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Timber Structure, Moisture Control Stress Grading

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Timber Structure, Moisture Control Stress Grading

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    1. Timber Structure, Moisture Control & Stress Grading Week 9

    2. Introduction Timber is a product cut & machined from trees & is a natural material A tree is capable of supporting its crown, conduct mineral solutions & store food material There are about 30,000 different species of tree Timber is an extremely variable material

    3. Definition Timber as a material can be defined as a low-density, cellular, polymeric composite It has high strength performance & is relatively low cost Timber is the worlds most popular fibre composite

    4. Environmental Considerations Availability Widespread ( not always well managed) Extraction Environmental implications Energy used Low Health & safety Few problems (wood dust & treatments) Recyclability Many options

    5. Structural Variation Four orders: 1.) Macroscopic 2.) Microscopic 3.) Ultra-structural 4.) Molecular

    6. Macroscopic Increasing crown diameter linked to diameter of trunk Conduction & storage restricted to the outer region of the trunk – sapwood Area in which this function is no loner carried out – heartwood

    7. Sapwood Width varies from species to species, rate of growth & age of tree Except for very young trees (sapwood = whole radius) sapwood typically represents 20 to 50% of the total radius

    8. Heartwood Heartwood advances to include former sapwood cells The acidity of the heartwood increases & extractives are formed & colouration changes take place Resistance to fungal & insect attack increases Many timbers develop gums & resins in the heartwood

    9. Growth Trunk grows by division of cambial cells immediately beneath the bark & crown size increases – enlargement of branches & production of new ones When cambium cells divide into two the new cell formed on the inner side increases in size & thickness of its wall until it is a fully developed wood (xylem) element Process continues through the growing season

    10. Cont’d Cells in the outer side of the cambium develop into new phloem (inner bark) Radial growth of the trunk must accommodate existing branches – Knot formation Live branches fuse with truck – live knots Dead branches – trunk grows around branch – dead knots

    11. Cell Structure

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