Timber as a modern construction material

Martti Kiisa
Professor, Vice-Rector for Academic Affairs
TTK University of Applied Sciences
Estonia

Karin Lellep
Lecturer
TTK University of Applied Sciences
Estonia

Timber is nowadays used for both simple as well as architecturally and technically complex structures. The heights of timber buildings exceed 50 m and spans of roofs and bridges can be over 100 m. Timber has numerous positive but also many negative properties that must be skilfully taken into consideration when designing. The claim that timber is not a considerable construction material because it rots and burns, is outdated and distorted. Decisions regarding timber structures can be made only by those who are up to date in this field and rely on research-based knowledge.

Among the many advantages of timber, it can be emphasized that it has a relatively high strength compared to its density. Although timber is most commonly used for simple elements (columns, beams, arcs, slabs) it is also possible to construct rather complex structures such as trusses. Thanks to the continuous development of technology the selection of timber products is very diverse. The biggest contribution to the development has been made by adhesives that allows to produce elements with very big spans and special cross-sections. Engineered wood products are based on sawn timber boards (glued laminated timber, cross-laminated timber), veneers (laminated veneer lumber, plywood) and strands, chips or fibres (oriented strand board, etc). If to add here different built up (like I-beams) and composite structures (combined mostly with concrete or steel), it is clear that there are a lot of options to create modern and impressive structures. Timber structures can undoubtedly compete with almost any construction material from both the structural aspect as well as solution diversity.

When using timber as a construction material, its disadvantages must also not be disregarded. Timber as a construction material has been investigated thoroughly – scientific research on its physical properties started already more than 250 years ago. A smart approach enables to consider the special characteristics of timber and the necessary applied instructions already exist. The unique structure of wood that develops when growing is more complex than any other construction material. The main factors that need to be taken into consideration are moisture content, duration of the load to be applied and in which direction, compared to the grain, the load is applied. One of the biggest challenges has turned out to be the insufficient moisture protection of timber, but there are many constructive and chemical solutions to fix this problem. E.g., it is possible to change the properties of timber in a wide range by modifying the cell wall polymers using acetylation, furfurylation and heat treatments. The main focus of these modifications and research is to increase biological resistance and dimensional stability and also to replace toxic wood preservation chemicals like creosote, arsenic, copper and chrome. In recent years a lot of research have been carried out to develop moisture-proof timber constructions.

The second biggest challenge is the fire safety of timber buildings. It is obvious that timber is a combustible material and can be destroyed in a fire. But actually, timber burns at a relatively slow rate and in a controlled form. A lot of effort has been put into the development of both appropriate measures as well as regulations. According to regulations and guidelines structures should limit all risks related to fire (including spread of fire, spread of fire gases, evacuation) and the safety of firemen is also taken into consideration. The most common methods to secure fire safety are adequate cross sections, sheathing, sprinklers and material modifications.

The fact that timber is the only widely used renewable construction material in the construction sector must surely be considered when making smart decisions. Approximately 4 billion hectares of land is classified as forest in the world and 25% of these forests are located in Europe. Timber is an ecologically clean material, and its use reduces the amount of CO2 that is cast into the atmosphere. In addition, the manufacturing of timber products takes quite a small amount of energy and effects the environment minimally.

Timber as one of the oldest construction materials that has been used already during the stone age, is undergoing a new awakening. Although there have been considerable changes taking place for over the last quarter century, the biggest developments have above all taken place in the last decade. Timber is by far not only a finishing material anymore, but also a considerable material for load-bearing structures. It is therefore important to understand the essence of timber as a material, trying to consciously and wisely take advantage of its strengths but at the same time skilfully avoiding its weaknesses. The increasing use of timber structures is a considerable option in environmental protection and ensuring sustainability.

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