The footbridge built from thin-walled and only three-centimeter thick concrete is a unique combination of a static solution, material technology and first-class production, designed by scientists from the KI of the CTU. The material has a unique composition, workability, and unusually high mechanical strength in compression and tension among cement materials. This opens up space for the design of previously unfeasible structural elements.
From a bold dream to reality
The story of the unusual footbridge, installed in 2019 in Vrapice near Kladno on a pedestrian path between the village and a cemetery, begins in 2015. The architects’ vision materializes in the design of a shell-arched footbridge with a U-shaped cross-section, about 10 meters long and curved in space in both vertical and horizontal directions.
The creators wanted to achieve a perfect harmony of the landscape, the local statue of the “Guardian”, and the footbridge itself when working on the first designs already. The idea of the shell counted on the thickness of the walls and the walking board in the range from 30 to 50 mm. A typical material for such a design would have been steel previously, but here the use of UHPC was considered from the beginning.
New and untested procedures
There were doubts from the beginning – the implementation of the architects’ ideas was not easy. The static design, material technology, formwork and production of the footbridge were provided at the Klokner Institute of the Czech Technical University, but without close cooperation with KŠ Prefa company, the thin-walled arched footbridge would not have been created. The production process itself was laborious. It was necessary to create new technological procedures that are significantly different from the methods used so far.
The proposed shape of the footbridge was subjected to non-linear static analyses by the KI staff that confirmed the suitability of the structural system. In terms of material, the UHPC bearing function is unique, reinforced only with scattered fiber reinforcement, not with steel bars as it is usual in concrete structures. The footbridge is special also from this point of view.
Material reliability requirements
The first complicated task turned out to be the production of complex formwork to ensure the smooth round curves of the footbridge. It was necessary for the construction material to ensure the filling of the mold without caverns and macropores, and to create smooth and aesthetically pleasing surfaces. This seeming trifle was the subject of a series of practical tests before an acceptable result was achieved.
This was followed by the production of the footbridge itself, well actually, of two of them. The first was a trial footbridge on which the casting technology was tested. The result was stunning. The thirty-millimeter wide railing walls and the arch bottom were perfectly filled with UHPC. The result of the additional load test was also very good and met expectations. Subsequently, concrete casting of footbridge number two took place.
Footbridge no. 2
The task was implemented successfully and footbridge no. 2 was ready for the stress test. It proved fit for use. The elegant black footbridge made of coloured UHPC, with surface patinated by etching, was installed in 2019 by KŠ Prefa s.r.o. to its current location in Vrapice, and has been serving its purpose ever since.
The unique shape and the material used have attracted a lot of attention and there is huge interest in the footbridge. In 2019, it was nominated for the Czech Architecture Award and won the prestigious prize. The comment of one of the judging experts says it all: “It seems to be floating due to its extremely thin form. When you come to the footbridge, you will immediately feel the compulsion to touch its sides and will be amazed at how they feel like leather.” Therefore, the creators managed to upgrade an ordinary footbridge into a work of art and its simple design into an architectural gem.
- Authors of the footbridge: Petr Tej / Klokner Institute, Ondřej Císler / AOC
- Author of the sculpture: Jan Hendrych
- Production technology: Jiří Kolísko / Klokner Institute, Implementation: KŠ Prefa, Klokner Institute of Czech Technical University in Prague
- Photo: Boys Play Nice