objectives behind the Standard AS 3959– Determine the Bushfire Attack Level (BAL) of a site– Identify the construction requirements in BAL–LOW to BAL–FZ – Outline alternative solutions for a timber system– Understand additional requirements in NSW

For architects CPD - AACA Competencies:– Design– Documentation


This Presentation

• Modes of bushfire attack• AS 3959 - Objectives & Determining BAL• Timber solutions for each BAL• Summary of construction solution• NSW requirements and implications


AS 3959: History

• AS 3959 Construction of buildings in bushfire-prone areas

• AS 3959—1991: ember protection only – 1 level• AS 3959—1999: introduction of 2 more levels –

higher level of protection generally required to address exposure to radiant heat

• AS 3959—2009: 6 levels (BALs) of exposure including “flame zone”– Amendment One – Late 2009– Amendment Two – Late 2010


Modes of Bushfire Attack


AS 3959—2009: Design Objectives

• Withstand ember attack prior to the arrival of the fire front with minor levels of assistance from occupants

• Provide a safe refuge whilst the fire front passes

• After front passes any residual burning of elements should be capable of being extinguished easily by occupants

• Consistent with the primary focus on life safety in the Building Code of Australia

• Intended for Class 1, 2 and 3 buildings only (residential)

– Some special purpose buildings such as unattended facilities serving critical functions and buildings housing the aged and people with disabilities may require higher levels of resistance to ignition (NSW RFS planning for bushfire protection guide has requirementsfor these buildings)


• Step 1: Determine the relevant Fire Danger Index (FDI)

• Step 2: Determine the classified vegetation type(s)

• Step 3: Determine the distance of the site from the classified vegetation type(s)

• Step 4: Determine the effective slope(s) under the classified vegetation type(s)

• Step 5: Determine the BAL from the appropriate table

AS 3959—2009: Determining the BAL


Step 1: Relevant Fire Danger Index (FDI)


Step 2: Vegetation Classifications


Step 2: Vegetation Classification - Grassland

• Grassland classification reintroduced in amendment 2 of AS3959 2009

• Grasslands have their own FDI – Grassland Fire Danger Index (Amendment 2 correlates GFDI to FFDI)

• Likely to be referenced in BCA 2011• Not yet known if this will be adopted in NSW


Step 3: Distance of Site from Classified Vegetation


Step 4: Effective slope of land under the classified vegetation


Step 5: Determine BAL


• BAL—LOW

• BAL—12.5

• BAL—19

• BAL—29

• BAL—40

• BAL—FZ (“flame zone”)

Bushfire Attack Level

BAL—12.5

BAL —19BAL —29

AS 3959—2009: BALs

BAL—LOW

BAL —FZ

BAL —40


Timber Solutions for each BAL

Timber can be used normally for the following applications (no restrictions) in any BAL:• Anywhere the timber is protected from

exposure to bushfire attack, e.g.: – house frame – internal flooring – Internal joinery or lining


• BAL—LOW

• BAL—12.5

• BAL—19

• BAL—29

• BAL—40



Timber Solutions for each BAL


BAL–LOW

• Most metropolitan and suburban blocks are defined as BAL–LOW

• very low risk of bushfire attack• no special requirements• Build as usual once BAL–LOW is confirmed by the

site’s approving authority• All traditional timber framing products and systems

can be used, along with the usual timber species and treated pine for decks, balustrades, handrails, finials, pergolas, etc.


BAL–LOW

• Vegetation of any type >100m from site• Single areas of vegetation <1ha and >100m from classified

vegetation• Multiple areas of vegetation <0.25ha and >20m from site• Strip of vegetation <20m in width regardless of length >20m

from site• Non vegetation areas (waterways, roads, etc)• Low threat vegetation (inc. managed grassland, maintained

lawns, golf courses, maintained public reserves and parkland, botanical gardens, vineyards, orchards, nature strips and wind breaks, commercial nurseries, etc.)


