Timber is among the ancient materials used for construction. Nowadays, its primarily replaced or complemented by steel, concrete, and plastics. Nonetheless, the application of timber remains quite extensive. There are two classifications of wood for carpentry: hardwoods and softwoods. Softwoods emanate from trees that have needle-like leaves. Their durability is lower unless protected and enhanced by preservatives. Hardwoods have broad leaves and are deciduous and are as well durable and can last for long without the application of preservatives (Aydin, Yardimci " Ramyar, 2007).
The mechanical properties of popular timber species such as pine, fir, poplar, and hornbeam have their flexural strength, compressive strength and toughness to be parallel and perpendicular to the grain. The Modulus of elasticity of these specimens of timber is also parallel to the grain when it comes to compressive test and perpendicular to the grain when measured against the flexural test. The direction of loading affects the mechanical properties of timber remarkably. The parameters of timber such as cell length, microfibrillar angle and density vary greatly according to the species of the tree. The variations are established upwards in the tree, within the growth ring and are quite apparent during the performance of mechanical tests (Aydin et a., 2007).
Physical Properties of Timber
Moisture Content
Water exists inside wood as either bound or free water. Free water exists in a cell cavity as a liquid. During drying, free water easily gets removed. The movement of the free water is through the neighboring and connecting cells. The bound water moisture gets absorbed within the cell wall. The water is bound to the molecules of the wood in the cell. Shrinkage occurs when bound water gets removed. The removal of bound water increases the strength properties of wood (Aydin et a., 2007).
Density
The quantity of the wood substance for a particular volume constitutes density whereas the woods that have more weight for a given volume possess more density than timber whose weight is less. Both volume and weight of wood are affected by the quantity of moisture it contains. For instance, the density of pine like is the case in most conifers is 480kg/cu (Aydin et a., 2007).
Dimensional Stability
The dimensional instability in wood reduces its applications in the construction industry and furniture. Dimensional stability of wood is affected by the constant release of water and adsorption that results in shrinking and swelling causing the physical degradation of timber. The dimensional stability of timber, hardness, and stiffness can be improved using a variety of treatment methods. The epoxide treated wood has better qualities of water repellency and anti-swelling efficiency as a characteristic of dimensional stability compared to untreated wood (Heräjärvi, et al., 2014)
Thermal and pyrolytic (fire) properties
Although timber falls under the classification of combustible material, the proper design of timber structures reduces the susceptibility of catching fire or getting burnt. Light timber construction resources improve their resistance to fire by the treatment of fire-resistant cladding materials. Heavy materials of timber used in the construction industry have good natural fire resistance because the timber forms a char layer that reduces the penetration of heat. The exposure of timber members to fire makes the exposed members of the wood to attain fire temperature, leading to ignition and burning.
Strength Properties
Bending
The bending property of wood is the ability to contain shock with some permanent deformation with severe or even less injury to the specimen. The work to maximum load in construction is a measure of the combined toughness and strength of wood experiencing bending stresses.
Compressive Strength
Compression of the grain relates to transverse shear and hardness. There are two methods in which wood experiences stress of this kind: The load might act over the whole area of the specimen or the weight may have concentration over a small part of the area. In most circumstances in the construction sites, most loads act on a portion of the timber. For example, a post lying on a horizontal sill signifies a load acting on a given section of the wood.
Shear strength
The ability of timber to resist internal slipping of one portion upon another in the direction of the grain. Faults, cracks, and knots that appear in wood weaken the shear strength of wood. All layered materials such as wood manifest shear in comparison of the lengthwise properties that is the grain and parallel aspects. Majority of failures in the structures of timber are due to shear. Wood products that have a higher shear capacity and engineered products that have the cross-linked configuration of composite reduce the chances of failure in the construction site.
Tensile Strength
The tensile strength of wood could be parallel or perpendicular to the grain. The tensile strength that is perpendicular to the grain is whereby the resistance of wood to the forces that are exerting pressure across the grain leading to the splitting of the member. The tensile strength that is parallel to the grain occurred when the maximum stress experienced along the direction of the grain. The modulus of rupture sometimes acts as the conservative or low estimate of tensile strength for straight grained wood pieces.
Torsion strength
According to the torsion theory, the deformation nature of many materials is dependent on the orientation. The strain-stress response of a sample obtained from the material in one direction will not be the same with the sample taken in a different direction. The term anisotropic describes such phenomena. Wood is very anisotropic because of the elongated shapes constituting wood cells. Therefore, the insights on the torsion strength of timber on the stress distribution in the anisotropic materials in very crucial in the evaluation of the high-performance wood the applications of structural strength (Amulu, Ezeagu " Obiorah, 2016)
Species of Timber
There are two important classifications of timber: the hardwood species and softwood species. The type of wood used in construction is hardwood. The following are some of the timber species available: poplar, fir, pine, and hornbeam (Ramage et al., 2107).
Conclusion
The compressive strength of timber under the load in parallel to grain is higher than that of timber experiencing loading in the perpendicular grain. The ration of the two values of strength is between 9.6 to 12.4 for pine, fir and poplar species. The corresponding figure for hornbeam is 4.8 because of its lower anisotropy.
There is a robust connection between density and compressive strength. For instance, hornbeam has a higher density as well as maximum mechanical properties. The properties are a flexural strength, compressive strength along the direction of the grain and the toughness in a perpendicular direction to the grain. Furthermore, there is a high correlation between flexural strength perpendicular to the grain and compressive strength parallel to the grain ((Aydin et a., 2007).
References
Amulu, C. P., Ezeagu, C. A., " Obiorah, S. M. O. (2016) Analytical Effects of Torsion on Timber Beams.
Aydin, S., Yardimci, M. Y., " Ramyar, K. (2007). Mechanical properties of four timber species commonly used in Turkey. Turkish Journal of Engineering and Environmental Sciences, 31(1), 19-27.
Heräjärvi, H., Möttönen, V., Reinikkala, M., " Stöd, R. (2014). Absorption–desorption behaviour and dimensional stability of untreated, CC impregnated and pine oil treated glulam made of Scots pine and Norway spruce. International Biodeterioration " Biodegradation, 86, 66-70.
Ramage, M. H., Burridge, H., Busse-Wicher, M., Fereday, G., Reynolds, T., Shah, D. U., ... " Allwood, J. (2017). The wood from the trees: The use of timber in construction. Renewable and Sustainable Energy Reviews, 68, 333-359.
