Wood in the cities – there are a number of arguments in its favor. The material is CO2-neutral, has good insulating properties and is a renewable raw material. Architect and civil engineer Wolfgang Winter would design any new building out of wood. Sufficient material and the technology to build upwards are available.
Wood in the cities – there are a number of arguments in its favor. The material is CO2-neutral, has good insulating properties and is a renewable raw material. Architect and civil engineer Wolfgang Winter would design any new building out of wood. There is enough material and the technology to build upwards.
Baumeister: Mr. Winter, we are confused: on the one hand, we hear about a renaissance in timber construction, but on the other hand, timber construction in the city has declined. Which is true?
Wolfgang Winter: A stable market segment has emerged for single-family houses in Central Europe. In multi-storey construction, it is more complicated: in the 70s to 80s, i.e. after the war, there was a market share of zero. In Austria, Germany and Switzerland, state-subsidized campaigns were created at the time to accommodate the returnees from Russia – building was done with wood. These campaigns caused the market share to rise to five percent in the short term. The fact that this figure is now weakening again is due to the lack of funding. The question is: Can ecological measures that cost more than concrete construction be justified at all? This brings up the concept of affordable housing, because expensive construction is not socially sustainable. Then we just build in concrete again. From this perspective, social sustainability excludes ecological sustainability.
B: Does timber construction necessarily have to be more expensive?
W W: In the short term, yes. A cubic meter of concrete costs 50 euros. Wood, on the other hand, costs 400 euros per cubic meter. So if you replace concrete with wood in an equivalent construction project, it is more expensive. That is of course a disadvantage of wood.
B: Where does this big price difference come from?
W W: A cubic meter of tree, as it comes from the forest, costs 100 euros. The price is determined by the forester who cuts the wood and the forest owner who waits 100 years for the tree to grow. If the tree is sawn down, 50 percent is lost through the waste products. This means that a cubic meter costs 200 euros. The wood then has to be dried and glued, tempered and quality sorted. This is always a high cost for a natural product.
B: The solution?
W W: You have to build intelligently. For timber construction in the city, you need a well thought-out system and a quality-assured product. This is not possible in this DIY niche with a regional, “cute” timber construction culture. For large-scale industrial projects with 200 residential units that need to be completed within six months, you need prefabricated products. In terms of price, timber is competing with in-situ concrete poured on site. At the moment, it is still losing this battle.
B: So timber has a lot of competition. Until 1800, things were different – every building was made of wood, at least in part. When exactly did the turning point come?
W W: Until 1800, all construction was “self-build”. People built with the materials that were available on site. Carpenters and bricklayers built without architects. A big break came with industrialization. The crafts disappeared. The railroad, steel and cement arrived.
B: What’s more, in the 19th century there was simply no more wood…
W W: That’s when the laws for sustainable forestry were introduced. From the second half of the 19th century, they stipulated that if a tree was felled, two new ones had to be planted.
B: So we would have enough wood again today. And the “paperless office” will surely ensure even more wood…
W W: The paper thing is not so easy to conclude. In fact, the yields from forests have increased enormously. This is due to properly managed forests. Until the 18th century, yields were five cubic meters per hectare. With forest management, the figure climbed to 10-15 cubic meters per hectare. Due to climate change and the high CO2 content in the air, forests are becoming even more productive.
B: So we would have enough wood to theoretically build entire cities with?
W W: Yes. There is more wood growing than we need. If we wanted to, we could build every new construction project in wood.
B: How high could we build with wood?
W W: Wood has a compressive strength of 30-40 newtons, concrete also has 30 newtons. Of course, it has a lower tensile strength than steel. But this can be compensated for with a higher cross-section. And timber is still relatively light. Pure timber buildings of up to ten storeys are technically possible without any problems, even when fire protection requirements are taken into account. Fire protection is actually a question of escape routes and access and not the combustible material.
B: Especially when we’re talking about urban areas, isn’t there a great risk of fire spreading from one building to another?
W W: Every fire is started by mobile fire loads – the furniture, the curtains. Wooden buildings don’t burn any more than other buildings. Wood does not ignite more quickly, nor is the risk of a fire starting greater than with other building materials. The most important fire protection measure is the escape routes.
B: Timber construction seems to reach its limits at ten storeys. Why then want to build even higher? Shouldn’t we think about the material according to its use?
W W: The tensile forces are the problem. But you can use timber steel for that.
B: Wooden steel?
W W: When we talk about timber-steel construction – steel clad with wood – then it’s the same principle as with reinforced concrete: you have a large cross-section consisting of compression elements, in this case made of wood, and inserted flat bars or angles that absorb the tension. From a structural point of view, all skeleton structures that are currently made of reinforced concrete could be made of wood.
B: What are the biggest advantages of timber in the city?
W W: Wood is an excellent raw material that can be used to make various products. It is easy to process. It also has low thermal expansion due to its high porosity. With other materials, you have to leave more space during installation, or the adhesive has to compensate for the expansion. Wood also has good thermal insulation properties. The advantages in the city lie in building gaps and extensions. The material is light and can be lifted into urban structures by crane.
B: Another major advantage of timber in the city is the high degree of prefabrication. Does this impose restrictions on the design?
W W: I think you can design very freely with wood. Nowadays, wood is machined and glued together. Robots mill out holes and join the wood together. So you can produce parts industrially and individually.
B: No disadvantages?
W W: Of course, it’s clear that if an architect builds monolithically beforehand, this allows for different building forms and requires different thought structures than if you put together an additive system from rods. Prefabricated timber construction requires a certain level of awareness on the part of the architect. If the architect has this knowledge, however, there is certainly freedom of design. The prefabrication of timber and steel is equivalent in the construction process. But wood has a few additional advantages.
B: Sustainability, for example. However, the word is now used everywhere. Has it lost any of its strength as an argument for timber construction as a result?
W W: A lot has been smuggled into the term sustainability: architectural quality, beauty and ecology. Now we no longer talk about sustainability, we talk about resource efficiency. Timber construction itself is clearly resource-efficient. And since we change our building fabric in relatively short cycles, resource efficiency also means what the material makes possible in terms of later use. The monolithic cast construction cannot be dismantled and rebuilt elsewhere. Steel and wood are easier to recycle.
B: Do you think that in a world surrounded by technology, we are longing for a natural building material?
W W: Yes, that is certainly part of it. On the one hand, there is this useful timber construction, but it doesn’t claim to be a statement. Our urban buildings have many half-timbered structures that were subsequently clad. Today, of course, things are different. Since concrete was the building material of the 20th century, if you offer an alternative, you also have to work with a feeling: We now live in a material that is closer to nature. But that will certainly only remain a niche. Eco-awareness is a decisive factor for a maximum of 20 percent of the population. The others don’t care if they live in a concrete building.
B: You said that concrete was the dominant building material of the 20th century. Is wood the building material of the 21st century?
W W: Wood has everything it takes to become the building material of the 21st century. Concrete was the building material of the 20th century, especially in Europe. This has to do with our specific history, with the Second World War. You could argue that the population’s growing environmental awareness is the basis for wood becoming the material of the 21st century. But, of course, you have to see how strongly wood is being fought over by the forestry, paper and pellet industries. The competing players for this natural material must agree that it makes the most sense to build with wood.
Read more in Baumeister 9/2013
Photos: Roman Mensing, artdoc.de
