The collapse of the Tacoma Narrows Bridge is more than the story of a bridge failure – it’s a story of how the lessons of the past were forgotten.
The collapse
On 7 November 1940, the Tacoma Narrows Bridge, spanning 853 m across Puget Sound in Washington State, began twisting violently in the wind.
Throughout the morning, the magnitude of the undulations grew, with the spectacle attracting sightseers, reporters and a film crew.
One of the reporters, Leonard Coatsworth, drove across the undulating structure, only for his car to stall partway across the bridge. He abandoned the car and, with the bridge heaving so intensely that he couldn’t walk, was forced to crawl off it on all fours.
The twisting continued until the bridge deck tore itself apart, flinging debris and Coatsworth’s car into the waters below.
In simple terms, the cause of the collapse was wind-induced aerodynamic forces, where the wind interacted with the structure to create vibrations that grew over time – like pushing a child’s swing with perfect timing so that it goes higher and higher.
This failure is often cited as the catalyst that set the structural engineering profession on the path to better understand the effects of wind-induced aerodynamic forces on structures. This is true – but it’s also the story of how the profession forgot that it had already solved the problem decades before.
Suspension bridges
The Tacoma Narrows Bridge was a suspension bridge, and at the time of its construction in the 1930s, suspension bridges had been in use for more than 100 years. In this type of design, the bridge’s deck is supported by suspender cables. These hang from the bridge’s main cables, which drape over the bridge’s towers.
Because it’s suspended in the air, the bridge’s deck is inherently flexible. And as far back as the 1820s, the engineering profession was well aware of the dangers posed by aerodynamic forces to flexible decks. For example, in 1826, Thomas Telford’s Menai Strait suspension bridge began oscillating and was damaged in a gale, just a month after it opened. In 1854, the Wheeling Suspension Bridge, then the longest span in the world, began twisting in a storm and was destroyed.
It was bridge engineer John Roebling who solved the problem in the mid-1800s. Roebling realised that stiffening the bridge deck – by adding trusses and diagonal cables – was the key. If the deck was more rigid, it would move and twist less in the air, despite being hung from flexible cables. His approach would prove highly successful, culminating in Roebling’s masterpiece, the Brooklyn Bridge.
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Drifting into failure
Bridge design, however, like all forms of design, does not stand still, and in the years that followed the completion of the Brooklyn Bridge, there was an unrelenting drive to build longer and more aesthetically pleasing structures. So suspension bridge span lengths increased and the designs became more slender – two factors that reduced deck stiffness and increased the risk posed by aerodynamic forces.
And there were warning signs of this increased risk. For example, there were flexibility problems with the Bronx-Whitestone Bridge, completed just one year prior to the Tacoma Narrows collapse, which required rectification to address its issues. But the profession didn’t recognise these warning signs for what they were: that aerodynamic forces were no longer being adequately considered by designers.
The reason for this lack of consideration was, in a perverse way, due to John Roebling. His solution to manage this problem had proven so successful that it bred an entire generation of engineers who forgot the problem existed. And for every slightly longer and more slender suspension bridge built that didn’t have aerodynamic issues, bridge designers pushed the envelope a little further. They designed longer bridges and further reduced the deck stiffness.
Each step took us one step closer to Tacoma Narrows.
Tacoma Narrows
The design of the Tacoma Narrows Bridge took the quest for slenderness way beyond anything that had been attempted previously, driven by its designer, Leon Moisseiff, an advocate for aesthetically pleasing structures. Moisseiff even removed the deck trusses, which had been such a feature of Roebling’s design. When questioned about the slim design, Moisseiff brushed off concerns and claimed all would be well.
But the issues began during construction and persisted after the bridge was opened. It undulated in the wind, and these undulations were so pronounced that when driving across the bridge, other cars would disappear and reappear from view, like ships at sea. It became a tourist attraction and earned the nickname “Galloping Gertie”.
Then, on the morning of 7 November 1940, four months after it opened, there was a failure at the top of one of the suspender cables, likely as a result of the bridge’s large vertical motions. This failure unbalanced the deck, allowing it to twist; with each twist, aerodynamic forces pushed it a little further, until it tore apart.
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Lessons
In the aftermath of the collapse, the designers of the Tacoma Narrows Bridge were not held liable – they had done what any other engineer would have done at the time: failed to consider wind-induced aerodynamic forces. While these forces had been an active concern prior to the 1850s, they had been so successfully addressed that engineers forgot how deadly they were. And over the decades, driven by the desire for more slender structures, engineers stripped away the very protections John Roebling had put in place, culminating in Tacoma Narrows: a slender deck with the trusses removed.
When the Tacoma Narrows Bridge was rebuilt in 1950, gone was Moisseiff’s slender deck. The bridge deck was deeper and the stiffening trusses had returned – all very reminiscent of John Roebling’s Brooklyn Bridge.
This article was originally published on LinkedIn. Read the original post.
About the author
Dr Sean Brady FIEAust CPEng is a forensic engineer and Managing Director of engineering consultancy Brady Heywood. In 2020, he completed the Brady Review, which was tabled in parliament and made 11 recommendations to the regulator and mining companies on how to improve safety. In 2024, he completed the technical and organisational investigation into the 25 May 2021 incident at Callide C Power Station in Queensland.
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Interesting article. Hopefully these days the lessons learnt are embedded in design requirements and Australian Standards so they don’t have to be re-learnt.
Informative and valuable.