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Timber New

4.3.1 Describe the structure of natural timber.

Natural timber is a natural composite material comprising cellulose fibres in a lignin matrix. The tensile strength of timber is greater along the grain (fibre) than across the grain (matrix).
Hardwood and softwood Structure Under the microscope

Wood is a fibrous material. The structure of wood similar to a bunch of parallel straws (the cellulose fibres), which are bonded together with a glue (lignin matrix). The fibres are long and slender and are aligned with the long axis of the trunk which gives it an interesting property behaviour.

When load is applied parallel to the axis of the fibres, they are very strong in tension and have reasonably good compressive strength until they start to buckle.

When the the load is applied perpendicular to the axis of the fibres, they will tend to crush under compression and are weakest in tension, where the “glue” bond fails and the straws literally tear apart.

4.3.2 Outline that timber can be classified according to the conditions needed for tree growth.

Consider temperate and tropical conditions. A general knowledge of the geographical distribution of world timber resources is required.

Study the distribution of forest map of the world below. Temperate forests tend top be in cooler regions and tropical tends to be between the Tropic of Cancer and Capricorn.

4.3.3 Outline that conifer trees are referred to as softwoods and that these grow only in temperate regions.

Recognize the characteristics of softwood trees.
Conifers
Any of various mostly needle-leaved or scale-leaved, chiefly evergreen, cone-bearing trees or shrubs such as pines, spruces, and firs.

Characteristics of softwood trees are ...

  • Softwood trees have several characteristics that make them different from hardwood trees.
  • The wood from these trees is generally softer. (That's where the name comes from.)
  • Softwoods reproduce* by cones.
  • Softwoods have needles.
  • They do not lose their needles in the fall. They are sometimes called evergreens because the needles are green year round.
  • Examples included are pine, cedar, and cypress.

4.3.4 Outline that deciduous trees are referred to as hardwoods and that these grow in both temperate and tropical regions.

Recognize the characteristics of hardwood trees.

Characteristics of hardwood trees are ...

  • Hardwood trees have several characteristics that make them different from softwood trees.
  • The wood from these trees is generally harder. (That's where the name comes from.)
  • Hardwoods reproduce* by flowers.
  • Hardwoods have broad leaves.
  • Many lose their leaves every autumn and are dormant* in the winter.

Some examples of hardwood trees includes eucalyptus, elm, maple, oak, and beech.

Pine tree - Softwood Eucalyptus - Hardwood

4.3.5 Discuss the issues relating to the consideration of timber as a renewable resource.

Consider time to reach maturity, soil erosion, greenhouse effect and extinction of species. The issues should be placed in local, national and international contexts.
  • time to reach maturity, e.g. Mahogany trees takes about 100 years to mature
  • soil erosion ... the roots of the tree hold the soil in
  • greenhouse effect ... less trees to remove the greenhouse gases
  • extinction of species ... destroying animal, insect and plant life
  • deforestation
  • underlying causes
  • deforestation @ wikipeadia.
  • Malaysian issues.
  • BBC on slowing of deforestation.

4.3.6 List two examples of composite timbers.

Consider particle board (chipboard) and plywood.
Particle Board Plywood Pinewood

4.3.7 Compare the characteristics of particle board, laminated woods (for example, plywood), pine wood (a softwood) and mahogany (a hardwood).

Consider composition, hardness, tensile strength, resistance to damp environments, longevity and the aesthetic properties of grain, colour and texture. The ability to produce sketches showing cross-sectional views of the structure of the materials is expected.
Particle Board Plywood Pine Wood
Composition small pieces mixed with glue Sheets (veneer) of wood glued together solid piece
Hardness V. Hard medium medium to soft
Tensile strength low high all directions high along the grain
Resistance to damp environments low Medium High
Longevity Short Long Long
Grain None Has Has
Colour Natural can see small pieces Of the veneer Natural Pine
Texture Rough Smooth Smooth

4.3.8 Outline criteria for the selection of timber for different structural and aesthetic design contexts.

Consider timber for buildings, furniture and children’s toys.

Buildings - often the timber rafters, studs, beams are hidden from view so there is no need for wonderful looking hardwoods but rough sawn pine is fine. If within a house there is flooring then aesthetics plays a role and a hardwood would be selected.

Furniture and children's toys need to withstand some amount of wear and therefore need to be durable. In furniture aesthetics plays an important role as well. In toys the wood may be painted or stained.

4.3.9 Describe the reasons for treating or finishing wood.

Consider reducing attack by organisms and chemicals, enhancing aesthetic properties and modifying other properties.
Organism attack Sanding Finishing

4.3.10 Explain three differences in the selection of timbers for flooring if it were made of a hardwood, a softwood or a composite material.

Consider durability, ease of maintenance and aesthetics.
Hardwood Softwood Composite (flooring) material
Durability Very suitable in high traffic areas like kitchens due to its higher hardness Not suitable in high traffic areas like kitchens due to its lower hardness. May need a special plastic finish to improve hardness. Highly durable
Ease of maintenance When finished very easy to clean. Will need to have a finished reapplied over time. When finished very easy to clean. Will need to have a finished reapplied over time. Very easy to clean.
Aesthetics Usually darker colours. usually pines are white or yellowy in colour. People like the feeling of warmth it gives. Colour and features can be designed in, may have a cool feeling to it

Relative hardness for timber used in flooring

References

Bulleted list and italicised paragraphs are excerpted from Design Technology: guide. Cardiff Wales, UK: International Baccalaureate Organization, 2007.

Images are clickable links to its location.

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Page last modified on February 23, 2009, at 01:33 AM