Wood is the most utilized building material after stone. The various properties of wood and its nature make it perfect for use as an enduring building material. However, don’t get confused between timber and wood; timber is a wood, which is prepared for building construction or carpentry application while the word wood is used to describe the non-structural items and furniture. Wood and timber are also known as lumber in North America.
Qualities of good timber depend upon the type of treatment the tree has received during its growth period, the time of felling, nature of the soil in which it has grown, etc. Selection of an excellent timber for house construction is not an easy task because woods are of different types with varying properties. Here we will be discussing some of the essential features of wood that will let you know the major factors to check in the selection of good quality timber.
Characteristics of Good Timber
01. Appearance:
A freshly cut surface of wood should exhibit hard and shining appearance.
02. Color:
The color of a good timber should be preferably dark.
03. Odor:
The good timber should have a sweet smell. The unpleasant smell mostly indicates a decayed wood.
04. Sound:
A good timber should give clear ringing sound when struck.
05. Grain and Texture:
The term grain and texture are used in explaining the texture of wood. Grains are often used to describe the relative sizes and distributions of cells in the texture of the log, it is explained either as coarse grains or as fine grains; this use of grains is roughly synonymous to the strength of the wood. It is also used to indicate the direction of fibers as in straight grains, spiral grains or interlock grains.
The texture is another word used to describe the size of the cells in wood. Based on the size of cells, it is said to be even textured or uneven textured wood.
06. Fiber:
The fiber is comparatively narrow and long cells of wood with closed ends. A good timber should have straight fibers.
07. Shape:
A quality timber should be capable of retaining its shape during the seasoning or conversion period.
08. Structure:
The structure of good timber should be uniform.
09. Hardness:
The hardness of timber is its capacity to resist penetration through a harder body. Good wood should be hard and is capable to resist penetration on it.
10. Toughness:
The toughness of timber is a material property which indicates the energy required to break the wood. Good lumber should be tight and capable of offering resistance to shocks due to vibration.
11. Abrasion:
Wooden pavements and floors are the subject to continuous traffic loads. Therefore, timber should not quickly deteriorate due to abrasion or mechanical wear.
12. Strength:
Strength of timber is its ability to resist forces, stress or pressure when it is used as columns and beams. A good quality timber should be sufficiently reliable for working as a structural element like a beam, column, joists, trusses, arches etc.
13. Elasticity:
Elasticity of timber is its ability to restore its original form and dimensions after the load is removed. The timber should return to its original shape when the load is removed from it.
14. Permeability:
Permeability is the quantity of water filtered or passing through a unit surface area of a specified thickness of the wood. A good timber should have low permeability.
15. Workability:
Timber should be easily workable and which can be shaped and cut in the required shape.
16. Durability:
For timber, the term durability denotes the resistance to fungal or insect attack. Good wood is expected to be durable and capable of resisting the damage of termite, fungi and other insects.
17. Fire Resistance:
Fire-resistance of timber is defined as its ability to withstand contact with fire without a major change in properties. It should be fire-resistant. Generally, the wood having dense texture offers better resistant to fire.
Usually, the timber has low heat conductivity, and it depends on various factors like moisture content, the orientation of fiber, porosity, moisture content, bulk density, etc.
18. Defects:
A timber defect is an imperfection that makes it weak and unsuitable for building construction work. A timber should be free from defects like knots, flaws, shakes etc.
Properties of Wood
01. Chemical Composition
02. Hygroscopic Properties, which includes
a) Moisture Content
b) Fiber Saturation Point
c) Equilibrium Moisture Content
d) Swelling and Shrinkage
03. Physical Properties, and it includes
a) Directional Properties
b) Dimensional Changes
c) Density
d) Specific Gravity
04. Mechanical Properties, which include
a) Strength Properties
i) Compression
ii) Tension
iii) Shear
iv) Bending
v) Energy Absorption Resistance
b) Elastic Properties
i) Modulus of Elasticity
ii) Shear Modulus
iii) Poisson’s Ratio
05. Decorative Properties
06. Acoustical Properties
07. Electrical Properties
Let us discuss step by step all the above mentioned timber properties.
01. Chemical Composition
Wood fibers or cells are primarily cellulose cemented together with lignin. The wood structure is made of 70% cellulose, 12% to 24% lignin, and up to 1% ash forming materials.
These constituents give excellent hygroscopic properties, its strength and susceptibility to decay. The bond between individual fibers is so definite that when tested in tension, they commonly tear apart rather than getting separate.
02. Hygroscopic Properties of Wood
Wood is hygroscopic meaning that it expands when it absorbs moisture and shrinks when it dries or loses moisture. This property affects the end-use of wood. The various hygroscopic properties of timber are moisture content, fiber saturation point, equilibrium moisture content, swelling and shrinkage etc.
a) Moisture Content of Timber
The moisture content of timber is the weight of the water it contains, expressed as a percentage of the weight of the wood when oven dried.
The moisture content of wood is an essential property for both construction and design of timber structures.
b) Fiber Saturation Point
Water in the timber is generally found in two forms:
- As free water in the cell cavities.
