Tacoma Narrows Bridge

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{{Infobox_Bridge |bridge_name=Second Tacoma Narrows Bridge |image=Tacoma bridge 1950 loc habs.jpg |carries=Washington State Route 16 |crosses=Tacoma Narrows |open=October 14, 1950 |mainspan=2,800 ft (853 m) |length=5,979 ft (1822 m) }}

The Tacoma Narrows Bridge is a mile-long (1600 meter) suspension bridge with a main span of 2800 foot (850 m) (the third-largest in the world when it was first built) that carries Washington State Route 16 across the Tacoma Narrows of Puget Sound from Tacoma to Gig Harbor, Washington. The first version of the bridge, nicknamed "Galloping Gertie," was designed by Clark Eldridge and altered by Leon Moisseiff. It became famous for a dramatic filmed structural collapse in 1940. The replacement bridge opened in 1950.

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First bridge

{{Infobox_Bridge |bridge_name=First Tacoma Narrows Bridge |image= |official_name= |carries= |crosses= Tacoma Narrows |locale= Tacoma to Gig Harbor, Washington |maint= |id= |design= Suspension |mainspan= 2,800 ft (853 m) |length= |width= |clearance= |below= |traffic= |open= July 1, 1940 |closed= November 7, 1940 }}

Desire for a bridge at this location 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. Several noted bridge architects, including Joseph B. Strauss, who went on to be chief engineer of the Golden Gate Bridge, and David B. Steinman builder of the Mackinac Bridge, were consulted. Steinman made several Chamber-funded visits culminating in a preliminary proposal presented in 1929 but by 1931 the Chamber decided to cancel the agreement on the grounds that Steinman was “not sufficiently active” in working to obtain financing.

The road to Tacoma’s doomed bridge continued in 1937, when the Washington State legislature created the Washington State Toll Bridge Authority and appropriated $5,000 to study the request by Tacoma and Pierce County for a bridge over the Narrows.

From the start, financing was the issue; revenue from tolls would not be enough to cover construction costs. But there was strong support for a bridge from the U.S. Navy, which operated the Puget Sound Naval Shipyard in Bremerton, and from the U.S. Army, which ran McChord Field and Fort Lewis in Tacoma.

Washington State engineer Clark Eldridge came up with a preliminary, “tried and true conventional bridge design,” and the toll bridge authority requested $11 million from the federal Public Works Administration (PWA). But, according to Eldridge, prominent “Eastern consulting engineers” — led by New York engineer Leon Moisseiff — petitioned the PWA to build the bridge for less.

Preliminary construction plans had called for 25-foot-deep (7.6 m) girders to sit beneath the roadway and stiffen it. Moisseiff, respected designer of the famed Golden Gate Bridge, proposed shallower supports — girders 8 feet (2.4 m) deep. His approach meant a slimmer, more elegant design and reduced construction costs. Moisseiff’s design won out. On June 23, 1938, the PWA approved nearly $6 million for the Tacoma Narrows Bridge. Another $1.6 million was to be collected from tolls to cover the total $8 million cost.

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Collapse

The collapse occurred on November 7, 1940. From the account of Leonard Coatsworth, a driver stranded on the bridge during this event: Image:Tacoma Narrows Bridge Falling.png

Just as I drove past the towers, the bridge began to sway violently from side to side. Before I realized it, the tilt became so violent that I lost control of the car… I jammed on the brakes and got out, only to be thrown onto my face against the curb… Around me I could hear concrete cracking… The car itself began to slide from side to side of the roadway.
On hands and knees most of the time, I crawled 500 yards [457 m] or more to the towers… My breath was coming in gasps; my knees were raw and bleeding, my hands bruised and swollen from gripping the concrete curb… Toward the last, I risked rising to my feet and running a few yards at a time… Safely back at the toll plaza, I saw the bridge in its final collapse and saw my car plunge into the Narrows.

The final destruction of the bridge was recorded on film by Barney Elliott, owner of a local camera shop. The Tacoma Narrows Bridge Collapse (1940) is preserved in the U.S. National Film Registry, and is still shown to engineering, architecture, and physics students as a cautionary taleTemplate:Fn.

No human life was lost in the collapse of the bridge. Theodore von Karman reported that the State of Washington was unable to collect on an insurance policy for the bridge, because its insurance agent fraudulently pocketed the insurance premiums.

On November 28, 1940, the U. S. Navy's Hydrographic Office reported that the remains of the bridge were located at geographical coordinates Template:Coor dms, at a depth of 30 fathoms (55 m).

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Film and Video of collapse

Film footage of the collapse was shown many times on the popular 1950's viewer request show You Asked for It.

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Cause of collapse

The bridge was solidly built, with girders of carbon steel anchored in huge blocks of concrete. Preceding designs typically had open lattice beam trusses underneath the roadbed. This bridge was the first of its type to employ plate girders (pairs of deep I beams) to support the roadbed. With the earlier designs any wind would simply pass through the truss, but in the new design the wind would be diverted above and below the structure. Shortly after its construction in July 1940 (opened to traffic on July 1), it was discovered that the bridge would sway and buckle dangerously in windy conditions. This resonance was longitudinal, meaning the bridge buckled along its length, with the roadbed alternately raised and depressed in certain locations — one half of the central span would rise while the other lowered. Drivers would see cars approaching from the other direction disappear into valleys which were dynamically appearing and disappearing. From this behavior the bridge gained the nickname "Galloping Gertie." However, the mass of the bridge was considered sufficient to keep it structurally sound.

