At 10 am on the morning of November 7th, 1940, Professor F. Bert Farquharson was one of the few people standing on the world’s third-longest bridge—the first Tacoma Narrows Bridge—as it bounced and twisted. He, probably better than anyone else, knew how strangely the bridge behaved in a gale. “We knew from the night of the day the bridge opened that something was wrong,” he later said. 

The first Tacoma Narrows Bridge was a suspension bridge in the state of Washington that spanned the Tacoma Narrows strait of Puget Sound between Tacoma and the Kitsap Peninsula. It opened to traffic on July 1st, 1940. Sleek and slender with a length of 7,392 feet, it was the third-longest suspension bridge in the world at the time, following the George Washington Bridge in New York City, and the Golden Gate Bridge in San Francisco. 

Construction of the bridge began in September 1938. Rather than the originally-proposed trusses, the bridge used two narrow plate girders to stiffen the deck, giving the bridge its iconic steel ribbon appearance across the Puget Sound. But from the time the bridge was built, it began to move vertically in windy conditions, so construction workers nicknamed it “Galloping Gertie.” 

Officials assured the public that the bridge was safe, and within months it became a central part of the local economic and military interests, cutting the two-and-a-half-hour drive between Tacoma, Washington and the Kitsap Peninsula down to 11 minutes, and connecting Seattle and Tacoma with the Puget Sound Naval Yard.  

But the motion felt during construction continued after the bridge opened to the public, despite several damping measures. Lightweight steel girders and thin layer of concrete permitted a certain unusual flexibility. But no one was quite sure why the bridge flexed in the wind, or what that might lead to. Four months later, they found out. 

On November 7, 1940, during 42mph winds, the bridge plummeted into Puget Sound. The collapse has been described as “spectacular,” and, in subsequent decades, it attracted the attention of engineers, physicists, and mathematicians. As horrified onlookers watched, the deck oscillated in an alternating twisting motion that gradually increased in amplitude until the deck tore apart. The disaster—which luckily took no human lives—shook the engineering community and forever changed the way bridges have been built around the world.

The desire for the construction of a bridge between Tacoma and the Kitsap Peninsula dates back to 1889 with a Northern Pacific Railway proposal for a trestle, but concerted efforts began in the mid-1920s. The Tacoma Chamber of Commerce began campaigning and funding studies in 1923. Washington State engineer Clark Eldridge produced a proposal for a conventional suspension bridge, and the Washington Toll Bridge Authority requested $11 million (equivalent to $185 million today) from the Federal Public Works Administration (PWA).  However, according to Eldridge, Leon Moisseiff, the noted New York bridge engineer who served as designer and consultant engineer for the Golden Gate Bridge, petitioned the PWA and the Reconstruction Finance Corporation (RFC) to build the bridge less expensively. 

Moisseiff argued for stiffening the bridge with a set of eight-foot-deep (2.4 m) plate girders rather than the 25-foot-deep (7.6 m) trusses proposed by the Eldridge—an approach that meant a slimmer, more elegant design that also reduced construction costs. Moisseiff’s proposal was approved.

Construction of the bridge took only nineteen months, at a cost of $6.4 million ($116.2 million today). Because planners expected fairly light traffic volumes, the bridge was designed with two lanes—a total 39-foot width. This was quite narrow, especially in comparison with its length. With only the 8-foot-deep (2.4 m) plate girders providing additional depth, the bridge’s roadway section was also shallow.

The decision to use such shallow and narrow girders proved to be the bridge’s undoing. The bridge’s deck had insufficient rigidity and was easily moved by wind; a mild to moderate wind could cause alternate halves of the center span to visibly rise and fall several feet over intervals of just seconds. This flexibility was experienced by the builders and workmen during construction, which had led workers to nickname the bridge “Galloping Gertie.” 

Even the public felt these motions on the day the bridge opened on July 1, 1940. “My grandparents would tell me that when they crossed the bridge, it undulated so much the car in front of you would disappear,” says Bill Baarsma, former Tacoma mayor and president of the Tacoma Historical Society. 

