Engineers achieve daring and elegant design
This is an excellent example of combining the structural engineer's skill with the vision of the architect - Paris based Explorations Architecture, to achieve a daring and elegant design. Advanced analytical techniques were used to achieve a very slender unbraced arch resulting in a high quality, attractive and innovative solution delivered within tight constraints.
The bridge carries a highway, cycle tracks and footways over the River Oise, and is an all steel structure to minimise weight and ease installation. Tight geometric constraints between the navigation clearance and highway tie-ins at each end demanded a very shallow deck, and the architect's desire to minimise the height of the arch to minimise the visual impact led to an innovative solution with very shallow arches in the main span and complimentary under-slung catenaries in the side span. The resulting form is a smooth and sinuous curve which responds to the topography of the site, keeping the arch below adjacent building heights. The cantilevered footways on either side maximise enjoyment of open river views for pedestrians and physically separate them from the busy roadway, while providing a very slender edge treatment.
To prevent buckling of the shallow slender unbraced arch, innovative flat plate hangers at 4m centres provide lateral restraint but still allow longitudinal articulation by virtue of their orientation. This enabled the arch depth to be kept very slender at only 300mm deep, in line with the architect's vision. In order to achieve this, the engineers used advanced second order analysis to verify stability, considering a range of equivalent initial imperfections which, when combined with the local out of plane distortional effects, permitted a very efficient design.
The result is not only elegant and highly popular, but also extremely economic. The £7.5M cost represents only £3,500/m2, despite the high quality of the finishes.