The Tacoma Narrows bridge was designed by the worlds foremost bridge designer of his time - Leon Moisseiff, and was completed in 1940 at a cost of US$6.4 million. The bridge was the third longest in the world at the time with a central span of 853m and two end spans of 335m. The selected design was chosen to save costs, the Washington Department of Highways had proposed a wind strengthened version that had a projected cost of US$11 million.
Moisseiff had implemented a theory known as "deflection theory" in his design to justify the substantial reduction in strengthening materials, believing that the dead weight of the bridge would suppress the vibrations caused by wind and use. As well, the bridge was quite narrow for its length, having only two lanes, and was only 39 feet wide.
The completed bridge was very flexible and suffered from extensive rolling down its length, to the extent that drivers could lose sight of other cars ahead of them on the bridge. This behaviour, which was nauseating to many drivers resulting in the bridge being nicknamed "Galloping Gertie". This notoriety lead to an increase in usage with twice as much traffic as originally designed for.
As a result of concern over the bridge's antics, the road authority added a variety of checking cables and devices and arranged for the University of Washington to study the bridge. Wind tunnel tests suggested a way of stabilising the bridge, however before the plan could be implemented, disaster struck.
On the 7th of November 1940 one of the checking devices failed in 40kmph winds and the bridge started twisting as well as the previous rolling. Trapped on the bridge with University of Washington engineering professor F. B. Farquharson was a logging truck and a car containing a reporter and a dog. The professor and reporter attempted to save the dog but the movement was so violent that they were forced to leave. The dog was the sole casuality of the bridges ultimate collapse.
Leonard Coatsworth, the reporter, gave this account of his experience:
"Just as I drove past the towers, the bridge began to sway violently from side to side. Before I realized it, the tilt became so violent that I lost control of the car... I jammed on the brakes and got out, only to be thrown onto my face against the curb."Around me I could hear concrete cracking. I started to get my dog Tubby, but was thrown again before I could reach the car. The car itself began to slide from side to side of the roadway.
"On hands and knees most of the time, I crawled 500 yards or more to the towers... My breath was coming in gasps; my knees were raw and bleeding, my hands bruised and swollen from gripping the concrete curb... Toward the last, I risked rising to my feet and running a few yards at a time... Safely back at the toll plaza, I saw the bridge in its final collapse and saw my car plunge into the Narrows."
The rolling motion of the bridge displaced the deck of the bridge by over 25 feet, shreding the supporting cables and distorting the support pylons. The collapse took less than thirty minutes. The professor was able to easily walk along the bridge by moving along the central yellow lines while studying the bridge's motion and was reportedly very surprised when the central span fell.
Interestingly, it was later learned that engineers had discovered the risks of long, narrow, suspension bridges over 100 years previously when 10 bridges collapsed between 1818 and 1889. While the lessons had been learnt and incorporated into subsequent designs, they had then been forgotten and were not taken into account when deflection theory was developed. Bridges designed to a more conservative standard by engineer John Roebling had overcome the poor reputation of suspension bridges and some are still in use. However his conservative approach was gradually superceded by a desire to have longer, more graceful bridges.
As well, when designing safety critical systems like bridges, testing is paramount. This means not just theoretical analysis but also actual trials and tests of the system prior to final implementation. A particular risk can be the adoption of new fads without a solid basis of experience, and software engineers like bridge builders need to be cautious when going beyond their existing experience, particularly when lives are at stake. It should be noted that more conservative engineers had repeatedly suggested a wider, less flexible design that might well have not failed.
Finally, it is clear that the cheapest option can turn out to be the most expensive, a lesson that has also been demonstrated locally with the INCIS and Terralink failures.
Holloway, C.M.
From Bridges and Rockets, Lessons for Software Systems
http://shemesh.larc.nasa.gov/people/cmh/ISSC99/cmh-issc-lessons.html
Smith, D.
A Case Study and Analysis of the Tacoma Narrows Bridge Failure
http://cee.carleton.ca/Exhibits/Tacoma_Narrows/DSmith/photos.html
Gig Harbor Peninsula Historical Society & Museum
A Tale Of Two Gerties
http://www.gigharbormuseum.org/nbonlinexhibit.html
Underwater investigation of the Tacoma Narrows bridge
http://www.nwrain.net/~newtsuit/recoveries/narrows/narrows.htm