A new landmark on the Frankfurt skyline: the Grand Tower.
Changing floor plans, limited space, limited crane times. This is how the difficult construction site conditions for the major Grand Tower project in Frankfurt can be summed up. A cleverly thought-out formwork concept and the use of Hünnebeck formwork and safety systems have nevertheless allowed the shell to grow rapidly in height. Included for the first time in Germany: the rentable windshield system "Safescreen" developed by Hünnebeck UK. Germany's tallest high-rise residential building, the 172-meter Grand Tower, has been under construction in Frankfurt's Europaviertel district since 2016. The client and project developer of this major project is gsp Städtebau GmbH, Berlin. The diamond-shaped tower construction comprises a total of 55.000 m² of gross floor area, divided into a basement, three technical floors and 47 floors with 401 apartments. The building is to be ready for occupancy as early as 2019.
Equalized: Leading core
For the structural engineers, this major project is a demanding task with many challenges: Extremely confined space, little crane capacity and floor plans that change from floor to floor… To get to grips with these conditions and develop a sensible approach, the construction company Karl Gemünden GmbH& Co.KG from Ingelheim in the Rhineland-Hesse region called in Hünnebeck's formwork experts to help them with this task. Together, they developed a continuous formwork concept that is adapted to the special construction site conditions.
The Grand Tower, Germany's tallest residential building, rises to 172 m.
The first decision to be made was to separate the structural work and construct the core of the building, consisting of the stairwell, elevator shafts and diamond-shaped ring wall, separately from the residential area – three to four stories in advance. The ceiling and wall areas of the varied apartment floor plans are being constructed later – fully screened behind a 13 m high wind shield, which Hünnebeck is using on this construction site for the first time in Germany. In principle, crane time is the most important resource in such a construction project. At the same time, the limiting factor on the construction site. If – as in Frankfurt – there is also a chronic lack of space (neither the site nor the building layout offer sufficient opportunity to store formwork temporarily), the situation is further exacerbated. Therefore, when designing the formwork, care was taken to ensure that as many working steps as possible could be carried out without the need for a crane. The outside of the ring wall is climbed and formed almost completely without the use of a crane. Here, brackets of the Hünnebeck SCF self-climbing formwork are arranged all the way round. The modular formwork system climbs hydraulically step by step with the structure, independent of the crane. In principle, SCF formwork is always unique, with the ratio of the number of brackets to the formwork area precisely matched to the geometry of the structure. All formwork and bracket components are designed as complete, self-supporting functional units – depending on the specific conditions on site.
German premiere: The 13 m high wind shield supplied by Hünnebeck extends over three full floors at Frankfurt's Grand Tower and overhangs the top ceiling by around 2.30 m.
On the Frankfurt construction site, the formwork is permanently installed on the platforms by means of a slide carriage. Due to the low floor height of only 3.25 m, only one trailing platform is required. The inner core area with its narrow shafts. Relatively thin interior walls, on the other hand, are not suitable for the use of self-climbing units. A mixed form of self-climbing brackets and clinker platforms was chosen for its manufacture. Manto panel formwork is used as the formwork system. Its compact design simplifies the operation of the formwork, which is suspended from a steel scaffold standing on the clinker platforms, especially in the narrow shaft geometries. This allows the formwork and platforms to be moved together in one lift – which in turn saves crane time. At the same time, the reinforcement can be pre-woven in large units on the ground and then inserted into the formwork from above, as there are no obstructing girders above the walls. This procedure also saves time, as concrete can be poured at the top of the structure at the same time as the reinforcement is produced on the ground below.
Complex anchoring problems
So much for the concept; but as is well known, the devil is in the details. A particular challenge is the anchoring of the SCF and clinker platforms in the ring wall. The constantly varying floor plan with differently positioned door and wall openings on each floor made it necessary to develop a system of various forms of anchoring for the automatic climbing formwork: In addition to the "normal" cone anchoring, Hünnebeck SCF specialists designed both a solution for anchoring in wall openings (climbing shoe on conical steel box) and anchoring above a door (bracket anchoring upwards and support via steel tube supports).