- As absorbed water within the cell fibers.
When wood dries, its cell fibers give off their absorbed water only after all the free water is gone, and the adjacent cell cavities are empty. The point at which the fibers get fully saturated, but cell cavities are found hollow, this point is known as the fiber saturation point.
Although the moisture content at fiber saturation point varies from one wood pieces to other within a specific spices, and also from one species to another species, a value of 30% moisture content is generally associated with the fiber saturation point. The significance of this condition is that it represents the point at which shrinkage begins.
c) Equilibrium Moisture Content (EMC)
After the tree is felled and cut into timber, its moisture content begins to drop as moisture in the wood gets lost to the surrounding air. The timber will then continue to give off or take on moisture until the moisture within the wood has reached a point of equilibrium with the moisture in the air. The moisture content of wood at this point is called equilibrium moisture content (EMC).
It is necessary to know equilibrium moisture content in a particular location because it permits to predict the moisture content of wood that it should attain in service there. To ensure that wood will experience only minor dimensional change, it should be seasoned at moisture content as close as possible to the equilibrium moisture content (EMC).
d) Swelling and Shrinkage
Swelling or shrinkage parallel to the grain (Longitudinally with the height of the tree) is practically negligible and has little significance in construction applications. However, across the grains, wood will shrink appreciably in both width and thickness. Shrinkage is the most significant factor which is observed in the direction tangent to the annual rings and it impacts almost one half to two-third or as much across these rings (radial) in the timber.
03. Physical Properties of Wood
The various physical properties of wood are directional properties, dimensional changes, density, and specific gravity.
a) Directional Properties
Wood is anisotropic building material due to its cellular structure. The structure of the wood in any particular log is considered to have three axes of symmetry such as,
- Longitudinal: Parallel to Grain.
- Radial: Perpendicular to Grain and Radial to Annual Rings.
- Tangential: Perpendicular to Grain and Tangential to Annual Rings.
Wood pieces sawn from logs are typically oriented with their long axis (or faces) approximately parallel to the longitudinal axis of the log, but other faces may seem indiscriminate with respect to the radial and tangential directions.
For practical purpose, directional properties of wood are distinguished between perpendicular to grain and parallel to grain (Longitudinal). Perpendicular to grain values mostly apply to assess its both tangential and radial properties.
b) Dimensional Changes
Dimensional changes occur in wood due to the changes in moisture content and temperature. Typically wood containing moisture responds to varying temperatures differently than other building materials.
As wood gets heated, it tends to expand (swell) due to temperature, and also shrinks due to the loss of moisture. Whenever the wood is dry initially, shrinkage due to moisture loss on heating will exceed thermal expansion. As a result, net dimensional changes occur in the wood.
In timber design, the dimensional changes due to temperature are considered small in comparison to the dimensional change due to changing in the moisture content.
Wood shrinks as it loses moisture and swells as it absorbs moisture between the fiber saturation point and in oven-dry condition. There is no dimensional change with variation in moisture content above the fiber saturation point. The amount of swelling and shrinkage differs in the radial, tangential and longitudinal directions of a piece of lumber or wood.
c) Density
Wood density is calculated by counting its weight per unit volume. Sometimes it is also known as bulk density and weight density. Density is related to the mechanical properties of wood as the density increases the strength of wood.
d) Specific Gravity
Wood fiber has a specific gravity of 1.5, and is heavier than water (Specific gravity of water is 1). However, dry wood of most species float in water because a proportion of the volume is occupied by air-filled cavities.
The range of specific gravity of most wood species is about 0.36 to 0.70 at the time of oven drying. Specific gravity is an approximate measure of solid wood substances and a general index of strength properties.
04. Mechanical Properties of Wood
The mechanical properties of wood are its ability to resist applied or external forces. It means that any outside effect of a given piece of material which tends to deform by an external force. It is the property of wood that make it suitable for the use of building materials and as furniture elements.
a) Strength Properties of Timber
Strength properties of timber describe the ultimate resistance of a material to applied loads. They include material behavior related to tension, compression, shear, torsion, bending, and shock resistance.
As with other timber properties, strength properties mostly vary in the three primary directions, i.e. Longitudinal, Radial, and Tangential. The differences between radial and tangential directions are relatively minor and randomized when a tree is cut into timber or lumber.
i) Compression: Timber can be subject to compression in direction perpendicular to wood grain, parallel to wood grain, and at an angle to the wood grain.
Perpendicular to Grain: When compression is applied perpendicular to the grain, it produces stress that deforms the wood cells perpendicular to their length. Wood cells collapse at relatively low-stress levels when load are involved in this direction.
Wood will deform to about half of its initial thickness before the phase of complete cell collapse occurs. According to the ‘Charles Arntzen’ (Author of book called Encyclopedia of Agricultural Science), for compression applied perpendicular to the grain, failure is based on the accepted performance limit of 0.04-inch deformation.
