Case study on a historical Civil Engineering failure
Introduction and Background
The 1940 Tacoma Narrows Bridge was a suspension bridge that spanned the Tacoma straits connecting the city of Tacoma with the Kitsap Peninsula. The bridge opened to traffic on July 1, 1940 and collapsed just 4 months later in to Puget Sound, an inlet of the Pacific Ocean. The Tacoma Narrows Bridge was the third longest suspension span in the world at the time, at 1810m long and 12m wide. The bridge was so narrow because relatively low volumes of traffic were to be expected because of the location of the bridge and so only 2 lanes were needed as a result. In comparison with its length, this design was very shallow, at a ratio of 1 to 72, when compared to other designs such as the Golden Gate Bridge, which was 1 to 47.
The original designer of the Tacoma Narrows Bridge was Clark Eldridge, who decided to use 25-foot deep stiffening trusses along the bridge. However federal authorities criticised the design as being too expensive and so employed Leon Moisseiff to come up with a design that was practical and cost effective. Leon Moisseiff (1872-1943) had experience on nearly every large suspension bridge in America before 1940. His fundamental change to the design was to use 8-foot plate girder supports, and as a result created a much lighter, more flexible bridge that fitted the specification of reducing the cost of the project. The design itself was supposed to have been revolutionary in its design, however it was known for its tendency to sway in a windstorm. (Bookrags (2003)) As a result it was nicknamed the ‘Galloping Gertie’ due to its evident swaying in high winds.
On the morning of November 7, 1940 the bridge had been swaying by as much as 5 feet in 40 mile an hour winds. As shown in the diagram to the right this swaying is obvious to the human eye. At around 10:30AM a cable-band at mid-span on the north cable slipped. This caused the cable to...