• BAL—LOW

• BAL—12.5

• BAL—19

• BAL—29

• BAL—40



BAL–12.5


• Possibility of ember attack has been identified due to:- proximity of vegetation- site itself- local conditions

• ‘12.5’ means external construction elements are not expected to be exposed to a heat flux greater than 12.5 kilowatts per square metre (kW/m²)

• All traditional timber framing products and systems can be used, along with the usual timber species and treated pine for decks (bearers and joists), balustrades, handrails, pergolas etc

• Further information is in the Timber Design Guide:- Building with timber in bushfire prone Areas – 04

BAL–12.5


Timber can be used normally for the following applications (no restrictions):

– Enclosed or Unenclosed subfloor space: • Subfloor supports (post, poles stumps)• Bearers and joists• Flooring

BAL–12.5



– Wall framing (studs and wall plates etc)– Internal timber (doors, joinery, wall linings, ceiling

linings, staircases, flooring over concrete slab etc.)

BAL–12.5



– Exterior wall cladding 400 mm or above from ground or slope less than 18 degrees – deck, roof

• Where exterior wall cladding is below 400 mm– Requires timbers to meet Appendix E2– Bushfire resisting timber

BAL–12.5


BAL–12.5


Definition of external horizontal surface– the ground, a deck, balcony, carport roof, awning, etc.– an angle less than 18 degrees to the horizontal– extending more than 110 mm in width from the door or window

BAL–12.5



– Timber Windows or Doors• Protected by screen or shutter

– Otherwise • Solid timber door

• Timber species from Appendix E

BAL–12.5



– Roof Framing• Main roof – under complying tiled or sheet roof

• Verandas, carports and awnings provided framing is protected by complying roofing material and a fibre cement ceiling linings

– Hand Rails and Balustrades • 125 mm or more from the building

BAL–12.5


Timber can be used as usual for following applications (no restrictions):

– Verandahs and Decks• Enclosed or unenclosed

– Supports, bearers and joists – no limitation– Decking

» More than 300 mm from glazing – no limits» Less than 300 mm from glazing

• Appendix E timber species• Bushfire resistant timber species

BAL–12.5


BAL–12.5


BAL–12.5: Appendix E Timbers

• Generally - any timber species that has a density of ≥750 kg/m3 (12% MC)

• For window and doors - timber species with a density at ≥650 kg/m3 (12% MC)


• BAL—LOW

• BAL—12.5

• BAL—19

• BAL—29

• BAL—40



BAL–19


• Increasing level ember attack• Burning debris ignited by wind-borne embers• The ‘19’ refers to an increasing heat flux, not greater than

19 kW/m²• All traditional timber framing products and systems can be

used, along with the usual timber species and treated pine for decks (bearers and joists), balustrades, handrails, pergolas etc

• All timber solutions for BAL-12.5 are applicable for BAL-19! NB: glazing requirements are different

BAL–19


• BAL—LOW

• BAL—12.5

• BAL—19

• BAL—29

• BAL—40



BAL–19


• Increasing level of ember attack and burning debris ignited by wind-borne embers.

• The ‘29’ refers to the chance of an increasing heat flux not greater than 29 kW/m²

• Naturally bushfire-resisting timbers and fire-retardant treated timbers are the only timbers to be used for exposed applications.

• For full window and glazed door protection, timber shutters (e.g. red ironbark) suitable to BAL–29 are available.

• Check external building materials and systems have been tested and comply with Standard requirements.

