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Design for Durability - Lunch & Learn


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This presentation, which includes termite management techniques, will help improve the specification and design performance of wood in construction.

The presentation uses structural engineering criteria, years of experience and field trials and predictive modelling to provide guidance on methods to increase the service life of woods used in Australia.

Also covered are the affects of natural variation between wood species, the effectiveness of wood preservative treatments and climatic conditions.

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Design for Durability - Lunch & Learn

  1. 1. Design for Durability<br />
  2. 2. Learn more about wood at UTAS<br />Centre for Sustainable Architecture with Wood<br />Graduate Certificate in Timber (Processing & Building)<br />4 units, part time, online<br />Areas covered include:<br />Wood science<br />Design for durability and service for life<br />Timber as a renewable resource<br />Sustainable design and construction<br />Engineered wood products<br />International technologies and developments<br />Plus, selected topics of individual interest<br />More information: Associate Professor Greg Nolan <br />(03) 6324 4478 or<br />
  3. 3. Learning Objectives<br />After this presentation you should be able to:<br />Understand the key timber hazards<br />Understand the fundamentals of timber durability<br />Understand the design factors that need to be considered to achieve satisfactory timber durability performance.<br />For architects - AACA Competency:<br />Design<br />Documentation<br />Elliot River Fire Tower, QLD<br />
  4. 4. Presentation Contents<br />Design process<br />Main timber hazards:<br />Decay, insects, corrosion of fasteners & weathering<br />Natural durability & treatment<br />Design detailing<br />Other hazards:<br />Marine borer, chemicals, fire<br />
  5. 5. Design Process: 1<br />
  6. 6. Design Process: 2<br />
  7. 7. Design Process: Discussion<br />To determine performance requirements you may need to consider:<br />Target life expectancy (minimum regulatory, standards or contractual requirements)<br />Level of reliability (life safety, cost or consequence of failure)<br />Costs (initial vs ongoing maintenance, repair or replacement)<br />In the context of building regulations, the Building Code of Australia (BCA) has implicit durability performance expectations:<br />see Durability in Buildings – Guideline Document at<br />
  8. 8. Design Process: BCA Design Life Guideline<br />For normal buildings, the design life for most structural timber members is 50 years and for moderately accessible members 15 years.<br />
  9. 9. Timber Hazards<br />
  10. 10. From a durability perspective, the main hazards that need to be considered are:-<br /><ul><li>In-ground and above ground decay (including hazard or ‘H’ levels etc)
  11. 11. Insects (inc. termites)
  12. 12. Corrosion (of fasteners)
  13. 13. Weathering
  14. 14. Marine borers
  15. 15. Chemical degradation (not usually an issue), and
  16. 16. Fire (subject of other resources) </li></ul>Hazard Classes (‘H’)<br />apply to these 3 agents <br />Above ground durability trials, Beerburrum QLD<br />Timber Hazards<br />
  17. 17. Decay<br /> The performance of timber with respect to decay is:<br /><ul><li>Highly related to the presence of free moisture in the wood above 20% moisture content (MC)
  18. 18. Different with respect to decay when in-ground vs above ground (different timber durability ratings apply in each case)</li></ul>Macro climatic decay hazard maps have been developed to address these fundamental differences<br />In-ground durability trials<br />‘Wedding Bells’, Nth NSW<br />
  19. 19. Decay: In-Ground Decay Hazard Zones<br />In-ground decay hazard zones for Australia<br />(Zone D has the greatest in-ground decay potential)<br />
  20. 20. Decay: Above Ground Decay Hazard Zones<br />Above ground decay hazard zones for Australia <br />(Zone D has the greatest decay hazard potential)<br />
  21. 21. Decay: Causes<br />To thrive fungi need:<br /><ul><li>Moisture > 20% MC in wood
  22. 22. Oxygen
  23. 23. Temperature >25o to <40o (ideal)
  24. 24. Food</li></ul>Remove any of these four key elements and fungal growth is either stopped or retarded<br />e.g. preservative treatment renders the ‘food’ (wood) immune<br />
  25. 25. Decay: Absence of Causes Example<br />Wood in an anaerobic condition (i.e. without access to oxygen) lasts indefinitely<br />e. g. Kauri dug out from the ground after 10,000 to 50,000 years<br />45,000 year old surf board<br />
  26. 26. Decay: Hazard Classes<br />A decay Hazard Class system has been developed for Australia that:<br /><ul><li>Primarily relates to the use of preservative treated timber but is also used with natural durability
