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Design for Durability
Design for Durability ©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 enquiries@arch.utas.edu.auwww.csaw.utas.edu.au
Design for Durability ©FWPA 2011
Learning Objectives
After this presentation you should be able to:
– Understand the key timber hazards– Understand the fundamentals of timber durability– Understand the design factors that need to be
considered to achieve satisfactory timber durability performance.
For architects - AACA Competency:– Design– Documentation Elliot River Fire Tower, QLD
Design for Durability ©FWPA 2011
Presentation Contents
• Design process• Main timber hazards:
– Decay, insects, corrosion of fasteners & weathering
• Natural durability & treatment• Design detailing• Other hazards:
– Marine borer, chemicals, fire
Design for Durability ©FWPA 2011
Design Process: 1
Design for Durability ©FWPA 2011
Design Process: 2
Design for Durability ©FWPA 2011
Design Process: Discussion
• To determine performance requirements you may need to consider:– Target life expectancy (minimum regulatory, standards or contractual
requirements)– Level of reliability (life safety, cost or consequence of failure)– Costs (initial vs ongoing maintenance, repair or replacement)
• In the context of building regulations, the Building Code of Australia (BCA) has implicit durability performance expectations:– see Durability in Buildings – Guideline Document at www.abcb.gov.au
Design for Durability ©FWPA 2011
Design Process: BCA Design Life Guideline
For normal buildings, the design life for most structural timber members is 50 years and for moderately accessible members 15 years.
Design for Durability ©FWPA 2011
Timber Hazards
Design for Durability ©FWPA 2011
From a durability perspective, the main hazards that need to be considered are:-• In-ground and above ground decay (including hazard or ‘H’ levels etc)
• Insects (inc. termites)
• Corrosion (of fasteners)
• Weathering
• Marine borers
• Chemical degradation (not usually an issue), and
• Fire (subject of other resources) Above ground durability trials, Beerburrum QLD
Hazard Classes (‘H’)apply to these 3 agents
Timber Hazards
Design for Durability ©FWPA 2011
The performance of timber with respect to decay is:
• Highly related to the presence of free moisture in the wood above 20% moisture content (MC)
• Different with respect to decay when in-ground vs above ground (different timber durability ratings apply in each case)
Macro climatic decay hazard maps have been developed to address these fundamental differences
In-ground durability trials‘Wedding Bells’, Nth NSW
Decay
Design for Durability ©FWPA 2011
Decay: In-Ground Decay Hazard Zones
In-ground decay hazard zones for Australia(Zone D has the greatest in-ground decay potential)
Design for Durability ©FWPA 2011
Decay: Above Ground Decay Hazard Zones
Above ground decay hazard zones for Australia (Zone D has the greatest decay hazard potential)
Design for Durability ©FWPA 2011
• Moisture > 20% MC in wood• Oxygen• Temperature >25o to <40o (ideal)• Food
Remove any of these four key elements and fungal growth is either stopped or retarded
e.g. preservative treatment renders the ‘food’ (wood) immune
To thrive fungi need:
Decay: Causes
Design for Durability ©FWPA 2011
Wood in an anaerobic condition (i.e. without access to oxygen) lasts indefinitelye. g. Kauri dug out from the ground after 10,000 to 50,000 years
45,000 year old surf board
Decay: Absence of Causes Example
Design for Durability ©FWPA 2011
A decay Hazard Class system has been developed for Australia that:• Primarily relates to the use of preservative treated timber
but is also used with natural durability
• Increases in severity from H1 to H6 Class
• Does not, at this stage, account for macro-climatic variations, but these can be addressed in the durability design process
Decay: Hazard Classes
Design for Durability ©FWPA 2011
H1 - fully protected indoors, not termites resistant
H2 – fully protected indoors plus termites resistant
H3 - exposed to weather, above ground,well ventilated
H4 - in ground (landscaping)
H5 - in ground (more critical)
H6 - marine piles30 deg
Exposed to weather
Protectedfrom weather
Decay: Hazard Classes (H1 – H6)
Hazard Classes as per AS 1604 for specifying preservative treatment
Decay: Hazard Classes
Design for Durability ©FWPA 2011
• Natural Durability Ratings apply to heartwood (or true wood) only – not sapwood
• Only sapwood can be effectively treated
• Limit non-durable timber to 20% cross-section (max)
Bark
Sapwood
Heartwood
Log Sawn Section
Heartwoodcannot be treated thereforedictated by natural durability class
Sapwoodcan betreated toappropriate‘H’ level
Decay: Resistance to Decay
Design for Durability ©FWPA 2011
