Tacoma Narrows Bridge (1940)

[Kristin Moore]

Tacoma Narrows Bridge (1940)

The 1940 Tacoma Narrows Bridge was the first incarnation of the Tacoma Narrows Bridge, 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. At the time of its construction (and its destruction), the bridge was the third longest suspension bridge in the world in terms of main span length, behind the Golden Gate Bridge and the George Washington Bridge.

Construction on the bridge began in September 1938. From the time the deck was built, it began to move vertically in windy conditions, which led to construction workers giving the bridge the nickname Galloping Gertie. The motion was observed even when the bridge opened to the public. Several measures aimed at stopping the motion were ineffective, and the bridge's main span finally collapsed under 40-mile-per-hour (64 km/h) wind conditions the morning of November 7, 1940.

Following the collapse, the United States' involvement in World War II delayed plans to replace the bridge. The portions of the bridge still standing after the collapse, including the towers and cables, were dismantled and sold as scrap metal. Nearly 10 years after the bridge collapsed, 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 resonance with the wind providing an external periodic frequency that matched the natural structural frequency, though its actual cause of failure was aeroelastic flutter.[1] Its failure also boosted research in the field of bridge aerodynamics-aeroelastics, the study of which has influenced the designs of all the world's great long-span bridges built since 1940.

[Kristin Moore]

Tacoma Narrows Bridge

The original Tacoma Narrows Bridge roadway twisted and vibrated violently under 40-mile-per-hour (64 km/h) winds on the day of the collapse
Other name(s)    Galloping Gertie
Design    Suspension
Total length    5,939 feet (1,810.2 m)
Longest span    2,800 feet (853.4 m)
Clearance below    195 feet (59.4 m)
Opened    July 1, 1940
Collapsed    November 7, 1940
Coordinates    47°16′00″N 122°33′00″W

[Kristin Moore]

[Kristin Moore]

Design and construction

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. Several noted bridge engineers, including Joseph B. Strauss, who went on to be chief engineer of the Golden Gate Bridge, and David B. Steinman, who went on to design 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. Another problem with financing the first bridge was buying out the ferry contract from a private firm running service on the Narrows at the time.

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 of the bridge was a problem: revenue from the proposed tolls would not be enough to cover construction costs, but there was strong support for the 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 near Tacoma.

[Kristin Moore]

Washington State engineer Clark Eldridge produced a preliminary tried-and-true conventional suspension bridge design, and the Washington Toll Bridge Authority requested $11 million from the Federal Public Works Administration (PWA). Preliminary construction plans by the Washington Department of Highways had called for a set of 25-foot-deep (7.6 m) girders to sit beneath the roadway and stiffen it.

However, according to Eldridge, "eastern consulting engineers" petitioned the PWA and the Reconstruction Finance Corporation (RFC) to build the bridge for less, about whom Eldridge meant the noted New York bridge engineer Leon Moisseiff, the designer and consultant engineer for the Golden Gate Bridge. Moisseiff proposed shallower supports—girders 8 feet (2.4 m) deep. His approach meant a slimmer, more elegant design, and also reduced the construction costs as compared with the Highway Department's design. 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 for the Tacoma Narrows Bridge. Another $1.6 million was to be collected from tolls to cover the estimated total $8 million cost.

Following Moisseiff's design, bridge construction began on September 27, 1938. Construction took only nineteen months, at a cost of $6.4 million, which was financed by the grant from the PWA and a loan from the RFC. The Tacoma Narrows Bridge, with a main span of 2,800 feet (850 m), 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, just north of San Francisco.[2] Moisseiff and Fred Lienhard, the latter a Port of New York Authority engineer, published a paper[3] that was probably the most important theoretical advance in the bridge engineering field of the decade.[4] 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.[4]

[Kristin Moore]

Using this theory, Moisseiff argued for stiffening the bridge with a set of eight-foot-deep plate girders rather than the 25 feet (7.6 m)-deep trusses proposed by the Washington Toll Bridge Authority. This change was a substantial contributor to the difference in the projected costs of the designs.

Because planners expected fairly light traffic volumes, the bridge was designed with two lanes, and it was just 39 feet (12 m) wide. This was quite narrow, especially in comparison with its length. With only the 8 feet (2.4 m)-deep 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 original Tacoma Narrows 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 center 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.

