Construction materials: Timber

Types

  • Softwood: conifers and evergreen
  • Hardwoods: broad leaved and deciduous
    • densest, strongest and most durable
    • some are actually soft
    • resins and oils make working hard and affect the hardeings of paints
    • cost is approx 1.5x softwoods in the UK
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Subsystems

The root: absorbs moisture containing minerals from soil to transfer via trunk to crown. Spread through soil is efficient anchorage enabling growing tree to resist wind forces

The trunk: provides rigidity, strength and height to maintain crown above ground level to function efficiently. Contains and protects growth cells, provides 2-way system to transport moisture up from root and sap down from crown. Stores manufactured food

The crown: chlorophyll in leaves and sunlight together with salt from soil and CO2 from air, food materials are produced by photosynthesis on which tree growth depends. Curl occurs at crotch where a large branch intersects trunk. Burrs occur when undeveloped buds are enclosed by subsequent.

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Growth

  • seasonal resulting in growth rings around the leading shoot usually 1 yr
  • rapid growth produces wide rings of low density/ strenght
  • each ring is made of fibrous cells cemented together
  • grain refers to general arrangement of vertically aligned cells which gives the direction of orientation
  • rings are made of early wood and late wood
  • late wood is slower in growth and darker in colour
  • new rings (sapwood) are formed as minerals are extracted from the inner rings
  • sapwood is mechanically similar to heartwood but it more vulnerble to attack by fungi or insects
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Production

Forestry: aim is to produce straight trees with few branches for ease of sawing and plaining

Felling:  aim is to strike a balance between age and cost for felling to be most efficient

Conversion: after felling, trees are converted to usable timber by saw mill. Most common are:

through and through: produces tangential boards

quarter sawn: produces radial boards

Seasoning: controlled drying of fresh timber with a high % of moisture. needs to be slow to prevent splitting and distortion. two methods:

air seasoning: natural and cheap but slow; little control on drying process

kiln seasoning: faster but expensive; full control on process

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Defects

  • Knots: occur where part of a branch becomes enclosed in a growing tree
  • Fungal decay: softening of fibres
  • Insect damage: some exit holes may be repairable by filling and painting
  • Resin pockets: affects painting
  • Reaction wood: denser and stronger than normal growth caused by lack of uniformity of cross-section due to sustained bending by wind or other forces
  • Wane: occurs when parts of bark or rounded periphery of the trunk appear
  • Slope of grain
  • Raised grain: from machining with high moisture content. deformed cells slowly gain original shape leading to irregular appearance after drying and painting
  • Seasoning defects: excessive or uneven drying, exposure to wind and rain and poor stacking and spacing during seasoning can all produce distortions or defects
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Grading

a) Visual: based on knot area ratio and other defects. a slow and conservative process. timber is graded as general structural or special structural. Hardwood, timber is graded as structural tropical or structural temperate

b) Machine: measures modulus of elasticity of pieces of timber and allocates grade based on set of relations between E and rupture. a rapid and reliable method and should be used for structural timber

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Moisture

  • If moisture content is over 27% then water is present in the cells.
  • if moisture content is below 27% then water is absorbed in cell wall only
  • point of inflexion is referred to as fibre saturation point
  • as water is removed below 27% strength increases and timber shrinks
  • compressive strength decreases with water content up to 27% then it plateaus
  • timber expands as moisture content increases and shrinks as it decreases
  • in dry atmospher timber will dry. in damp atmosphere dry timber will get wet.
  • time required to meet equilibrium moisture content is dependent on dimensions
  • in dry timber, adjoining cells bod directly, hence grip and strength
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Mechanical properties

  • behaviour is elastic for small stresses of short duration but creep is a major problem for sustained loads
  • elastic modulus and impact resistance are high so timber is stiff and tough
  • density varies widely although wood tissue is constant at 1500kg/m3
  • thermal insulation  is good and depends on the density and moisture content
  • thermal movement- coefficient of thermal expansion= 30-60x10-6 /C for I grain and 3-6x10-6 for II grain
  • strength varies widely and is affected by density, species, moisture content, grain direction 
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Protection and preservation

  • Use heartwood: less permeable and more durable than sapwood
  • use resistant species
  • keep moisture content below 20%
    • good seasoning
    • careful detailing
    • paint
  • use preservatives
    • tar oils
    • water borne
    • solvent type
  • application by brush or spray, dipping, pressure vacuum
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General use

Solid timber: beams, columns, piles, roof trusses etc

Laminated timber: glulam

Board materials: plywood, chipboard, hardboard

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Connections

Mechanical connectors: split ring, toothed plate, pattern nail plates, screws and nails, bolts

Adhesives: modern glues are stronger than timber but durability is vital, cleanliness essential and only possible under factory conditions

lap joints

scarf joints

finger joints

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Laminated timber

Advantages:

  • larger and longer members
  • greater strength
  • economical use of timber

Disadvantages:

  • factory process only
  • potential decay of glue
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Board materials

- plywood is the commonest-  thing sheets glued with alternate grains perpendicular

- moisture movement is low 

- strength is equal in two planes

- durability depends on glue

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