When the Tacoma Narrows Bridge opened to traffic in July 1940, it was celebrated as a masterpiece of modern slender suspension design. However, drivers quickly noticed that even moderate winds caused the span to roll in alarming wave-like vertical oscillations, earning the bridge the nickname Galloping Gertie. The fatal flaw lay in its aerodynamic design choice: engineers had replaced deep, open stiffening trusses with solid eight-foot plate steel side girders.
While open trusses allowed wind to pass harmlessly through the bridge deck, the solid plate girders acted like giant sails, trapping air currents and creating chaotic wind vortices above and below the roadway. On November 7, 1940, a steady 42-mile-per-hour wind struck the bridge at a critical angle. Instead of mere vertical movement, the trapped airflow induced a devastating aerodynamic phenomenon known as aeroelastic flutter.
As wind vortices shed alternately from the top and bottom edges of the solid steel girders, they matched the natural torsional frequency of the bridge structure. The deck began twisting violently back and forth, turning up to 45 degrees in opposite directions. The twisting forces eventually snapped the suspension cables, causing large sections of the concrete roadway to plunge into the Puget Sound.
The disaster revolutionized civil engineering, establishing wind-tunnel testing and aerodynamic stability as essential steps in suspension bridge design.