The Tacoma Narrows Bridge Collapse: A Lesson in Engineering History

The Tacoma Narrows Bridge was a suspension bridge that spanned the Tacoma Narrows strait of Puget Sound in Washington state, connecting the Olympic Peninsula with the mainland. It opened to traffic on July 1, 1940, and was hailed as a marvel of modern engineering. It was the third longest suspension bridge in the world at the time, with a main span of 2,800 feet (840 meters). It was also one of the most flexible bridges ever built, designed by Leon Moisseiff to withstand wind forces by allowing the deck to move up and down and side to side.

However, this flexibility proved to be its fatal flaw. On November 7, 1940, just four months after its opening, the bridge collapsed into Puget Sound during a windstorm that reached speeds of 42 mph (68 km/h). The bridge went into violent torsional oscillations, twisting and turning like a ribbon until it broke apart. The collapse was captured on film and became one of the most iconic images of engineering failure in history.

Background on the project

The idea of building a bridge across the Tacoma Narrows was first proposed in the 1920s by local improvement groups who wanted to boost the economy and accessibility of the region. In 1937, the Washington State Legislature authorized the construction of a toll bridge that would be financed by private bonds. The project was awarded to Clark Eldridge, a local engineer who had designed several other bridges in the state. Eldridge proposed a conventional suspension bridge with a main span of 2,600 feet (790 meters) and two side spans of 1,100 feet (340 meters) each. He also planned to use web trusses under the deck to stiffen it against wind forces.

However, Eldridge’s design was rejected by the Reconstruction Finance Corporation (RFC), a federal agency that provided loans for public works projects during the Great Depression. The RFC wanted a more economical design that would reduce the cost and weight of the bridge. They suggested hiring Leon Moisseiff, a renowned engineer from New York who had worked on several famous bridges, including the Golden Gate Bridge and the George Washington Bridge. Moisseiff agreed to join the project as a consultant and proposed a new design that increased the main span to 2,800 feet (840 meters) and eliminated the web trusses. He argued that his design would be more elegant and graceful than Eldridge’s, and that it would be sufficiently stable against wind forces due to its flexibility.

Moisseiff’s design was accepted by the RFC and approved by Eldridge, who remained as the chief engineer of the project. The construction began in November 1938 and was completed in July 1940. The total cost of the bridge was $6.4 million, which was $1.6 million less than Eldridge’s original estimate. The bridge was built by McClintic-Marshall Company of Pittsburgh as the general contractor, Bethlehem Steel Company as the steel fabricator, and Pacific Bridge Company as the erector.

The collapse

The Tacoma Narrows Bridge opened to traffic on July 1, 1940, with much fanfare and celebration. However, it soon became apparent that the bridge had a serious problem: it moved too much in the wind. Even in mild breezes, the bridge deck would sway up and down and side to side, sometimes as much as five feet (1.5 meters). The movement was so noticeable that drivers would slow down or stop to watch it. The bridge earned several nicknames, such as “Galloping Gertie”, “Fluttering Flo”, and “The Flying Bridge”.

The engineers tried to explain and fix the problem, but they did not fully understand its cause or severity. They attributed the movement to resonance, a phenomenon where an external force matches the natural frequency of an object and amplifies its vibration. They thought that adding some additional weight or damping devices would reduce the resonance and stabilize the bridge. They also assured the public that the bridge was safe and strong enough to withstand any wind force.

However, on November 7, 1940, their confidence was shattered by a catastrophic event. Around 10 a.m., a moderate windstorm hit the area, with gusts reaching 42 mph (68 km/h). The bridge deck began to twist violently from side to side, reaching angles of up to 45 degrees. The movement was so severe that it tore several suspenders loose and cracked the main cables. The bridge was closed to traffic and a few workers and a reporter remained on the bridge to observe the situation. One of them was Leonard Coatsworth, a Tacoma News Tribune editor who had left his car on the bridge with his dog, Tubby, inside.

At 11:02 a.m., the bridge could not take it anymore. The main span broke apart and plunged into Puget Sound, taking Coatsworth’s car and Tubby with it. The side spans remained intact, but were severely damaged. The collapse was witnessed by hundreds of people on both shores and was filmed by Barney Elliott, a local camera shop owner who had set up his equipment on the Gig Harbor side. His footage was later shown in newsreels and documentaries around the world, making the Tacoma Narrows Bridge collapse one of the most famous engineering disasters of all time.

