The Tacoma Narrows Bridge was designed by Leon Moisseiff in 1938, which was the

connection between Seattle and Tacoma with the nearby Puget Sound Navy Yard. In

Moisseiff's mind, he wanted to design a bridge that is cheaper, longer and sleeker. The bridge

earned its other nickname “Galloping Gertie” soon after completion because of the oscillation

of the deck during construction. Even after completion oscillation could be felt and seen even

in modest windy day. On 7th November 1940, four months after completion day, the world

third longest suspension bridge at that time collapsed under relatively slow wind compare

with the designed withstand. Luckily no fatality was caused by this incident. The Public

Works Administration (PWA) and Federal Works Agency (FWA) assembled a group of

engineers to investigate, which it is called Carmody Board. This report will give an overview

of the failure, the reasons for the failure and the lessons learned from the event.

Description of FailureOn the 7th of November 1940 the bridge displayed twisting (shown in figure 1) that gradually increased. The first sign of failure was recorded shortly before 11:00 a.m. when a chunk of concrete dropped out of the roadway. Shortly after the bridge collapsed in the manor displayed in figure 2.

The main cables were damaged due to the severe stress, many suspender cables were broken. The bridge deck had major structural damage with the middle section laying in the bottom of the narrows, the remainder was bent and overstressed. The towers showed slight buckling with permanent distortion due to the high stress.

All these parts were beyond repair and was turned into scrap metal.

Figure 1. Moments before the failure

Figure 2. Bridge failure

Lessons learnt The major error that happened during the design was the lack of importance of wind as a factor. Previous bridges before designers believed that failures occurred due to mainly due to heavy traffic loading and poor workmanship. Suspension bridges now are designed to be aerodynamically stable and are tested in wind tunnels. Ethically if theoretical knowledge is weak, adequate experimental knowledge should be supplemented if that is also weak it should be expanded. Designs should not be proposed if they lack an adequate and complete theoretical and/or experimental knowledge base.

http://failures.wikispaces.com/Tacoma+Narrows+Collapse

http://britton.disted.camosun.bc.ca/tacoma/tacoma.html

http://www.wsdot.wa.gov/TNBhistory/Machine/machine3.htm

http://www.engineergirl.org/File.aspx?id=4261

http://www.cedengineering.com/upload/Ethical%20Issues%20Tacoma%20Narrows.pdf

http://www.enm.bris.ac.uk/anm/tacoma/tac09.gif