Class 1 (highly durable) – ironbark, tallowwood
Class 2 (durable)– spotted gum, blackbutt
Class 3 (moderately durable)– brush box, rose gum
Class 4 (non durable)– all sapwood for all species, Victorian ash, Tasmania oak,
Oregon, radiata pine, slash pine, hoop pine
Examples of In-ground Durability
Design for Durability ©FWPA 2011
For an extensive list of natural durability ratings check AS 5604 Timber – Natural durability ratings, or download the Timber Service Life Design Guide, at www.woodsolutions.com.au
Class 1 (highly durable) – ironbark, tallowwood, river red gum, spotted
gum, blackbutt, cypress
Class 2 (durable)– rose gum, jarrah, yellow stringybark, Sydney blue
gum , brush box
Class 3 (moderately durable)– Victorian ash, Tasmanian oak, huon pine
Class 4 (non durable)– all sapwood for all species, Oregon, radiata pine,
slash pine, hoop pine
Examples of Above Ground, Exposed Durability
Design for Durability ©FWPA 2011
Natural Durability
Class
Heartwood Service Life (years) (1) (2)
H1 Fully Protected
H3 Above Ground exposed
H5 In- Ground
Class 1 50+ >40 25+
Class 2 50+ 15 – 40 15 - 25
Class 3 50+ 7 – 15 5 - 15
Class 4 50+ 0 - 7 < 5
(1) Based on in-ground graveyard stake trials and above ground ‘L’ joint trials
(2) Greater service life can be achieved with larger sections, relevant maintenance and/or preservative treatment
Service Life of Heartwood
Design for Durability ©FWPA 2011(1) Incising the timber may assist to obtain an envelope treatment
Preservative Treat in accordance with AS1604 or Timber Marketing Act
(NSW)
Immunise Lyctus Susceptible Sapwood
No Treatment required
When to Treat Timber?
Design for Durability ©FWPA 2011
TYPE HAZARD LEVEL
H1 H2 H3 H4 H5 H6
Water Boron ☺ ☺ CCA ☺ ☺ ☺ ☺ ☺ ☺(1) Copper
Azole ☺ ☺ ☺ ☺ ☺
ACQ ☺ ☺ ☺ ☺ ☺ Solvent LOSP ☺ ☺ ☺ Double CCA +
Creosote ☺
(1) Southern waters only
Suitable Preservatives
Design for Durability ©FWPA 2011
The main insects of commercial significance to the durability performance of timber are:– Lyctus beetles, and– Termites
Lyctus beetles only attack the sapwood of some susceptible hardwood species.
Termites can attack any cellulose based material and are more active and prevalent in northern Australia.
Furniture beetles are also present in Australia, but are low risk.
Termite damage to a non-termite resistant post in ground contact
Insects
Design for Durability ©FWPA 2011
There are two types of termites that can cause commercial damage to timber
• Drywood termites
– Do not require contact with the ground. They are present in and north of Cooktown (QLD). Naturally termite resistant or preservative treated timber should be used where drywood termites are prevalent.
• Subterranean termites
– Are by far the most significant insect pest for timber. They require contact with the ground (for moisture), therefore a range of termite management options are available including isolation from the ground.
Insects: Termites
Design for Durability ©FWPA 2011
Termite hazard zones for Australia (Zone D has the greatest termite hazard)
Insects: Termite Hazard Zones
Design for Durability ©FWPA 2011
Timber structures are best protected from insect damage through:
• correct design and construction practices• accurate specification and supply• appropriate species selection• preservative treatment (where necessary)
Timbers natural resistance to different insects varies widely from species to species
AS 5604 Timber – Natural Durability Ratings provides lyctus and termite resistance ratings for a wide range of species, as well as other natural durability ratings
Insects: Resistance
Design for Durability ©FWPA 2011
Examples from AS 5604
Design for Durability ©FWPA 2011
• Australian Standard grading rules, for decorative and structural timber, limit the amount of lyctus susceptible sapwood that can occur in different products. In addition, susceptible sapwood can be preservative treated to make it ‘immune’
• In NSW, consumer protection legislation prohibits the sale of lyctus susceptible timber, which if present, should be treated
Insects: Lyctus Beetle Protection
Design for Durability ©FWPA 2011
• The Building Code (BCA) requires termite protection for buildings in designated termite prone areas
• Effectively all States/Territories except Tasmania and some Victorian local authorities, are designated termite prone areas
• In termite prone designated areas, the BCA provides two options:– Either all ‘primary structural elements’ (definition varies from State to
State) are to be termite resistant (for timber refer to AS 3660.1), OR– Termite protection shall be provided in accordance with AS 3660.1 which
provides options and combinations including isolation, termite shields, physical and chemical soil barriers, termite resistant materials etc.