The cause

The collapse of the Tacoma Narrows Bridge stunned and shocked the engineering community and the public. How could such a modern and sophisticated bridge fail so spectacularly? What was the cause of the collapse? And how could it be prevented in the future?

To answer these questions, a team of experts was assembled to investigate the collapse. They included Eldridge, Moisseiff, Joseph Strauss (the chief engineer of the Golden Gate Bridge), Theodore von Karman (a leading aerodynamicist from Caltech), and Frederick Lienhard (a professor of civil engineering from the University of Washington). They conducted extensive tests and analyses on the bridge remnants and the wind conditions on the day of the collapse. They also reviewed the design and construction records and interviewed the witnesses and workers.

The investigation revealed that the cause of the collapse was not resonance, but aeroelastic flutter, a phenomenon where aerodynamic forces interact with structural vibrations and cause them to grow uncontrollably. Flutter occurs when two conditions are met: (1) there is a positive feedback loop between the aerodynamic forces and the structural deformations, and (2) there is a coupling between different modes of vibration, such as vertical and torsional. In other words, flutter happens when the wind pushes the bridge in one direction, causing it to bend or twist, which changes its shape and affects how the wind pushes it in another direction, causing it to bend or twist more, and so on.

The Tacoma Narrows Bridge was particularly susceptible to flutter because of its design features. It had a very slender and shallow deck that offered little resistance to wind forces. It also had solid plate girders instead of open web trusses that created large flat surfaces that acted like wings or sails. Moreover, it had very flexible cables and suspenders that allowed large displacements of the deck. These factors combined to create a positive feedback loop between the wind forces and the structural deformations, as well as a coupling between vertical and torsional modes of vibration.

On November 7, 1940, the wind speed reached a critical value that triggered flutter in the bridge. The wind gusts hit the bridge at an angle that caused it to twist from side to side. The twisting changed the angle of attack of the wind on the deck, creating lift forces that caused it to bounce up and down. The bouncing changed the tension in the cables and suspenders, affecting their stiffness and damping properties. The stiffness and damping changes altered the natural frequencies of vibration of the bridge, bringing them closer to each other and to the frequency of wind gusts. This resulted in a resonance-like condition that amplified the flutter until it exceeded the strength of the bridge.

Lessons learned

The Tacoma Narrows Bridge collapse was a wake-up call for engineers and designers. It exposed their lack of knowledge and understanding of aerodynamic effects on suspension bridges. It also challenged their assumptions and methods of analysis that were based on static or linear models that ignored dynamic or nonlinear phenomena. It prompted them to conduct more research and experiments on wind-bridge interaction and develop new theories and tools to predict and prevent flutter.

The collapse also led to important changes in suspension bridge design and construction. The plate girder was abandoned as a stiffening element for suspension bridges, as it was found to be ineffective and inefficient. Instead, web trusses or box girders were used to provide more stiffness and aerodynamic stability. The deck width-to-span ratio was increased to reduce flexibility and susceptibility to flutter. The deck shape was also modified to reduce drag and lift forces by adding fairings or vents. Moreover, wind tunnel tests were conducted for every major suspension bridge project to evaluate its aerodynamic performance under various wind conditions.

The Tacoma Narrows Bridge collapse was a tragic but valuable lesson in engineering history. It taught engineers how to design safer and more reliable suspension bridges that can withstand wind forces without compromising aesthetics or economy. It also inspired generations of engineers to pursue innovation and excellence in their profession.

Further reading

If you want to learn more about the Tacoma Narrows Bridge collapse, here are some links you can check out:

(1) Tacoma Narrows Bridge — Wikipedia. https://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge.

(2) Tacoma Narrows Bridge | Collapse, Disaster, Length, History, & Facts. https://www.britannica.com/topic/Tacoma-Narrows-Bridge.

(3) Tacoma Narrows Bridge collapses — HISTORY. https://www.history.com/this-day-in-history/tacoma-narrows-bridge-collapses.

(4) Tacoma Narrows Bridge (1940) — Wikipedia. https://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge_%281940%29.