Bridges and the Wind
For most of the history of engineering, the main concern in bridge design was weight: a bridge had to carry itself, its traffic and perhaps a load of snow. Wind was treated as a steady sideways push that a strong enough structure could simply resist. That view began to change when several long, light bridge decks were seen twisting and swaying in quite moderate breezes.
Dr Ana Ruiz, who studies the aerodynamics of bridge decks, argues that the danger lies less in the strength of the wind than in its rhythm. As air flows past a deck, it sheds small whirlpools, called vortices, first from one side and then from the other. If the rate at which these vortices break away matches the frequency at which the deck naturally sways, each one adds a little energy and the movement grows. “A gentle, steady wind can be more troublesome than a storm,” she notes, “because a storm is too turbulent to keep a regular beat.”
Professor Tomas Lindqvist approached the problem through shape. In wind-tunnel tests comparing models of flat-sided decks with decks given tapered, wing-like edges, he found that the streamlined versions shed far weaker vortices. He concluded that a deck's cross-section matters more than its mass in determining how it behaves in wind, a view that has influenced many long-span designs. Lindqvist has also cautioned that a model in a tunnel cannot reproduce every feature of a real site, such as the effect of nearby hills.
Not every solution involves redesigning the deck. Dr Kwame Hollis specialises in dampers, devices that absorb movement once it has begun. His team fitted a footbridge with heavy weights hung on springs and tuned to swing against the bridge's own motion; sway fell by more than two-thirds. Hollis points out that dampers can be added to bridges that are already standing, which makes them far cheaper than rebuilding. He admits, however, that they need regular adjustment, because a bridge's natural frequency can shift as its materials age.
Dr Ines Varga has looked at a different source of movement: the people on the bridge. Her measurements on crowded walkways showed that pedestrians unconsciously fall into step with a slight sideways sway, and that their footsteps then make the sway stronger. Designers who test only for wind, she warns, may miss this effect entirely. Varga agrees with Ruiz on one general point: regular, repeated forces are a greater threat to a structure than a single large one.
Today, sensors on many large bridges record their movement continuously. Lindqvist regards this data as the best way of checking whether predictions made in the wind tunnel hold true at full scale.