The 1940 Tacoma Narrows Bridge, the first bridge at this location, was a suspension bridge in the U.S. state of Washington that spanned the Tacoma Narrows strait of Puget Sound between Tacoma and the Kitsap Peninsula. It opened to traffic on July 1, 1940, and dramatically collapsed into Puget Sound on November 7 of the same year. The bridge's collapse has been described as "spectacular" and in subsequent decades "has attracted the attention of engineers, physicists, and mathematicians".Gianni Arioli and Filippo Gazzola. A new mathematical explanation of what triggered the catastrophic torsional mode of the Tacoma Narrows Bridge. Applied Mathematical Modelling, Jan 2015. doi.org Throughout its short existence, it was the world's third-longest suspension bridge by main span, behind the Golden Gate Bridge and the George Washington Bridge.
Construction began in September 1938. From the time the deck was built, it began to move vertically in windy conditions, so construction workers nicknamed the bridge "Galloping Gertie". The motion continued after the bridge opened to the public, despite several damping measures. The bridge's main span finally collapsed in winds on the morning of November 7, 1940, as the deck oscillated in an alternating twisting motion that gradually increased in amplitude until the deck tore apart. The violent swaying and eventual collapse resulted in the death of a Cocker Spaniel named "Tubby", as well as inflicting injuries on people fleeing the disintegrating bridge or attempting to rescue the stranded dog.
Efforts to replace the bridge were delayed by US involvement in World War II, as well as engineering and finance issues, 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 bridge that fell into the water now serves as an artificial reef.
The bridge's collapse had a lasting effect on science and engineering. In many physics textbooks, the event is presented as an example of elementary forced mechanical resonance, but it was more complicated in reality; the bridge collapsed because moderate winds produced aeroelastic flutter that was self-exciting and unbounded: for any constant sustained wind speed above about , the amplitude of the (torsional) flutter oscillation would continuously increase, with a negative damping factor, i.e., a reinforcing effect, opposite to damping. The collapse boosted research into bridge aerodynamics-aeroelastics, which has influenced the designs of all later long-span bridges.
Design and construction
Proposals for a bridge between Tacoma and the Kitsap Peninsula date at least to the Northern Pacific Railway's 1889 trestle proposal, but concerted efforts began in the mid-1920s. The Tacoma Chamber of Commerce began campaigning and funding studies in 1923. Several noted bridge engineers were consulted, including Joseph B. Strauss, who went on to be chief engineer of the Golden Gate Bridge, and David B. Steinman, later the designer of the Mackinac Bridge. Steinman made several Chamber-funded visits and presented a preliminary proposal in 1929, but by 1931 the Chamber had cancelled the agreement because Steinman was not working hard enough to obtain financing. At the 1938 meeting of the structural division of the American Society of Civil Engineers, during the construction of the bridge, with its designer in the audience, Steinman predicted its failure.
In 1937, the Washington State legislature created the Washington State Toll Bridge Authority and appropriated $5,000 (equivalent to $ today) to study the request by Tacoma and Pierce County for a bridge over the Narrows.Plaut, R.H. (2008). "Snap Loads and Torsional Oscillation of the original Tacoma Narrows Bridge". Journal of Sound and Vibration. doi: 10.1016/j.jsv.2007.07.057
From the start, financing of the bridge was a problem: Revenue from the proposed tolls would not be enough to cover construction costs; another expense was buying out the ferry contract from a private firm running services on the Narrows at the time. Nonetheless, there was strong support for the bridge from the United States Navy, which operated the Puget Sound Naval Shipyard in Bremerton, and from the United States Army, which ran McChord Field and Fort Lewis near Tacoma.
Washington State engineer Clark Eldridge produced a preliminary tried-and-true conventional suspension bridge design, and the Washington State Toll Bridge Authority requested $11 million (equivalent to $ million today) from the federal Public Works Administration (PWA). Preliminary construction plans by the Washington Department of Highways had called for a set of trusses to sit beneath the roadway and stiffen it.
However, "Eastern consulting engineers" — by which Eldridge meant 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 for less. Moisseiff and Frederick Lienhard, the latter an engineer with what was then known in New York as the Port Authority, had published a paperLeon S. Moisseiff and Frederick Lienhard. "Suspension Bridges Under the Action of Lateral Forces," with discussion. Transactions of the American Society of Civil Engineers, No. 98, 1933, pp. 1080–1095, 1096–1141 that was probably the most important theoretical advance in the bridge engineering field of the decade.
