The Tacoma Narrows Bridge

To be clear, this post isn't about the twin bridges between Tacoma and Gig Harbor, Washington on state route 16. Ten years before either of those bridges spanned Puget Sound, a single bridge over the Tacoma Narrows was completed in July 1940. The bridge was most famous for spectacularly and photogenically collapsing four months later, and you can read more about it here:
Tacoma Narrows Bridge (1940)

Text and photos aren't enough to convey the weirdness of how this structure behaved. Film captured between the bridge's construction and collapse shows it bucking and wobbling like a half-mile-long cartoon rubber band. It looks so impossible my mind refused to believe it was real when I saw the clips on Bill Nye the Science Guy back in the 1900s. It wasn't until my high school physics teacher solemnly described the aeroelastic resonance that brought down the bridge that I realized this really happened with a start:

Before World War II, aerodynamics was seldom considered when building large structures. For most buildings and bridges, this is acceptable since ground-based structures tend to be stiff relative to the wind forces they see. In this case, though, the winds of the the Tacoma Narrows were consistently harsh and the bridge deck was sufficiently compliant that the natural frequency of the main span wound up very close to the frequency at which a leeward vortex street was shed on blustery days. While the steel girders were strong enough to withstand jiggling like jell-o for a time, eventually fatigue cracks linked and the center span came crashing down.

The Pacific northwest has had some bad experiences with bridges. In addition to the Tacoma Narrows embarrassment, over the last 35 years not one but two floating bridges sank in western Washington, first in the Hood Canal and then in Lake Washington. Out of fears the 520 bridge between Seattle and Medina could suffer a similar fate, a billion-dollar effort is under way to replace it with a sturdier iteration as of the time of this writing. It's a shame Bill Nye couldn't have been of more help: