Engineering Disaster: Tacoma Narrows Bridge Collapse (1940)
About the Disaster
The Tacoma Narrows Bridge, built in 1940, spanned the Puget Sound in Washington and featured a longer, narrower take on the increasingly popular suspension bridge. The bridge was originally set to be designed by Clark Eldridge, Washington Department of Highway’s leading bridge engineer. His design was turned down for exceeding the budget, and design control was turned over to Leon Moisseiff, renowned suspension bridge engineer.
Moisseiff designed the bridge as the most flexible suspension bridge ever constructed. It failed on a windy day because aerodynamic forces were not properly taken into consideration during design. At the time of its construction, aerodynamic forces were rarely factored into bridge designs because it was thought that wind was a stress only capable of moving bridges laterally.
Luckily, there were no human fatalities from the accident. The only loss of life was someone’s dog that was trapped in a car abandoned on the collapsing bridge, which is still a sad outcome that could have been prevented with better design.
Engineering Perspective
Moisseiff had previously discovered “deflection theory” which was a set of mathematical formulas that proved heavy, costly stiffening trusses were not necessary for bridges because “dead weight” elements such as the weight of the deck, main cables, and suspender cables provided enough structural strength against the effects of wind. This theory made the poor assumption that aerodynamic forces could only move a bridge laterally.
Wires snapped, the main towers twisted, and the deck floor crumbled all because the bridge was too flexible and aerodynamic forces were poorly understood. According to a summary article published in Engineering News Record, "The fundamental weakness" of the Tacoma Narrows Bridge was its "great flexibility, vertically and in torsion." Excessive flexibility occurred because the deck was too light (1/350 ratio with the center span) and the side spans were too long, compared with the length of the center span. The cables were also anchored too far from the side spans and the width of the deck was extremely narrow compared with its center span length, an unprecedented ratio of 1 to 72.
Stress limits were exceeded in the bridge which led to the destructive twisting, torsional motion that caused slippage of the cable band on the north cable at mid-span. When the band slipped, the north cable split into two unequal pieces. The imbalance translated quickly to the thin, flexible plate girders, which twisted and soon resulted in progressive failure. Professor F. B. Farquharson concluded that the "cumulative effects of undampened rhythmic forces" had produced "intense resonant oscillation."
When answering the question of why this happened, most blamed the civil engineering profession as a whole. Earlier suspension bridges had a history of unreliability, but the risks seemed to be overlooked or forgotten in an effort to make aesthetically pleasing, cheaper bridges.
Lessons Learned
The biggest lesson learned from this event is the importance of understanding aerodynamic forces and to value safety over design looks when people’s lives are at stake. A lesson I personally took from researching the event is the importance of researching, learning from, and correcting failures that occurred in previous examples.
The Tacoma Narrows Bridge Collapse changed the engineering design process by ultimately ending an entire generation of bridge engineering theory and practice. It halted the trend in designing increasingly flexible, light, and slender suspension spans.
Resources
https://www.wsdot.wa.gov/tnbhistory/Machine/machine2.htm
https://www.kiro7.com/news/tacoma-narrows-bridge-collapses-on-november-7-1940/1006176170/
https://www.history.com/this-day-in-history/tacoma-narrows-bridge-collapses