This memo is a failure analysis report on the Tacoma Narrows bridge. The bridge collapsed on November 7th, 1940 just over four months after it was opened to the public on July 1st, 1940(Green, 2006). The only casualties(good word??) from the bridge collapse were reporter Leonard Coatsworth’s car and dog. The bridge’s design and failure will be discussed, as well as new suspension bridge design methods.
2. Description of Failure
Other similar built bridges such as the Golden Gate Bridge in San Francisco can have vertical oscillations with amplitudes of up to 2 feet and horizontal oscillations up to 6 feet during severe windstorms(Levy, 1992). Due to visible oscillations in other similarly built bridges, oscillations in the Tacoma Narrows bridge were expected. However, the magnitude of the oscillations the bridge experienced and the length of time it took the bridge took to damp them out was of concern to the engineers involved. The length of time Tacoma Narrows Bridge needed to damp out its oscillations was unlike other similarly built suspension bridges, namely the Golden Gate Bridge, whose oscillations were quickly damped due to its larger width-to-span ratio. (need connecting sentence)Just before the total failure of the bridge the oscillations changed from their standard vertical and horizontal to a violent twisting motion, this twisting motion, which reached almost 45°, was what brought the bridge down. After a momentary lull in motion the twisting started up again and the bridge started to break, beginning with smaller suspender cables connecting the bridge deck and the 2 large main support cables. The breaking suspender cables meant that nothing was holding up that part of the bridge deck and it began to fall. (See the figure on page 3.) Once part of the bridge deck began to fall a 600 foot section of the bridge broke free and fell into the water(Levy, 1992). Most of the middle section of the bridge between the two towers broke off.
3. Reasons for the Failure
The specific cause of the Tacoma Narrows bridge failure was the “slippage of the north cable tie at mid-span”(Goller, 1965). The imbalance caused by the broken cable tie initiated the twisting motion, which was the ultimate reason for collapse. The bridge was part of...