“Hybrid is the new way of building with timber”

Until recently, timber construction had almost completely disappeared from the urban landscape. Now, the development of new timber building materials as high-tech solutions has led to more and more buildings being built of wood, and in structures that are reaching ever-increasing heights. We are currently witnessing how urban space is being reclaimed. Alongside other biobased construction solutions, wood could be a key to achieving climate turnaround. “Reforest the planet, retimber the city,” says climate researcher Hans Joachim Schellnhuber, the visionary behind the initiative New European Bauhaus. It is an approach that Stefan Winter also wholeheartedly embraces – whether in his capacity as a professor at the Technical University of Munich, as an author of technical publications or – with his company bauart – as a structural engineer for multi-storey timber buildings such as Timber Peak, a current UBM Development project at the former customs port Zollhafen Mainz.

As the founder and managing director of bauart, you are responsible for the structural engineering of Timber Peak. It is the first high-rise office building in Mainz to be built using timber hybrid construction techniques. Does it make sense to use wood as a building material in urban areas?

STEFAN WINTER: Wood is an obvious choice for urban use, especially for multi-storey hybrid buildings, where we should be substituting as much material as possible with a more environmentally friendly material that can be produced with a low amount of primary energy. Of course, you can’t do that everywhere. With this project, we have an underground car park and structural elements that are in contact with the ground, where it doesn’t make much sense to build with timber. But it’s an outstanding material for every situation where it can be used. And it is wonderful that UBM Development is constructing Timber Peak, the first timber hybrid building in Mainz. That fits with the times. 

The book description for the “Manual of Multi-Storey Timber Construction”, which you co-authored, states: “Hybrid is the new way of building with timber”. What does this mean?

In hybrid building, that is, building with mixed materials, we actually distinguish between three levels. First, there is the purely material level. We now have building products composed of combined hardwood and softwood. For instance, there is glued laminated timber with outer layers of beech – which are stronger – and coniferous timber on the inside. Or wood-reinforced timber which has been developed by us with coniferous timber lamellae on the outside and reinforced with beech veneer on the inside to strengthen the wood using internal and constant transverse tension.

The next level is the component level. This includes, for example, hybrid building components like timber-concrete composite slabs, in which a layer of concrete is poured over a layer of solid wood. That significantly minimizes the use of concrete in the ceiling and combines the benefits of both materials. Concrete helps to create non-combustible layers, for instance.

And at the third level, we refer to hybrid buildings. For instance, we might have a staircase tower and elevator shafts made of concrete, with the floors built around this reinforcing core consisting of a solid timber and skeleton construction.

Hybrid construction is therefore encountered on various levels, and there are actually thousands of potential combinations.

In other words, it doesn’t make sense to use only timber in multi-storey buildings?

We will not be able to build modern buildings entirely out of timber, as we once did with log cabins – and this is precisely what led us to say in this book: Hybrid is the new way of building with timber. We are convinced that there is no point in building entirely with wood, it will always be combined with other materials. Our resources need to be reployed where it makes sense to do so. And it’s simply a very good idea to use them together in a coordinated manner.

At what point would we call something a genuine timber construction?

For our purposes, we have defined it as follows: If more than 50 percent of the volume of a building’s shell is made of wood, then we can call it a timber construction. That is the minimum requirement. It’s not the same if you build a tower out of reinforced concrete and then add a timber facade – you can’t call that a wooden high-rise.

The Scandinavians are a bit more radical as far as that is concerned, they are using cross laminated timber to build lift shafts. What do you think about that?

We have done that ourselves as well, for Kampa K8 in Aalen, for instance – but in Germany we are facing resistance. Purely for fire safety reasons, we have only managed to gain approval for CLT stairwells in Germany up to building category 4. I would like this to be extended to include building category 5 as well.

In Aalen – where we just stopped short of classification as a high-rise – both stairwell shafts are load-bearing and structural. However, buildings that are tall and slender need concrete as reinforcement. I believe that a timber-framed facade – like the ones for the timber high-rises Treet in Bergen and Mjøstårnet near Lillehammer in Norway – is not really the best solution, neither architecturally nor spatially.

But I’m torn in two here – seeing it from the perspective of a timber construction engineer on the one hand and a fire safety engineer on the other. And when true high-rises are built, fire safety engineers argue the case for a shaft that will cope with a catastrophe. But here I could imagine a combination of the two in future: using reinforced concrete for the shaft and cross laminated timber for everything else.

It seems as though the topic of multi-storey timber constructions and fire safety is being reinvented in each individual city – especially when they encounter this kind of project for the first time. Do you think existing knowledge could be shared here?

It’s always best to specify uniform rules. Right now, we are working on developing these uniform rules in Germany, called the Musterholzbaurichtlinie (model timber construction directive – supplementary specifications for the building regulations in Germany), as part of the evaluation of the research project TIMpuls. The new directive has been finished as a draft and it will hopefully be available soon. Discussions about fire safety will only die down when we have rules that are common knowledge.

