On November 7, 1940, the original Tacoma Narrows Bridge dramatically collapsed into Puget Sound after twisting violently in modest 42 mph winds. While famously nicknamed Galloping Gertie for its vertical bouncing, the fatal structural flaw stemmed from a design choice made in the name of aesthetics and cost savings: swapping traditional open stiffening trusses for solid steel plate girders. Lead engineer Leon Moisseiff championed the Deflection Theory, believing slender bridges were safe because gravity and cable tension would keep them stable.
To make the bridge cheaper and visually striking, Moisseiff replaced deep 25-foot open steel lattice trusses—which allowed wind to flow through harmlessly—with shallow eight-foot solid steel sheet girders. This choice proved catastrophic. Instead of permitting airflow, the solid vertical plate girders acted like rigid sails.
As wind struck the solid steel walls, it separated and formed alternating low-pressure vortices above and below the deck, a phenomenon known as vortex shedding. When these wind eddies matched the bridge's natural dynamic frequencies, the vertical rippling escalated into destructive torsional twisting, a complex physical instability known as aeroelastic flutter. The solid sheet steel girders trapped the wind and continually fed aerodynamic energy into the twisting motion rather than dissipating it.
As the deck pitched at extreme 45-degree angles, the steel and concrete tore themselves apart. The failure permanently reshaped civil engineering, demonstrating that aerodynamic forces—not just static weight loads—must govern long-span bridge design. Today, modern suspension bridges utilize open trusses, aerodynamic box girders, or wind vents to prevent solid surfaces from capturing turbulent wind.