Success Syndrome: The Collapse of the Dee Bridge

Scaling up existing successful designs can cause latent weaknesses to become dominant, leading to catastrophe. By studying past failures where this has happened--for example, the 1847 collapse of the Robert Stephenson-designed Dee Bridge in Cheshire, England--engineers can guard against overconfidence bred of success. The Dee Bridge was designed in an environment of success and confidence, for many bridges like it had been erected over the previous decade and a half and had been performing well. What distinguished the Dee from its predecessors was that it was geometrically at the outer limits of experience with the design class, which itself had evolved some novel features that distinguished in rather subtle ways newer examples from older ones. Furthermore, no doubt as a result of then-recent successful experience, the latest design had less rigidity and a lower factor of safety than its predecessors. These elements contributed to causing a failure mode that had been latent, and thereby of insignificant consequence in the design class, finally to dominate the behavior of the Dee Bridge.