Insects: Termite Protection
Design for Durability ©FWPA 2011
There are two types of corrosion:
Embedded
Typical installation of fasteners embedded in wood subjected to corrosion
Atmospheric
Typical fastener installation subjected to atmospheric corrosion
Note: red marks denote where corrosion is possible
Corrosion: Fasteners
Design for Durability ©FWPA 2011
Most metal fasteners for timber have a part that is in the timber and a part exposed to the atmosphere
Embedded portion
Corrosion of the embedded part will be dictated by:
• the moisture content of the timber
• the natural pH of the timber
• electrolytic action that may occur due to the presence of preservatives such as copper in CCA or ACQ treated timber
The sapwood in this pole has been treated with CCA and is causing accelerated corrosion of the galvanized plate
Corrosion: Fasteners
Design for Durability ©FWPA 2011
Exposed portion
Corrosion of the exposed portion of the fasteners will be influenced by:
• all of the embedded factors
• air-borne contaminants such as salt or other chemicals
Macro climatic hazard influences on corrosion need to differentiate between embedded and atmospheric corrosion, and apply separate hazard maps
The sapwood in this pole has been treated with CCA and is causing accelerated corrosion of the galvanized plate
Corrosion: Fasteners
Design for Durability ©FWPA 2011
Corrosion of fasteners needs to be considered for the following reasons:
– Breakdown of wood– Integrity of fasteners and connections– Aesthetics (rusting, stains etc.)
40 year old hot dipped galv. bolts in cross-arms
Corrosion: Fasteners
Design for Durability ©FWPA 2011
Hazard zones for embedded corrosion (Zone C is the most hazardous)
Corrosion: Hazard Zones – Embedded Corrosion
Design for Durability ©FWPA 2011
Coastal Zones Related to corrosion due to airborne salt(Zone E has the greatest hazard)
Corrosion: Hazard Zones – Atmospheric Corrosion
Design for Durability ©FWPA 2011
Resistance to corrosion is best provided by selecting and using material with the required resistance to corrosion, appropriate to the intended life of the structure – refer to the following table.
Cross-arm King bolt (hot dipped galvanized)after 35 years exposurein power pole.
Corrosion: Resistance
Design for Durability ©FWPA 2011
Corrosion: Resistance (cont.)
Design for Durability ©FWPA 2011
To help reduce deterioration of timber around metal fasteners where moisture is present, consider:
– Avoiding joint details that trap moisture
– Using non-corrosive or protected metals (galvanized, coated, stainless steel or monel metals)
– Avoiding placing dissimilar metals in contact with each other (e.g. copper, as in CCA or ACQ treatments, with zinc, as in galvanized coatings)
– Greasing, coating or sheathing fasteners in contact with CCA or ACQ treated timber with shrink wrap plastic or bituminous or epoxy paints
– Countersink and plug or ‘stop’ fasteners
Polymer coated screws.
Corrosion: Resistance (cont.)
Design for Durability ©FWPA 2011
Unprotected timber exposed to moisture and sunlight will undergo physical and chemical changes known as weathering
Weathering is the result of:
– surface erosion (this is slow – 6 mm to 13 mm per century)
– wetting/drying (causing swelling and shrinking)
– chemical change (caused by sunlight and oxygen)
– freezing and thawing in alpine areas
20 year old deck and retaining wall
Weathering
Design for Durability ©FWPA 2011
Unpainted or unfinished timber, when exposed for an extended period will:
– discolour on the surface
– check, crack and splinter
– become rough on the surface
Weathering may cause cupping and warping and colours to bleach to a silvery grey or show surface stains or mould growth
Horizontal surfaces are more prone to weathering than vertical surfaces
Weathering
Design for Durability ©FWPA 2011
Although hazard maps for weathering have not yet been developed, it is generally accepted that the effects of weathering are more severe and accelerated in harsher climates with greater extremes in temperature and rainfall
Note the delineation in degree of weathering owing to shelter provided provided by the roof.
Weathering (cont.)
