Important facts about wood
Updated: Dec 16, 2021
In an earlier post, I addressed the "renaissance of wood". Following that I will present the facts that I believe are the most important to consolidate some of the arguments in favor of the use of wood in architecture.
Wood is used by man for the merit of its own qualities, but the arguments that are presented to convince us to choose wood do not always appear as impartial. Wood companies, in particular, announce it as the eighth wonder of the world. It is true that we are dealing with an extraordinary material, but from the point of view of Architecture also steel, concrete, and brick are beautiful materials.
Although I am an adept of wood, I am not Xylo-fundamentalist. I am well aware that wood does not solve all the architectural problems. So, I do not believe in a “brave new world”, in which this excellent material replaces all the other materials. I even consider that wood is sometimes not a good choice and it will almost always have to be used in conjunction with other materials. Concrete foundations, for example, are required in almost all constructions. In most wood construction systems of wood, we can not work without connectors and the anchors and fasteners in steel. Concrete, ceramics, steel, and textiles are almost unavoidable in Architecture.
The beauty and comfort of architectural forms and spaces are the results of a correct balance between the different materials. Each material stands for Architecture as the good ingredients stand for an exceptional dish. Also in an Orchestra the "woods" benefit from the company of "metals", "strings" and "percussion".
I think it is clear that I do not intend to sell wood, but I have to confess that I like wood a lot, I like nature, I like forests, I like trees and I am worried about the destruction of the environment. So I decided to present some facts that may help us understand the validity of wood when we have to prescribe the materials for our projects.
What, then, are the reasons that can lead us to the unconventional choice of wood as a structural material? Although I could present ten, thirty or sixty facts, in a synthesis effort I present only a small selection organized into five groups:
Comfort, Environment, Economy, Efficiency, and Knowledge. Probably because these adjectives are so generic they tell us absolutely nothing. Let us then go to the facts:
1st - Wood is beautiful and comfortable
Typically, wood fans argue with the hygrothermal and acoustic comfort and the good indoor air quality provided by wood solutions.
The most relevant fact that we can indisputably point out is precisely the insulating quality of wood relative to rival structural materials. Roughly, wood is 400 times more resistant to thermal conductivity than steel and 10 times more thermally resistant than concrete or brick. This quality can be decisive when the goal is to avoid thermal bridges or when we want to use load-bearing exterior walls. Also the role of wood as a regulator of moisture in interior spaces, due to its hygroscopicity, can be a comfort factor. Interior wood elements and finishes allow indoor humidity fluctuations to be reduced up to 50% when compared to painted plasterboard solutions.
The broad concept of wellbeing encloses some subjective dimensions as aesthetics and as visual comfort. But it is well known that even these dimensions if confined to a specific culture and groups, can be codified, objectified and studied. Thus, according to several investigations about the relationship between psychology and environment, the use of wood in interior spaces has psychological benefits because of the emulation effect of staying in outdoor spaces. Feelings of comfort provided by wood may have an influence on blood pressure and heart rate, contributing to moderate stress and anxiety.
It is necessary to warn, that products responsible to promote comfort from one point of view can be harmful from another perspective. This is the case of wood products containing adhesives that emit harmful gases. An interesting case occurred in a long-term experiment in Riga comparing different types of walls. A construction with “wood log walls” and thermal insulation proved to be the best, both from the perspective of energy consumption and of hygrothermal behavior. However, this same solution turned out to be the one with the highest concentration of VOCs (Volatile Organic Compounds) due to the varnishes used. Also, the use of adhesives, containing urea-formaldehyde resins, especially in agglomerates and pressed products such as particle board, plywood and especially MDF, can contribute to asthma symptoms, fatigue, and allergic reactions. The same experiment also served to acknowledge the risk of overheating in wooden buildings when subjected to direct sun exposure. These weaknesses can, of course, be overcome by expertise and through correct design decisions.
On the attractive power of wood, we might mention Michael Green who once said that he had never seen anyone in his buildings hugging a concrete or a steel column, but that was exactly what he saw one day doing in one of his wood buildings. The relationship between wood and man is described by Frank Lloyd Wright in this way: "It is the most humanly intimate of all materials. Man loves his association with it, likes to fill it under his hand, sympathetic to his touch and to his eye. Wood is universally beautiful to man. "
2nd - Wood is environmentally friendly
Wood is a renewable, recyclable and reusable material. Its use in construction is economically beneficial to forests and it is an incentive for forest sustainable management. When wood is used efficiently, it reduces waste and energy consumption on site, while at the same time saves resources and costs in possible changes and demolition processes.
