Timber as a Structural Material

The use of timber as a structural material is not new, in fact, it predates the development of concrete and steel as structural members, dating back many centuries. With the passage of time, advances in the various types of timber element available and their use in different structural forms have taken place, modern advanced timber products are now available which allow structural engineers to achieve the performance and efficiency needed in the construction industry of the 21st century.

The word ‘Timber’ is often used to describe the structural products of wood. Timber is categorised as either ‘softwood’ or ‘hardwood’. Softwood is obtained from coniferous trees and hardwood comes from broad-leaved trees. Softwood and hardwood are botanical terms and do not necessarily refer to the density or hardness of the wood. For example, Balsa, which is known to be soft and used for building lightweight models, is a hardwood whereas Douglas Fir is softwood with good durability and high strength properties.

Softwood is widely used for timber structures (Figure 1), because it is readily accessible, efficiently used, of relatively low cost and its high growth rate provides a constant supply from regenerated forest areas. Hardwoods are usually used in exposed frameworks and cladding where toughness and particular aesthetics, such as colour or grain pattern, are needed.

Strength of Timber

Timber strength comes from the wood’s natural characteristics, and how the tree grows (Figure 2). Normal wooden joists are sawn from the root of a tree. The trunk consists of cells arranged axially around the roots, and are long in proportion to their width. Both timber joists and beams sawed from the tree’s trunk have these cells parallel to their circumference, supplying both axial and flexural support. Because of the inherent properties of these cell configurations, the stress and strain power of timber parallel to the grain is even greater than that perpendicular to the grain.

The strength of sawn timber is a function of nature, width, size and member shape, moisture content and length of loading added along with strength-reducing characteristics such as grain slope, knots, fissures and wane. Strength grading methods were developed to distinguish timber using either visual force grading methods or machine strength grading methods.

Timbers with common strength properties are grouped into a set with resistance groups described in BS EN 338:2009. This simplifies the design and development process by allowing projects to be built on specified strength class limits without having to classify and source a particular mixture of species and grades. The strength groups are described as ‘C’ (coniferous) prefixed softwoods, and ‘D’ (deciduous) prefixed hardwoods.

Moisture Content of Timber

Immediately after the felling, the water in the tree sits inside the walls and voids of the cells mentioned above. As the timber dries, water is then removed from inside the cell voids and there is no dimensional adjustment as the moisture content decreases (m/c is the weight of water to oven-dry timber) before the ‘fibre saturation stage’ (about 25 per cent m/c) is achieved and water starts to be removed from the cell walls. Dimensional change, otherwise known as shrinkage, then commences. Likewise, when timber attracts moisture it will swell until the fibre saturation is reached.

Timber moisture content can influence strength and rigidness properties and typically improves timber strength with decreasing values of moisture content. The quality of the construction referred to in BS EN 338:2009 relates to the characteristic strength of the wood with a moisture content compatible with the specifications of Service Classes 1 and 2. At above 20% moisture content (Service Class 3) timber is classed as wet and stresses are reduced with increasing moisture content up to 30%.

Dimensional change in timber perpendicular to the path of the grain is generally calculated as 1 percent change in scale with every 4 percent change in the quality of moisture. For practical purposes shrinkage is common enough parallel to the grain to be neglected

Creep of Timber

An significant aspect of timber that affects its efficiency of serviceability is ‘creep.’ Timber deforms elastically when first loaded but over a period of time, more deformation occurs.

The degree of creep deformation would increase with the timber’s increased moisture content and the load applied for longer durations. For eg, the deflection of a sawn timber joist in Service Class 2 could increase by as much as 80 per cent of the initial deflection due to the creep.

Durability of Timber

Timber materials do have different degrees of natural durability and this depends on the plant. Timber rot can typically occur where the moisture content is over 22 per cent over an extended period of time, e.g. when building structural deficiencies or inadequate upkeep cause moisture to accumulate up to this level.

Where a non-sustainable specie requires durability against the possibility of deterioration or insect attack, chemical preservative treatments may be applied. Preservatives are also used as a second line ‘insurance’ against defects in design or production that may lead to moisture content above 22%.

Examples where timber preservative treatment according to BS 8417 (taken from NHBC Standards Appendix 2.3A) is needed are:

• Roof timbers in areas where house longhorn beetles (Hylotrupes bajulus L) are prevalent (see Building Regulations Paper A accepted Table 1)
• Flat roof joists and tiling battens which are exposed to condensation risk
• Soleplates above damp proof course level
• Exterior wall studs

Alternatively, for a particular project where natural durability is necessary, the structural engineer can follow a specific timber species, such as exposed timber columns, bridges, and water-containing structures.

However, the majority of timber structures use timber materials which are considered not to be durable and it is thus the construction of the structure itself that plays the most important role in ensuring that the timber is not vulnerable to high levels of moisture in use, thereby increasing the capacity for decay.

Fire Resistance of Timber

Wood is a combustible material and the rapid spread of flame across its surface can occur when it ignites. However, carbon build-up on the surface (in the form of charred wood) limits the supply of oxygen to the underlying wood and acts as an insulator, making the wood below the charred level relatively cool and retaining its structural integrity.

Therefore, solid timber of broad cross section can be built without additional fire protection in accordance with BS EN 1995-1-2:2004 by considering the properties of the degraded section below the charred timber. Alternatively, the wood can be covered by fire-resistant linings such as plasterboard.

The ignitability and surface spread of flame of exposed timber surfaces can be reduced by surface coating or impregnating chemical treatments.

Merits of using timber for the 21st century structures

Timber has outstanding ecological qualities, as a sustainable and green construction material. It functions as a drain of carbon and possesses low energy embodied. Thus, the energy required to turn trees into the wood and consequently into structural timber is considerably smaller than other construction materials, such as steel and concrete. Figure 3 contrasts the demands of lightweight concrete block and timber frame walls for energy production. This argument is further proven by comparison of the energy needs for steel, reinforced concrete and timber beams, as seen in Figure 4.

Depending on species, timber can also have good durability characteristics and good insulating properties against heat and both airborne and impact sound.

Timber has a very high strength-to-weight ratio. This is illustrated as a comparison between steel and timber in Figure 5. Also, softwood timber has a low density relative to other structural materials. This means that it can offer light structural solutions resulting in advantages such as reduced loads on the foundation and ease of lifting prefabricated elements during transportation and assembly.

The use of timber as a structural material encourage pre-fabrication. Many new structures are prefabricated in off-site materials. As a result, there is a general provision for fewer on-site activity and a shortened service time for on-site work. Prefabrication in turn also offers an incentive for quality management and removes the vagaries of temperature and site conditions.

This post is just an introduction. More information on timber specifically in relation to its design and construction will be put out in the subsequent posts.

Also: See: Classification of Steel Cross-section

Sources

Porteous J. and Karmani (2008). Structural Timber Design to Eurocode 5, Chichester, John Willey & Sons.

Lancashire R. and Taylor L. (2011). Timber Frame Construction (5th Ed). High Wycombe, Trada.

The Institution of Structural Engineers (2012), Technical Guidance Note 18 (level 1), The Structural Engineer 90(11)

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