Tacoma Narrows Bridge

The Tacoma Narrows Bridge is a pair of suspension bridges over the Tacoma Narrows strait of Puget Sound in Pierce County, Washington, USA. They link the city of Tacoma with the Kitsap Peninsula and carry State Route 16. The name is best known for the first bridge, which the wind tore apart on November 7, 1940, a failure that changed how long-span bridges are designed worldwide.

History of the first bridge

The first bridge opened on July 1, 1940. It was designed by the New York engineer Leon Moisseiff. It was about 1,810 m (5,939 ft) long with an 853 m (2,800 ft) main span and a narrow deck of about 11.9 m (39 ft). It cost 6.4 million dollars, and the toll was 55 cents, later 50. Moisseiff assumed such a light, slender bridge would not move much in the wind, yet on windy days the deck rolled in waves up to about 1.5 m high, which earned it the nickname "Galloping Gertie".

On November 7, 1940, at around 11:00, in a wind of about 65 km/h (40 mph), the main span fell into the strait. Traffic was light. The last person on the bridge, driver Leonard Coatsworth, abandoned his car and made his way off the heaving deck; his dog Tubby, left in the car, was lost. The collapse was filmed on 16 mm Kodachrome colour film, and the footage became the documentary The Tacoma Narrows Bridge Collapse (1940).

Causes of the collapse

Textbooks often blame resonance, as if the gusts matched the bridge's natural frequency. In fact the cause was aeroelastic flutter, a self-sustaining twisting motion of the deck. The bridge had been designed for static loads, wind included, but not for the aerodynamic action of the wind. The structure was too flexible and poorly damped: the deck twisted about its long axis, and vertical motion of the cables made it worse. Once the hangers of the main span snapped, the side spans sagged and the towers leaned. The failure set off research into the aerodynamics and aeroelasticity of structures.

The 1950 and 2007 bridges: length and height

The remaining towers and side spans were taken down by May 1943. The replacement reused the old anchorages and tower foundations and opened on October 14, 1950, then the third-longest suspension bridge in the world. It is about 1,822 m long with an 853 m main span. Instead of solid girders its deck got open stiffening trusses, along with expansion joints and vibration dampers. Today it carries westbound traffic.

In 2002–2007 a second bridge was built alongside for eastbound traffic: about 1,646 m long, with an 853.4 m main span and towers 155.4 m (510 ft) high.

Good to know

The wreckage of the 1940 bridge lies on the floor of the strait and is listed on the National Register of Historic Places. The bridges are on SR 16 between Tacoma and Gig Harbor. Good views are from War Memorial Park at the Tacoma end and from Narrows Park on the west shore.