Wood as a Construction Material: History, Properties, Use

Introduction

For as long as mankind has been civilized, wood has remained an important material for the construction of housing and boats. Up until the close of the nineteenth century, virtually all the boats were constructed using wood (Hoadley, 1980). Today, wood is still a common material in the building of boats and houses. The use of wood in the construction of the frames of a majority of domestic houses is also common.

Even in a case whereby a building has been constructed with the help of such other materials as concrete, wood somehow finds use in some parts of the construction too, such as in the construction of interior door frames, roofs, or even in the exterior cladding (Crawford, 2001).

Traditionally construction of houses in America such as the building of log cabins is notably one area in which wood has been utilized in construction. This shows that the need for use of wood in housing goes way back in history. In addition, the era of timber framing coincides with sawmill development.

The first-ever water-powered sawmill was located in Maine, California, as early as 1620. Later on, balloon framing as a form of construction was introduced, at just about the same time that the powered steam sawmills were being developed. With the growth of veneer panel products, the design of western platform framing was also formed (Hoadley, 1980). This was at the start of the 190s. Wood, being amongst the most natural and vital materials of engineering, is historically viewed as a wonderful product.

Among the largest ancient bridges ever built, some of them were almost entirely constructed using wood, with only modest use of iron fastening. Traditionally, platform and balloon framing have been some of the light-frame methods of construction that have been in use. On the one hand, balloon framing was popular at the start of the 20^th century. The method of construction is made up of wall framing that is full-height, and which is usually applicable in the construction of two-story buildings.

Another construction method, platform framing, was however to become dominant in the latter stages of the 20^th century. The method still finds valuable use in light industrial and commercial constructions as well. Before the start of the twentieth century, a majority of the commercial and residential structures that were being erected in North America utilized wood as the chief construction material.

As wood was in abundance then, it was only fair that it should also form the basic structure for a majority of the brides, commercial buildings, and residential houses. Utility poles also greatly utilized the use of wood (Hoadley, 1980). Currently, most of the industrial buildings, residential and commercial houses all utilize wood as a structural material.

There are a variety of mechanical and physical properties, just as there are a lot of wood species. The weight-to-strength ratio of wood is also outstanding. As a renewable resource, wood finds wide usage in the construction industry (Nunnally, 2007). This is because of its versatility, and also because its useful end-products tend to be extremely cheaper in comparison to concrete, plastic, or steel.

Properties of wood

Physical properties

These refer to wood quantitative parameters, as well as how wood reacts to external factors, besides applied forces. As such, the physical properties of wood would include moisture content, dimensional stability, fire properties, electrical, density, chemical, thermal, as well as decay resistance.

Directional properties

Wood serves both as an anisotropic and orthotropic material. Based on how the fibers of wood have been oriented, as well as the fashion in which the diameter of a tree increases with age, its directional properties also tend to differ alongside the radial, longitudinal, and tangential axes.

The longitudinal axis is that which happens to run comparably to the direction of the fibers. On the other hand, the radial axis is that which runs regularly to the growth rings. Finally, the tangential axes are those that run tangentially to the expansion rings, as well as at right angles to the route taken by the fibers. A majority of the wood properties shall often vary based on the three properties.

Moisture content

As a hygroscopic material, wood is able to absorb water under humid conditions, as well as lose the same when the atmosphere becomes dry. Consequently, the water content of wood is dependent on the temperature and relative humidity that surrounds it. The balance between the moisture content of wood and that of its surrounding represents the equilibrium moisture content.

Under the structural applications, variations in humidity and temperatures also ensure that the moisture content of wood keeps changing. Such variations come about due to both the short-term and gradual fluctuations on wood’s surface. Whereas it would be impossible to combat the variations in moisture content, they can however be reduced by either treating the surfaces of wood, or even by coating it.

Dimensional stability

When the moisture content of wood falls below its fiber saturation point, then such wood tend to shrink. When the moisture content is above this, then swelling occurs. As a result of such dimensional changes, checking, splitting and warping shall also occur.

Thermal expansion

Heating moist wood leads to its expansion, as its moisture is reduced by the rise in temperature. When the moisture content of wood is between 8 and 20 percent, then such wood tends to expand upon heating. Later, it shrinks to occupy a much lesser volume than it did before heating had occurred, owing to a loss of moisture.