Their theory of elastic distribution extended the deflection theory that was originally devised by the Austrian engineer Josef Melan to horizontal bending under static wind load. They showed that the stiffness of the main cables (via the suspenders) would absorb up to one-half of the static wind pressure pushing a suspended structure laterally. This energy would then be transmitted to the anchorages and towers. Using this theory, Moisseiff argued for stiffening the bridge with a set of plate girders rather than the trusses proposed by the Washington State Toll Bridge Authority. This approach meant a slimmer, more elegant design, and also reduced the construction costs as compared with the Highway Department's design proposed by Eldridge. Moisseiff's design won out, inasmuch as the other proposal was considered to be too expensive. On June 23, 1938, the PWA approved nearly $6 million (equivalent to $ million today) for the Tacoma Narrows Bridge. Another $1.6 million ($ million today) was to be collected from tolls to cover the estimated total $8 million cost ($ million today).
Following Moisseiff's design, bridge construction began on November 23, 1938. Construction took only nineteen months, at a cost of $6.4 million ($ million today), which was financed by the grant from the PWA and a loan from the RFC.
The Tacoma Narrows Bridge, with a main span of , was the third-longest suspension bridge in the world at that time, following the George Washington Bridge between New Jersey and New York City, and the Golden Gate Bridge, connecting San Francisco with Marin County to its north.Henry Petroski. Engineers of Dreams: Great Bridge Builders and the Spanning of America. New York: Alfred A. Knopf/Random House, 1995.
Because planners expected fairly light traffic volumes, the bridge was designed with two lanes, and it was just wide. This was quite narrow, especially in comparison with its length. With only the plate girders providing additional depth, the bridge's roadway section was also shallow.
The decision to use such shallow and narrow girders proved the bridge's undoing. With such minimal girders, the deck of the bridge was insufficiently rigid and was easily moved about by winds; from the start, the bridge became infamous for its movement. A mild to moderate wind could cause alternate halves of the centre span to visibly rise and fall several feet over four- to five-second intervals. This flexibility was experienced by the builders and workmen during construction, which led some of the workers to christen the bridge "Galloping Gertie". The nickname soon stuck, and even the public (when the toll-paid traffic started) felt these motions on the day that the bridge opened on July 1, 1940.
Attempt to control structural vibration Since the structure experienced considerable vertical oscillations while it was still under construction, several strategies were used to reduce the motion of the bridge. They included:Rita Robison. "Tacoma Narrows Bridge Collapse." In When Technology Fails, edited by Neil Schlager, pp. 18–190. Detroit: Gale Research, 1994. 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. Finally, the structure was equipped with hydraulic buffers installed 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 State Toll Bridge Authority hired Frederick Burt Farquharson, an engineering professor at the University of Washington, to make wind tunnel tests and recommend solutions to reduce the oscillations of the bridge. Farquharson and his students built a 1:200-scale model of the bridge and a 1:20-scale model of a section of the deck. The first studies concluded on November 2, 1940—five days before the bridge collapse on November 7. He proposed two solutions: To drill holes in the lateral girders and along the deck so that the airflow could circulate through them (in this way reducing lift forces). To 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 the chosen one, but it was not carried out, because the bridge collapsed five days after the studies were concluded.
Collapse
On November 7, 1940, at around 9:45 a.m. PST, especially strong winds caused the bridge to sway wildly from side to side. At least two vehicles were on the bridge at the time – a delivery truck driven by Ruby Jacox and Arthur Hagen, employees of Rapid Transfer Company, and a vehicle driven by Leonard Coatsworth, editor at The News Tribune. The truck tipped over due to the swaying, while the car lost control and began to slide from side to side. Jacox, Hagen, and Coatsworth exited their respective vehicles and got off of the bridge on foot. Coatsworth's daughter's dog Tubby, with only three legs, was left inside the car.
Coatsworth later described his experience.
Traffic was stopped to prevent additional vehicles from entering the bridge. Howard Clifford, a photographer for the Tacoma News Tribune, walked onto the bridge to try to save Tubby, but was forced to turn back when the span began to break apart in the center. At approximately 11:00 a.m., the bridge collapsed into the strait.
Coatsworth received $814.40 (equivalent to $ today) in reimbursement from the Washington State Toll Bridge Authority for his car and its contents, including Tubby the Cocker Spaniel.
Film of collapse