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Timber Properties


Introduction

The notes below provide identify the factors which affect the properties of timber.    More detailed information on wood types and properties is provided on webpage Wood   

The timber used for construction, carpentry, joinery etch is obtained from the trunks of trees.    This part of the tree includes features required to structurally support the tree its branches and leaves and withstand natural forces such as wind.    The truck also includes a cellular structure to transport moisture containing minerals and nutrients, sap between the roots and the upper branches and leaves.

The trunk is composed of long tubular cells made up of cellulose which are bonded together by lignin.     Most of these tubular cells run along the trunk and result in the grain structure of the wood. some cells run radially out from the centre of the trunk and these are identified as rays.     In temperate countries, trees produce a new layer each growing season under the bark and this results in the annular rings.     In tropical regions which have continuous growth the trunk includes and essentially uniform section.

The central, normally darker, area of the trunk is called the heartwood and the outer area is called the sapwood. The heartwood performs a mechanical support function and stiffness to the tree.    The sapwood transports the fluids along the trunk.    The heartwood is generally stronger and more durable .

Density...

The density of is a very important factor affecting the strength of wood.    The densest woods are generally the strongest.    The density is also related to the stiffness and hardness e.g both these properties increase as the timber density increases.

Softwoods...

Softwoods are generally evergreens (coniferous) with needle-like leaves.    They have the following characteristic features

They grow relatively quickley and trees softwood trees can be felled after 30 years
They have, generally, poor durability qualities unless treated with preservatives.
They are readily available and comparitively cheap

Hardwoods...

Hardwoods are generally broad-leaved (deciduous) that lose their leaves at the end of each growing season.     The cell structure of hardwoods is more complex than softwoods with thick walled cells called fibres providing the structural support and thin walled cells called vessels providing the meddium for food condution along the trees.     Hardwoods have the following characteristics affecting their use.

The grow relatively slowly ,resulting in denser, stronger timbers.     These tress take over 100 years to mature.
They have, generally, good durability qualities with less need for preservatives.
They are obtained from tropical areas and are comparatively expensive. .

Moisture Content

Timber has high moisture content when first processed from the tree.    The moisture content is reduced prior to the timber being used as a structural material.    The drying off timber can be achieved using either of two methods

1)     Air seasoning ...The timber is stacked in open sided weatherprrof containers is such the air can freely circulated to all surfaces of the timber to promote drying. This process can take considerable time and much space is required.

2)    Kiln Drying..The timber is dried out in heated , ventilated and dehumidified ovens. This process requires special equipment and requires high energy energy use.     However the process is controlled and rapid.

Because of the nature of timber poor control of the drying process can result in a range of defects including bowing, cupping, twisting and cracking.

The moisture content of wood w() is defined using the equation

w = (m1 - m2 ) / m2

w = moisture content expressed as a percentage.
m1 mass of test piece before drying (g)
m2 mass of test piece after drying (g)

Directional Properties / Anisotropy

The properties of timber are different in different directions.    The elastic modulus and strength of wood is much greater in a direction along the grain than across the grain.    If the grain is sloping at an angle to the direction of loading this can also have a marked effect of the strength of the timber.



Stress/Strain relationship

At low stress levels the strain is proportional to the strain.    The strain is greater for a given stress at high moisture contents.     However timber does have significant creep characteristics i.e over time at high constant stress values the strain will increase.     In calculating strength of structures the duration of the loading is one of the factors to be considered.



Thermal Properties

Timber is a poor conductor of heat.    The denser timbers are better conductors than the lighter timbers. Timber expands when heated but this is offset by the reduction in section resulting from the loss of moisture.



Fire Resistance

Timber compares favourably with other construction materials in terms of fire resistance.     Small timber section may ignite easily and reduce to ash.    However larger sections have significant resistance to fire because the surfaces initially in contact with the flames burn to charcoal which resists the further spread of the flame.


Durability

Timber does not corrode.     It is however subject to weathering, chemical attack, and fungal attack.    The durability of timber varies depending on the wood species    If timber is kept dry or is continuously immersed in fresh water then decay shoud not be a problem.    Timber immersed in seawater is at high risk of damage resulting from attack by molluscs.

There are a number of preservative treatments available to prevent the deterioration of timber resulting from fungi, insects or molluscs.     The types of wood presevative treatments available are briefly described bel

1)     Tar oil penetration - Creosote is a a typical tar oil presevative.    This group is used generally for external systems such as telegraph poles , fences, railway sleepers, timber enclosures (garden sheds) etc.

2)     Organic solvents.. These are widely used and are applied using simple techniques such a brushing spraying or dipping.

3)    Water borne solvents are the most widely used for industrial applications and are normally applied into the timber under pressure.


Links Providing information on Wood
  1. Chapter 4..Mechanical Properties of Wood ..Very informative American Download.
  2. Woodzone..Informative site providing general information
  3. Woodweb..Woodworking Industry Information
  4. Wood Handbook -- Wood as an Engineering Material..Downloads ..Comprehensive Document (American ) Excellent
  5. Wood Guide.. Friends of the Earth Guide - Including very useful information
  6. Canadian Wood Council ..Excellent site on Wood Engineering - my words
  7. Timber Trade Federation ..The Timber Trade Federation is the official voice of the UK timber trade.

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Last Updated 8/05/2009