Why Wood?

Stefan Behnisch

In recent years, wood has experienced a renaissance as building material. Not only for interiors, surfaces, and cladding, but also for construction. In the past, the use of wood was limited—structurally and technically. Fire, structure, weakness due to a limited unitized length, durability but also change in appearance and form limited the way architects and engineers considered wood for construction.

Several aspects of the above elaborated have changed. Experience has taught us that wood—used in dimension in the right way—can be very well fire-resistant, especially structural wood or engineered wood. Structurally, wood has great advantages: it is elastic, flexible, and light and has unique qualities. We just need to consider the way we construct differently. Compared to steel and concrete, wood breaks easier, but longitudinally it can withstand high pressure. Today, we need a smarter way to construct, we have to relearn the way to construct with wood. It is a more sophisticated approach than construction with concrete or steel requires. Regarding durability and appearance we have developed a different expectation. We appreciate if a building ages well. And regarding technical durability we learned that a well-detailed wood structure ages very well and lasts for centuries if maintained thoroughly. In Europe, we have old wooden structures that had been constructed 500 or 600 years previously. A massive wooden structure, as for instance the “Gradierwerk” (Saline Evaporation Structures) in Bad Kösen, is a great example of durability under climatic and environmentally challenging conditions. Consequently, it is well established nowadays that we can responsively build and construct in wood.

But why should we?

Contrary to some official political wisdom, we do face substantial climatic change with unknown challenges. The building sector is responsible for a remarkable portion of pollutants, especially fine dust and carbon dioxide arising in the energy-intense concrete and steel construction.
Together with some of our colleagues and engineers, we have studied the impact of the building industry on energy consumption and pollution. We reached the conclusion that only if we consider the gray energy—the energy embedded in the construction process—we will be able to contribute to a more sustainable environment.

While wood is growing, it embeds CO2, and is a massive CO2 storage. Thus, wood, if used properly, has several substantial advantages. It is a regrowing resource. If used correctly we will not harvest more than we can regrow in the same time. Wood also has a second life after being used in construction. When demolished, wood can either be reused, or in the worst case, used as an energy provider by being burned. Wood structures help us rebalancing our energy and CO2 management. Right now, our main challenge is that we ignore the natural balance of producing and using fossil energy. We burn in several seconds the fossil energy our planet produces in one day.
Hence, we should use natural materials we take from the natural cycle and return it if we don’t need it anymore. These materials include wood, earth, clay, straw, natural stone, and bricks. All of these are modern materials, well-accepted in modern societies and also readily available in rural areas of the economically less spoiled regions of our world. We can learn from each other.

Over the last 15 years, our offices have designed various buildings in wood. The conference hall for the World Intellectual Property Organization (WIPO) in Switzerland is a very good example. The project of a conference hall for 900 people in a rather tight spot atop an existing parking garage called for a light and flexible structure. Furthermore, a conference hall of this magnitude is only occupied several hours per week. The light structure with little thermal mass has advantages here. It can be heated and cooled down with relatively limited energetic effort, since no thermal mass has to be tempered with. We designed a building with a wooden structure, wooden exterior, and wooden interiors. The structural wood were prefabricated beams and plates, joined with steel plates, maximized in dimensions. The voids in the structure are the air ducts providing the advantage to keep these areas dry. Structurally, wood did provide us with the necessary fire rating and bomb blast security. It withstands blasts better than steel or masonry and compensates better than concrete, causing less damage in the vicinity. Overall, wood is, beyond its sustainable qualities, an ideal material for these types of buildings.

If one needs to compensate for the lack of thermal mass for example in housing, we consider either prefabricated clay elements in the ceiling or rammed earth for walls or gypsum walls with embedded phase-change materials for the walls. This is especially important in offices, in housing, or any live-in buildings. We have just finished the conceptual design for a rather large office building. Explaining this rather complex concept of energy calculation per capita would, however, lead too far in this context. But the concept is interesting, very demanding, and a good political means. Our design concept requires us to look at the life cycle of all building materials, on the harvesting or production of the building itself, on the maintenance, on heating and cooling but also on demolishing a building again. Here we proposed a full wooden structure with a walled clay ceiling. These clay elements have embedded tubes for radiant heating.

In Nuremberg we are designing a 17-story-high rise building, all wood (structure, walls, cladding, shear walls). Only the elevator shafts and escape stairs are positioned outside and are built in concrete. This structure will be one of the tallest occupied wooden structures so far. Our new administration building for the National Center for Tumor Diseases (NCT) in Heidelberg will also incorporate an all wood structure. While a high-rise wooden structure still poses some challenges—in the same way steel structures did in the late 19th century—we hope we will be able to better control it in a few years. Today, especially in a high-rise structure it is still a form-driving material. At WIPO’s salle de conference and the NCT administration building we were able to work in a formally progressive manner with the wood. Not the material drove the design but the quality of the material allowed for new boundaries in our design approach. The social housing project in Ingolstadt for the same client as the high-rise building in Nuremberg, the social service of the Catholic Church, is another successful example of a wooden structure. Structure, walls, façades, are all wood. The building created a new benchmark in low-energy construction considering the implementation and energy consumption for heating and warm water. It is more efficient than a regular “Passivhaus” (passive house).

Generally, there are different types of wooden structures. On the one hand, the full wood structures like the projects described above. On the other hand, the hybrid structures where the wood load-bearing system is concrete and all walls (interior and exterior) are built in wood. Our condominium project in Eichstätt, a hybrid structure for a private client, has just been finished. These structures are also very comfortable to live in, have a good climate, and a good thermal mass but are still felt and perceived as a wooden building.

Our first wooden construction was realized in Herbrechtingen, Germany, a school for mentally and physically challenged children. It is a one-story building with a wooden roof and an all-wood façade.

It is our goal to design and realize more wooden structures, not only in Germany and Switzerland but also in the United States. Here in a country with vast forests and excellent wood the acceptance for larger wooden structures is just developing.