Why can twisting vibrations be dangerous for a bridge deck?

Twisting, or torsional, vibrations are dangerous for a bridge deck because they represent dynamic oscillations that can lead to structural failure. When a bridge deck lacks sufficient torsional and vertical stiffness, wind-induced forces can cause the structure to move in ways that exceed its design limits.12

These movements are particularly hazardous because they can escalate rapidly, as seen in the 1940 collapse of the Tacoma Narrows Bridge. In that instance, a relatively moderate wind triggered wild twisting and vertical motions because the deck was too thin and lacked the necessary structural bracing to resist aerodynamic forces.2

To prevent such failures, engineers must ensure that a suspension bridge deck is either sufficiently heavy or stiff—or both—to minimize excessive movement under loading. Modern designs often incorporate aerodynamic shaping or additional bracing, such as trusses, to ensure the structure can withstand dynamic wind loads without developing these destructive, self-reinforcing oscillations.312