It is not the first of its kind, but it is definitely the best, Holzbau Amann is convinced. The new observation tower in the Frankfurt City Forest is the youngest of three wooden towers which were all built in the same place. The first tower was built in 1867. It was 22 meters high and had to be demolished after the First World War because it had become dilapidated. In 1931, the city of Frankfurt built a new, 43-meter-high tower made of pine, beech and oak timber. With 196 steps leading up to the platform, the so-called Goetheturm was the highest publicly accessible timber building in Germany until 1999. From 1981 to 1982, it was extensively renovated, impregnated with creosote to protect it from the weather and reopened during the Goethe Tower Festival. This festival took place each year until the celebrated building was completely destroyed by arson on the night of October 12, 2017. Ironically, the impregnation accelerated the spread of the flames. For tower number three, the people of Frankfurt again chose wood as a building material. The structure is also largely identical to the previous tower. However, it is state-of-the-art and takes current building regulations and findings into account.
43-meter-high tower made of block-glued chestnut wood
Knapp connectors: A 250-ton crane lifted the observation deck module to the top of the wooden tower © Holzbau Amann (project pictures) and Knapp GmbH (product foto)
The tower is a use class 3 building. The weather protection is achieved mainly by the construction itself with its overlapping sheet metal covers, stainless-steel connectors installed at a distance from each other and plastic spacers. Thanks to several approvals, the timber construction could be built outdoors without chemical wood protection – and using block gluing. The building rests on four corner supports, each of which consists of four cylindrical wooden columns whose diameter is 39 cm at the base and only 25 cm at the top. Stainless-steel connectors connect the supports at the corners, while connection lugs made of duplex-coated steel connect the horizontal truss elements with the supports. Thanks to this structure, it is possible to exchange single load-bearing elements if necessary. The roof was built following a similar principle.
In order to connect the rafters to the hip rafters, Holzbau Amann, which was responsible for the wood and steel construction, used 90 stainless-steel Ricon 60/30 EA connectors and 4 stainless-steel Ricon 60/40 EA connectors. “This type of connector was already pre-assembled in the factory. We only had to screw it into place on the construction site,” project manager Tobias Döbele recalls who was responsible for the development and structure of the tower at Holzbau Amann. “It made assembly a lot faster and it was possible to have an invisible connection. Also, these stainless-steel connectors are resistant to corrosion and they don’t discolor the chestnut wood,” he adds.
Fast tower construction thanks to modular design
The observation deck at 43 meters of height offers a spectacular view of the surrounding nature © Holzbau Amann (project pictures) and Knapp GmbH (product foto)
In addition to Ricon connectors and rod dowels, coach screws were also used for the reversible assembly of the timber construction elements of the tower. First, Amann pre-assembled the supports in a length of 13 meters before connecting them to form three tower segments on the construction site.
The first segment, which rests on the renovated original foundation, was connected directly to the concrete, while the other two segments were connected with the respective substructure after assembly. Last, Amann used a 250-ton crane to lift the 16-ton observation deck with its glue-laminated timber corner supports and a tent roof consisting of hip rafters, rafters and sheet metal roofing up to a height of 40 meters, where it was fastened to the tower.