But you are right, of course. It is because training is based more on reinforced concrete and steel construction. In the follow-up project TIMpuls dissemination, we are working specifically on further training programmes for professional fire departments, fire brigades, and local building authorities. All parties involved have to be trained and educated. The greater number of buildings built with wood in the future, the simpler everything will become.

It is like that everywhere. I know the people who did the engineering for Mjøstårnet (Ed. – until autumn 2022 the world’s tallest timber high-rise). You mustn’t think it was all done without discussions.

Timber Peak is situated on filled ground at the tip of the harbour basin. Did that present a particular challenge?

The subsoil was indeed quite challenging. But the foundation piles required by this situation will be used for heating and cooling, which kills two birds with one stone. If you have to drill down deep anyway, you can also tap into geothermal energy, which is an excellent idea. Anything that provides multiple benefits is ultimately more efficient, cost-effective and environmentally friendly as well.

Another good idea – which was to equip all the UBM buildings at the Zollhafen with a central photovoltaic system – could not be achieved.

The land-use plans are often 20 or 30 years old. And the fact that these plans are upheld sometimes makes us despair. By now, everyone will have heard the shot being fired. We’re in a hurry, and in the construction sector we have a huge influence on what happens to the climate as a whole. But it still needs a lot of development in society, and here we need a change in policy. We need to start convincing people, to attract a broad public. Each and every one of us needs to understand that we must change the way we behave, and that we also need to rethink approaches to urban planning. I am convinced that in future we will design and develop buildings according to their ability to generate energy and cooling.

Timber construction is seen as a highly promising way of achieving the green building revolution and decarbonizing the construction industry. The EU has paved the way for this with the Green Deal and the New European Bauhaus initiative. In practice, however, there is often a lack of skilled workers and the producers of timber building materials are unable to keep up with the growing demand. How can we turn this situation around?

For one thing, the market naturally regulates a great many things. And the market has also already begun this adaptation process, with many traditional building companies now focusing very intensively on timber construction. People recognize the direction that this development is taking and are investing accordingly. This won’t happen overnight, but we are in the process of massively ramping up production capacity.

Moreover, the timber industry fortunately has relatively few recruitment problems compared to bricklayers and concrete workers. Timber construction is still an attractive career choice for young people. They are attracted by the craftsmanship aspect and the natural beauty of wood. At the same time, today’s carpentry profession offers digital work processes with software-controlled CNC machines – that’s a brilliant mix. Timber construction is without a doubt the building sector that has made the most progress in terms of prefabrication and digitalization.

If timber is used as a resource for more construction projects, could the supply dry up?

Firstly, the market here is influenced by global developments. Two years ago, the price of timber went through the roof because the U.S. suddenly screamed out for wood and a lot was shipped overseas.

However, it is naturally right to ask whether we can continue the product portfolio in this way, especially in the area of mass timber, i.e. cross laminated timber. We have been working on a research project called LaNaSys (Ed. – taken from the German name Laubholz- & Nadelholzsystem, meaning soft- and hardwood system) in which the cross laminated timber is further developed using a middle layer of hardwood. As the characteristics of hardwood are much better than those of softwood, you need less wood overall in order to achieve the same performance characteristics.

Timber construction must begin to use resources more efficiently. After all, sustainable forestry is also limited in its growth. At the moment there is a lot of planting around the world, but it naturally takes a certain amount of time before the wood is fit for cutting.

And so you are not among the optimists who say that we could theoretically use timber to build everything?

I am a bit more sceptical because wood is not just a resource for construction purposes, we shouldn’t forget that. In the German state of Saxony-Anhalt, for example, UPM Biochemicals has built a factory where biochemicals are manufactured on the basis of wood. Their products include PET bottles, for example, and also organic soot for car tyres. In other factories wood is used to make textiles. About 1.5 million cubic metres of wood per year are needed for the first building phase of such a chemical plant, rising to 10 million cubic metres in the final phase. This would be about one seventh of our standard felling in Germany.

But I still believe there is enough wood for building if you try to conserve resources at the same time.

What about the state of our forests?

We have had widespread bark beetle damage not only in Germany, but also in Austria and Switzerland. We will have to consider new forest management measures for the future. But I remain highly optimistic that no matter what the forest yields, with modern processing technologies we can produce materials that, when used over long periods, can store carbon for 50, 100 or even 150 years.

The most unsatisfactory approach would be to say that we merely have to let the forests grow by themselves, without any help. Scientists have already provided evidence that the opposite is true. With ecological forest management and sensible use of wood in long-term products, there is a realistic chance of actually combatting climate change and reducing the amount of CO₂ in the air.