Pyrolysis

This refers to the thermal degradation of wood. Wood not only generates, but also retains heat. Timber is thus highly susceptible to burning, but only in the presence of an externally-generated flame. Furthermore, timber is capable of generating a layer of char made up of wood combustion residue. In the case of heavy timber, the char layer opposes combustion by forming a cushion between the source of flame and uncharred wood.

Density and Specific Gravity

A material’s density refers to the mass of such a material for every single volume of the same material, under precise situations. Wood, being a hygroscopic material means that its density is reliant on two factors: the moisture that such wood is able to retain, and its weight. With differing moisture contents, the density of wood shows significant variation too (Nunnally 2007). In order for such densities to be of valuable meaning, their reporting ought to be undertaken under certain conditions. On the other hand, the specific gravity of wood is used to refer to that index used to determine the amount of wood substance that is contained in a given wood sample. As such, specific gravity of wood acts as a ratio of the volume of wood that has undergone oven draying, relative to a similar volume of water (Hoadley 1980).

For engineering purposes, the specific gravity of wood is often determined when the moisture content of such wood is at 12 percent. For instance, for a volume of wood at some particular percentage relative humidity, a wood sample would record a density equivalent to 500 kg/m3, while its specific gravity would usually record as 0.50.

Decay Resistance

Below moisture content of 20 percent, woods are not susceptible to decay. Additionally, wood shall also not decay if it is repeatedly immersed into great depth of water for a substantial amount of time. Decay thus occurs in between this continuum. The most important parameter to help avoid wood decay is the maintenance of its moisture content.

However, in case this proves impossible, then either the treated timber or the durable varieties can be used. The ability of wood to resist decay is usually a function of the species of such wood, as well as its structural characteristics. On the whole, the external region (sapwood) of all wood species easily renders itself to rapid deterioration.

On the other hand, the natural durability of the heartwood is dependent wood species. The rapid death of the sapwood, leads to the formation of heartwood. For some wood species, some of their sugar cells are transformed to extractives that usually tend to be extremely toxic.

These then gets placed inside the wall of wood cell. For a majority of the wood species, these are able to produce heartwoods that are durable, and these include redwood, western red and black locust. Nevertheless, durability differs both between the various trees of a given species, as well as within a specific tree. In an attempt at augmenting durability, the use of chemical preservatives with EPA approval could as well be used in the treatment of wood.

Resistance to chemicals

The levels of resistance of wood to a wide variety of chemicals are remarkably high. As such, wood is more preferred as a construction material. Thanks to its high resistance to mild acids, wood is more preferred compared to both steel and concrete.

Although iron work better in alkaline conditions, wood renders itself to a wide range of preservatives, thus improving its performance under these conditions.

With respect to chemical attack, the sapwood is more susceptible that the heartwood is. This is because the heartwood tends to offer a greater resistance to liquid penetration. To augment liquid penetration, there are a number of preservative treatments that could be utilized, such as the use of pentachlorophenol, or creosote. In addition, such treatment also serves as a deterrent to chemical attack.

Electrical Resistance

Wood acts as a better insulator in comparison to a majority of other construction materials such as steel. Nevertheless, there still exists a small element of electrical conductivity. Wood is a good electrical insulator. However, considerable disparities are usually exhibited in as far as conductivity is concerned.

Such conductivity variations tend to have a correlation to temperature, the orientation of the grains, and the moisture content of such wood. On the longitudinal axis, wood conductivity is about two times that recorded on the tangential or radial axes. As the temperature of wood rises by about 10 degrees Celsius, it has been observed that its electrical conductivity also tends to increases twofold.

By and large, discrepancies in conductivity associated with the wood species and density are usually deemed as being minor. The association connecting moisture content and electrical resistivity are the two parameters that are utilized in the functioning of moisture meters, but the measurements tend to be effective only for readings ranging from 5 to 25 percent.

Mechanical properties

Elasticity

This is used in reference to the ability of a material to withstand a deformation as a result of an applied force, while at the same time also being able to revert to its original shape once such a stress has been withdrawn. The elastic properties of wood are not ideal, meaning that it may not return to original shape upon threw removal of a stress. Nevertheless, wood is still regarded as an elastic material from the point of view of engineering applications.

Strength

This means the definitive resistance a material offers to a load applied. In the case pf wood, this strength shall; often vary, with respect to the conditions of loading, species the duration of the load, as well as on the basis of environmental aspects, and an assortment of other materials.

