Bridges Wobble Slightly by Design, Not by Accident

Modern suspension bridges are designed to flex and sway — rigidity would cause them to crack and fail. The Tacoma Narrows Bridge collapse in 1940 wasn't caused by a gust of wind but by resonance: the wind matched the bridge's natural frequency and amplified the oscillations until it tore apart. Every bridge since has been designed to avoid that frequency.

Source: Engineering Mechanics — various structural engineering texts