[Kristin Moore]

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[5]

    * 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 Toll Bridge Authority hired Professor Frederick Burt Farquharson, an engineering professor at the University of Washington, to make wind-tunnel tests and recommend solutions in order to reduce the oscillations of the bridge. Professor 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 some holes in the lateral girders and along the deck so that the air flow 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.[4]

[Kristin Moore]

Collapse

The wind-induced collapse occurred on November 7, 1940, at 11:00 a.m. (Pacific time), because of a physical phenomenon known as aeroelastic flutter.[1]

From the account of Leonard Coatsworth, a Tacoma News Tribune editor who was the last person to drive on the bridge, he was driving with his dog, Tubby, over the bridge when the bridge started to vibrate violently. Coatsworth was forced to flee his car:

    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 (460 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.[6]

No human life was lost in the collapse of the bridge. Tubby, a black male cocker spaniel, was the only fatality of the Tacoma Narrows Bridge disaster; he was lost along with Coatsworth's car. Professor Farquharson[7] and a news photographer[8] attempted to rescue Tubby during a lull, 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.[9] Coatsworth had been driving Tubby back to his daughter, who owned the dog. Coatsworth received US $450 for his car and $364.40 in reimbursement for the contents of his car, including Tubby.[10]

[Kristin Moore]

Inquiry

Theodore von Kármán, the director of the Guggenheim Aeronautical Laboratory and a world-renowned aerodynamicist, was a member of the board of inquiry into the collapse.[11] He reported that the State of Washington was unable to collect on one of the insurance policies for the bridge because its insurance agent had fraudulently pocketed the insurance premiums. The agent, Hallett R. French, who represented the Merchant's Fire Assurance Company, was charged and tried for grand larceny for withholding the premiums for $800,000 worth of insurance. The bridge, however, was insured by many other policies that covered 80% of the $5.2 million structure's value. Most of these were collected without incident.[12]

On November 28, 1940, the U.S. Navy's Hydrographic Office reported that the remains of the bridge were located at geographical coordinates 47°16′00″N 122°33′00″W / 47.2666667°N 122.55°W / 47.2666667; -122.55, at a depth of 180 feet (55 meters).

[Kristin Moore]

Film of collapse

The collapse of the bridge was recorded on film by Barney Elliott, owner of a local camera shop. The film shows Leonard Coatsworth leaving the bridge after exiting his car. In 1998, The Tacoma Narrows Bridge Collapse was selected for preservation in the United States National Film Registry by the Library of Congress as being culturally, historically, or aesthetically significant. This footage is still shown to engineering, architecture, and physics students as a cautionary tale.[13] Elliot's original films of the construction and collapse of the bridge were shot on 16 mm Kodachrome film, but most copies in circulation are in black and white because newsreels of the day copied the film onto 35 mm black-and-white stock.

[Kristin Moore]

[Kristin Moore]

http://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge_(1940)

http://commons.wikimedia.org/wiki/Category:Tacoma_Narrows_Bridge

http://www.enm.bris.ac.uk/research/nonlinear/tacoma/tacoma.html

[Kristin Moore]

Commission of the Federal Works Agency

A commission formed by the Federal Works Agency studied the collapse of the bridge. It included Othmar Ammann and Theodore von Kármán. Without drawing any definitive conclusions, the commission explored three possible failure causes:

    * Aerodynamic instability by self-induced vibrations in the structure
    * Eddy formations that might be periodic in nature
    * Random effects of turbulence, that is the random fluctuations in velocity and direction of the wind.

[Kristin Moore]

Cause of the collapse

The original Tacoma Narrows 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 construction finished at the end of June (opened to traffic on July 1, 1940), it was discovered that the bridge would sway and buckle dangerously in relatively mild windy conditions that are common for the area, and worse during severe winds. This vibration was transverse, one-half of the central span rising while the other lowered. Drivers would see cars approaching from the other direction rise and fall, riding the violent energy wave through the bridge. However, at that time the mass of the bridge was considered to be sufficient to keep it structurally sound.

The failure of the bridge occurred when a never-before-seen twisting mode occurred, from winds at a mild 40 miles per hour (64 km/h). This is a so-called torsional vibration mode (which is different from the transversal or longitudinal vibration mode), whereby when the left side of the roadway went down, the right side would rise, and vice versa, with the center line 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. Two men proved this point by walking along the center line, unaffected by the flapping of the roadway rising and falling to each side. This vibration was caused by aeroelastic fluttering.

[Kristin Moore]

Fluttering is a physical phenomenon in which several degrees of freedom of a structure become coupled in an unstable oscillation driven by the wind. This movement inserts energy to the bridge during each cycle so that it neutralizes the natural damping of the structure; the composed system (bridge-fluid) therefore behaves as if it had an effective negative damping (or had positive feedback), leading to an exponentially growing response. In other words, the oscillations increase in amplitude with each cycle because the wind pumps in more energy than the flexing of the structure can dissipate, and finally drives the bridge toward failure due to excessive deflection and stress. The wind speed that causes the beginning of the fluttering phenomenon (when the effective damping becomes zero) is known as the flutter velocity. Fluttering occurs even in low-velocity winds with steady flow. Hence, bridge design must ensure that flutter velocity will be higher than the maximum mean wind speed present at the site.

Eventually, the amplitude of the motion produced by the fluttering 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 that broke in turn until almost all of the central deck fell into the water below the span.