• Compression

The parallel application of a load will often result in a shortening of the cells of such a wood. This is because such an action tends to generate a stress deforming force. When such a stress is applied in a parallel direction to that taken by the grain cells of wood, instability and weaknesses occurs inside the walls of the cells. Increased stress leads to a folding of the cells of the wood, resulting in surface wrinkles.

• Tension

The tension of wood tends to be extremely strong along its grains. In wood, failure happens in two fold: failure of the cell wall, and inter-cell slippage. Slippage come about following a sliding between tow individual cells (Constantine 1975). On the other hand, a cell wall failure is as a result of a rupture inside the wall of a cell, followed by no observable deformation before a total failure has occurred.

• Bending

Bending is a vital property of wood. The stresses of bending get induced following the usage of a wood material as a beam, like in the case of a rafter system, or even a floor. Douglas-fir has been shown to have a bending strength of 52.6 MPa, while that of Loblolly pine is given as 50.3 MPa.

On other hand, the elasticity modulus of the two types of wood is 10.7 and 9.7 GPa, in that order. Owing to the variation in compressive and tensile strength corresponding to grain, the bending strength of wood thus tends to exceed its tension. In contrast, this bending strength also tends to be less than during compression.

• Shear

During its application as a beam, wood gets exposed to both tensile and compressive stresses from on either surface. Consequently, horizontal shearing occurs due to contrasting stresses. For loblolly pine and Douglas-fir, this horizontal shearing has been recorded as being 5.9 and 6.2, in that order. Equally, the perpendicular application of stress and corresponding to the grains causes cells of the wood to roll onto one another (Constantine 1975).

• Resistance to energy absorption

Also known as shock resistance, this is an index of a material‘s ability to both absorb and give away energy, by way of deformation. In this respect, wood exhibits outstanding resilience.

• Fatigue

In construction management, the resistance properties of wood to fatigue become important. Just like a majority of other fibrous materials, wood offer a resounding resistance towards frequent loading effects. In comparative analysis, fatigue strength of wood has been shown to be a few times greater than that of a majority of the metals (Hoadley, 1980).

• Hardness

This is a measure of the ability of wood to overcome marring and indentation. The comparative determination of hardness is on the basis of that force which is needed for the embedding a 1.5 diameter of an 11.3-mm ball inside wood (Constantine, 1975).

Uses of wood

 Application in cladding and sheathing purposes

The use of wood products for purposes of cladding has especially been buoyed by its exceptional performance. According to the Wood Council in Canada, the wood used for both sheathing and cladding purposes has made it possible for application in a variety of building designs and types (Sullivan & Bennet 2008).

Additionally, wood could as well be integrated into high-performance covering congregations, like in the case of rainscreeens. The Wood Council of America has also noted that wood-based products renders themselves to such structural resistances as cannot be matched by any other building material.

Such resistance is applicable to seismic loads, as well as a resistance to wind. Moreover, wood is also an excellent barrier to infiltration by air. As a cladding material, numerous architects have applauded wood owing to ease of adaptability to the conditions in the field, as well as its malleability (Sullivan & Bennet 2008). In contrast, wood as a construction material may not be of the same strength as steel.

Even in a case where by architectural protections such as substantial hangings and pressure treatment have been utilized, wood that has been treated so will have to degrade in the long-run. The secret then is to ensure that such wood has been explicitly coated with a material that hinders both exposure to water, and ultraviolet light.

In light of this, the Building Science Consulting, a group of building and constriction experts who are based, in Westford, UK, strongly recommends that wood meant for either sheathing or cladding be first sealed on all its six surfaces, to overcome the effects of rainwater.

Wood products that have been engineered afford stiffness and strength properties, are readily available, and also enjoy durability and insulating properties (Crawford 2001).

Besides, wood sheathing has been shown to perform better as opposed to either concreted buildings, or those made of masonry, in the face of a pending earthquake.

Construction of floors

Floors made of wood are popular owing to their durability and aesthetics appeal. Not only do wooden floors perform better with radiant systems of heating, they are also warm to the feet. By rejuvenating them with refinishing and sanding, and which only has to be done for say, every 15 years or so, floors made of wood have been shown to last longer (Nunnally 2007).

What is more, a floor made of wood renders itself to a complete biodegradable process. According to market studies, it can now be revealed that such floor choices as plank and parquet are quite popular and appealing.

