Tacoma Narrows Bridge
Coordinates: 47 ° 15 '59 " N , 122 ° 32' 59" W.
| Tacoma Narrows Bridges | ||
|---|---|---|
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| use | Washington State Route 16 | |
| Crossing of | Tacoma Narrows ( Puget Sound ) | |
| place | Washington , USA | |
| construction | two parallel suspension bridges | |
| overall length | 1645.92 m (new bridge) | |
| Number of openings | each 3 | |
| Longest span | 853.44 m (new bridge) | |
| height | 155.45 m (new bridge) | |
| completion | 1950 and 2007 | |
| toll | toll only in the east direction (new bridge) | |
| location | ||
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The Tacoma Narrows Bridge (English Tacoma Narrows Bridge ) is a suspension bridge in the US state of Washington .
The collapse of the first Tacoma Narrows Bridge on November 7, 1940 was the reason to consider the dynamics in the construction of bridges and to test models in the wind tunnel before building larger bridges .
location
It leads the freeway-like State Road 16 from Tacoma over the Tacoma Narrows, a branch of the Puget Sound , to Gig Harbor and thus opens up the Kitsap Peninsula, which is only connected to the mainland in the south-west, with the capital Bremerton from the south-east. The bridge consists of two parallel structures, each of which accommodates traffic in one direction of travel. Before the second bridge was put into operation in July 2007, all traffic flowed over the structure, which was completed in 1950.
The 1940 bridge
description
The first Tacoma Narrows Bridge was also built as a suspension bridge at the same location in 1938–1940. It became famous for its spectacular collapse after only four months of operation.
The bridge designed by Leon S. Moisseiff had only two lanes and one narrow walkway each; but with a main opening of 853 m at the time of its completion it had the third largest span of all suspension bridges worldwide (after the Golden Gate Bridge and the George Washington Bridge ).
Its two pylons , standing in 37 m deep water, caused the greatest problems during construction. They had to be built with caissons down to a depth of 68 m for one pylon and 54 m for the other - at that time one of the deepest foundations ever. The pylon foundation consumed 40% of the bridge's total budget.
Moisseiff, who first applied the deflection theory founded by Joseph Melan to suspension bridges and then developed it further, envisaged an extremely low and slim construction of the roadway girder as a solid steel wall girder . Fritz Leonhardt is said to have initiated this development with the planning of the Rodenkirchen Bridge in Cologne, but in the USA it was also in line with the trend towards ever lighter bridges made possible by the deflection theory. Before that, suspension bridges were always built with significantly higher and stiffer trusses . The new concept was still in place before construction began on the Rodenkirchener Brücke u. a. was taken over and continued by Othmar Ammann for the Bronx-Whitestone Bridge in New York City , on which Moisseiff had also worked. Moisseiff took this route even further to narrow and light bridges at the Tacoma Narrows Bridge. However, his design gave no cause for doubt. At wind speeds of 96 km / h, it provided for the bridge deck to move to the side by 2.8 m, which would have been barely noticeable given the span of 853 m. At 161 km / h, the deck would have dodged 6.1 m. These values satisfied everyone involved, but they were based on the calculation of static wind loads , aerodynamic effects were not known in the structural calculations at the time.
Bethlehem Steel quickly erected the pylons, which were not very rigid in the longitudinal direction of the bridge. Even John A. Roebling's Sons , who had just significantly improved their air-jet spinning process for the Golden Gate Bridge , did not need long to manufacture the 44 cm thick suspension cable. Bethlehem Steel assembled the bridge decks from solid wall girders in just six weeks. The construction time of only 19 months for the entire bridge was a record - and the bridge had cost less than 6.6 million US dollars.
Even before completion, workers had pointed out the movements of the bridge. The client had commissioned Frederick Burt Farquharson from the University of Washington with investigations, which although causing vibrations on a model of the bridge, could not yet explain them.
The bridge opened on July 1, 1940.
Soon afterwards it was nicknamed "Galloping Gertie" because of its swinging up and down and became a tourist magnet. Some drivers came especially to “ride a roller coaster”. Others preferred to take the detour via Olympia in the southwest, which the bridge was supposed to save. At the end of July 1940, a camera was installed on the roof of the toll booth, which registered waves in the bridge deck up to an amplitude of 60 cm at 25 vibrations per minute. However, the bridge behaved completely differently at different wind speeds.
Comparison of the first suspension bridges with solid wall girders
| bridge | Span l | Structure height h | Structure width b | Slimness h / l | Slimness b / l |
|---|---|---|---|---|---|
| Tacoma Narrows Bridge | 853 m | 2.4 m | 11.9 m | 1: 350 | 1:72 |
| Bronx-Whitestone Bridge | 701 m | 3.4 m | 22.6 m | 1: 209 | 1:31 |
| Rodenkirchen Bridge | 378 m | 3.3 m | 22.6 m | 1: 114 | 1:17 |
The table shows that the Tacoma Narrows Bridge was significantly longer than the other two bridges, but had the flattest and narrowest girder and is therefore at the top of both indices for slenderness.
This slimness led to a very low weight, but also to a very low rigidity. Together with the aerodynamically unfavorable shape of the girder, this made the bridge very sensitive to wind. Even in light winds, a Kármán vortex street formed behind the girder , the vortices of which separated with about the same frequency as the natural frequency of the vertically oscillating bridge. Although these processes were theoretically described at the time, but hardly known with regard to their effects on the road surface, they were not the cause of their collapse.