Some studies indicate that in the residential sector, wood residential solutions account for less embodied energy than steel solutions (16% less) and reinforced concrete (17% less). Based on the construction of a square meter of a wall, Polish researchers concluded that wood frame solutions consume only 270 MJ compared to 875 MJ for masonry solutions. Another study, comparing the life cycle of a single-family house in Light Steel Frame (LSF) with another in wood frame, concluded that the latter presents a more favorable behavior. Joseph Kolbe in his book "Systems in Timber Engineering" compared the energy required to produce a 3-meter high column in different materials, bearing the same loads. He concluded that a wood column weighing 60 kg and a consumption 60 KWh of energy is equivalent to a steel column of 78 kg and 561 KWh and a reinforced concrete column of 300 kg and 227 KWh, and yet a column of ceramic bricks of 420 Kg and 108 KWh.
Wood has also the advantage of reducing Carbon emissions of buildings, taking into account their energy efficiency and the carbon storage effect. It is considered that, due to photosynthesis, each cubic meter of wood growth absorbs one ton of CO2 and releases about 727 kg of O2. Thus, instead of being released into the atmosphere, CO2 is stored in building elements. In addition, as is normal practice in well-managed forests, felled trees are replaced by others, that will store more CO2.
3rd - Wood means economy
In North America or Australia or even Scandinavia, there is no doubt that low rise solutions are cheaper when built in wood and are therefore mostly used. There are significant percentages of their use in this type of building in countries such as the United States of America, with 90% to 94%, Canada with 76% to 85%, Nordic countries with 80 to 85% and Scotland with 60%.
In Australia, for example, the use of wood is said to reduce construction costs by up to 25 percent. In Portugal, there are no conclusive studies, although some clues may be given. An investigation we conducted with LNEC support in 2012, "Characterization of wooden houses supply in Portugal", questioned the companies of wooden houses on the comparison between the prices of wooden houses and current construction. In company’s responses, 40% of companies said that the price of wooden houses would be lower than the price of current construction houses, 36% considered that it would be the same and 24% said it would be higher. Recently Rusticasa responded as follows to a pricing question: "(...) to give you an estimate of values, we have houses between 450 € and 750 € per square meter (based on a 3 room’s house, with 200m2)". Also on this matter, Architect Telmo Cruz mentioned at the Conference “C-6 Building in wood "(it took place on 10.12.2010) that when designing a wooden dwelling he got prices of around € 750 / m2, which is considered better than other structural solutions. The same architect also stated that in another project (a marketplace) the choice of the wood structure was justified not only by the costs but also by its overall efficiency: the wood solution was 152 tonnes lighter than the concrete solution and 110 tonnes lighter than the steel and concrete solution.
An economy that leaves no doubt, especially in the Portuguese context, is the one we get in construction time. A pre-fabricated wooden house will is built faster than a construction that uses traditional processes. Of course, we have to admit that this is not exclusively due to the wood choice. The good performance is obtained here because of the pre-fabrication. However, if we compare prefabricated systems in steel and wood, the lightness of the latter will be a very advantageous factor. Wood is easier to transport, to handle on site and it allows for easy adjustments and changes to the work in place.
4th - Wood is structurally and constructively effective
Steel and reinforced concrete offer great creative freedom, having been welcomed with great enthusiasm by the Architects since the 19th century. The extraordinary overhangs obtained by these materials, so much appreciated by modern Architects, are not the strong point of the wood. But wood has many trump cards to present, namely those derived from its already mentioned, lightness.
Safety in the event of an earthquake was evident for example in the Hyogo-ken Nambu earthquake, Kobe, Japan, in 1995, with an intensity of 6.8 (Richter scale), in which 6300 people lost their lives. Of about 8000 buildings of wood frame reached by the earthquake, in no one there were collapses or losses of human lives. The lightness associated with flexibility enables wood components to bend without collapse, while the horizontal and vertical forces are dissipated by the numerous components and connectors in the system. The multiple elements composing the frames are responsible for a structural redundancy that provides greater resistance.
