The project for a footbridge located in Roche-sur-Yon was commissioned as a collaborative work in between HDA Paris, who has a previous experience with the footbridge they did in Turin for the Olympic Village in 2006 and Bernard Tschumi, who recently finished the Acropolis Museum.
The program for the extension of the TGV network in southern France includes a passage through the town of la Roche sur Yon. The town is modernizing the train station and replacing an 1890’s footbridge over the railway tracks. The town is separated by the railway tracks into two parts: the historical central neighborhood, which contains the ‘Pentagon’ planned by Napoleon and it’s contemporary counterpart with its modern facilities (stadium, school and residential zones).The ambition of the town, is not only to create a symbolic link between the two neighborhoods, but equally to celebrate the arrival of the TGV.
La Roche sur Yon is the birthplace of Robert le Ricolais, engineer, architect, poet and painter, known for his theoretical research on trellis structures and tensegrity during the 1950’s. This heritage, both intellectual and historical, has inspired the design of the new footbridge by attempting maximum lightness. During the design process therefore HDA combined structural optimization with the architectural concepts by creating a full height filigree lattice tube, that provides not only a support for safety meshes as required by the railway authority, but also maximum structural inertia.
The diagonal lattice design recalls the existing old riveted footbridge. At the support points, the stresses are mainly shear, in the predominantly vertical direction, and at mid-span, the stresses become principally bending and the direction tends towards the horizontal. The natures of the forces are highlighted by ‘T’ or ‘H’ section profiles for compression and simple rod ties for those in tension. The transition between supports and mid-spans is also underlined by the presence of vertical circles that recreate links for the shear force transfer. The architectural result is an expression of the natural forces.
It is interesting to consider that the structural optimization process that permits, saving tons of steel material is not only driven by practical, aesthetic and economic objectives, but also has an ecological dividend. Steel itself is a recyclable material to begin with. From the initial analyses, where all steel sections were identical, the tonnage was reduced considerably using subsequent iterative analyses, and notably by replacing the tension members with thin rods. This also significantly contributed to a delicateness in the architectural quality.
A real scale prototype is now complete and the footbridge construction is in progress and the final delivery is planned during 2009.