Wood and timber

No other material has all the advantages of wood. One material may equal wood in insulating quality but lack its abundance and low cost. Another may rival it in strength but fail on the point of workability. A third may rank with it in workability but fail to measure up in durability. If wood were a newly discovered material, its properties would startle the world.

Background

Since the human race first started to build crude shelters at the dawn of civilization, wood has been available as a construction material. It has long been used in the construction of buildings, bridges, and boats. As technology developed, wood also found a variety of less readily recognizable forms, such as paper, films, and pulp products, many of which are mainstays of daily life.

Woody material is produced in many plants, but its most useful manifestation is in the limbs and trunks of trees. There is a great diversity of tree species, and most climate zones have at least one that has adapted to the prevailing conditions within that area. Thus, wood is generally available in most inhabited regions of the world. Wood has played a dominant role as a construction and engineering material in human society, yet humankind has lived with this material for so long that its significance is easily overlooked.

Hardwoods and Softwoods

Trees are broadly classified into hardwoods and softwoods. These terms can be misleading, since they are not connected to the actual hardness of the wood. Hardwoods are broad-leaved deciduous trees, while softwoods have narrow, needlelike leaves and are usually evergreen. Oak, birch, and basswood are common hardwood species; longleaf pine, spruce, and cypress are softwoods. Some hardwoods, such as oak, are actually hard. Many others, such as basswood, are actually softer than the average softwood. In fact, balsa is classified as a hardwood even though it is one of the softest woods in the world.

By far the majority of timber used in building structures comes from the softwood category. Douglas fir, southern pine, and redwood are some of the important softwood species widely employed in structural applications. They are relatively strong and can be used in structural elements such as joists, beams, and columns. By comparison, the stronger hardwood species, such as oak, are relatively heavy, hard to handle, and hard to nail. As far as construction is concerned, their utility is limited; they are generally used only in flooring, cabinetry, and furniture.

Supply and Disposal

Wood is a renewable resource. It does not exist in finite quantities; rather, it is constantly produced in growing trees. If forests are carefully managed, timber can be harvested on a sustained-yield basis year after year. Wood is also a reusable resource. The recycling of timber from old buildings is well documented. The ease with which wood can be cut, joined, and worked into various shapes permits the extension of its functional life beyond that of many other construction materials.

Wood is a biodegradable natural product, meaning that it can be reduced to its constituent carbohydrates and extractives through degradation. After wood has reached the end of its useful service, it can be disposed of with little damage to the environment. Unlike plastics or chemicals, timber has a very low pollution potential. A study quantifying the pollution potential of various construction materials found that steel is five times more polluting than timber, while aluminum and concrete blocks are respectively fourteen and twenty-four times more polluting. From an environmental standpoint, timber is recognized as the most appropriate construction and engineering material.

Logging

Tremendous quantities of timber are consumed each year throughout the world. An average of about 3.5 billion cubic meters of timber are harvested annually. The majority of hardwood harvest is used for fuel, while softwoods are primarily used in construction and manufacturing. To produce the large quantities of timber needed annually, logging operations have become highly organized and technologically advanced. When trees are removed in harvest, steps are taken to provide for forest renewal and to prevent soil erosion. Such steps include leaving some trees to produce seeds, transplanting young trees, and other methods of reseeding. Sometimes a “prelogging” operation is undertaken before the main harvest. In this phase, the small trees are removed for conversion into poles, posts, and pulpwood. During harvest, various types of machinery are used to cut trees close to the ground. The limbs are then removed from the fallen trees and the trunks are bucked into various lengths and transported to sawmills for further processing. The remaining tree limbs are converted into chips for sale to pulp and paper mills. Frequently roads are built to facilitate the transportation of trunks and the deployment of heavy logging equipment. At the conclusion of harvest, refuse should be disposed of so that it will not interfere with the growth of new trees.

Owing to careful management of forests and improved efficiency of logging operations, the supply of timber in the United States currently renews itself at a higher rate than the removal level. It must be pointed out, however, that growth in world population will inevitably bring about an increase in timber consumption. The adequacy of timber supply will be a matter of concern in the future.

Physical Structure and Strength

As a material of botanical origin, wood is composed of hollow, elongated fibers. These fibers are usually arranged parallel with one another in the direction of the length of the trunk. They are cemented together by a substance known as lignin. The fibers in softwoods are longer than those in hardwoods. The length of the fibers, however, is not a criterion of the strength of the wood. Owing to the parallel arrangement of their fibers, wood possesses different mechanical properties in different directions and is said to be anisotropic. As an example, timber is five to ten times as strong in compression parallel to the grain as it is perpendicular to the grain. The varying strength of timber in different directions must be taken into consideration in construction design. By contrast, metals are isotropic, having the same characteristics in any direction.

The strength of timber is affected by its moisture content. Wood in a living tree typically contains more moisture than the surrounding atmosphere. When a piece of timber is cut from the log and exposed to air, its moisture content decreases to an equilibrium value determined by the temperature and relative humidity of ambient air. Should wood dry below a value called the “fiber saturation point,” it becomes stronger and stiffer. That is why higher design stresses are allowed for timber which is used under relatively dry conditions, such as a girder in a building, than for timber used under relatively moist conditions, such as in a waterfront house or in a bridge.

