Galloping Gertie

The collapse of the original Tacoma Narrows Bridge on November 7, 1940, was a pivotal event in the field of engineering and bridge design, offering profound lessons that have since influenced the construction of all long-span bridges.

Known as “Galloping Gertie,” the bridge was at the time the third-longest suspension bridge in the world, following the Golden Gate and George Washington Bridges. Despite its impressive length, it suffered from structural issues that led to its collapse in a 40-mile-per-hour wind, a moderate speed that should not ordinarily have threatened such a structure. The bridge’s design exhibited little resistance to torsional (twisting) forces due to its large depth-to-width ratio, making it extremely flexible. A critical event on the morning of the collapse was the slippage of a cable band at mid-span on the north cable, which changed the movement of the bridge deck from vertical to torsional. This was exacerbated by “vortex shedding,” where the wind created a vortex as it struck the side of the bridge’s deck. The structure’s movements became self-generating, leading to a self-excited motion that eventually exceeded the bridge’s ability to resist, causing failure.

The collapse highlighted the inadequacy of the deflection theory, which at the time considered the effect of wind on suspension bridges to be minor. As a result, the engineering community shifted their focus to include the dynamic effects of wind forces on bridge designs. It became clear that engineers had forgotten lessons from past suspension bridge failures and had not sufficiently understood the impact of aerodynamic forces, an oversight that contributed to the disaster. The event abruptly ended the era of designing flexible, light, and slender suspension spans without adequate consideration for wind forces.

In the aftermath of the Tacoma Narrows Bridge disaster, the field of bridge engineering underwent significant changes. Wind tunnel testing for aerodynamic effects on bridge designs has become a standard practice, and the United States government now requires that all bridges built with federal funds undergo such testing. The principles of the Deflection Theory still remain a part of suspension bridge engineering, but they are now supplemented with aerodynamic stability analysis, which is supported by advanced computational methods like finite element analysis. These practices have led to the design of bridges that are either aerodynamically streamlined, stiffened against torsional motion, or both.

Othmar Amman, a renowned bridge engineer, commented on the failure by acknowledging the invaluable information gained from the incident, which has steered the industry toward safer and more economical designs against wind action. The loss of the Tacoma Narrows Bridge, while tragic, has thus significantly advanced our understanding and methodology in bridge design, ensuring such a failure would not be repeated.