Bridges are often considered to belong to the realm of the engineer rather than that of the architect. But the architecture of infrastructure has a powerful impact on the environment and the Millau Viaduct, designed by Foster and Partners in close collaboration with structural engineers, illustrates how the architect can play an integral role in the design of bridges.
Located in southern France, the bridge, the highest elevated roadway in the world, connects the A75 motorway networks of France and Spain, opening up a direct route from Paris to Barcelona, across the Massif Central. The bridge crosses the River Tarn, which runs through a spectacular gorge between two high plateaux: the northern “Causse Rouge” and the southern “Causse Larzac”. The motorway has two lanes in each direction with emergency hard shoulders. It takes approximately one minute to cross the bridge by car, and a toll plaza marks the northern part of the bridge. Interestingly, alternative readings of the topography suggested two possible structural approaches: to celebrate the act of crossing the river or to articulate the challenge of spanning the 2.46 kilometres from one plateau to the other in the most economical manner. Although historically the river was the geological generator of the landscape, it is very narrow at this point, and so it was the second reading that suggested the most appropriate structural solution.
A cable-stayed, masted structure, the bridge is delicate, transparent, has the optimum span between columns. Each of its sections spans 342 metres and its columns range in height from 75 metres to 245 metres (almost equivalent to the height of the Eiffel Tower), with the masts rising a further 90 metres above the road deck. To accommodate the expansion and contraction of the concrete deck, each column splits into two thinner, more flexible columns below the roadway, forming an A-frame above deck level. The tapered form of the columns both expresses their structural loads and minimises their profile in elevation. The bridge not only has a dramatic silhouette, but crucially, it also makes the minimum intervention in the landscape.