User:Wsterrit

The Tacoma Narrows Bridge was originally built in 1940 and collapsed within four months of its completion. Its design pushed the limits of engineering knowledge in order to create a cheaper, longer bridge. Two similar bridges were built around the same time and were retrofitted in response to this disaster: the Deer Isle Bridge and the Bronx-Whitestone Bridge. Notable structural artist who are connected to this project are John Roebling (whose company supplied the cables) and Othmar Ammann (who investigated the bridge's collapse).

Structural description
The Tacoma Narrows Bridge (1940) spanned the Puget Sound connecting Tacoma to Gig Harbor in the state of Washington. The bridge had a main span of 2800 ft and a sag ratio of 1:12 (cable sag divided by main span length). It was the third longest suspension bridge in the world at the time of its completion behind the Golden Gate Bridge and the George Washington Bridge. Washington State bridge engineer Clark Eldridge suggested a traditional design with an estimated cost of $11 million. The nationally prominent New York bridge engineer Leon Moisseiff approached the Public Works Administration with plans to build the bridge for $6.4 million. The cost reduction was a result of a narrower and lighter bridge. The most significant design change was replacing the 25 ft deep trusses with 8 ft feet deep girders.

Qualifications as Structural Art
Moisseiff’s design is a failure of structural engineering and therefore cannot be defined as successful structural art. The bridge still carries significance as a learning point for engineers everywhere of the possible consequences of pushing the understanding of bridge dynamics into unknown territories.

Scientific
As in all suspension bridges, loads in the direction of gravity were carried from the main deck through suspender cables in tension to the four main cables. These main cables then carry the load in tension to the nearest tower which transmits the load to the ground in tension. Wind loads were also supported by the suspender cables, main cables, and towers as described by Josef Melan. The bridge’s shallow girders (eight feet deep), lack of a stiffening truss, and relatively narrow width with respect to its length gave the bridge a very slim look. This elegant approach was very expressive of how loads were carried by the cables instead of the deck. The bridge failed due to the deck's lack of stiffening members as the deflections due to wind loading eventually caused a main cable to snap. The loss of a main cable caused the displacements to increase dramatically and a section of the main deck broke loose, falling into the water below.

Social
With the nation looking at its defensive strategies in preparation for WW II, a bridge to connect McChord Air Base south of Tacoma with the Puget Sound Navy Shipyard in Bremerton was needed. This BLANK made the bridge’s construction a priority, but the effects of the Great Depression kept cost as a primary concern. Therefore when Moisseiff suggested a design that was $4.6 million cheaper than Eldridge’s design, WSDOT chose the former. For a short time surrounding communities experienced economic benefits of bridging the sound. The bridge created a reasonable route between Mount Rainier National Park and Olympic National Park which led to more opportunities for locals and tourists. Linking the Olympic Peninsula to nearby Tacoma and Seattle encouraged timber companies and farmers to increase their business in the area bringing in more economic activity. Fortunately there was no loss of human life during the bridge's failure and the city had taken out an insurance policy on the bridge worth 85% of its value. The original piers were also used for the bridge's replacement ten years later.

Symbolic
The Tacoma Narrows Bridge of 1940 is remembered as a monumental failure of structural engineering. Its oscillations during and after construction earned it the nickname of "Galloping Gertie". These oscillations and subsequent collapse spurred research on the effects of wind on suspension bridges and resulted in much of what we know about this type of bridge. As a result of its failure to support wind loading, wind tunnel testing for aerodynamic effects is now required for any federally funded suspension bridge project.