The rock 'n' roll bridge

Unlike rock 'n' roll high school or even Rock 'n' Roll McDonalds, the rocking 'n' rolling done by the infamous bridge known as "Galloping Gertie" that spanned the Tacoma Narrows was fun for exactly no one. Soon after it was built in 1940, the half-mile suspension bridge that linked Tacoma, Washington to Gig Harbor began to exhibit a strange wavelike motion rippling up and down its length during light breeze conditions, even though it was built to withstand winds of up to 120 MPH. Thanks to Damn Interesting for the cogent explanation as to why:

"Even in a light breeze, Gertie's undulations were known to produce waves up to ten feet tall. Sometimes these occurrences were brief, and other times they lasted for hours at a time. But the matter wasn't considered terribly urgent because the winds were causing longitudinal waves along Gertie's center span"“ waves which traveled back and forth along the length"“ which did not put undue stress on the roadbed. The structure was not at risk, nor did it create unsafe driving conditions."

At least, that's what they thought until the morning of November 7, 1940, when a wind blowing a mere 42 MPH caused the bridge to "abandon her usual rippling action in favor of a never-before-seen twisting motion which increased in intensity at an alarming rate," eventually collapsing into the water 195 feet below. You can see it for yourself in this newsreel clip of the incident:

Exactly why the bridge behaved this way remained a mystery until sometime after the fact. We'll explain, after the jump.

"Examination of the film, the bridge's remains, and tests with scale models determined that resonance was responsible for Gertie's demise. Due to the design decision to replace the lattice supports with "I" beams, the wind was unable to pass under the structure as readily as it it passed over it, causing a difference in pressure much like an airplane wing. Once there was sufficient sway to tilt the deck slightly, aerodynamics caused the roadbed to twist to the point that it sprang back, causing a repeating cycle of back-and-forth twisting. During this process, the steadily blowing wind added more and more energy to the vibrations."