Unique Properties of Timber

 

Timber is a naturally occurring organic material. There are more than 300,00 species of tree throughout the world, producing timber with a wide range of properties, providing the designer with an almost unlimited choice for both structural and decorative applications.

 

Specific details of the properties of common species used in Australia can be found in AS1720.2 Timber Structures Code Part 2: Timber Properties.

 

Appearance and physical properties

• Colour
• Texture
• Figure
• Grain
• Density
• Thermal properties
• Acoustic properties
• Chemical resistance
• Fire resistance
• Termite resistance
• Electrical resistance
• Mechanical damping

 

 

Direction of strength and stiffness

 

Creep

 

Duration of Load

 

Appearance and physical properties

 

The appearance and physical properties that interest designers and users of timber are functions of the microstructure of the wood. Timber offers designers many opportunities because it is a natural material with unique properties:

 

• Colour – Most timbers show variation in colour between species and within species. It can also vary within a single piece. Colour descriptions usually relate to the heartwood of the species and may be significantly different from that of the sapwood. (Sapwood is always white to very light brown.) Colour can vary with use, age and by the application of finishes. Timber exposed to light will change colour, and unprotected timber exposed to the weather will eventually become silvery grey in colour.

 

    

 

 

 

  

  

• Texture – of timber may be described as being coarse, fine, even or uneven. The differentiation between coarse and fine texture is determined by the size and arrangement of the wood cells. Softwoods are usually fine textured, while hardwoods may be either. For example, mountain ash is a coarse textured hardwood, but brush box is a fine textured hardwood. The main process affecting texture of the timber is the finishing applied during fabrication. Planing will produce a fine texture, sanding or brushing – a fine to coarse texture, rough sawing or splitting a coarse to uneven texture. Surface finishes will also affect the texture, high build, smooth coatings will give the finest texture.

 

• Figure – refers to the pattern produced on the surface of the timber. The pattern is determined by the type of grain, the arrangement and size of cells, colour variations and sawing patterns. Designers can use these features to produce striking effects in different lights. Where particular grain patterns are required, this will need to be investigated very carefully with the supplier, and may require special milling.

 

Further information on species:

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   Grain – refers to the general direction, size and arrangement of wood fibres. Grain can be described as sloping, straight, spiral, irregular, wavy etc.

 

   Hardness – refers to the resistance of the timber to penetration. This is an important parameter for flooring, with harder species wearing better and requiring less maintenance than softer species. It may also be an important parameter for some cabinetry and joinery.

 

   Density – is influenced by cell structure and size, thickness of the cell walls and moisture content. The density of timber at a specific moisture content (usually 12%) is the amount of wood substance in a given volume, expressed as kilograms per cubic metre. Density is one of the most reliable indicators of stiffness, joint strength, hardness, ease of machining, fire resistance and drying characteristics.

 

• Thermal properties – timber is a natural insulator. Air pockets within its cellular structure make timber a natural barrier to heat and cold. As thermal conductivity increases with density, lightweight timber is a better insulator than dense timber. Thermal conductivity also varies slightly with moisture content, and natural characteristics such as checks, knots and grain. Air spaces between building elements, such as studs in a framed wall, are effective heat barriers and are considered when determining the thermal resistance of building assemblies. An advantage of timber framed construction is that additional insulating material can be placed in the spaces between framing members without increasing wall, ceiling, roof or floor thickness.

 

• Acoustic properties – an important property of timber is its ability to damp vibrations. Its cellular network of minute interlocking pores converts sound energy into heat energy by frictional and viscous resistance within these pores and by vibration of their small fibres. Because of this high internal friction, wood has more damping capacity than most other materials. This damping reduces the tendency of structures to transmit vibrations long distances and is suitable for use in applications requiring acoustic separation such as MRTFC. Timber also reduces the magnitude of resonant vibrations, so is used extensively where good acoustics are required eg concert venues, music suites, halls and meeting rooms. Some acoustic panelling may have arrangements of holes to further increase damping.