Collapse of the bridge in 1940
On November 7, 1940, the bridge collapsed due to strong torsional vibration . This oscillation was caused by aerodynamic flutter (a self-exciting oscillation ) caused by strong winds.
The exact sequence was as follows: The wind came from the southwest, across the bridge. This caused the bridge to experience torsional vibrations (independent of vertical vibrations) . Due to its changing position in the wind, the twisting road girder was able to extract more and more energy to amplify the oscillation through flow resistance and dynamic buoyancy , completely independent of the frequency of a Kármán vortex street, which would now have been a factor of five above that of the torsional oscillation . After three quarters of an hour, the ropes broke at a wind speed of 68 km / h (wind force 8) and the roadway fell into the Tacoma Narrows with an abandoned car and a dog. The event was filmed by engineers from the University of Washington, who had systematically observed the bridge for a long time due to the vibrations. As a result of these investigations, it was planned to equip the deck girder with sheet steel wind deflectors for the following days.
People were not killed in the accident, as the bridge was closed to public traffic some time before it collapsed. The car on the bridge seen in the film before the collapse belonged to one of the experts involved in monitoring the bridge, who, as can also be seen, leaves the bridge shortly after two other people on foot.
After the collapse
At first, one puzzled about the causes, since the aerodynamic effects on bridges were not understood at all at the time. It took many years, many wind tunnel tests and calculations, until the dynamic effects of wind on bridge structures and the effects of aeroelastic flutter were reasonably understood.
The immediate effects were that Ammann's slender Bronx-Whitestone Bridge was subsequently stiffened with trusses to calm the ( toll-paying ) motorists, although it had significantly better key figures than the Tacoma-Narrows Bridge. As a counter-reaction to the deflection theory, which favored slender carriageway girders, the new construction of the Tacoma Narrows Bridge, completed in 1950, and above all by David B. Steinmans in 1957, the Mackinac Bridge was provided with high and already visually solid-looking lattice girders. Othmar Ammann also used high and stiff trusses for the Throgs Neck Bridge (1961); with the Verrazzano-Narrows Bridge (1964) the problem did not arise because of the two-story construction.
The remains of the crashed carriageway are still in place under water today; they were placed under monument protection in 1992 . The owner of the car that fell into the water with the Cocker Spaniel was compensated by the state bridge administration. He received $ 450 for the car and another $ 364.40 for the contents of the car, including the dog. The Tacoma Narrows Bridge was rebuilt with new pylons on the foundations of the old bridge and with conventional trusses. Ten years after the collapse, the new bridge reopened on October 14, 1950.
The construction with trusses should last a long time worldwide. Although solid wall (with wind-deflecting sheet metal in front of it) or box girders ( principally very torsionally stiff ; wind deflection with a trapezoidal cross-section) were also available early on, the Severn Bridge in England was not completed until 1966 as the first large suspension bridge with box girder.
The film of the collapse is often used as an illustration of the aerodynamic and vibration-related processes. It is the only known film of a suspension bridge collapse. As a culturally and historically significant film document, this film, which is available in many other adaptations, for example for the contemporary American newsreel, was included in the US National Film Registry in 1998 .
The most significant consequence of the catastrophe was that since then, in addition to the statics , the dynamics have also been taken into account in the construction of bridges. In addition, a model of the bridge is tested in the wind tunnel before larger bridges are built .
Web links
- Washington State Department of Transportation sites to the Tacoma Narrows Bridge
- Pictures of the collapse
- Background information and footage
Individual evidence
- ↑ a b c Tacoma Narrows Bridge. In: Structurae
- ^ Tacoma Narrows Bridge Toll Rates. Washington State Department of Transportation, accessed August 12, 2013 .
- ↑ a b c d e f Richard Scott: In the wake of Tacoma, suspension bridges and the quest for aerodynamic stability . ASCE Press, Reston, Va. 2001, ISBN 0-7844-0542-5 , p. 41 f.
- ↑ A clue is that on the day of the catastrophe the oscillation frequency was 1 Hz, but the typical frequency of the oscillation ( Strouhal number ) generated by separating eddy roads was 0.2 Hz. K. Yusuf Billah, Robert H. Scanlan : Resonance, Tacoma Narrows bridge failure, and undergraduate physics textbooks . In: American Journal of Physics . tape 59 , no. 2 , 1991, p. 118–124 , doi : 10.1119 / 1.16590 ( PDF file [accessed October 3, 2009]).
- ^ Bernard J. Feldman: What to Say About the Tacoma Narrows Bridge to Your Introductory Class . In: The Physics Teacher . tape 31 , February 2003, p. 92–96 , doi : 10.1119 / 1.1542045 ( PDF file [accessed August 12, 2013]).
- ↑ The lattice girders attached to the Bonx-Whitestone Bridge were only able to dampen vibrations a little during storms; it was not until the aerodynamically shaped cladding that was attached in 2004 that the bridge itself withstood Hurricane Sandy without major vibrations.
- ^ Tacoma Narrows Bridge: Weird Facts . $ 814.40 in 1940 equals $ 14,549 in 2018 monetary value .