  27. 27. Increases in severity from H1 to H6 Class
  28. 28. Does not, at this stage, account for macro-climatic variations, but these can be addressed in the durability design process</li></li></ul><li>Decay: Hazard Classes (H1 – H6)<br />H1 - fully protected indoors, not termites resistant<br />H2 – fully protected indoors plus termites resistant<br />H3 -exposedto weather, aboveground,<br /> well ventilated<br />H4 - in ground (landscaping)<br />H5- in ground (more critical)<br />H6 - marine piles<br />30 deg<br />Protected<br />from weather<br />Exposed to weather<br />
  29. 29. Decay: Hazard Classes<br />Hazard Classes as per AS 1604 for specifying preservative treatment<br />
  30. 30. Decay: Resistance to Decay<br />Bark<br />Sapwood<br />can be<br />treated to<br />appropriate<br />‘H’ level<br />Heartwood<br />cannot be <br />treated therefore<br />dictated by <br />natural durability <br />class<br />Sapwood<br />Sawn Section<br />Heartwood<br />Log<br />Natural Durability Ratings apply to heartwood (or true wood) only – not sapwood<br />Only sapwood can be effectively treated<br />Limit non-durable timber to 20% cross-section (max)<br />
  31. 31. Examples of In-ground Durability <br />Class 1 (highly durable) <br />ironbark, tallowwood<br />Class 2 (durable)<br />spotted gum, blackbutt<br />Class 3 (moderately durable)<br />brush box, rose gum<br />Class 4 (non durable)<br />all sapwood for all species, Victorian ash, Tasmania oak, Oregon, radiata pine, slash pine, hoop pine<br />
  32. 32. Examples of Above Ground, Exposed Durability <br />Class 1 (highly durable) <br /><ul><li>ironbark, tallowwood, river red gum, spotted gum, blackbutt, cypress</li></ul>Class 2 (durable)<br /><ul><li>rose gum, jarrah, yellow stringybark, Sydney blue gum , brush box</li></ul>Class 3 (moderately durable)<br /><ul><li>Victorian ash, Tasmanian oak, huon pine</li></ul>Class 4 (non durable)<br /><ul><li>all sapwood for all species, Oregon, radiata pine, slash pine, hoop pine</li></ul>For an extensive list of natural durability ratings check AS 5604 Timber – Natural durability ratings, or download the Timber Service Life Design Guide, at<br />
  33. 33. Service Life of Heartwood<br />Based on in-ground graveyard stake trials and above ground ‘L’ joint trials<br />Greater service life can be achieved with larger sections, relevant maintenance and/or preservative treatment<br />
  34. 34. When to Treat Timber?<br />No Treatment required<br />Immunise Lyctus Susceptible Sapwood<br />Preservative Treat in accordance with AS1604 or Timber Marketing Act (NSW)<br />(1) Incising the timber may assist to obtain an envelope treatment<br />
  35. 35. Suitable Preservatives<br />(1) Southern waters only<br />
  36. 36. Insects<br />The main insects of commercial significance to the durability performance of timber are:<br /><ul><li>Lyctus beetles, and
  37. 37. Termites</li></ul>Lyctus beetles only attack the sapwood of some susceptible hardwood species.<br />Termites can attack any cellulose based material and are more active and prevalent in northern Australia. <br />Furniture beetles are also present in Australia, but are low risk.<br />Termite damage to a non-termite resistant post in ground contact<br />
  38. 38. Insects: Termites<br />There are two types of termites that can cause commercial damage to timber <br />Drywood termites<br />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.<br />Subterranean termites <br />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.<br />
  39. 39. Insects: Termite Hazard Zones<br />Termite hazard zones for Australia <br />(Zone D has the greatest termite hazard)<br />
  40. 40. Insects: Resistance<br />Timber structures are best protected from insect damage through:<br /><ul><li> correct design and construction practices
  41. 41. accurate specification and supply
  42. 42. appropriate species selection
  43. 43. preservative treatment (where necessary)</li></ul>Timbers natural resistance to different insects varies widely from species to species<br />AS 5604 Timber – Natural Durability Ratingsprovides lyctus and termite resistance ratings for a wide range of species, as well as other natural durability ratings<br />
  44. 44. Examples from AS 5604<br />
  45. 45. Insects: Lyctus Beetle Protection<br />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’<br />In NSW, consumer protection legislation prohibits the sale of lyctus susceptible timber, which if present, should be treated<br />
  46. 46. Insects: Termite Protection<br />The Building Code (BCA) requires termite protection for buildings in designated termite prone areas<br />Effectively all States/Territories except Tasmania and some Victorian local authorities, are designated termite prone areas<br />In termite prone designated areas, the BCA provides two options:<br />Either all ‘primary structural elements’ (definition varies from State to State) are to be termite resistant (for timber refer to AS 3660.1), OR<br />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. <br />