BAL–29



– Enclosed subfloor space • Subfloor supports (post, poles stumps)• Bearers and joists• Flooring

BAL–29



– Unenclosed subfloor space more than 400 mm above finished ground:• bearers and joists• flooring

– Unenclosed subfloor space less than 400 mm above finished ground:• Bearers, joists and flooring – bushfire resistant timbers

BAL–29



– Wall framing (studs and wall plates etc)

– Internal timber (doors, joinery, wall linings, ceiling linings, staircases, flooring over concrete slab etc)

BAL–29



–Timber windows or doors:• If a door is completely protected externally by a

complying screen

Otherwise: • Timber window or door to be bushfire-resisting

timber

BAL–29



– Roof framing• Main roof – under a complying tiled or sheet roof

• Verandas, carports and awnings, provided framing is protected by complying roofing material and fibre cement ceiling linings

– Hand rails and balustrades • 125 mm or more from the building

BAL–29


BAL–29: Bushfire Resisting Timber

• Bushfire-resisting timber can be:– Naturally bushfire resisting; or– Impregnated with fire-retardant chemicals; or– Covered by fire-retardant coatings or substrates

• Materials are under development - available soon• Always check to see if test reports nominate timber is

artificially weathered before the test if used in “non-protected” external application



• Natural bushfire-resisting timbers: – Must pass a prescribed

test in Appendix F AS3959– Blackbutt, red ironbark,

merbau (kwila), river red gum, silvertop ash, spotted gum, turpentine



• Impregnated with fire-retardant chemicals; or• Fire-retardant coatings or substrates• Materials are under development - available

soon• Always check to see if test reports nominate

timber is artificially weathered before the test if used in “non-protected” external application.



Protected:

30 deg

Exposed to weather

Protectedfrom weather

“and they are well detailed and maintained (painted or stained and kept well ventilated)”


• BAL—LOW

• BAL—12.5

• BAL—19

• BAL—29

• BAL—40



BAL–40


• Ember attack• Burning debris ignited by wind-borne embers• Increased likelihood of exposure to bushfire flames.• ‘40’ comes from an increasing heat flux not greater

than 40 kW/m²• System solutions: AS 1530 Part 8.1 - Radiant heat and

small flaming sources.

BAL–40




– Unenclosed subfloor • Bearers, joists and flooring protected by a non-

combustible lining e.g. fibre cement, roof sheet

BAL–40



– Wall framing (studs and wall plates etc.)– Internal timber (doors, joinery, wall linings, ceiling

linings, staircases, flooring over concrete slab etc.)

BAL–40


Timber can be used as usual for following applications (no restrictions):

– Timber windows • if a window assembly is completely protected by a complying

bushfire shutter

– Timber doors• If a door is completely protected externally by a complying screen

Otherwise• Meets AS1530 8.1 test

BAL–40



– Roof framing• Main roof – under complying tiled or sheet roof

• Verandas, carports and awnings provided framing is protected by complying roofing material and a fibre cement ceiling linings


BAL–40


For a timber solution in BAL–40 all other applications require the system tested and comply to:

AS 1530 Part 8.1 - Radiant heat and small flaming sources

BAL–40


BAL–40: Standard Test Configurations


BAL–40: Principle of AS 1530.8.1(bushfire testing standard)


Time (mins) -15s to 0 0-1010-

20mins20-

60minsExposure Condition

Ignited Crib

Radiation

Small Gas Flame

Performance Criteria

Formation of through-gaps greater than 3 mm

Sustained flaming for 10 s on the non-fire side

Flaming on the fire-exposed side at the end of the 60 min test period

Radiant heat flux 365 mm from the non-fire side exceeding 15 kW/m2

Mean and maximum temperature rises greater than 140 K and 180 K

Radiant heat flux 250 mm from the specimen, greater than 3 kW/m2 between 20 min and 60 minMean and maximum temperature of internal faces

exceed 250°C and 300°C respectively

BAL–40: Summary of Exposure and Criteria AS 1530.8


BAL–40: AS1530 8.1 Complying Products


• BAL—LOW

• BAL—12.5

• BAL—19

• BAL—29

• BAL—40



BAL–FZ


BAL–FZ

• Very bushfire-prone! • Direct exposure risk to flames• ember attack • heat flux greater than 40 kW/m²• building envelope needs extra reinforcement in the most

vulnerable places:– leading edges of roofs– glazed areas and– decks where flames, embers and hot air can penetrate.

