What is so special about timber constructions?

Quite wrongly, timber is normally associated with low durability. The truth is that there are examples demonstrating that timber constructions can last for centuries. Under certain conditions, the durability of timber constructions can be high. The key degradation agent is the water. The most severe is undoubtedly fire. But the most frequent is water. However, water in itself is not the degradation agent. Actually, in the presence of water, the wood’s water content rises, creating ideal conditions for the appearance of degradation agents such as fungi, insects and termites. To put it more simply, it is well-known that if we keep the water content of timber members below 20%, the risk of degradation is minimal. This should be our main prevention measure. Then, we know that some wood species show a natural durability that is more appropriate for particular applications. The designer is not only responsible for making the safety calculations, but also for selecting the most appropriate wood species for a given application. If the natural durability is not sufficient for the level of aggression represented by the application’s environment, then the wood must be treated. However, not all wood species can be treated, or permit the same level (depth) of treatment. The durability of a timber construction is greatly associated with the construction details. The detailing should boost the ventilation of the wood and prevent the accumulation of water in contact with the timber. The design should seek to reduce direct the wood’s contact, in particular on its longitudinal direction (this is the direction along which the fibres are aligned, so as to facilitate the transport of water inside the fibres in this direction) with all possible water sources.

Timber is easy to work with, to cut and drill holes in. This advantage is a double-edged sword, as it can sometimes be a disadvantage. By being easy to work with, it often leads to a certain carelessness on-site, or lack of appreciation of its true value. It is commonly assumed that even someone with limited construction or even manual work skills would not have much difficulty in building with timber. This less technological, more archaic aspect is very damaging for building with wood and is not entirely wrong. Nowadays, timber construction uses the most modern technological and digital processes available. When we currently talk about the importance of prefabrication, digital imaging and for example BIM, the timber industry emerges as an advocate in these areas. Timber construction is by definition prefabricated. Nowadays, all timber construction members are designed and produced using computer-aided design (CAD), computer-aided manufacturing (CAM) and computer numerical control (CNC) technology. Rigour is in the order of one millimeter.

Bearing in mind the analogy presented above of wood composition being fibres aligned longitudinally with the tree trunk, this concept becomes easy to understand. Wood shows different physical and mechanical performances according to the direction in relation to the fibres. Along the direction of the fibres, its mechanical resistance, tensile strength and compression is high. It is in this direction that the porosity is higher as this direction facilitates the circulation of water in its interior. However, in the perpendicular direction, resistance to tension is very low. Resistance to perpendicular compression to the fibres is lower than that in the parallel direction, but shows an interesting ductile performance, wisely explored by master carpenters in the design of joinery with notches, steps, tenon-mortise, etc.

A joint represents the union of various members. These members are normally presented in different directions, inclinations, etc. Since we know that timber is an anisotropic material, whose physical and mechanical properties depend on direction, it becomes easier to accept that the joints represent the weakest members of the timber construction. From the start, we have members with distinct performance because they are submitted to stress in different directions in relation to the fibres. The connectors most used in timber construction are of the dowel-type – screws, nails, bolts, etc. These connectors essentially works under shear stresses. Nowadays, there are joints that convey part of the stress by axial forces, but the design of the joints should take into account that the transfer of stress is essentially by shear (with deformation of the connector and localised crushing of the wood).

What makes building with wood so special is that, in order to be successful, it is necessary to understand the material we are using. For example, we could have conducted the most rigorous and reliable calculations, but we selected a wood species that does not have the appropriate durability for the application, with the certain outcome that we will end up with a construction with a short useful life and flawed performance. But when we are armed with this knowledge, building with wood is fantastic.