BLACKPRINT: Mr Sterl, with HoHo Wien you have realised one of the most internationally renowned projects of contemporary timber construction. How did the project come about?

Oliver Sterl: I need to give a bit of background here. Our roots are very much in urban design, and that shaped the development of HoHo Wien. The site sits on a former airfield from 1911 that remained unused for a long time after the Second World War and was later used as a private airport and as a venue for car races. As early as the 1990s we had developed what we called an "urban score" for the area, which was also shown at the Venice Biennale. The goal was to create a framework of rules that would allow the sustainable development of the urban area over time.

BLACKPRINT: But the approach was not implemented at the time...

Oliver Sterl: No, the zoning procedure was too complex, so no one wanted to take on the risk back then. You have to know that the population of Vienna kept shrinking until 1990 because the city sat on the edge of Western Europe. After the fall of the Iron Curtain, however, the city suddenly grew strongly again, so new areas for urban development had to be opened up. The central challenge was that Vienna is almost entirely surrounded by nature reserves. New construction was therefore only possible on existing brownfield sites. Thanks to its existing infrastructure, the former airfield seemed ideal.

BLACKPRINT: Despite this need, it was not until 2005 that an urban planning competition was held for the site, which Tovatt Architects from Sweden ultimately won.

Oliver Sterl: Exactly. Their design envisaged a lake in the middle of the district, a ring around it and, in between, larger and smaller plots with different programmes. In 2012 an urban development procedure followed for the Seeparkquartier south of the lake. We took part with the same approach we had used in 1995 and produced a framework of rules to predict the development of the district over time and to define degrees of freedom.

BLACKPRINT: And how did the idea for the timber high-rise HoHo Wien come about?

Oliver Sterl: At first it was just a not entirely serious idea. But after one of the investors, Kerbler Holding, had bought a Styrian timber construction company, it took shape more and more concretely.

Oliver Sterl: The building sits directly at the terminus of the U2 underground line, where it plays a key role: as one of three prominent high points, it sets a vertical focal point within the otherwise flat district and at the same time marks the entrance to the area. The square in front of it acts as a spatial "gateway" into the Seestadt. This urban situation is reflected directly in the building's structure. The complex consists of two interlocking towers and an additional low-rise. While the volume facing the lake acts as a striking landmark, on the side facing the station it responds with a stepped silhouette and a horizontal canopy in order to create a human scale. Urban design, high-rise logic and timber construction interlock and together define the form. And the functional mix of offices, serviced apartments, hotel and wellness areas is made possible by a flexible structural system.

BLACKPRINT: HoHo Wien demonstrates the advantages of timber-hybrid construction: bracing concrete cores accomodate the building services and circulation cores, with the timber structure docked on for open use. From the ground floor up, the share of timber is around 74 %. How did you manage to realise the project in this form and at this height?

Oliver Sterl: When we started planning in 2014, the normative and regulatory framework was still well behind the requirements of such a project. So we first systematically catalogued all relevant high-rise regulations and examined how these could be applied to a timber high-rise. A central topic was fire safety: load-bearing components in a high-rise have to be classified as non-combustible — a requirement that timber as a material does not meet by itself. On this basis we developed a hybrid construction concept step by step and continuously discussed our solutions with the City of Vienna. After several rounds of coordination, it became clear that the approach was technically and compliant with the regulatory requirements. The city then set up its own task force in which fire safety, structural engineering and other disciplines were bundled. Within this framework our concepts were discussed and on that basis we developed, tested and refined them further.

BLACKPRINT: And what did that mean specifically for the development of the structure?

Oliver Sterl: A decisive step was carrying out fire tests. We built a component mock-up consisting of a column, a beam and a slab and tested it under extreme conditions: 90 minutes at around 1,000 degrees. Afterwards the surface of the timber components was heavily charred, the concrete partly spalled, but the load-bearing structure remained structurally intact at its core. This very clearly showed the protective effect of the charring layer and proves that timber structures can be reliably calculated in the event of fire. On this basis we were able to develop the project step by step in close dialogue with the city.

BLACKPRINT: And what emerged is not only a key project for the Seestadt Aspern, but also a reference model for timber high-rise construction worldwide.

Oliver Sterl: Yes, but height was never the objective in itself — it grew out of the urban situation. And quite fundamentally, timber high-rises are about the material and the right protective concepts. In our case, the approval process nevertheless took around two years up to 2016. The close coordination with the authorities ultimately also led to standards and regulations being developed further. For HoHo Wien the necessary special verifications could be provided. As a rule, however, in Vienna today only buildings of up to six storeys may still be constructed in timber without such verifications.

BLACKPRINT: Which insights from this project are particularly relevant for you in retrospect?

Oliver Sterl: A central insight is: you don't create such projects alone. You need an environment that is willing to go down the path with you. At the time of HoHo Wien this environment was favourable and the people involved were both capable and willing to support the project. Otherwise the undertaking would not have been possible in this form.

BLACKPRINT: And what about the economic side — does such a project actually pay off?