  47. 47. Corrosion: Fasteners<br />There are two types of corrosion:<br />Embedded<br />Typical installation of fasteners embedded in wood subjected to corrosion<br />Note: red marks denote where corrosion is possible<br />Atmospheric<br />Typical fastener installation subjected to atmospheric corrosion<br />
  48. 48. Most metal fasteners for timber have a part that is in the timber and a part exposed to the atmosphere <br />Embedded portion<br />Corrosion of the embedded part will be dictated by: <br /><ul><li>the moisture content of the timber
  49. 49. the natural pH of the timber
  50. 50. electrolytic action that may occur due to the presence of preservatives such as copper in CCA or ACQ treated timber</li></ul>The sapwood in this pole has been treated with CCA and is causing accelerated corrosion of the galvanized plate<br />Corrosion: Fasteners<br />
  51. 51. Exposed portion<br />Corrosion of the exposed portion of the fasteners will be influenced by:<br /><ul><li>all of the embedded factors
  52. 52. air-borne contaminants such as salt or other chemicals</li></ul>Macro climatic hazard influences on corrosion need to differentiate between embedded and atmospheric corrosion, and apply separate hazard maps<br />The sapwood in this pole has been treated with CCA and is causing accelerated corrosion of the galvanized plate<br />Corrosion: Fasteners<br />
  53. 53. Corrosion: Fasteners<br />Corrosion of fasteners needs to be considered for the following reasons:<br />Breakdown of wood<br />Integrity of fasteners and connections<br />Aesthetics (rusting, stains etc.)<br />40 year old hot dipped galv. bolts in cross-arms <br />
  54. 54. Corrosion: Hazard Zones – Embedded Corrosion<br />Hazard zones for embedded corrosion <br />(Zone C is the most hazardous)<br />
  55. 55. Corrosion: Hazard Zones – Atmospheric Corrosion<br />Coastal Zones Related to corrosion due to airborne salt<br />(Zone E has the greatest hazard)<br />
  56. 56. Corrosion: Resistance<br />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.<br />Cross-arm King bolt <br />(hot dipped galvanized)<br />after 35 years exposure<br />in power pole.<br />
  57. 57. Corrosion: Resistance (cont.)<br />
  58. 58. To help reduce deterioration of timber around metal fasteners where moisture is present, consider:<br />Avoiding joint details that trap moisture<br />Using non-corrosive or protected metals (galvanized, coated, stainless steel or monel metals) <br />Avoiding placing dissimilar metals in contact with each other (e.g. copper, as in CCA or ACQ treatments, with zinc, as in galvanized coatings)<br />Greasing, coating or sheathing fasteners in contact with CCA or ACQ treated timber with shrink wrap plastic or bituminous or epoxy paints<br />Countersink and plug or ‘stop’ fasteners<br />Polymer coated screws.<br />Corrosion: Resistance (cont.)<br />
  59. 59. Weathering<br />Unprotected timber exposed to moisture and sunlight will undergo physical and chemical changes known as weathering<br />Weathering is the result of:<br /><ul><li> surface erosion (this is slow – 6 mm to 13 mm per century)
  60. 60. wetting/drying (causing swellingand shrinking)
  61. 61. chemical change (caused by sunlight and oxygen)
  62. 62. freezing and thawing in alpine areas</li></ul>20 year old deck and retaining wall<br />
  63. 63. Unpainted or unfinished timber, when exposed for an extended period will:<br /><ul><li>discolour on the surface
  64. 64. check, crack and splinter
  65. 65. become rough on the surface</li></ul>Weathering may cause cupping and warping and colours to bleach to a silvery grey or show surface stains or mouldgrowth<br />Horizontal surfaces are more prone to weathering than vertical surfaces<br />Weathering<br />
  66. 66. Weathering (cont.)<br />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<br />Note the delineation in degree of weathering owing to shelter provided provided by the roof.<br />
  67. 67. Weathering: Resistance<br />Protection from weathering can be maximised by:<br />Applying appropriate finishes, including paints, stains and water repellants<br />Regular maintenance with appropriate finishes<br />Architectural and design detailing that specifies overhangs, capping, verandahs and shading<br />Considering ventilation, water shedding and drainage during design and construction<br />
  68. 68. Weathering: Paints, Stains and Water Repellents<br />The following should be considered in respect of finishes:<br /><ul><li>Pale colours absorb less heat, therefore drying effects and accelerated decay due to raised temperature are minimised