Do you also see potential for decarbonization in other building sectors?

There are of course also exciting developments with other materials. Construction companies are working intensively on concretes with lower CO₂ emissions, with other aggregates and with low-density concretes. It’s always in crisis situations that people start to focus their engineering acumen on finding new solutions. Fortunately, there is also a great deal of engineering expertise today among young women who are interested in environmentally relevant topics and bring new creativity to the table. I am confident that we will see a host of inventions and advancements in the coming years.

Currently, timber hybrid high-rises are being built that surpass the 100-metre mark. What further developments do you expect to see in urban timber construction?

I can imagine hybrid high-rises being built up to the 300-metre threshold. From a purely technical point of view, I can’t think of any limitations. Wood is a material with an extremely good strength-to-weight ratio, which means that its weight is extremely low in relation to its structural properties. So I can envision wood primarily playing a role in the densification of existing urban landscapes. In Central Europe, at any rate, this will be a major focus for the industry, with additions, extensions, added storeys and other expansion projects. Our office has a regular influx of such projects – it’s like performing open-heart surgery, without the building being vacated during the construction work.

Most buildings in cities have a height of between six and ten storeys, which is a range where timber construction is a good choice – and not just in this part of the world. I also hope that the opportunity will be taken to make Ukraine one of the first truly sustainable countries when it is rebuilt.

What other challenges will the construction industry have to contend with in the future?

We will have to deal intensively with urban mining, in other words, with recovering materials that we have already used, including timber. In a large-scale research project with the Karlsruhe Institute of Technology, we are currently analysing how to evaluate used timber for its performance properties. This also applies to other building materials.

Surveys have shown that at present wood waste usually ends up on a rubbish tip because it is supposedly too expensive to perform structural engineering calculations and check for chemical treatment.

Costs are always relative and also dependent on availability. If I have enough green wood and it is also inexpensive, then that will be my first choice. But if I am faced with a shortage, then it becomes more attractive to reuse resources. If twenty-five years ago somebody like Karl Moser (Ed. – the first industrial manufacturer of cross laminated timber) had decided production was much too expensive, then we wouldn’t have 1.5 million cubic metres of cross laminated timber being manufactured in Europe today, and that number is increasing.

Of course, old wood from the 1950s that was treated with arsenic can’t be reused for construction purposes. But luckily, we got rid of chemical treatment in the early 1990s. Nowadays, we know this isn’t necessary, and that technical drying and sensible structural wood preservation are entirely sufficient.

In Portland, Oregon, an architectural firm recently salvaged 80-year-old wood from an industrial building and reused it in a different one (Project: Redfox Commons). The wood was in pristine condition.

Wood is actually indestructible as long as it stays dry. There is no carbonation like you get with concrete, and no corrosion like with steel. The best evidence here are the stave churches made of wood in Norway, and also the temples in Japan. They are around a thousand years old.

The current high demand for wood – as a ray of hope for the climate – is coinciding with a record level of global logging and deforestation. How will humanity overcome this conflict?

Sometimes humans can be really stupid, there’s no other way to say it. The power shift in Brazil gives us reason to hope that the reclamation of land for arable farming and cattle ranching will be curbed. At the end of the day, however, humanity will need to be convinced that our forests, when they are managed correctly, are an infinite resource. This needs to be achieved by means of educational measures, research work and training on the one hand, and through the political will to actually implement this on the other. Considering that is such a mammoth task, trying to face it on your own can sometimes be overwhelming. Any attempt to make progress needs you to take one small step at a time.

Can you give an example?

At Munich Technical University we have submitted a project called the “Intra-Africa Education Team for Sustainable Construction”. We would like to work with the KNUST (Ed. – Kwame-Nkrumah-University of Science and Technology) in Ghana, Johannesburg University and a university in Tanzania in order to set up relevant degree courses and create awareness for sustainable construction and ecological forestry. We have to understand that our life in Europe is at an insanely high standard, and that we have to use all our strength from the outset to support those who want to improve their own lives, so they can do this in a way that is environmentally sound.

What small steps can each individual take?

We have to work on reducing our carbon footprint. This is also a matter of education. Sometimes it is very difficult to teach people that a little less meat is healthy for them and good for the environment. Or that taking the train is in fact better than driving an SUV. We each have to start with ourselves.

So we can achieve this?

I wouldn’t say that humanity is completely incapable of failing here. But wood could be a key to tackling climate change. To quote Hans Joachim Schellnhuber, Director Emeritus of the Potsdam Institute for Climate Impact Research: “Reforest the planet, retimber the city.” I find this an excellent approach that I am happy to continue to support.

Interview: Gertraud Gerst
Translation: Rosemary Bridger-Lippe
Photos: Philipp Horak, Munich Technical University, Jeremy Bittermann, Lever Architecture
Visualizations: Sacker Architekten