Safe working behind a full-surface wind shield made of rentable components. The outer cladding of the windshield is made of telescopic steel frames, which are covered with a perforated aluminum plate.
The many openings in the core ring wall not only make it difficult to anchor the SCF self-climbing units on the outside of the wall, they also affect the anchoring of the clinker platforms on the inside. Several variants also had to be worked out for this – from simple to highly complex, for example in the door area, where vertical loads had to be transferred to the lintel below the door opening using tubular steel supports. Ultimately, around 350 different special parts were required for the climbing platforms of the building core. Since the anchorages change on each floor, the Hünnebeck climbing formwork experts prepared a plan for each floor showing the type and the steel parts to be used for each individual anchor point. The effort was worth it. Pays off in a smooth construction process.
Floor plan change
This also applies to the construction of the apartments, which quickly follows the construction of the building core. The slab formwork used for this job is the manually operated Topec modular system, the Topmax steel-frame slab formwork table and the Gass aluminum support system, as well as load frame and steel tube props. All wall surfaces are formed with the Manto large-frame formwork resp. Special formwork elements (H20 wall formwork) were produced.
The support arms for the Safescreen windshield were mounted on brackets made of MK2 girders.
A complicating factor: Although the floor plans are repeated in various rhythms (for example, every four floors between the 8th floor and the first floor), they are not identical. And 29. Storey. The formwork planning developed by Hünnebeck addresses these conditions. Enables the economical use of the formwork. The formwork planning prepared by Hünnebeck addresses these conditions. Enables economical use of the formwork.
German premiere: Hünnebeck "Safescreen" wind shield
The Safescreen system, which was used on the Frankfurt construction site for the first time in Germany, ensures a high level of safety and a pleasant working situation even at great heights. In addition to a leading side protection, the system also provides light weather protection. The lightweight safety screen system is a development of the British sister company Hünnebeck UK. Has already been used successfully on the island for ten years. The outer cladding of the windshield is made by telescopic steel frames covered with a perforated aluminum plate.
A working platform is mounted on each ceiling level for better accessibility of the work at the ceiling edge. The platforms were brought to the site as pre-assembled elements including the wind screen.
These rentable cladding elements can be continuously adjusted in width between 2.90 m and 5.00 m. The balconies on the facade of the building are made of precast elements. These precast elements are not capable of transferring the loads from the Safescreen windshield and the next level balcony precast elements. Hünnebeck supplied brackets made of rentable MK2 girders, which bridge the precast elements and absorb the loads from the wind shield and safely transfer them to the slabs without putting any load on the balconies. In addition, the precast elements for the balconies above were supported on these brackets.
Pre-assembled working platforms
On the Frankfurt construction site, the 13 m high wind shield extends over three full floors and overhangs the uppermost slab by around 2.30 m. In addition, a working platform is mounted on each slab level for better accessibility of the work on the slab edge.
The balconies on the facade are not able to carry the superimposed loads from the wind shield. Therefore, the balconies were bridged by brackets.
The platforms were brought to the construction site as pre-assembled elements including a 2.90 m wide wind shield. A deliberately chosen dimension, because a platform less than 3 m wide can be transported "normally" fully assembled without any further effort. The time- and space-saving advantage for the construction site: The complete pre-assembly and later also the dismantling of the working platforms can be carried out in the warehouse.
Shell construction on schedule
Ultimately, all detailed considerations and measures were aimed at not further burdening the spatially tight conditions on site and at constructing the architecturally sophisticated object as smoothly as possible and thus quickly. "Thanks to good planning and cooperation in a spirit of partnership, this was an outstanding success" – this was the joint conclusion of the Gemünden construction team and Hünnebeck Projektentwicklung shortly before the shell construction work was completed on schedule.