A new footbridge has connected the Wien river and the U4 underground line since September 2010. It joins together Astgasse in the 14th with Fleschgasse in the 13th Vienna municipal district. It offers pedestrians barrier-free passage, and also connects cyclists with the Wien river bike path.
Rudolf Brandstötter and Gonzalo Espinosa Ortega drafted the footbridge blueprint. They both received first place for their entry Link 27 in the Student Concrete Trophy 2007. The jury explained their decision as follows: “The independent draft combines a highly appropriate response to the location with an elegant solution. It not only achieves a symbolic effect, but also represents harmonious integration in the surrounding area.“ To keep maintenance costs low the building was designed as an integral bridge; i.e. it is constructed without supports or joints between the bearing structure elements.
Construction time window
Executing the plan required a great deal of creativity on the part of the ALPINE project team. For example, the prestressed concrete elements for the asymmetrical bearing structure were produced directly on site, despite the space restrictions. On a 45 x 10 m form table. Right in the middle of Vienna.
After completion a crane was used to position the approximately 40 m long beams. First, however, they had to be lifted from the formwork and turned in mid air, a feat only possible through the use of two additional cranes. All of this work was carried out in just four hours. The time allotted for the project was not only affected by the metro operating hours, but also the weather: The cranes had to be set up in the river bed of the Wien, which is subject to extreme water level fluctuations in rainy weather. Construction planning thus also required close coordination with the ZAMG (Central Institute for Meteorology and Geodynamics).
Perfect white
Above all, the new footbridge obtains its unique aesthetics from its brilliant white colour, which gives the construction a light, dynamic feel and truly stands out against a relatively grey metropolitan backdrop. But the colour presented the ALPINE team with an enormous challenge. “White cement is more difficult to produce than regular cement,” says construction engineer Martin Ofenböck. “In addition, prefinished cement samples showed that the desired colour effects could not be achieved by adding white cement alone.“
After intensive research and discussions with suppliers and VÖZ (Austrian Cement Industry Association) they finally obtained the perfect mixture of cement, white cement and additional colour pigments. The bearing structure elements were produced at the same time in order to achieve homogeneous colour quality. Here the key was ensuring that the formwork was absolutely clean in order to prevent any type of contamination of the white cement.
Therefore a number of challenges had to be met before the footbridge could span its white arch across the Wien. The fact that this work was honoured with the 2011 SOLID Bautech Prize on 6 April 2011 in Vienna is a positive confirmation for the dedicated ALPINE project team.
Picture gallery „Steg Astgasse“
>> Link 27 – Competition entry
>> 2011 SOLID Bautech Award – The winners