Movie S2.

The input bridge video is concatenated with two motion-magnified videos revealing different modal shapes of the bridge. Motions within a 1.6- to 1.8-Hz frequency band are amplified 400 times to produce the video on the right, in which the first bending mode is revealed. Motions within a 2.4- to 2.7-Hz frequency band are amplified 250 times to produce the video on the bottom, in which the first torsional mode is revealed. The impact of the central span (not shown in the video) occurs ∼5 s after the video’s start.