Walking over water Footbridges on Dusuzeau and Austerlitz channel, Strasbourg
The 3 footbridges are walkways. They continue the urban tissue and link the pier with the city. Besides linking two parts of the city it is the experience of the promenade, the discovering of new perspectives and views, the closeness to the water which guides the design of the bridges. The architectural ambition is to make them part of the urban landscape, integrating them naturally. Lightness and transparency are major issues. The bridges are opposing the dense urban tissue. They contrast the monumentality of the surrounding buildings by their freely adapted geometry. The bridges want to be part of the natural environment, inspired by the presence of water. Like insects they stretch out their members to reach the quay sides offering a multiple choice of crossing – fast and slow, for bicycles and disabled on flat ramps. The ramps are part of the crossing, part of the design. In the centre the different connexion add to form a plaza over the water forming a privileged viewpoint. Local oak wood as deck covering, the transparent railing made of a stainless steel cable net and the integrated lighting of the bridge create the specific atmosphere on the bridge. Structure is a main part of the form. It is clearly expressed, details are carefully designed and tell the way forces are led to the banks.
The site A former industrial pier is being transformed into a new urban area given it’s attraction by the new media centre, a recently adapted storage building. The presence of water on three sides of the pier isolates the site. The new footbridges (3) connect the site to the city, a rediscovered neighbourhood with a high density of housing and offices.
The 3 bridges are related to each other. They are part of the same family being adapted to the specific site conditions of each. A steel box forming an arch articulated on the banks is the major structural element. Carrying and to be carried is the Leitmotiv of the design. The second walkway joining the arch is being carried by the steel box. The strong compact primary element is completed by a light transparent structure leaning on the box. The ramp over the water is fixed on the ground by a pair of inclined columns. Dynamics issues are being resolved by the integration of a damper into the steel box. The geometry allows ships to pass under the bridges (16m x 3m85). The structure was prefabricated in the factory, divided into 2 parts, brought to site with a ship and mounted on site. Special pillar foundations are provided to respond to the efforts introduced by the arch. The geometry of the arch is adapted to respond perfectly to structural needs transforming the pure circular shape into an organic form.
The ambition of the structural concept was to generate a slender construction, considering difficult geometrical boundary conditions (the structure gauge for shipping traffic) and dynamical requirements.
The main structural components can be described as follows:
1. Arc structure: with its span of 42m, width of 1.8m and height of 0.55m the arc structure describes the main part of the total construction and is also used as walkway for pedestrians. The cross section of the arc was designed following the bending line and describes a welded box girder (picture 1).
2. Cantilever bars / deck: the main deck is located parallel to the arc consisting of cantilever beams which are connected one-sided to the arc. Further more longitudinal beams build up the pedestrian deck (picture 2).
3. Ramp: in progress to the main deck ramps close the span to the landside and are geometrical parallel to the landside. The supporting columns of the ramp deck build a triangular structure which is connected with a pile foundation (picture 3). The dynamical properties of the structure were important for the structural design.