 

• Chemical resistance – timber offers considerable resistance to attack by a wide variety of chemicals including organic materials, hot or cold solutions of acid or neutral salts or dilute acids.
  Resistance to chemical attack is greater in softwoods than in hardwoods. Timber is commonly used for vats and tanks for chemical storage and for structural members in factories where corrosive vapours are present.
  Direct contact with caustic soda should be avoided. Strong acids and alkalis will destroy timber in time, but the process is relatively slow.

 

   Fire resistance – is influenced by density, and type of extractives. It can be enhanced by various treatments including pressure application of fire retardants or surface application of intumescent coatings. These treatments are not common, and may require special production runs. Plenty of lead time needs to be given for the supply of these products.

 

   Termite resistance – is influenced by cell size, and type of extractives. Termite resistance can be improved by treatment processes .

 

   Electrical resistance – varies greatly with moisture content. Moisture meters use this property to measure the moisture content of timber. Seasoned timber is normally regarded as a non-conductor for most practical purposes. Timber can be heated by subjecting it to a high frequency electrical field. Some adhesives can be heat cured by this process and are used in the manufacture of laminated timber and plywood.

 

       Mechanical damping – This property is important in the evaluation of vibration in structures, and in determining earthquake response. Timber itself has relatively high internal damping due to its cell structure as discussed under “Acoustic properties” above, but in normal framed construction, the large number of semi-flexible nailed connections lend further damping to the assembled structure.

 

 

Direction of strength and stiffness

 

Wood itself is fibrous. Cells are long and slender and are aligned with the long axis of the trunk. It is these fibres that give the grain in the wood, not the growth rings. They also make the properties of wood quite anisotropic with much higher stiffness and strength parallel to the grain than across the grain.

 

 

 

 

The structure of wood can be likened to a bunch of parallel straws (representing the fibres or grain of the wood), which are bonded together using a weak glue. When load is applied parallel to the axis of the straws (a), they are very strong in tension and have reasonably good compressive strength until they start to buckle. However, if the load is applied perpendicular to the axis of the straws (b), they will tend to crush under compression and are weakest in tension, where the “glue” bond fails and the straws literally tear apart.

 

Creep

 

Creep is the term used to describe the changes in microscopic structure of wood that causes deflection of timber over a given time under a given load. (It is not the instantaneous deflection that occurs due to the changes in the level of load.

 

Because the individual wood fibrils are only loosely bonded to form wood fibres, the fibrils can move relative to each other under stress. Some fibres change their structure, others slide past each other, and some others may fail. The amount of deflection, or creep, that occurs is a function of the magnitude of the load and the time that has elapsed since the change in load level.

 

Creep deformations increase significantly with changes in moisture content. It is the change in moisture content that is more important than the actual moisture content. Under normal use conditions, the ambient humidity changes, sometimes throughout the day, but certainly between seasons. The changing moisture loading of the air means that moisture will move into or out of the timber. This can lead to substantial increase in creep. Even under constant relative humidity conditions, unseasoned timber will gradually lose moisture over time. The moisture movement through the timber will contribute to higher creep deflections. This is shown in Figure (a). The elastic deformation will occur instantaneously as soon as the load is applied. The deformation will slowly continue to increase, even though the load remains constant. Unseasoned timber shows more creep deformation than seasoned timber.

 

 

Duration of Load

Timber has time-dependent properties. Under long-term stresses, the fibres in the wood stretch and move relative to one another. These movements are in addition to the short-term elastic response to load, and they result in creep under long-term loads. The movement of fibres can cause damage on a microscopic scale. Over a very long period of loading, this can lead to a loss of strength. Standard testing of timber typically takes less than 5 minutes. This loading period is taken as the base strength. Longer periods of sustained load will lead to lower strengths being achieved.

Reference: NAFI Timber Manual Timber Datafile P1 Timber species and properties

HB108-Timber Design Handbook.

 

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