Parallel to Grain: When Compression is applied parallel to the grain, it produces stress that deforms the wood cells along their longitudinal axis. At that time, each wood cell acts as an individual hollow column that receives lateral support from adjacent cells and its internal structure. As a result, the large deformation occurs from the internal crushing of the complex cellular structure at failure.
An Angle to Grain: Compression applied at an angle to grain produces stress acting both perpendicular and parallel to the grain. Therefore, the compressive strength at an angle to grain is taken as the intermediate value of parallel and perpendicular to the grain.
ii) Tension: Wood acts strong in tension as parallel to its grain and failure occurs by a complex combination of these two modes such as
- Cell wall failure
- Cell to cell Slippage
Slippage occurs when two adjacent cells slide pass to one another while cell wall failure involves the rupture within the cell wall. In both the modes of failure, little or invisible deformations are found occurring before completing the failure.
iii) Shear: The shear acts on wood in three ways, i.e. horizontal, vertical, and rolling.
The horizontal shear is the most important shear in wood that acts parallel to the grain. It produces a tendency for the upper portion of the specimen to slide about the lower part by breaking intercellular bonds and deforming the cell structure of wood.
Vertical shear is generally not considered because another failure i.e. compression perpendicular to grain almost always occurs before cell wall breaks in vertical shear.
In addition to horizontal and vertical shear, a less common type of shear, known as rolling shear, also develops in wood. It is caused by loads acting perpendicular to the cell length in-plane parallel with the wood grain.
According to the USFS Timber Bridge Manual developed by United States Forest Service (It is an agency of US Department of Agriculture), wood has low resistance to rolling shear, and failure is preceded by large deformations in the cell cross-sections.
iv) Bending: When wood is loaded by bending, the portion of the wood on one side of the neutral is stressed in compression parallel to grain while the other side is noted in tension parallel to the grain.
Bending also produces horizontal shear parallel to grain and compression perpendicular to wood grain at the supports. A standard failure sequence in simple bending is the formation of compression failure followed by the development of visible compression wrinkles. This effectively increases the compression zone and decreases the tension zone, which is eventually followed by tensile failure.
v) Energy Absorption Resistance: Energy absorption or shock resistance of wood is its ability to absorb and then dissipate energy via deformation quickly. Timber is remarkably resilient in this respect and that is why often acts as a preferred material for shock loading.
b) Elastic Properties of Timber
Elastic properties of timber are related to its resistance to deformation under applied stress and the ability of the timber to regain its original shape or dimension when stress is removed.
As an isotropic material with equal properties in all directions, elastic properties of wood is described by three elastic constants such as modulus of elasticity, shear modulus and Poisson’s ratio.
i) Modulus of Elasticity: Modulus of elasticity of timber is related to the stress applied along one axis to the strain occurring on the same axis.
ii) Shear Modulus: Shear modulus of a timber is related to the shear stress and shear strain.
iii) Poisson’s Ratio: Poisson’s Ratio of timber is related to the applied parallel stress to the lateral strain.
05. Decorative Property of Wood
A decorative property of wood depends upon its color, luster, texture, patterns, structure, finishes, stains, ability to take filler, and the method of cutting or sawing. It is impossible to give a detailed color description of the various types of wood because the wood has a combination of colors and the multiplicity of shades texture.
06. Acoustical Properties of Wood
Acoustical properties of wood are determined by its sound absorption and sound insulation abilities. Sound absorption ability is the amount of incident sound on a surface that is not reflected by the surface. The sound insulation ability is the reduction in the intensity of sound when it passes through a barrier.
Sound insulating values for construction materials are measured in decibels at various frequencies. Wood alone does not provide excellent sound insulation as most other common building construction materials.
According to Timber Construction Manual developed by the American Institute of Timber Construction, when the wood is combined with other construction materials, it will provide a pleasing sound-insulating ability. The sound absorption value of wood changes with variation in density, moisture content, and the direction of the grain.
07. Electrical Properties of Wood
The most important electrical properties of wood are its dielectric properties and resistance to electric current. The electrical resistance of wood changes with the temperature, moisture content and direction of travel of electric current concerning the direction of the grain.
The dielectric properties of timber are utilized in the drying of wood and the high-frequency curing of adhesives in glued-laminated members. The electrical resistance property of wood is used in electric moisture meters to determine the moisture content. Also, it changes with modification in moisture content, especially below the fiber saturation point, electrical resistance decrease with an increase in moisture content.
Summing up, Timber is one of the oldest and most reliable building materials that have been used for various types of construction works since ancient times. The use of wood as a building material has several advantages over other building materials. It is readily available at renewable mode, it exhibits durability and functional strength, and it is light-in-weight as compared to brick and concrete construction materials.
Worldwide construction industries are focusing on the development and utilization of environment- friendly building materials so that their production does not cause any environmental hazard. Wood is a renewable and environmentally friendly material, and hence is one of the most desired materials used in construction.
Before choosing wood as a building material, you should consider the qualities of wood following the above mentioned guidelines. Based on the essential characteristics and properties of timber, it can be regarded as one of the best building materials for house construction.