Design for Durability ©FWPA 2011
Protection from weathering can be maximised by:
– Applying appropriate finishes, including paints, stains and water repellants
– Regular maintenance with appropriate finishes
– Architectural and design detailing that specifies overhangs, capping, verandahs and shading
– Considering ventilation, water shedding and drainage during design and construction
Weathering: Resistance
Design for Durability ©FWPA 2011
The following should be considered in respect of finishes:
– Pale colours absorb less heat, therefore drying effects and accelerated decay due to raised temperature are minimised
– Oil based paints are better ‘vapour’ barriers than acrylics
– Paint systems should include quality primers and undercoats – they are designed to seal the timber and provide a key for top-coats
– Stains and water repellants require more frequent application to maintain their effectiveness
– Sawn surfaces provide a better ‘key’ for stains and water repellants than dressed surfaces – particularly denser species
The following table provides a summary of different finishes and maintenance.
Weathering: Paints, Stains and Water Repellents
Design for Durability ©FWPA 2011
NOTES:(a) Table is compilation of
data from many researchers
(b) Using top quality acrylic latex paints
(c) With or without added preservatives for mildew/mould
(d) Common water repellant preservatives including copper naphthenate
Weathering: Paints, Stains and Water Repellents
Design for Durability ©FWPA 2011
Appropriate design detailing can help reduce the impact of weathering and improve durability performance by:
– Shielding timber with overhangs, pergolas, capping, flashing, fascias and barges
– Isolating timber using sarking, cladding and DPC’s
– Avoiding moisture traps by using adequate ventilation, free draining joints and water shedding surfaces, particularly when end grain is involved
– Ensuring ventilation and insulation addresses condensation potential in warm and cold climates by using vapour permeable sarking or foil
Weathering: Design Detailing
Design for Durability ©FWPA 2011
Weathering: Good Details
Design for Durability ©FWPA 2011
Weathering Hazards: Good Details
Design for Durability ©FWPA 2011
• Make allowance for shrinkage and minimise shrinkage restraint to avoid splitting
• For weather exposed horizontal members (decking, handrails etc.) limit open defects on the top surface and keep members ‘stocky’ – preferred breadth to depth ratio approximately 3:1 to 4:1
• Limit the use of wide boards – narrow boards expand and shrink less in response to moisture changes
Weathering: Other Detailing
Design for Durability ©FWPA 2011
The satisfactory performance of timber structures will often depend on adequate maintenance programs.
Maintenance regimes for finishes have already been covered.
Other maintenance issues to consider are:
– Termite barriers, inspection and replenishment of treatments
– Maintaining sub-floor ventilation
– Tightening bolts and fasteners
– Re-applying supplementary preservatives and sealants
– Cleaning - sweep or blow, don’t hose away (moisture issues)
Maintenance
Design for Durability ©FWPA 2011
Where timber (piles, braces etc.) is in contact with marine waters (ocean, bay and tidal), specific hazards such as marine borers and organisms need to be considered.
Factors that affect the level of marine borer hazard include:
− Macro-climatic hazard zone (warmer waters, higher hazard)
− Water salinity
− Sheltering effects
− Construction detailsAquaculture is a valuable wood use.
Marine Borers
Design for Durability ©FWPA 2011
Marine borer hazard zones (Zone G is the most hazardous)
Marine Borers: Hazard Zones
Design for Durability ©FWPA 2011
Marine piles and timber in marine contact are best protected by:– Using species with high natural resistance such as
turpentine or in cooler southern waters, swamp box, river red gum or white mahogany
– Using preservative treated timbers (‘H6’) with wide sapwood bands such as spotted gum and plantation softwoods
– Using mechanical barriers and floating collars
Piles encased in sand filled concrete pipe
Marine Borers: Resistance
Design for Durability ©FWPA 2011
AS 5604 provides an extensive list of species with their marine borer resistances where known – as seen in the extract below.
Marine Borers: Resistance
Design for Durability ©FWPA 2011
• Timber is resistant to mild acids but strong acids (pH<2) and alkalis (pH>10) can cause degradation.
• Timber is ideally suited to special applications such as:
– swimming pool buildings
– chemical storage
– reservoir roofs etc.
Chemicals
Design for Durability ©FWPA 2011
• Fire is covered by two separate presentations:
– Using Wood in Bushfire-prone Areas
– Fire Safety and Performance of Wood in Multi-Residential and Commercial Buildings
Fire
Design for Durability ©FWPA 2011
• Timber has the ability to deliver design service life in a wide range of applications
• There are significant issues that need to be considered to appropriately design, specify and detail timber structures to ensure satisfactory durability
• These issues are well known and understood
Conclusion
Design for Durability ©FWPA 2011
Technical Design GuideTimber Service Life Design
More Information
Design for Durability ©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 enquiries@arch.utas.edu.auwww.csaw.utas.edu.au