10.07.2018 Frankfurt’s Grand Tower: Germany’s tallest residential building on the up-and-up
Varying floor ground plans, lack of space, and limited crane time: These, in a nutshell, are the difficult site conditions faced by the large-scale Grand Tower construction project in Frankfurt. An ingenious shuttering strategy and the use of Hünnebeck formwork and enclosure systems have nevertheless allowed the building fabric to advance swiftly upward. In operation for the first time in Germany was the rentable Safescreen enclosure system developed by Hünnebeck UK.
At a height of 172 m, the Grand Tower under construction in Frankfurt’s Europaviertel (European Quarter) since 2016 will be Germany’s tallest residential high-rise. The client and project developer of this large-scale project is gsp Städtebau GmbH, Berlin. The lozenge-shaped tower block comprises a total of 55,000 m² of gross floor space, distributed among 1 sublevel, 3 technical floors, and 47 above-ground levels containing 401 apartments. The building is scheduled for occupancy in 2019.
Staggered process with advancing core
For the fabric constructors, this major project is a testing task with many challenges: extremely confined space, little crane capacity, and ground plans changing from one floor to the next. To get a grip on these conditions and develop an appropriate procedure, contractor Karl Gemünden GmbH & Co.KG from Ingelheim in Rhine-Hesse brought in the services of the formwork experts from Hünnebeck. Together they developed a comprehensive shuttering strategy geared to the special site conditions. This included, first, the decision to stagger work on the fabric and pour the building core comprising the stairs, lift shafts, and lozenge-shaped perimeter wall separately from the living quarters – with a lead of three to four stories. Construction of the floor and wall areas of the diversified apartment ground plans trails behind – fully screened off behind a 13 m high enclosure system that Hünnebeck is employing for the first time in Germany on this site.
Saving crane time
During such a construction project, crane time is the key resource and at the same time the limiting factor on the site. If this is coupled, as in Frankfurt, with chronic shortage of space (neither the site nor the building’s ground plan offer sufficient space for intermediate formwork storage), the situation is compounded. In the planning of the formwork assignment, efforts were made to organize as many work steps as possible without the need for crane assistance. Ascent and shuttering of the outside of the perimeter wall are thus being performed almost entirely without the use of a crane.
This is where brackets of Hünnebeck SCF (Self Climbing Formwork) are arranged in a continuous row. The modular formwork system climbs hydraulically in pace with the building, without the need for a crane. Basically, SCF is always a one-off configuration in which the ratio of number of brackets to shuttered surface area is precisely adapted to the building geometry. All the formwork and bracket components are designed as complete, self-supporting functional units matching the specific conditions on site. On the Frankfurt site, the formwork is permanently attached to the platforms via slides. Owing to the low story height of only 3.25 m, only one trailing platform is required.
The inner core zone with its narrow shafts and relatively slim inner walls, on the other hand, is unsuitable for the use of climbing units. For its construction, a mix of self-climbing brackets and latch platforms has been chosen. The selected formwork system is Manto frame formwork. In the tight shaft geometries particularly, its compact design simplifies the handling of the formwork, which is mounted on a steel scaffold supported on latch platforms. As a result, the formwork and platform can be raised together in a single lift – and this in turn saves crane time. At the same time, the rebars can be connected together in large cages on the ground and then lowered from above into the formwork as there are no obstructive beams over the walls. This procedure also saves time, as concrete can be poured on the building while the reinforcement is assembled at the same time on the ground.
So much for the principle. But, as we know, the devil is often in the details. The tying of the SCF and latch platforms to the perimeter wall poses a particular challenge. The constantly changing ground plan with differently positioned door and window openings on each floor makes it necessary to develop a system of different types of tie to anchor the self-climbing formwork. In addition to the “standard” cone tie, the Hünnebeck SCF specialists have designed both a solution for tying in wall openings (climbing shoe on a conical steel box) and for tying above a door (bracket tie upward and support with tubular steel props).
The many openings in the perimeter wall of the core not only hamper the tying of the SCF units on the outside of the wall, but also affect the tying of the latch platforms on the inside. For this, several variants have also had to be produced – ranging from simple to highly complex, particularly at the door openings where vertical loads have to be discharged via tubular steel props placed under the door lintels. Ultimately, some 350 different custom parts have been required for the climbing platforms of the building core. Since the ties change on each level, the Hünnebeck climbing formwork experts have drawn up a plan for each floor itemizing the type of tie and steel parts used in every single tying position. The effort has paid off, translating into a no-hassle construction process.
Changing ground plan
The same also applies to the construction of the apartments which follows hard on the heels of the building core. In operation here for the slabs are the manually handled modular Topec system, Topmax steel-frame floor table, and aluminum Gass support system, as well as load frame supports and tubular steel props. All the wall surfaces are being shuttered with Manto large-frame formwork and custom formwork elements (H20 wall formwork). The aggravating factor is that although the ground plans are repeated at regular intervals (every four floors between the 8th and 29th stories, for example), they are always different on successive floors. The formwork plan devised by Hünnebeck responds to these limitations and permits cost-effective formwork use.
German premiere for Hünnebeck Safescreen enclosure screen system
A high standard of safety and pleasant working conditions at altitude are provided by the Safescreen system in use on the Frankfurt construction site for the first time in Germany. While providing advancing edge protection, it also provides lightweight weather protection. The lightweight enclosure screen system has been developed by the British affiliate Hünnebeck UK and has been in successful use in Britain for the last ten years.
The outer cladding of the screen is enabled by telescoping steel frames covered with perforated aluminum sheet. These rentable cladding elements can be continuously width-adjusted from 2.90 to 5.00 m.
The balconies on the building’s façade are built using precast elements. Since these elements are incapable of discharging the loads from the Safescreen perimeter enclosure system and from the next level of precast concrete elements, Hünnebeck has supplied brackets of rentable MK2 beams that bypass the precast elements and take up the loads from the enclosure screen, discharging them dependably into the slabs without burdening the balconies. The precast elements for the balconies above were also supported by these brackets.
Pre-assembled work platforms
On the Frankfurt construction site, the 13 m high enclosure screen extends over three full floors and projects some 2.30 m above the top floor. For better access to the work on the edge of the slab, a work platform is fitted on each floor level. The platforms are brought to the construction site as pre-assembled elements inclusive of the 2.90 m wide enclosure screen. A deliberately chosen dimension, as a platform less than 3 m wide can be transported fully assembled in the normal way without any extra effort. The time - and space-saving advantage for the site is that the work platforms can be fully assembled and later dissembled in the warehouse.
Punctual completion of the building fabric
Ultimately, the aim behind all the detailed considerations and measures was to not put any extra strain on the limited space on site and to build this architecturally adventurous building as smoothly and swiftly as possible. "Thanks to good planning and cooperation in a spirit of partnership, we’ve achieved just that," is the sentiment shared by the Gemünden site team and Hünnebeck’s project developers shortly before punctual completion of the building fabric.
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