Leon Moisseiff’s 1940 Narrows Bridge was built to look like a blade. Same 2,800-foot main span the 1950 replacement still uses — but the deck was only 39 feet wide and stiffened by a pair of solid 8-foot plate girders running the length of the roadway, one along each edge. Those vertical webs sat parallel to traffic. A southwest wind did not go through them. It hit them like a sail.

Air sliding past a blunt, solid face peels off in alternating swirls — a Kármán vortex street. Each swirl is a pocket of low pressure. Left, then right. Up, then down. The HAER failure sheet draws that as a lift-and-drag couple on the girder section (figs. 3–4). When the shedding beat matched a natural bounce of the span, the deck began to gallop. Workers named her long before she fell.

Past about 40 mph the picture changed. The deck’s own twist reshaped the flow, feeding energy back into the motion — self-excited torsional flutter. On November 7, under ~42 mph southwest wind, suspenders snapped and the main span folded into the Narrows. The Carmody Board (1941) blamed “excessive flexibility, shallow girders, and aerodynamic forces like vortex shedding and self-excited torsional flutter.”

The 1950 rebuild kept the span and threw out the sail: a deeper open stiffening truss, a wider deck, hold-downs, later hydraulic dampers. Wind-tunnel models became standard. Gertie’s wreck is still on the bottom — an artificial reef, and the first page of every aeroelastic textbook. (Grokipedia: Tacoma Narrows Bridge (1940))