Double-curved roof for Mariendom Linz with innovative concrete formwork technology

The construction of concrete shell structures is one of the most demanding tasks in architecture. Geometry, statics, and aesthetics convey lightness and create an atmosphere of transcendence through targeted lighting. They convey contrasts such as ascent and descent, heaviness and lightness. Concrete shell structures, as well as double-curved shell structures made of concrete, known as hyperbolic paraboloid shells or hypar shells, are not only technical masterpieces, but also spiritual symbols of modern architecture. Well-known examples include Kenzo Tange‘s cathedral in Tokyo from 1966, Ulrich Müther‘s hypar shell in Magdeburg from 1969, Jørn Utzon‘s Sydney Opera House from 1973, and Santiago Calatrava‘s Auditorio de Tenerife, built in 2011. Now they are also coming to Linz.

Innovative digital and manufacturing processes

Freely formed concrete shell structures are not only challenging to manufacture, but also require complex formwork, which is why they have rarely been built in recent years. Today, however, innovative digital and manufacturing techniques are making a renaissance possible. The first step in this rediscovery is the extension to Mariendom in Linz, Austria. The new Cathedral Center Linz, which features a double-curved concrete roof, was designed by architects Peter Haimerl from Munich and Clemens Bauder from Linz.

Mariendom in Linz and the idea of a concrete extension

Mariendom in Linz, the largest church in Austria, was built between 1862 and 1924. With its neo-Gothic architecture, it dominates the cityscape. However, despite being the city‘s landmark, it has increasingly lost its significance as a meeting place due to the structural changes in our society and the accompanying decline in the social importance of churches. This is a circumstance that many sacred buildings suffer from today.

On the occasion of the cathedral‘s centenary in 2024, the Diocese of Linz decided in 2022, as part of a workshop, to counteract this by opening the cathedral building to new target groups and making it more attractive. The initial plan was to create new exhibition spaces and a bookshop within the cathedral building. During the workshop, architects Peter Haimerl (responsible for the architecture) and Clemens Bauder (responsible for planning the exhibition spaces) developed this original idea further in the form of an extension to the cathedral church: The new cathedral center is intended to serve as an inviting entrance to the church, comparable to the foyer of a museum or concert hall. Visitors will be welcomed by a modern café and an information and book point, from where they will be guided through a new sequence of rooms to the exhibitions and the historic entrance area of the church in the north. The Cathedral Center thus combines tourist and pastoral services while also offering a flexible space for church and non-church events.

According to Peter Haimerl, „we pursued two basic ideas for the entrance, which opens up the historic church building on its eastern side to the city as an interface between sacred space and urban life. On the one hand, we planned to build a tent-like structure in front of the church from the outside. On the other hand, it was to correspond to the reversal of the pointed arches in the side aisles of the historic cathedral.“ The architect reports that he was also inspired by the Catalan architect Antoni Gaudi for this idea. Gaudi, known among other things for his church „Sagrada Familia“ in Barcelona, also attempted to simulate rib vaults using suspended structures. „Which, incidentally, doesn‘t work – I had to learn that here too,“ adds Haimerl.

Planning, materials, and construction of the concrete roof shells

According to Peter Haimerl, two variants were initially developed for the realization of the tent-like structure of the extension, which resembles a canopy: a wooden construction and a robot-woven mesh of hemp ropes, which would have been even closer to Gaudi‘s ideas. However, according to Haimerl, neither variant met with much approval from the Federal Monuments Office responsible for Mariendom. Instead, the monument authority wanted high-quality architecture in keeping with the cathedral‘s construction, „which is sustainable and will last for centuries.“ In line with this request, Haimerl says, a concrete construction was chosen that was technologically sophisticated, freely formed, and in keeping with the cathedral‘s stone shell, allowing for a slim profile.

For reasons of monument preservation, the extension was structurally decoupled from the cathedral, which presented another special challenge: three canopy-like shell structures, composed of several precast concrete elements, each rest balanced on a support in the center and rise up in front of the historic façade without touching it. The slender supports on the long facade take on a tensile function in a rare load condition to prevent them from tilting towards the cathedral. The load-bearing lower shell of the structure is curved in three dimensions to create a soft, pleasant atmosphere inside. At the same time, it creates an exciting spatial atmosphere. The upper shell, which is placed on intermediate insulation, is only curved in two dimensions and thus corresponds to the inversion of the two-dimensional pointed vaults in the side aisles of the historic cathedral.

Realization of the double-curved concrete roof

The architectural design of the extension and the choice of materials in the listed context were coordinated closely from the outset of the project with the Federal Monuments Office and the responsible cathedral builders Wolfgang Schaffer and Michael Hager, as well as the Bishop Rudigier Foundation.

Puracrete GmbH from Übelbach in Styria, Austria, which specializes in complex formwork for in-situ concrete and precast concrete elements, among other things, was commissioned with the detailed planning and production of the double-curved concrete roof of the cathedral center. „The shapes of the concrete roof designed by architect Peter Haimerl,“ says Gernot Parmann, managing director of Puracrete, „were continuously optimized and redesigned during the planning and manufacturing process. The data required for production was extracted from the architectural firm‘s CAD models. The design was implemented in Rhinoceros 3D, a software environment for the use of parametric tools and the processing of free-form surfaces.

With such geometrically complex surfaces, even the smallest changes have an extreme impact: a small change in the overall shape requires adjustments to the reinforcement layers, the built-in parts, the formwork scaffolding, and the structural model. In this specific case, changes were made to the shells during production, which repeatedly affected the entire planning process.

Innovative formwork techniques

The three canopy-like concrete shell roofs of the Domcenter were realized as double-shell prefabricated constructions. Each of these consists of a lower load-bearing shell with a thickness of 6 to 36 cm and an upper layer, known as the cover shell, which is 6 cm thick and made of ultra-high-strength concrete (UHPC).

Special formwork elements developed by Puracrete were used to produce the highly complex formwork, which not only replicate the formwork skin but also the entire load-bearing formwork segment. Despite its high load-bearing capacity, this innovative material is freely formable. The formwork segments taken from the positive molds had a nominal thickness of between 8 and 10 cm and a standard size of 90 x 200 cm. The resulting overall formwork was designed for a concreting height of up to 4.20 cm. All formwork segments were recycled after concreting and returned to the material cycle.

Concreting and assembly

The load-bearing shell elements were concreted using white cement SCC C50/60. The individual segments weighed up to 14 t with a length of 7.20 m and a height of 4.20 m. The elements were transported from the factory to Mariendom on flatbed trucks, and transport was minimized thanks to specially selected segmentation. With the exception of the uppermost attachment parts, the prefabricated parts were transported in their concreting position. They were rotated into their installation position on site during assembly or when being lifted from the truck onto the falsework. The anchors required for this were planned from the outset.

The final assembly took place on two base scaffolds and a movable formwork frame. After the precast elements had been laid and aligned, the joint plates were welded. For structural reasons, the space between the plates was filled with synthetic resin, and the welded joints on the outside were then filled and leveled with the concrete. The final connection of the prefabricated parts was achieved by welding the respective connecting elements.

Contemporary design language

The architecture of the new Domcenter picks up on historical references and transforms them into a contemporary design language. The roof structure, consisting of three innovative concrete canopies, continues the tradition of lightweight tent roofs. The innovative digital and manufacturing processes used here offer the opportunity to rediscover hyperbolic paraboloid shells made of concrete.