Moisseiff worked his way from New York City’s Department of Bridges into a busy private practice as a consulting engineer—the role that had brought him to Tacoma. By then, he was widely known as a leading authority on a series of mathematical formulas called “deflection theory,” which stated, in short, that the weight of longer suspension bridges meant that they did not need trusses built beneath the road to hold the bridge steady through wind and traffic. This called for less steel, which meant cheaper construction. It had worked back east, as in the lithe and symmetrical George Washington Bridge, built in 1931, with Moisseiff serving as a design consultant. 

After it experienced considerable vertical oscillations during construction, several strategies were employed in an effort to reduce the motion of the bridge:

  • attachment of tie-down cables to the plate girders, which were anchored to 50-ton concrete blocks on the shore. This measure proved ineffective, as the cables snapped shortly after installation.
  • addition of a pair of inclined cable stays that connected the main cables to the bridge deck at mid-span. These remained in place until the collapse, but were also ineffective at reducing the oscillations.
  • Installation of hydraulic buffers between the towers and the floor system of the deck to damp longitudinal motion of the main span. The effectiveness of the hydraulic dampers was nullified, however, because the seals of the units were damaged when the bridge was sand-blasted before being painted.

The Washington Toll Bridge Authority hired Professor Frederick Burt Farquharson, an engineering professor at the University of Washington, to conduct wind-tunnel tests and to recommend solutions for reducing the oscillations of the bridge. His studies concluded on November 2, 1940—five days before the bridge collapse on November 7.  He suggested to:

  1. Drill holes in the lateral girders and along the deck so that the air flow could circulate through them (in this way reducing lift forces).
  2. Give a more aerodynamic shape to the transverse section of the deck by adding fairings or deflector vanes along the deck, attached to the girder fascia.

The first option was not favored because of its irreversible nature. The second option was chosen, but it was not carried out because the bridge collapsed five days after the solution was proposed.

On November 7, 1940, high winds buffeted the area and the bridge swayed considerably. The first failure came at about 11:00 am, when concrete dropped from the road surface. Just minutes later, a 600-foot section of the bridge broke free. By this time, the bridge was being wildly tossed back and forth. At one point, the elevation of the sidewalk on one side of the bridge was 28 feet above that of the sidewalk on the other side. Even though the bridge towers were made of strong, structural carbon steel, the bridge proved no match for the violent movement, and it finally collapsed. 

Clark Eldridge, Project Engineer, Washington State Toll Bridge Authority ran to the bridge at 10:00 am, after receiving a call that it “was about to go.” According to Eldridge, the center span swayed wildly, and it became possible to see the entire bottom side of the roadway as it swung into semi-vertical position. He further described his eyewitness observations: 

 I observed that all traffic had been stopped and that several people were coming off the bridge from the easterly side span. I walked to tower No. 5 and out onto the main span to about the quarter point observing conditions. The main span was rolling wildly. The deck was tipping from the horizontal to an angle approaching forty-five degrees. The entire main span appeared to be twisting about a neutral point at the center of the span in somewhat the manner of a corkscrew. 

Eldridge said that at that time, it appeared that if the wind were to have died down, the span would perhaps have come to rest. “I resolved that we would immediately proceed to install a system of cables from the piers to the roadway level in the main span to prevent any recurrence…” he said.  “I was then informed that a panel of laterals in the center of the span had dropped out and a section of concrete slab had fallen…” 

Why had the Tacoma Narrows Bridge been so unstable to the point of collapse?  And was Moisseiff actually to blame?             

The Investigation and Causal Factors in the Collapse

The FWA appointed a 3-member panel of top-ranking engineers. Their report to the Administrator of the FWA, John Carmody, became known as the “Carmody Board” report.  In March 1941, the group announced its findings:  

  1. The principal cause of the Narrows Bridge’s failure was its “flexibility.”
  2. The solid plate girder and deck acted like an airfoil, creating “drag” and “lift.” 
  3. Aerodynamic forces were little understood and engineers needed to test all suspension bridge designs thoroughly using models in a wind tunnel.

For a long time, resonance was blamed for the collapse, with Gertie’s well-known vibrations deepened by just the right wind through the Narrows. Now, the prevailing thought is that it was the result of “aeroelastic flutter.”  The failure of the bridge occurred when a never-before-seen twisting mode occurred, from winds at 40 miles per hour. This is a so-called torsional vibration mode, whereby the two halves of the bridge twisted in opposite directions, with the center line of the road remaining motionless. This vibration was caused by a phenomenon known as “aeroelastic fluttering.”

