Beyond the functional link they represent, footbridges belong to the public space they shape. This paper will focus on the approach we developed in Saint-Denis, involving a bridge spanning over 48 railway tracks between the two railway stations, extending over a length of more than 300 meters. To limit the height for trains passing by, the structure extends as a superstructure above the deck, creating a suspended square in the center of the structure that opens up to the railway river and the Paris skyline. We proposed a structure composed of multiple arches that shape two large naves, each 120 meters long and 11 meters high, which simultaneously accommodate outdoor public activities and a station connected to the lower-level railway tracks. Thus, we decided to develop a structure that constitutes a true, usable space: a way to inhabit the structure, to be inside the void that characterizes the inertia of this supporting structure. These naves, by creating a virtual, usable volume, offer a true public space while freeing up a volume for appropriation. We organized a longitudinal system of multiple arches and a transverse system of parallel arches, forming a Vierendeel grid (with no diagonal stabilizing elements or shear force transfer), and the whole behaves as a very large three- dimensional Vierendeel beam within which space is offered for public activities. The longitudinal system is supplemented by a Y-shaped section supported by a continuous steel surface, three-dimensionally modeled. The connection is made here between the theory of structures and the development of a unique form designed to serve a generous urban intent centered around the quality of public space.
Marc Mimram (Wed,) studied this question.