The Tacoma Narrows Bridge collapsed on November 7 1940 in a wind of only 64 km/h - well within its design limits. The problem was not wind strength but frequency. The wind created vortices that shed alternately from each side of the deck at a rate matching the bridge's natural oscillation frequency. Resonance. The bridge amplified the motion until it tore itself apart. Every bridge designed after 1940 tests for aerodynamic flutter. The Tacoma Narrows taught engineers a failure mode that 50 years of bridge building had never encountered at this scale.