Filigree reinforced concrete net shapes and supports office tower

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The downtown area of the Polish capital of Warsaw has a new architectural highlight. It’s not the 70 m height and shape that makes Prosta Tower unique, but rather the front of the building – a glass-and- concrete structure comprising individual diamond-shaped RC sections in a perfect architectural concrete finish that characterizes the appearance of this office block. The delicate concrete façade, sloping forwards and backwards, has been erected in the form of a net over the external glazing. In addition, it serves as a structural component.

Architectural and logistical challenges

The challenges for the Warbud construction team included, on the one hand, ensuring that the form of the complicated concrete structure was accurately realized despite the high degree of reinforcement as well as having to take into consideration a formwork pressure of 90 kN/m² and achieving a perfect surface finish with no tie marks. On the other hand, the inner-city location with no storage space available plus the resulting site facilities with only one crane meant that detailed site logistics operations were required to suit the construction progress. Furthermore, the floor plans along with the cross-section of the reinforced concrete structure changed repeatedly from floor to floor.

In close consultation with the site management, PERI’s Polish engineers designed a comprehensive formwork and scaffolding solution tailored exactly to all requirements. By using test assemblies to build reference structures, the architectural concrete team was able to coordinate the detailed combination of formwork, scaffolding and concrete technology in advance. This resulted in optimized results and speeded up construction progress without any detrimental impacts, which meant subsequent surface treatment could therefore be avoided.

Special formwork from system components

For constructing the reinforced concrete façade, the TRIO panel formwork served as an inexpensive as well as a simple and quick-to-assemble basic form with a formwork height of 3.60 m. Double-layer form liners ensured attractive and virtually joint-free concrete surfaces; Fin-Ply Maxi form liners were the preferred choice for this. Due to the special panel arrangement as well as the use of walings to distribute the loads, the tie arrangement could be determined so that anchors did not have to penetrate the concrete itself.

Custom box-outs were used to ensure millimetre-exact shaping of the structure. The basis for this was formed by system components such as steel walings, heavy-duty spindles and standardized connectors – all easy to hire and all taken from the extensive PERI product portfolio. The special formwork elements were completely pre-assembled at the PERI formwork assembly shed in Warsaw and subsequently delivered to the site ready to use. All 28 elements required for the standard and special cross-sections were equipped with a specially designed striking mechanism. As a result, striking could be carried out without any damage to formwork or concrete structure. In addition, the PERI concept for the internal formwork took into consideration the massive reinforcement connections at the level of the floor slabs.

Formwork + scaffolding = complete solution

A working scaffold that extended continuously in line with progress on the basis of the PERI UP Rosett modular scaffolding system supplemented the PERI complete solution. This greatly contributed to maintaining the construction schedule as well as improving safety for site personnel. The scaffold construction fulfilled two purposes here: it served as a supporting structure for the respective topmost working level with sufficient space available for forming, placing reinforcement and concreting operations as well as façade scaffolding for the subsequent assembly work. Bracket extensions on the inside allowed maximum adaptation to the façade structure and this guaranteed optimum, safe accessibility.

Prosta: not the easiest of tasks

The name of this 24-storey office complex – "Prosta Tower” – with over 8000 m² of high-quality floor space is derived from the street of the same name: Prosta Street in downtown Warsaw. The translation for the Polish word "Prosta” is "straightforward” or "simple”, but this is not necessarily true when referring to the new structure. On the contrary, implementation of the architectural requirements was extremely demanding and the building with its filigree reinforced concrete net seems anything but straightforward and simple!