Wood has a high strength-to-weight ratio. Compared with many other construction materials, wood, pound for pound, is stronger. For instance, in bending tests, Douglas fir has a strength-to-weight ratio which is 2.6 times that of low-carbon steel. Wood also has very high internal friction within its fibrous structure and is therefore a good absorber of vibrations. It has much greater damping capacity than other materials, particularly the metals. That explains why wood is the preferred material for construction of houses in earthquake-prone regions. Finally, timber structures can be designed to withstand impact forces that are twice as large as those they can sustain under static conditions. Materials such as steel and concrete do not permit such increase in the applied forces. This exceptional impact strength of wood is used in timber structures such as bridges and the landing decks of aircraft carriers.

Insulation and Fire Resistance

Because of its fibrous composition, wood has excellent insulating properties. At a low moisture content, wood is classified as an electrical insulator. This is what makes wood such a common material for high-voltage power-line poles and for tool handles. Wood is also an effective thermal insulator. The thermal conductivity of timber is only a fraction of that of metals and other common construction materials. For example, bricks are about 6 times more conductive than timber, and glass and steel are respectively 8 and 390 times more conductive. Thermal insulation of timber structures can be further enhanced using stud walls or layers of spongy materials. In addition, timber structures may be designed to provide a degree of acoustic insulation. Sound is transmitted through vibration of air particles. Because of its high vibration-damping capacity, wood is also a good acoustic insulator.

It is well known that wood is combustible. On the other hand, wood that is thick enough is also fire resistant. Because of the low thermal conductivity of wood, the high temperatures of a fire cause a temperature rise for only a short distance into the wood from the surface exposed to the fire. This is the reason larger timber members may continue to support a structure in a fire long after an insulated steel member has collapsed because of elevated temperatures. In fact, buildings framed with large timber members have been given the highest rating by fire underwriters among all common buildings erected.

Fabrication and Workability

Wood may be cut and worked into various shapes with the aid of simple hand tools or with power-driven machinery. It therefore lends itself not only to conversion in a factory but also to fabrication on site. It is the latter fact that principally keeps conventional wood-frame construction fully competitive with any method of prefabrication of houses yet employed.

Timber can be joined with nails, screws, bolts, and connectors, all of which require the simplest kinds of tools and produce strong joints. Timber may also be joined with adhesives, which can produce a continuous bond over the entire surface to which they are applied and develop the full shear strength of the solid timber. This use of adhesives provides a means of fabricating timber members of different shapes and almost unlimited dimensions. The prefabrication of large wood trusses, laminated beams and arches, and stress-skin panels has permitted wood to remain extremely competitive as a building and engineering material.

Durability

Wood is remarkably resistant to decay and is inert to the action of most chemicals. It is widely used in facilities for bulk chemical storage; the timber may be in direct contact with the chemicals. When wood is exposed to atmospheric conditions, it slowly erodes under the action of weather at a rate of about 0.64 centimeter per century. There is no reason why, if properly used, wood should not last for a long time. Decay and insect damage are often significant problems, but these can be minimized by following sound methods of design in construction and by using properly seasoned timber. In situations where biological wood-destroying agencies are difficult to control, the decay resistance of timber can be maintained by impregnation with suitable preservatives.

Untold numbers of well-designed bridges and buildings made entirely or partly of wood have served satisfactorily for long periods with little maintenance. Many that are more than a century old are still in service, whereas others, although in satisfactory condition, have been altered or replaced to meet more stringent modern building codes. If years of satisfactory service are a measure of durability, few construction materials can rival timber.

Significance of Wood

Wood has remained a primary construction material for thousands of years essentially because no competitive material has all the advantages of wood. No other natural substance can meet the ever-increasing demands of modern society for paper and other pulp products. It is also unlikely that a synthetic material can be made that can compete economically with wood as a source of pulp, particularly in the light of the limited supply and high cost of petroleum. On the other hand, methods for converting wood into various chemicals are continually in development. There is potential for using wood as a raw material to produce chemicals that are now obtained from petroleum.

Future Prospects

Tremendous progress has been made in transforming wood from a material of craftsmanship to one of engineering. Reliable structural grading, improved fastenings, efficient fabrication, and glue-laminating have all contributed to making wood a modern construction material. Timber connectors and other improvements in fastenings have permitted the use of small timber members for larger spans. It is expected that even better methods of fastening will be developed so that long, clear-span timber trusses will become common sights in new buildings.

The increasing popularity of glue-laminating is of particular significance. A glue-laminated timber member typically has greater strength than a solid sawed member of the same size. It may also have superior surface properties such as higher fire resistance. The laminated arches used in churches and buildings are common examples of this application. Other examples include the exterior waterproof laminations in such structures as bridges and ships.

Bibliography

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Flynn, James H., Jr., editor. A Guide to More Useful Woods of the World. Forest Products Society, 2007.

Green, Harvey. Wood: Craft, Culture, History. Viking Penguin, 2006.

Hoadley, R. Bruce. Understanding Wood: A Craftsman’s Guide to Wood Technology. Rev. and updated ed., Taunton Press, 2000.

"Timber Resources." National Geographic, 19 Oct. 2023, education.nationalgeographic.org/resource/timber-resources/. Accessed 29 Dec. 2024.

United States, Department of Agriculture, Forest Service, Forest Products Laboratory. The Encyclopedia of Wood. 1999. Skyhorse Publishing, 2007.

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