Design:Suspension Longest span 853.4 m Total length 1810.2 m Opening date July 1,1940  Collapse date Nov 7 1,1940

The original Tacoma Narrows Bridge opened on July 1, 1940 and dramatically collapsed on November 7 of the same year. At the time of its construction (and destruction) it was the third longest suspension bridge in the world, behind the Golden Gate Bridge and George Washington Bridge.

Tacoma Narrows Bridge Collapse: Movie 1

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 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 for the area. This vibration was transverse, 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 that dynamically appeared and disappeared. 

Because of this behavior, a local humorist gave the bridge the nickname Galloping Gertie  from its rolling, undulating behavior. Motorists crossing the 2,800-foot center span sometimes felt as though they were traveling on a giant roller coaster, watching the cars ahead disappear completely for a few moments as if they had been dropped into the trough of a large wave.

However, the mass of the bridge was considered sufficient to keep it structurally sound.

Cause of collapse: 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 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. 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.

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 a 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 stresses. The wind speed that causes the beginning of the fluttering phenomenon (when the effective damping becomes zero) is known as the flutter velocity. 

Tacoma Narrows Bridge Collapse: Movie 2

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.

Effect of collapse on Bridge Engineering: 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, even though its real cause of failure was aeroelastic flutter. 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.

End Note: No human life was lost in this accident. Tubby, a black male dog, was the only fatality of the Tacoma Narrows Bridge disaster. Leonard Coatsworth, a Tacoma News Tribune editor, was driving with the dog over the bridge when the bridge started to vibrate violently. Coatsworth was forced to flee his car, leaving Tubby behind. Professor Farquharson and a news photographer 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. Coatsworth had been driving Tubby back to his daughter, who owned the dog.

The replacement of this bridge was opened in the same location in 1950, and a second, parallel bridge opened in 2007