Oliver Sterl: A good question. Two points need to be considered. First, the timber components used must be very massive, especially in high-rises. At HoHo Wien, for example, the ground-floor columns have dimensions of 1.20 × 0.40 m, in some cases even doubled. That takes up a lot of space. But the more net floor area is lost in relation to the gross area, the more expensive the project becomes. A second point is material availability: timber is a natural material. Spruce only grows optimally under certain conditions. And pests such as the bark beetle or climatic fluctuations influence supply. Alternatives such as silver fir are more resistant but take around 80 years to mature. Added to this is the complexity of the construction method: it requires highly qualified specialists, precise planning and perfect execution. Even small mistakes quickly lead to delays. Particularly critical are water, condensation, wind and weather — four factors that strongly influence construction. At HoHo Wien, for example, huge wall elements of 4.80 × 3.50 m had to be installed. Wind alone can lead to unpredictable delays. Timber high-rises are therefore technically exciting and show what is possible. But they are not a universal solution for more sustainability.

BLACKPRINT: Another pioneering achievement of your practice some years earlier was the residential and commercial building on Wiedner Hauptstraße, where façade greening was implemented in Vienna for the first time. What challenges did you face?

Oliver Sterl: The project is located on an extremely narrow lane that heats up strongly in summer due to the high degree of sealing. Our answer to this was façade greening. At the time there was very little precedent for this type of project, which is why we had to provide technical answers to questions about insects, daylight or fire risks. At the same time our calculations showed that the perceived temperature in summer reduced by about ten degrees, partly due to the shading effect, partly due to reduced heat storage in the surroundings. Building on this experience, we now regularly plan façade greening, particularly in densely built-up, hot inner-city locations. Selection of species is important: which plants thrive, which attract pests, which animals feel comfortable? Equally decisive is whether the greening is ground-based, so that the plants absorb enough energy, or whether it involves wall-mounted systems that have to be specially constructed. Either way: when implemented correctly, façade greening is a highly effective means of improving the microclimate. In winter the plants have no leaves and therefore do not block daylight; in summer they act as highly effective natural solar shading.

BLACKPRINT: You are currently realising the Biotop Wildquell project in the south of Vienna. Which urban and programmatic questions are at the forefront there?

Oliver Sterl: Biotop Wildquell is a former industrial site with an adjacent park of around 30,000 square metres, which is being newly developed as an urban ensemble with several construction projects. Four basic principles are at the forefront for us: a highly landscaped, car-free urban environment; a base structure oriented to the north that provides noise protection towards the railway; and high-rise residential buildings that extend into the surrounding green space — but not in long rows like in the seventies, but staggered and interlocked. Together this creates more daylight, differentiated open spaces and scales that are friendly to people.

BLACKPRINT: And how are you implementing the topic of sustainability there?

Oliver Sterl: A large part of the project consists of subsidised housing. The cost pressure is correspondingly high, so timber construction was not feasible here. A central aspect is instead social sustainability: the park is publicly accessible and open to everyone, not just the residents. In this way we create added value for the neighbourhood and foster urban coexistence.

BLACKPRINT: An exciting aspect that ties in directly with Vienna's tradition of social housing. Another major topic for your practice is the reuse of existing buildings. A good example is your project on Vienna's Siccardsburggasse. What opportunities do you see in refurbishment compared to new construction, especially with regard to climate targets and resource consumption?

Oliver Sterl: Buildings from the 1980s generally have poor thermal insulation and oil or gas heating systems. They are therefore ideal candidates for a comprehensive refurbishment programme. On Siccardsburggasse we faced various options for thermal refurbishment, from the classic demolition of the façade through to adding an additional wood-fibre board layer on top of the existing structure. After weighing all the pros and cons it was clear to us: if we want to take circular economy and sustainability seriously, we have to make the best possible use of existing building substance. In the end, due to higher costs, we were not able to retain and enhance the existing façade substructure, but we did use a façade insulation made from renewable raw materials (hemp). Unfortunately, perfection is not so easily achievable; it's a matter of trade-offs.

BLACKPRINT: Looking at the current development of RLP Rüdiger Lainer + Partner Architekten ZT GmbH: which topics, typologies or questions will shape your work in the years to come?

Oliver Sterl: I see three central pillars for our work: urban design, new construction and the further development of existing structures. If we want to seriously get a grip on the climate crisis, then the primary structure of buildings plays a central role above all. And regardless of whether they are made of wood, concrete or steel: long-lasting primary structures bind CO₂ over decades and allow flexible use over time. Timber currently plays a special role here because it binds CO₂ over the long term. That does not solve the climate problem on its own, but it buys us valuable time. What is decisive, however, is that the buildings are not only designed to be sustainable but also adaptable. Spatial layers should be designed in such a way that various functions can be accommodated within them, from residential and workplace functions to public and community uses. Only in this way can CO₂ be saved in the long term and flexibility ensured. In addition, windows, façades or installations must be replaceable and easy to maintain. If we implement these principles, then buildings can be created that can last 200 years or more.

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