• For system solutions: AS 1530 Part 8.2 - Large flaming sources.


BAL–FZ



– Unenclosed subfloor • Bearers, joists and flooring protected by a FRL 30/30/30

or 30 minutes resistant to incipient spread of fire system


BAL–FZ


– Wall framing (studs and wall plates etc)– Internal timber (doors, joinery, wall linings, ceiling

linings, staircases, flooring over concrete slab etc)


BAL–FZ


– Timber windows • if a window assembly is completely protected by a

complying bushfire shutter to AS1530 8.2

– Timber doors• If a door is completely protected externally by a

complying screen to AS1530 8.2


BAL–FZ


– Roof framing• Tested to AS1530 8.2 (more later)



For a timber solution in BAL–FZ all other applications require the system tested and comply to:

AS 1530 Part 8.2 - Large flaming sources

BAL–FZ


Exposure Condition

Time (mins) 0-30 30-60 60-90

Performance Criteria

Formation of through-gaps greater than 3 mm

Flaming on the fire-exposed face

Radiant heat flux 365 mm from the non-fire side exceeding 15 kW/m2

Mean and maximum non fire side temperature rises greater than 140 K and 180 K

Radiant heat flux 250 mm from the specimen, greater than 3 kW/m2

Mean and maximum temperature of internal faces exceed 250°C and 300°C respectively

0

200

400

600

800

1000

0 30

Fu

rnac

e T

em

pe

ratu

re (

C)

BAL–FZ: Summary of Exposure and Criteria AS 1530.8.2


75mm

BAL–FZ: Timber Roof Systems


Up to BAL 29: Timber is permitted for use in various applications if it is:

– “Bushfire-resisting timber” (natural and treated with fire-retardant)

Internal

External

All BALs: All timber and wood products permitted (e.g. studs, wall plates, staircases, ceiling lining, flooring etc)

Up to BAL 19: Common natural timber species with medium to high density utilisable for all applications

Beyond BAL 29: Test elements or entire building systems (e.g. windows, doors, service penetrations, eaves/gutter details, roofs, verandas and decks, exposed structural elements)

Summary of Timber use up to BAL 29


Summary of Timber use in BAL—40 and FZ

• Systems approach• BAL—40:

– Must comply with AS 1530.8.1• BAL—FZ:

– Must comply with AS 1530.8.2; or– Designated FRLs


Standard Update– AS 3959—2009 has now been issued and adopted

by the BCA 2010 with variations in NSW

– BAL—FZ timber-framed roof systems have been tested and solutions are available. These including sheet metal and concrete tile coverings.

Going Forward ….


NSW: Rural Fire Service (RFS)

• Planning for Bushfire Protection: Appendix 3 - Amended in May 2010

• BAL—29 requirements for BALs 12.5 & 19 for main flooring and deck structures: – Sub-floors supports– Sub-floor framing and flooring

• NSW RFS don’t recognize AS 3959 BAL—FZ DTS solutions


NSW: Rural Fire Service (RFS)

• Always check NSW RFS information for interpretation to the standard as there are many

• Advice:– Always try and safely lower the BAL before trying

to provide alternative solutions


Conclusion

Timber can be effectively used in buildings that are located in bushfire-prone areas by complying to AS 3959


Learn more about wood at UTAS

Centre for Sustainable Architecture with WoodGraduate Certificate in Timber (Processing & Building)• 4 units, part time, onlineAreas covered include:• Wood science• Design for durability and service for life• Timber as a renewable resource• Sustainable design and construction• Engineered wood products• International technologies and developments• Plus, selected topics of individual interestMore information: Associate Professor Greg Nolan (03) 6324 4478 or [email protected]


Technical Design Guide:Building with Timber in Bushfire-prone areas

More Information

building with timber in bushfire-prone areas - lunch & learn

Self Improvement