Towers for tower

Built in 1942, the Flak Tower originally served as a bunker during the Second World War offering the residents of Hamburg protection from air raids. Today, it is home to manycultural and media enterprises.

The new construction will featurea public garden offering panoramic views across the Hanseatic city, a memorial dedicated to the victims of the Nazi regime, various cultural and exhibition areas, and a hotel.

Construction work has been underway for nearly a year involving four Wolff cranes commissioned by the civil engineering firm Georg Bähr. The proximity of the bunker to the Heiligengeist festival ground—home to the Hamburger Dom, Northern Germany's largest public fun fair, three times a year—posed a challenge at the start of the project.

“Assembling the cranes during the fair was not an option due to the lack of space. Given that the fair is held on twelve weeks of the year, coordinating the time slots for the assemblies with the city authorities and the contractor was quite a task,”said Carsten Druske, head of rental operations for Wolffkran in Germany and branch manager of Wolffkran Dortmund.

The first cranes, two flat-top Wolff 6031.8clear, were assembled in four days in the summer of 2019 using a 500t mobile crane with a luffing jib. Both cranes were erected from the same mobile crane bay, as it was difficult to find a place for the large mobile crane in the space-restricted St. Pauli district. The third crane, a Wolff 6031.8clear, was erected in fall last year and the fourth, a Wolff 6020.8 clear, in January of this year.

All four cranes were assembled with their final freestanding hook heights of 88.4m, 78m, 70.2 m and 59m, respectively. The high freestanding height are necessary since the construction site is located at a height of about 30m on the roof of the bunkerand the cranes could not be tied to the historically protected building. As a result, they have been erected on cross frames with 200t of central ballast. “In this way we were able to achieve high freestanding tower heights and cover the entire construction site with four cranes,” said Druske.

Thanks to their flat-top design, the Wolff cranes can slew very closely above one another and the overall crane concept was thus kept relatively low. “The jibs are each only about 10m apart, saving both assembly time and costs,”Druske added.

“The limited space around the construction site is also a key challenge with regard to the huge amounts of formwork, prefabricated parts and construction material that has to handled,” said site manager Falko Stephan of Ingenieurbau George Bähr. Additionally, the Flak Tower is fully occupied by tenants, posing further logistical and safety challenges.

Upon completion of the five-levelextension in 2021, around 4,700 trees, shrubs, hedges, bushes and climbing plants will be planted high above the roofs of the city, providing the residents of Hamburg with an impressive new landmark and a natural oasis.