Analysing the mechanical properties of wood and steel in relation to their weight, it is observed that wood, free of defects, with the same weight of steel is 1.6 times more resistant. However, we have to admit that for classified wood with some type of defects, the behavior of both materials turns out to be very similar. To get a clearer idea of the lightness of the wood, we can compare specific building elements such as a slab and a wall. According to Portuguese researchers, a floor with wood beams and floor weighting 40Kg /m2 corresponds to a concrete slab with 315Kg /m2. In the case of walls, a 75 kg /m2 wood solution can be compared with a traditional masonry wall with 274 kg /m2.
The lightness of wood is important for the economy of transport and installation. Due to this feature, wood structures require smaller foundations since the loads they support are naturally inferior to the ones of steel or concrete solutions. According to the International Residential Code 2006, the minimum width dimensions of 3-story wooden foundations should be between 12" to 23", but for masonry buildings, these dimensions rise to values between 16" and 42".
5th - Wood is a material whose weaknesses can be overcome
The fire and the attack of xylophages represent the two greatest fragilities of wood.
As for fire, we know now the sufficient to design wooden structures taking into account their safety in case of fire. If we don’t want to protect wood structures with sacrificial (e.g. protective) elements, the desired fire resistance should result from an appropriate calculation of the section of wood components. Thus, the residual section, after the expected combustion time, must be sufficient to withstand the considered structural forces. After 4 to 5 minutes after the start of a combustion, a carbonized layer is formed, which has a good insulation property and half the specific heat of the wood. With the increasing of wood thickness, the time to reach the interior of a wood part increases quadratically, limiting the progression of fire and the degradation of its mechanical properties. It is possible to achieve a fire resistance of 90 minutes by means of a correct dimensioning, a good design of the details and a special attention to the connections between components.
For the pre-dimensioning of wood structures, theoretical combustion ratios can be taken into account, (available in Eurocode 5). Although knowing that these ratios lead to conservative solutions, for each face exposed to fire and without fire protection, the values of 0.8mm /min for softwoods, 0.7mm /min for softwood glulam and LVL and 0, 55min /mm for hardwood and glulam hardwood.
Many countries have seen in recent years an evolution of regulations against fire hazards. For example in Australia, until 2016, when someone wanted to build wooden buildings with more than 3 floors it was necessary to use a complex and very expensive solution. Right now wood construction up to 8 floors, or 25 meters, is already anticipated in the National Construction Code (NCC). In recent years, largely due to the development of Glulam and CLT structures, there is a very interesting race to the record of the tallest wooden tower. In Norway, the “Treet” tower was built with 14 floors and 49 meters high. In Vancouver, the “Brock Commons”, a student residence, presents itself with 18 floors and 53 meters high. In London, an 80-story, 300-meter tall timber tower, named “Oakwood”, is being designed. Tokyo wants to win the competition with the 350-foot “W350 tower”, with 90 percent wood in its hybrid structure.
But how long does a wooden structure last? Durability requires, first of all, a careful design that takes into account the access of water to the components of the building. Keeping the wood dry and ventilated should be therefore one of the fundamental concerns of designers. Structural elements in contact with the external environment must be protected from rain or from contact with the ground, and only when this is not possible should additional treatment be used. For exterior sidings and finishes, chemical treatments can be avoided, by paints or by using thermo-treated wood or durable species. Some species are historically recognized as being the most durable. In North America, for example, Cedar has a fame of durability that led it to be chosen as a standard for the shingles of traditional roofs. The durability of Red Cedar (Western Red Cedar) and Pacific Coast Yellow Cedar is due to the extractives (natural chemicals) that prevent the growth of fungi. In Portugal, although we know of durable species such as Chestnut, Casuarina, Olive, Yew, Cypress, or Cedar, there is no significant commercial production that allows us to take advantage of them right now.
Wood is then a sensitive material whose durability depends on the sensibility of the various actors involved in an architectural work. The owner must ask for a durable work. The Architect should “design for wood protection". The Engineer, in addition to the knowledge of structural calculation, should complement and inform the concerns of the Architect. The builder has not only the responsibility for the proper building process and for the protection of wood materials on site, but he is also accountable for the verification of the rationality of the proposed solutions. The inhabitant, who will ultimately be the permanent inspector of the finished work, must have a culture of inspection and maintenance.
In the recommendations of the Canadian Wood Council, "Tips for Durable Wood Building Envelopes", it is said that we should not believe that the buildings we build are going to be demolished in 30 years, rather we must design so that buildings can last 200 years.