In historical footage of the collapse, it can clearly be seen that right before failure, the bridge did not oscillate vertically, but rather in a twisting (torsional) motion. The reason for this change in oscillation is still debated, but one of the best suggestions relates to the aerodynamics of the bridge. Rather than a truss through which wind could flow, the shape of the Tacoma Narrows Bridge, with the large steel plates on either side creating some strange interactions with the wind. Any amount of twist in the bridge created vortices (areas of low pressure) in locations that actually amplified the twisting motion. 

As the bridge returned to its natural state, its momentum twisted it in the other direction where the wind could catch it and continue the twisting—aeroelastic flutter.  It’s the same reason that a strap or sheet of paper vibrates in the wind. 

Eventually, the amplitude of the bridge’s motion produced by the fluttering increased beyond the strength of a vital part—the suspender cables. As several cables failed, the weight of the deck transferred to the adjacent cables, which became overloaded and broke as torsional flutter eventually created too much stress in the suspension cables, and the bridge failed and fell into the water below the span. 

Engineers K. Yusuf Billah and Robert H. Scanlan stated in 1991 that, in fact, many physics textbooks have incorrectly explained that the cause of the failure of the Tacoma Narrows bridge had been externally-forced mechanical resonance, saying: 

Resonance is the tendency of a system to oscillate at larger amplitudes at certain frequencies, known as the system’s natural frequencies. At these frequencies, even relatively small, periodic driving forces can produce large amplitude vibrations, because the system stores energy. For example, a child using a swing realizes that if the pushes are properly timed, the swing can move with a very large amplitude. The driving force, in this case, the child pushing the swing, exactly replenishes the energy that the system loses if its frequency equals the natural frequency of the system.

For resonance to occur, it is necessary to have also periodicity in the excitation force. In this case, the most tempting candidate of the periodicity in the wind force was assumed to be the so-called vortex shedding. This is because bluff bodies (non-streamlined bodies), like bridge decks, in a fluid stream will shed wakes, whose characteristics depend on the size and shape of the body and the properties of the fluid. These wakes are accompanied by alternating low-pressure vortices on the downwind side of the body—the so-called “von Kármán vortex street.” Eventually, if the frequency of vortex shedding matches the natural frequency of the structure, the structure will begin to resonate and the structure’s movement can become self-sustaining. Therefore, Billah and Scanlan conclude that the culprit in the Tacoma disaster was the von Kármán vortex street.

The After-Effects of the Disaster

Eighty years after the first Tacoma Narrows Bridge collapse, her legacy is cemented in local lore as the tall, slender, elegant bridge that crumpled into the Puget Sound in 1940. Photos and film of the last moments of Galloping Gertie are iconic to this day. The bridge’s fall is still taught in schools. 

After the collapse, the portions of the bridge that were still standing, including the towers and cables, were dismantled and sold as scrap metal. Efforts to replace the bridge were delayed by World War II, but in 1950, a new Tacoma Narrows Bridge opened in the same location, using the original bridge’s tower pedestals and cable anchorages. The portion of the original bridge that fell into the water now serves as an artificial reef. In 1992, the sunken remains were placed on the National Register of Historic Places to protect them against theft.

The bridge’s collapse had a lasting effect on science and engineering, boosting research into bridge aerodynamics-aeroelastics, which has since influenced the designs of all later long-span bridges. Following the Tacoma Narrows Bridge collapse, engineers took extra caution to incorporate aerodynamics into their designs, and wind-tunnel testing of designs was eventually made mandatory. 

A key consequence of the disaster was that suspension bridges reverted to a deeper and heavier truss design, including the replacement Tacoma Narrows Bridge (1950), until the development in the 1960s of box girder bridges with an aerofoil shape such as the Severn Bridge.  

And every year, civil engineers honor one of their field’s greatest advances with the Moisseiff Award. It stands as an extraordinary afterlife, especially considering that Moisseiff lived for less than three years after Galloping Gertie’s collapse and designed no new bridges in that time.