  69. 69. Oil based paints are better ‘vapour’ barriers than acrylics
  70. 70. Paint systems should include quality primers and undercoats – they are designed to seal the timber and provide a key for top-coats
  71. 71. Stains and water repellants require more frequent application to maintain their effectiveness
  72. 72. Sawn surfaces provide a better ‘key’ for stains and water repellants than dressed surfaces – particularly denser species</li></ul>The following table provides a summary of different finishes and maintenance.<br />
  73. 73. NOTES:<br />Table is compilation of data from many researchers<br />Using top quality acrylic latex paints<br />With or without added preservatives for mildew/mould<br />Common water repellant preservatives including copper naphthenate<br />Weathering: Paints, Stains and Water Repellents<br />
  74. 74. Weathering: Design Detailing<br />Appropriate design detailing can help reduce the impact of weathering and improve durability performance by: <br />Shielding timber with overhangs, pergolas, capping, flashing, fascias and barges<br />Isolating timber using sarking, cladding and DPC’s<br />Avoiding moisture traps by using adequate ventilation, free draining joints and water shedding surfaces, particularly when end grain is involved<br />Ensuring ventilation and insulation addresses condensation potential in warm and cold climates by using vapour permeable sarking or foil<br />
  75. 75. Weathering: Good Details<br />
  76. 76. Weathering Hazards: Good Details<br />
  77. 77. Weathering: Other Detailing<br />Make allowance for shrinkage and minimise shrinkage restraint to avoid splitting<br />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<br />Limit the use of wide boards – narrow boards expand and shrink less in response to moisture changes<br />
  78. 78. Maintenance<br />The satisfactory performance of timber structures will often depend on adequate maintenance programs.<br />Maintenance regimes for finishes have already been covered.<br />Other maintenance issues to consider are:<br />Termite barriers, inspection and replenishment of treatments<br />Maintaining sub-floor ventilation<br />Tightening bolts and fasteners <br />Re-applying supplementary preservatives and sealants<br />Cleaning - sweep or blow, don’t hose away (moisture issues)<br />
  79. 79. Marine Borers<br />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. <br />Factors that affect the level of marine borer hazard include:<br /><ul><li> Macro-climatic hazard zone (warmer waters, higher hazard)
  80. 80. Water salinity
  81. 81. Sheltering effects
  82. 82. Construction details</li></ul>Aquaculture is a valuable wood use.<br />
  83. 83. Marine Borers: Hazard Zones<br />Marine borer hazard zones (Zone G is the most hazardous)<br />
  84. 84. Marine Borers: Resistance<br />Marine piles and timber in marine contact are best protected by:<br />Using species with high natural resistance such as turpentine or in cooler southern waters, swamp box, river red gum or white mahogany<br />Using preservative treated timbers (‘H6’) with wide sapwood bands such as spotted gum and plantation softwoods<br />Using mechanical barriers and floating collars<br />Piles encased in sand filled concrete pipe<br />
  85. 85. Marine Borers: Resistance<br />AS 5604 provides an extensive list of species with their marine borer resistances where known – as seen in the extract below.<br />
  86. 86. Chemicals<br />Timber is resistant to mild acids but strong acids (pH<2) and alkalis (pH>10) can cause degradation. <br />Timber is ideally suited to special applications such as:<br />swimming pool buildings<br />chemical storage <br />reservoir roofs etc.<br />
  87. 87. Fire<br />Fire is covered by two separate presentations:<br />Using Wood in Bushfire-prone Areas<br />Fire Safety and Performance of Wood in Multi-Residential and Commercial Buildings<br />
  88. 88. Conclusion<br />Timber has the ability to deliver design service life in a wide range of applications<br />There are significant issues that need to be considered to appropriately design, specify and detail timber structures to ensure satisfactory durability<br />These issues are well known and understood<br />
  89. 89. More Information<br />Technical Design GuideTimber Service Life Design<br />
  90. 90. Learn more about wood at UTAS<br />Centre for Sustainable Architecture with Wood<br />Graduate Certificate in Timber (Processing & Building)<br />4 units, part time, online<br />Areas covered include:<br />Wood science<br />Design for durability and service for life<br />Timber as a renewable resource<br />Sustainable design and construction<br />Engineered wood products<br />International technologies and developments<br />Plus, selected topics of individual interest<br />More information: Associate Professor Greg Nolan <br />(03) 6324 4478 or<br />