Findings of a national survey that had been commissioned by the National Association of Wood Flooring revealed that about 90 percent of agents of real estates concurred that those houses that are made of wooden floor tends to sell much more easily (Sullivan & Bennet 2008).

In addition, such houses have also been shown to sell at a premium price, as opposed to the ones whose floors are quite plain and exposed. It is also the opinion of many building scientists that wood floors assist house occupants overcome some of the more common problems that comes about as a result of carpeting. Such problems would include toxins, dust, and bacteria.

Construction of wooden windows and doors

Thanks to increased demands fro aesthetics, a lot more of the institutional, commercial, and multifamily houses are turning towards the use of wood for the construction of both doors and windows. Des Plaines, an association of doors and windows makers in both the United States and Canada provides that wood has over the years become quite versatile owing to its composition, botanical structure, and its physical and chemical properties.

Wood as a material for construction tends to exhibit relatively high strength when compared to the weight. In addition, wood also exhibits elevated electrical properties, while its heat conduction parameters are quite poor. Moreover, the acoustical properties of wood are quite remarkable. Wood also tends to impart a warmth feeling, some thing that one may not experience with other construction materials such as plastics and metals.

Timber bridges

Before the 20^th century was ushered, both the railroad and highway bridges were mainly constructed using timber. With the development of reinforced concrete and steel came other two options for use in the construction of bridges. To date, the two alternatives have become the chief building materials for a wide variety of bridges (Sullivan & Bennet, 2008). Nevertheless, there are still a large number of bridges in the united stars that are still built with the help of timber as the primary raw material.

In light of this, efforts are being directed towards technology transfer ands research with a view to augmenting the performance and design of wooden bridges. Consequently, the early 1990s saw hundreds of wooden bridges being built across the regions in the United States (Nunnally, 2007). A majority of these were constructed with the help of innovative materials and designs.

The construction of bridges consists of both a superstructure, and a substructure. The later is made up of piers, abutments, or even pilings. It is this substructure that offers reinforcement to the superstructure. On the other hand, the superstructure consists of deck and stringers.

The deck has asphalt as a wearing surface covering it. More often that not, timber gets combines with the rest of the materials that forms a superstructure. For instance, a timber deck could be laid over stringers made of steel (Sullivan & Bennet, 2008). Covered bridges were quite popular. Nevertheless, this popularity has waned over the years, as they tend not to be feasible in the long-run.

Conclusion

Historically, wood as a construction material has gained immense popularity, mainly as a result of it’s readily availability, in comparison to other construction materials such as steel. Moreover, wood was quiet abundant in the previous years. As such, a majority of the structures, ranging from log cabin, to boats, and to residential and commercial houses were built using wood as the primary raw material.

Wood has also historically been utilized in the construction of bridges, a trend that has remained to this day. Wood has remarkable aesthetic appeal, like in instances whereby it has been used in the construction of floors, as well as its use for cladding and sheathing purposes. There are numerous physical and chemical properties of wood that normally renders it to become a popular material of construction. For one, wood acts as a good insulator, meaning that it finds quite a wide range of usage in the construction of doors, and windows.

Wood also exhibits remarkable resistance to the conduction of electricity. Wood is also resistant to decay, at moisture content below 20 percent. Furthermore, wood also tends to be resistance to a wide variety of chemical, and can also be treated with such chemicals as creosote to make it so.

Furthermore, wood also tends to exhibit outstanding compression, tension and bending properties, and these are some of the properties that construction engineers are on the look-out for; in as far as construction materials are concerned. Little wonder, then, that wood has time and again proved to be the construction material of choice.

Works cited

1. Anderson, L. O. Wood- Frame House construction. London: Minerva Group, 2002.
2. Architectural education journal (2008). “A general introduction to wood construction”. Journal of architectural education.
3. Constantine, A. Know Your Woods. New York, Charles Scribner’s Sons, 1975.
4. Crawford, J. (2001) “The wonders of wood: the importance of forest products in a sustainable society”
5. Hoadley, Bruce. Understanding wood: a fine woodworking book. Newtown, CT, The Taunton Press, 1980.
6. Nunnally, S. Construction Methods and Management (7th Ed). New York: Prentice Hall, 2007.
7. Sullivan, C. C. & Bennet, Barbara. (2008). “Using wood for sustainable design and construction”.