Tacoma Narrows Bridge

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Tacoma Narrows Bridge Collapsing

The Tacoma Narrows Bridge is a mile-long (1600 meter) suspension bridge with a main span of 2800 ft/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, designed by Clark Eldridge and altered by Leon Moisseiff, became famous for a dramatic filmed structural collapse in 1940.

Contents

Collapse of first bridge

The event

The collapse occurred on November 7, 1940. From the account of a driver stranded on the bridge during this event:

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 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. 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. However, Tubby, a cocker spaniel dog, was left in one of the cars on the bridge. Two people attempted to rescue the dog, but he was too terrified to leave the car and bit one of the rescuers. The dog died when the bridge fell, and neither his body nor the car was ever recovered. [1]

Origins of the bridge

The road to Tacoma’s doomed bridge began 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 Navy Yard 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. 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 girders to sit beneath the roadway and stiffen it. Moisseiff, respected designer of the famed Golden Gate Bridge, proposed shallower supports — girders 8 feet 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.

Mechanics

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 (also see torque) whereby when the left side of the roadway went down, the right side would rise, and vice-versa, with the centre line of the road remaining still. In fact, 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 a 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, it is a poor example of resonance since resonance normally implies forced resonance (as from the periodic motion induced by a group of soldiers marching in step across a bridge). However, 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.

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 long — 40 feet longer than its predecessor and 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. It currently handles 90,000, and is projected to handle 120,000 by 2020.

Construction of second span

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.

Media

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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!'" [2]

Trivia

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 northbound span) is called "The Current Bridge". The new southbound span being built is called "The 2007 Bridge".

See also

External links

Historical

Second span project

de:Tacoma-Narrows-Brücke nl:Tacoma Narrows Bridge ja:タコマナローズ橋 pl:Most Tacoma

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