The failure of the bridge occurred when a never-before-seen twisting mode occurred. This is called a torsional, rather than longitudinal, mode (see also torque) whereby when the left side of the roadway went down, the right side would rise, and vice-versa, with the centerline of the road remaining still. Specifically, it was the second torsional mode, in which the midpoint of the bridge remained motionless while the two halves of the bridge twisted in opposite directions. A physics professor proved this point by walking along the centre line, unaffected by the flapping of the roadway rising and falling to each side. This vibration was due to aeroelastic flutter. Flutter occurs when a torsional disturbance in the structure increases the angle of attack of the bridge (that is, the angle between the wind and the bridge). The structure responds by twisting further. Eventually, the angle of attack increases to the point of stall, and the bridge begins to twist in the opposite direction. In the case of the Tacoma Narrows bridge, this mode was negatively damped (or had positive feedback), meaning it increased in amplitude with each cycle because the wind pumped in more energy than the flexing of the structure dissipated. Eventually, the amplitude of the motion increased beyond the strength of a vital part, in this case the suspender cables. Once several cables failed, the weight of the deck transferred to the adjacent cables which broke in turn until almost all of the central deck fell into the water.

The bridge’s spectacular self-destruction is often used as an object lesson in the necessity to consider both aerodynamics and resonance effects in structural and civil engineering. However the effect which caused the destruction of the bridge should not be confused with forced resonance (as from the periodic motion induced by a group of soldiers marching in step across a bridge). In the case of the Tacoma Narrows Bridge, there was no periodic disturbance. The wind was steady at 42 mph (67 km/h). The frequency of the destructive mode, 0.2 Hz, was neither a natural mode of the isolated structure nor the frequency of blunt-body vortex shedding of the bridge at that wind speed. The event can only be understood while considering the coupled structural and aerodynamic system.

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Replacement bridge

The bridge was redesigned and rebuilt with open trusses and stiffening struts and openings in the roadway to let wind through. The new bridge opened on October 14, 1950, and is 5,979 feet (1,822 m) long — 40 feet (12 m) longer than its predecessor. It is currently the 5th-longest suspension bridge in the United States.

Modern suspension bridges built with steel plate now use sharp entry edges rather than the flat plate sides used in the original Tacoma Narrows Bridge (see the suspension bridge article for an example).

The bridge was designed to handle 60,000 vehicles a day. As of 2005, it handles 90,000, and is projected to handle 120,000 by 2020.

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Construction of second span

Image:TacomaNarrowsBridges2006.jpg In 1998, voters in several Washington counties approved an advisory measure to create a second Narrows span. Construction of the new span, which will run parallel to the current bridge, began on October 4, 2002, and is scheduled to be completed in 2007. The state intends to recoup construction costs by adding a toll to the eastbound (new) span. The existing span is currently toll-free.

NB Washington State Department of Transportation refers to each of the three spans differently. The original bridge built in 1940 is called "The 1940 Bridge" or just "Gertie". The current bridge (and future westbound span) is called "The Current Bridge". The new eastbound span being built is called "The 2007 Bridge".

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Footnotes

Template:Fnb "The effects of Galloping Gertie's fall lasted long after the catastrophe. Clark Eldridge, who accepted some of the blame for the bridge's failure, learned this first-hand. In late 1941 Eldridge was working for the U.S. Navy on Guam when the United States entered World War II. Soon, the Japanese captured Eldridge. He spent the remainder of the war (three years and nine months) in a prisoner of war camp in Japan. To his amazement, one day a Japanese officer, who had once been a student in America, recognized the bridge engineer. He walked up to Eldridge and said bluntly, 'Tacoma Bridge!'" [1]

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Trivia

  • The Canadian rock band The Tragically Hip refer to the bridge in their 2004 song, Vaccination Scar. "It went down like a bad card table leg, like the Tacoma Narrows Bridge".
  • In the animated series Drawn Together, the Double Hemm Productions logo at the end of each episode shows a portion of the film clip where the bridge is about to collapse.
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Tubby the dog

Tubby, a cocker spaniel dog, was the only fatality of the Tacoma Narrows Bridge disaster. Leonard Coatsworth, Tubby's owner, was driving with the dog over the bridge when it started to vibrate violently. Coatsworth was forced to flee his car, leaving Tubby behind. Two people attempted to rescue Tubby, but the dog was too terrified to leave the car and bit one of the rescuers. Tubby died when the bridge fell, and neither his body nor the car were ever recovered. [2]

Coatsworth received US $364.40 in reimbursement for the contents of his car, including Tubby. In 1975, Coatsworth's wife claimed that Tubby only had 3 legs and was paralyzed.

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Reference

  • K. Billah and R. Scanlan (1991), Resonance, Tacoma Narrows Bridge Failure, and Undergraduate Physics Textbooks, American Journal of Physics, 59(2), 118--124 (PDF)
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External links

Template:Commons Template:Geolinks-US-cityscale

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Historical

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Second span project

de:Tacoma-Narrows-Brücke nl:Tacoma Narrows Bridge ja:タコマナローズ橋 pl:Most Tacoma sl:Most Tacoma Narrows zh:塔科馬海峽吊橋

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