The official description for what went wrong was “synchronous lateral excitation” from pedestrians. It was the people walking on the bridge who caused it to wobble. Getting something as massive as the Millennium Bridge to start to wobble using brute force is a near-impossible challenge for a bunch of pedestrians, but this bridge was accidentally tuned to make it easy. Most people walk at about two steps per second, which means their body swings side to side once per second. A human walking is, for all bridge intents and purposes, a mass vibrating at 1 Hertz—which was the perfect rate to get
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