Les Cèdres footbridge. Integral bridge

After winning the Competition with a stress ribbon typology, the Federal Roads Office (Switzerland) reconsidered its initial agreement with the proposed drainage system and required that it not be carried out on the highway that the bridge had to cross (link).

The new proposal for the bicycle and pedestrian footbridge maintains the fundamental ideas that presided over the formal design in the competition and schematic design phase, minimising the visual impact and allowing for a fast construction sequence, with the lowest possible interference to the motorway traffic. To achieve these goals, a slender arched-shaped frame is designed, with a rise-to-span ratio equal to 1:16. The structure is constituted by a main span and two lateral spans, leading to an integral footbridge whose total length is equal to 62 m and with a clear distance equal to 56.50 m between pier bases.

The 35 m long main span is fixed in the lateral structural systems formed by the inclined piers, the deck lateral spans and the prestressed abutments. This system generates, at the front part of the abutment, compressions resisted by piles and, at the rear one, tractions compensated by the self-weight of the foundation. The horizontal component of the pier reaction is balanced, within the abutment, through tensile forces on the lateral span deck. In the foundation, the piles are moved towards the highway to maximize the lever arm and thus reduce the number of required piles.
The deck is formed by a trapezoidal steel box, of variable height, and an upper concrete slab of 0.23 m maximum thickness, constituting a composite structure of total structural height of 1.13 m and 1.66 m at mid-span and over the inclined piers respectively, resulting in slenderness values of L/50 and L/34.

The box-girder webs, with constant inclination, are extended into the piers, giving a minimum width at the start of the latter of 1.10 m. The deck is of double composite action, provided with a bottom slab in the areas of hogging bending moments and thus avoiding any local buckling of the steel plates.