Theory of Structure and Public Space

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.