Home / Trivia / Buildings & Engineering
What caused the Tacoma Narrows Bridge to collapse in 1940?
It twisted and fell just months after opening, and engineers changed how bridges are tested.
The Tacoma Narrows Bridge opened in July 1940 and fell on 7 November 1940. The state transportation department reports winds of roughly 38 to 42 miles per hour that morning, which is strong but not extreme for a major bridge. Around 10 a.m. the deck began a twisting motion, and within minutes the sides of the roadway were moving up and down by many feet. The middle of the span broke apart at about 11 a.m.
The design was slender. The deck was stiffened by solid plate girders, not open trusses, so wind could not pass through and instead struck the deck broadside. This made the bridge unusually flexible, and it had earned the nickname Galloping Gertie because it moved even in ordinary winds during construction. The investigation by Professor Farquharson of the University of Washington identified the solid girders as the probable source of the instability.
The cause is now described as aeroelastic flutter, a feedback loop in which the movement of the structure changes the airflow around it, which in turn feeds energy into the movement. A widespread misconception, repeated in many textbooks for years, is that the bridge failed because of resonance with vortices shed by the wind, a so-called von Karman vortex street. Engineers and physicists have since argued that this explanation is incomplete, since flutter is a self-excited process rather than a simple matching of frequencies. After the collapse, wind-tunnel testing of bridge models became a standard step in design, and later suspension bridges used open trusses or streamlined box shapes.