How does a cable-stayed bridge like the Millau Viaduct differ from a suspension bridge?

A cable-stayed bridge holds its deck up with cables that run straight from a tall tower down to the road, like the strings of a harp. A suspension bridge does something different: its deck hangs on vertical cables from giant main cables that run between the towers and are anchored at both ends. The Millau Viaduct in southern France, completed in 2004, is a famous cable-stayed example. It crosses the gorge valley of the Tarn with a structural height of 343 metres, and its design team was led by engineer Michel Virlogeux and English architect Norman Foster.

In a cable-stayed bridge the towers are the main load-bearing structures that send the bridge's loads to the ground. Each stay pulls on the deck at an angle, to the side rather than straight up, so the deck itself has to resist a squeezing force along its length. The reward is that you do not need firm anchorages in the ground to hold giant main cables. Cable-stayed bridges are best suited to spans longer than cantilever bridges and shorter than suspension bridges.

Millau began as a traffic problem. In the 1980s, heavy road traffic, especially summer holiday traffic from Paris to Spain, was causing congestion near Millau. After wind tunnel tests, the shape of the road deck was altered, and the calculations covered winds of up to 225 kilometres per hour.

The two ideas have even been mixed: early bridges such as the Brooklyn Bridge combined suspension and cable-stayed features, and their designers found the combination created a stiffer bridge.