Bridge Collapse

posted: 04/11/12
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As seen in "MythBusters: Breakstep Bridge"


Explanation: On April 14, 1831, a brigade of soldiers marched across the Broughton Suspension Bridge in Manchester, England - and broke it. MythBusters Jamie Hyneman and Adam Savage wondered whether bad vibes were to blame for the architectural disaster, so naturally, they built a miniature bridge and small army of mechanized feet to stomp across it.

Theoretically, if the vibrations from the footfalls matched the bridge's natural vibrations, the structure would break down. By that same science principle, glasses can shatter if someone belts out the correct high-pitched note. While the MythBusters' bridge indeed collapsed under the 12 pairs of fake feet, it wasn't the vibrations from the synchronized marching that did it in. Since the suspension cables failed to quiver and snap, Jamie and Adam initially concluded that the broken bridge could only be blamed on over weighting, not violent vibrations.

But when the MythBusters later retested the breakstep bridge with a simpler structure, and with Adam as the lone soldier, the results changed. Adam lightly bounced along a plank bridge to find that elusive resonant frequency. As Jamie removed the planks from the bridge one by one to amplify the oscillations from Jami's bouncing, the bridge eventually broke, deeming the redone myth plausible.

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