Fig. 3.

The motion microscope reveals modal shapes of a lift bridge. (A) The outer spans of the bridge are fixed while the central span moves vertically. (B) The left span was filmed while the central span was lowered. A frame from the resulting video and a time slice at the red line are shown. (C) Displacement and noise SD from the motion microscope are shown for motions in a 1.6- to 1.8-Hz band at the cyan, green, and orange points in B. Doubly integrated data from accelerometers at the cyan and green points are also shown. A time slice from the motion-magnified video is shown (Movie S2). The time at which the central span is fully lowered is marked as “impact.” (D) Same as C, but for motions in a 2.4- to 2.7-Hz band.